COMPOSITION OF ICE CREAM MIXES
Ice cream is composed of a mixture of food materials, such as milk products, sweetening materials, stabilizers, emulsifiers, flavours, or egg products which are referred to as ingredients. The effect of these ingredients upon the finished product is influenced by the constituents of these ingredients. An ice cream mix is the unfrozen blend of the ice cream ingredients, and consists of all the ingredients of ice cream with the exception of air and flavouring materials. The composition of ice cream is usually expressed as percentage of its constituents, i.e., percentage of milk fat, milk solids not fat, sugar, egg yolk solids, stabilizer, emulsifier and total solids.
Composition standards are influenced by demands of customers, volume of operation, quality of ingredients and ingredient costs.
Although the methods of processing and filling influence the characteristics of the mix and the finished product, the effect of constituents supplied by the ingredients is also important. Therefore, the role of each constituent (fat, MSNF, sweeteners, egg solids, stabilizers, emulsifiers, total solids, salts, optional ingredients, flavour and colours) is important in contributing to the characteristics of the ice cream.
THE ROLE OF THE CONSTITUENTS
Milk fat is of major importance in ice cream. It contributes a rich flavour to ice cream, is a good carrier for added flavour compounds and promotes desirable tactual qualities. It is essential to use the correct percentage of milk fat to balance the mix properly as also to satisfy legal standards). It tends to retard the rate of whipping without lowering freezing point. High fat content limits consumption, will increase the cost and increase calorific value. (Generally, the fat content of a good average ice cream is considered to be 12%. The best source of milk fat is fresh cream. Other sources are frozen cream, plastic cream, butter, butter oil and condensed milk blends.)
Milk Solid Not Fat (MSNF)
MSNF is the solids of skim milk. It includes the proteins, the milk sugar lactose and the mineral matter. MSNF is approximately 36.7% protein, 55.5% lactose and 7.8% minerals. It contributes to body and texture, and storage properties. It is inexpensive, high in food value, and adds very little to flavour, except indirectly by improving the body and texture. Lactose supplements the sweet taste largely produced by added sugars. The minerals impart a slightly salty taste which rounds out the flavour of the finished ice cream. The proteins in MSNF make the ice cream more compact and smooth and tend to prevent a weak body and coarse texture. However, excessive amounts of MSNF result in a salty or cooked flavour and soggy or sandy body and texture defect. A sandy body and texture defect is caused by high concentration of lactose. Although statistically it is impossible to state the limit of MSNF that may be used in ice cream mix, the following thumb rule is adopted to calculate the maximum serum solids content. Subtract from 100 the sum of the percentage of all the solids of the mix, except MSNF and divide by a factor of 6.4-7.4 depending on whether the turnover is expected to be rapid or slow respectively.
Sweeteners are added to ice cream to enhance palatability as also to improve handling properties. The most common sweetener used in ice cream is cane sugar (sucrose). The sugar is used in liquid or dry form. Of all the sweeteners available, sucrose imparts the most desirable properties. However, many good sugar blends are available. Blends of sucrose with low medium or high conversion corn solids are also used to gain solids and maintain product properties and sweetness. The proportion of sweetening agent to be used alongwith sucrose will depend on :
- the total solids content of the mix;
- the desired concentration of sugar in the mix;
- the effect on the properties of the mix, such as viscosity, freezing point and whipping ability, and
- the concentration and relative sweetening power of the sweetening agent itself.
Generally, 14-16% sugar seems most desirable. Sugars increase the viscosity and the total solids concentration of the mix. This improves the body and texture characteristics provides the total solids content does not exceed 40%, or the sugar content does not exceed 16%. Beyond these limits the ice cream tends to become soggy and sticky. The sugars, being in solution, depress the freezing point of the mix. This results in slower freezing and requires a lower temperature for proper hardening.
Egg Yolk Solids
Egg yolk solids are high in food value but greatly increase the cost of ice cream. They impart a delicate flavour, but even slight off flavours in egg products are easily noticeable in ice cream.
They increase the viscosity, have almost no effect on the freezing point and improve considerably the body and texture characteristics. Egg yolk solids improve whipping ability, and this is attributed to lecithin which exists as a lecithin-protein complex. They can be advantageously used in mixes where the fat is obtained from butter, butter oil or in mixes having low total solids concentration.
Stabilizers are used to prevent the formation of objectionable large ice crystals in ice cream. They have high water holding capacity which is effective is giving smooth body and texture to the finished product. Besides, they prevent ice crystal formation in storage, give uniformity of product, give desired resistance to melting and improve handling properties. They increase viscosity, have no effect on the freezing point. The amount of stabilizers to use varies with its properties, with the solids content of the mix, with the type of processing equipment, and other factors.
Generally, stabilizers are added at the rate of 0.2 to 0.3% of the mix. Stabilizers commonly used are sodium alginate, CMC (Sodium carboxyl methyl cellulose), guar gum, locust bean gum, carrageenan, gelatin and pectin. It is not necessary to age the mix when alginates are used. CMC produces a chewy characteristic in the finished product. Gelatin produces a thin mix and requires an ageing period. Pectin is used alone or in combination with gums as a sherbet or ice stabilizer. Addition of excessive amounts of stabilizers results in soggy or heavy body and high resistance to melting.
Emulsifiers are used to produce ice cream with a smoother body and texture, to impart dryness and to improve whipping ability of the mix. Emulsifiers extensively used are monoglycerides or diglycerides, sorbitan esters and polyoxyethylene sorbitan esters (polysorbates) and are added at the rate of 0.1 to 0.4% of the finished product. Egg yolk solids are also used as emulsifiers. Excessive amounts of emulsifiers result in ice cream having slow melting characteristics and body and texture defects.
Total solids replace water in the mix and, thereby, increase the viscosity, and improve the body and texture of ice cream. Addition of sweet cream buttermilk solids, dextrin and eggs significantly improves the body and texture of ice cream. However, the total solids content should not be too high; when it is above 40-42% a heavy, soggy product is obtained.
Water and Air
Ice cream is a physico-chemical system having a gas (air) dispersed in a liquid (water), a solid or a mixture of liquid and solid. Thus, a partly frozen emulsion with ice crystals and solidified fat globules embedded in unfrozen water phase constitute a continuous phase. The source of water in the ice cream mix is mainly from fluid dairy products or added from the water supply.
The amount of air in ice cream is important because it influences quality, profits and legal standards. In order to maintain quality, it is important to have a uniform amount of air.
Flavour is generally considered the most important characteristic of ice cream. The kind of flavouring material to be added is influenced by the quality of the ice cream mix since slight off-flavour in it can obscure the delicate flavour of the flavouring material to be added. Local preference of the consumers will determine the type and intensity of flavour to be added.Natural and synthetic flavour substances are available for the flavouring of ice cream.
IMPORTANCE OF FLAVOUR
Flavour is generally considered the most important characteristic of ice cream. It is easily confused with taste, which includes the "feel sensation" of body and texture as well as the true flavour. The flavour of ice cream is the result of blending the flavours of all the ingredients. Some of which may not be sufficiently pronounced to be recognizable, although each contributes to the final effect. This makes it difficult to predict the effect of a certain ingredient upon the flavour of the ice cream.
Flavour has two important characteristics : type and intensity. Flavours that are delicate and mild are easily blended and tend not to become tiresome even when very intense, while harsh flavours soon become tiresome even in low concentrations. As a general rule, therefore, delicate flavours are preferable to harsh ones; but in any case the flavour should be only intense enough to be easily recognized and delicately pleasing to the taste.
Ice cream should have a delicate, attractive colour which can be readily associated with the flavour. Most colours are of chemical origin. Colours are available in liquid or powder form. Dry colours are more economical since these can be dissolved in boiling water as needed.
Several special ingredients are used for their desirable effects in the preparation of the mix and on the finished products. Ordinary salt is sometimes used in ice cream. This is usually unnecessary, except in certain flavours such as custards and nut ice creams. Some believe that a small amount of salt (-[0.1%) improves the flavour of ice cream. Perhaps this is a carryover from earlier times when ice cream formulations contained a lower percentage of NMS and thus less natural milk salts. In any case, a salty flavour should be avoided.
The caseinate derivatives, especially sodium caseinate, are effective in increasing the whipping rate and overrun; however, they may so stabilize the emulsion that insufficient fat is destabilized during freezing. In this case the product may lack dryness and stiffness.
The mineral salts, including citrates and phosphates, the calcium and magnesium salts, affect mix and finished-product qualities. Mineral salts are usually used in limited amounts, and affect handling properties and appearance of the product. The citrates and phosphates are good casein solvents and increase the hydration of the casein. They also impart stability to the mix during heating and processing.
Calcium salts may decrease the stability of the protein, but they also give the mix and finished product a creamy, rich appearance. Calcium sulfate in small quantities affects the dryness and stiffness of the frozen product as it is drawn from the freezer.
Specially prepared low-lactose milk solids are available for increasing the total milk solids (TMS) without causing an excessive lactose content. These products contain the milk protein and mineral salts and impart their nutritive value and properties to the product.
THE BALANCED MIX
A balance mix is one in which the proportion of the ingredients will produce a satisfactory finished product - a frozen dessert in which the defects, if any, cannot be further corrected by any change in the composition or ingredients of the mix.
Defects such as rancid flavour, feed flavour, or uneven color cannot be corrected by changing the concentration of the constituents. Therefore, they do not indicate a poorly balanced mix. However, other defects, such as (1) lack of flavour-insufficient concentration of flavouring , (2) lack of richness-insufficient concentration of fat, (3) sandiness-too high concentration of lactose, or (4) weak body-low total solids or low stabilizer, may be corrected by changing the composition of the mix. These defects do, therefore, indicate that the mix is incorrectly balanced.
Conditions That Limit the Balancing of a Mix
Balancing is done to give desirable results under certain limited conditions of processing and handling the mix or of handling the finished ice cream. For example, a mix may be properly balanced for a finished ice cream that is to have a rapid turnover, but the components might cause sandiness if the ice cream were to be stored for an extended time. Another mix may be properly balanced for freezing in a batch freezer but not in a continuous freezer. A mix may be thrown out of balance by changing the source of the constituents. For example, if the fat in the mix is obtained from butter, the mix may need added egg yolk solids to improve its whipping ability and to give it the proper balance, but if the mix is made with sweet cream, the egg yolk solids would not be necessary. A knowledge of the role of each constituent together with its advantages and limitations is necessary in selecting a desired composition and in properly balancing a mix. Usually an ice cream mix that is properly balanced for average commercial conditions will have between 36-42% TS and between 20-26% TMS (obtained by adding the percentage of fat to the percentage of NMS). This does not apply to a mix for ice cream with lowered fat content, a sherbet, or ice.
The ice cream mix represents a complex colloidal system, of which many of the properties have not been fully investigated. In the mix, some of the substances occur in true solution (the sugars, including lactose, and the salts), others are colloidally suspended (casein, stabilizers, insoluble sweetener solids, and some of the calcium and magnesium phosphates), and the fat globules are in coarse dispersion. Although the whey proteins are dissolved, they have little effect on the freezing point.
The substances in true solution are small molecules or ions and have a strong affinity for water. The substances in colloidal suspension typically have particles with an opposite electrical charge to that of the solvent, and the mutual attraction keeps them together in suspension. The like electric charges on the particles keep them apart, which helps to maintain the suspension, as do collisions between particles in suspension with those of the solvent.
Occasionally, the substances in suspension may not have sufficient attraction for the solvent and there may not be sufficient viscosity to keep them suspended. Different substances also have differing affinities for water. Some particles, the hydrophobic colloids, have so little affinity for water that if there is no charge on the particle precipitation occurs. On the other hand, substances with high degrees of affinity for water, the hydrophilic colloids, may remain in suspension even with no electrical charge. Substances in coarse disperson or suspension do not stay uniformly dispersed, but settle or rise depending on their density relative to the suspending medium.
Ice cream mixes consists of (1) an aqueous phase (58-68% water) in which carbohydrates, whey proteins, and minerals are dissolved, (2) casein and the substances associated with it are colloidally suspended, and (3) milkfat is emulsified. This complex emulsion can withstand the stress of freezing mechanical agitation, and concentration. Aeration is remarkable considering the inherent instability of the fat globules, casein micelles, and lactose under these conditions.
Mix properties of practical importance include stability, density, acidity, surface tension, interfacial tension, viscosity, absorption, freezing point , and whipping rate.
Mix stability refers to the resistance to separation of the milk proteins in colloidal suspension and the milkfat in emulsion. Instability results in separation of (1) protein particles as coagulated or precipitated material (2) whey from melted mix , or (3) syrup in the mix upon aging.
Homogenization, mix acidity, dehydrating salts, ratio of fat to TMS, heat-treatment, freezing, ageing time, and the extent to which the water in the mix is bound all affect mix stability.
FLAVOURS AND COLOURS-MATERIALS AND PREPARATION
Frozen desserts are valued mainly for other pleasing flavour and their cooling and refreshing effects. The many kinds of flavouring material and the many brands and market categories under which they are sold make it useful to understand their sources and to select and buy with great care. Among the important flavouring substances for frozen desserts are vanilla, chocolate and cocoa, fruits and fruit extracts, nuts, spices, and sweeteners.
Flavour is considered to have two important characteristics: type and intensity. Generally, the delicate flavours are easily blended and tend not to be objectionable at high concentrations. Harsh flavours tend to be objectionable, even at low concentrations. In any case flavours should be only intense enough to be easily recognized and to present a delicate, pleasing taste.
Choosing a mix composition and the ingredients in often less of a problem for the manufacturer than is standardization of flavouring material for several reasons :
- *The many flavouring materials available make it difficult to make a proper choice.
- " The supply of flavours may vary from time to time in quality and availability.
- " Serving conditions affect how pronounced a flavour will be perceived.
- " No two consumers have exactly the same sense of taste, so choice of flavor varies widely.
Because flavour is so important in influencing consumer acceptance, it is easy to lose sales when a product is poorly flavoured. Defects associated with flavour additions include too much or too little flavouring, unnatural or a typical flavouring, and lack or excess of sweetness. Flavourings also can affect appearance: lack or excess of particles, particles too large or too small, uneven distribution of particles or variegate, ribbon that is too thick or thin, and wrong ingredient or colour.
Federal Standards of Identity divide flavouring materials and the labels of frozen desserts are divided into three categories:
- Pure extracts and flavours
- Pure extracts and flavours that dominate over synthetic components(s)
- Artificial flavour that predominates over natural flavour components(s)
Economy brands of ice cream commonly contain predominantly artificial flavouring as well as lower solids content than the average trade brand. The latter commonly contains pure and artificial flavour with pure flavouring predominating. Premium and superpremium products contain only pure extracts and flavours to complement the relatively high content of dairy solids and the very high qualities of all of the ingredients. The usage rate of flavorings is usually high in premium and super premium products.
Establishing favorable product acceptance by consumers is vital to increasing product sales. The use of regular flavour evaluation clinics and consumer taste panels are good ways to help achieve this objective.
FLAVOURS FOR FROZEN DESSERTS
An abundant variety of flavouring substances has been provided by nature, and flavour chemists and flavorists have attempted to duplicate and expand this variety. Natural flavouring substances are sometimes limited in supply, but the chemically produced types have been made available in almost unlimited quantities and at relatively low cost.
Flavourings of naturally and chemically produced origins are available mainly in mixtures for the proper flavouring of foods. Natural flavourings useful in frozen desserts derive from citrus and noncitrus fruits, tropical fruit, sugar-free fruit, natural flavourings from botanicals, spices, cocoa and chocolate, coffee, natural flavourings from vanilla beans, and nuts. The Code of Federal Regulations, paragraph 101.22 of Title 21, defines "natural flavour" or "natural flavouring" as the essential oil, oleoresin, essence or extractive, protein hydrolysate, distillate, or any product or roasting, heating, or enzymolysis that contains the flavouring constituents derived from a series of materials, which it then lists. The synthetic flavourings include aromatic chemicals and artificial flavours, Liquor flavourings include alcohol whiskey and distilled beverages, fruit brandy distillate, brandy flavour essence, and fruit liquors.
The delicate mild flavours imparted by natural flavouring materials usually produce a pleasing flavour even at high concentrations, whereas the overuse of imitation flavourings usually results in a harsh, objectionable flavour.
In addition to the great importance of flavouring in determining consumer preferences for ice creams, there are several other factors. Important among them are : mix composition, processing methods, serving temperature, and color hue and intensity.
Milk solids and sweeteners contribute substantially to flavour. In those frozen desserts containing fat, churning that takes place during freezing causes fat to concentrate around the air cells where it strengthens the air cells and imparts a sense of smoothness to the consumer's mouth. Nonfat milk solids (NMS) contribute cooked and slightly salty flavours to ice cream because of the release of sulfhydryl groups during pasteurization and the natural milk salts contained in these solids, respectively.
Sweetening at the sucrose equivalencies of 15% for plain ice creams and 17-18% for bulky flavours of ice cream usually results in optimal acceptance.
The most popular of all ice cream flavours is vanilla. This delicate flavour makes up more than one-fourth of the total volume of ice creams sold in the United States. Of course, not all of those products are flavoured with vanilla extract. Some contain a mixture of pure vanilla and synthetically produced vanillin while others contain artificial vanilla.
Natural vanilla powders are made by mixing finely ground vanilla beans with sugar, or by incorporating the vanilla extractives with a dry carrier, evaporating the solvent, and drying. The amount used would correspond by weight to the number of ounces of a standard strength extract.
Vanilla paste is made by mixing the concentrated extractives with a dry carrier to form a paste. The amount used is the same as for powders.
Although vanillin is the primary flavourful substance of vanilla, the taste threshold for it is must higher than for several other components of vanilla. For example, guaiacol concentration is about two-thousands that of vanillin, but the taste intensity is one thousand times higher than that of vanillin. Similarly, the taste threshold for anisaldehyde is three thousand times lower than that of vanillin.
Imitation Vanilla Flavourings
Artificial vanilla flavourings have less than half of their flavouring derived from vanilla beans; some may contain no natural bean extractives. These preparations may contain water, ethanol, propylene glycol, vanillin, ethyl vanillin, propenyl guaethol, anisyl aldehyde, and heliotropine (piperonal).
Consistency in Vanilla Quality
Vanilla beans vary in their composition, and the methods of fermentation, curing, extraction, and blending introduce more variation. Therefore, it is quite important that the beans meet specifications for colour, flavour, and moisture, and that extraction be done at the lowest practical temperature in a closed system for the minimal exposure time. Chill proofing and clarification by centrifugation or filtration are important for removing sediment and microbial spores. Ageing for several weeks permits formation of esters from the acids and alcohols in the extract.
Vanilla Ice Cream
The plain ice cream mix is processed, cooled, aged at least 4 hr, then flavoured by thoroughly blending the flavouring just prior to freezing. The amount of flavouring used depends on the composition of both the flavouring and the mix. The quantity of vanilla must be increased as the concentration of milkfat is lowered and as the content of nonfat milk solids is increased. When the concentration of sweetener is low, vanilla must be increased in quantity, but at high intensities of sweetness variations in quantity of vanilla make comparatively small differences in consumer preference.
CHOCOLATE AND COCOA
Chocolate and cocoa are among the most popular flavours of frozen desserts. They are obtained from the cocao bean, the fruit of the Theobroma cacao tree, which grows in tropical regions about 10 degrees north and south of the equator. The major producing countries are the Ivory Coast, Brazil, Malaysia, Ghana, Indonesia, Nigeria, Cameroon, Ecuador, the Dominican Republic, and Mexico.
There are two main types of cocoa beans, criollo and forastero. Criollos are lightly coloured and mildly nutty in flavour. Forastero cocoas are dark brown, strongly flavoured, slightly bitter, and comparatively high in fat content. Foresteros dominate the market, especially in their subtype, amelonado. The almond-sized cacao beans or seeds develop in a pulpy pod with 30-40 beans to the pod. The ripened pods, rich golden red in color, are cut from the trees, gathered in piles, and left to ripen further for about 48 hours, after which they are slashed open. The beans are removed and placed in bags, boxes, or vats to heat and ferment for about 10 days or until the characteristic flavour and cinnamon-red colour develop. Pulp surrounding the beans is first degraded by yeasts and bacteria that convert sugars into alcohol and carbon dioxide. Subsequently, anaerobic bacteria convert the remaining sugars into alcohol and lactic acid. The Acetobacter convert ethanol to acetic acid. By this time the beans have lost their viability and are susceptible to several types of enzymatic degradation. Flavour precursors and pigments are formed and oxidation and condensation of polyphenolic compounds occurs. The beans are then washed clean of the dried pulp, dried slowly and sufficiently to prevent mold growth, and then sorted and graded prior to shipment to manufacturers of chocolate and cocoa. Bean quality is affected by genetics, environment, exposure. To enhance the flavour of some cocoa products some manufacturers add aromatic substances such as cinnamon, oil of cloves, oil of bitter almond, or vanillin. Small quantities of these substances can impart desirable flavour notes to cocoa and chocolate. Among the desirable flavour notes of cocoa products are cocoa (the basic flavour note), bitter, rich, bouquet, sour, astringent, and acrid. Undesirable flavour notes include burnt, earthing/moldy, hammy, smoky, metallic, rancid, cardboard, and raw.
Chocolate Ice Cream
Chocolate products used in flavouring ice cream are cocoa, chocolate liquor, blends of cocoa and chocolate liquor, chocolate syrups, and extracts from cocoa nibs. Chocolate extracts are made with alcohol and water so they contain little fat. However, it is permissible to substitute vegetable fat for milkfat in cocoa products provided the words vegetable fat appear in the names of the products.
The amount of chocolate flavouring to use in ice cream depends upon the desired strength of flavour and intensity of colour. Consumer preference tests used to determine there parameters. Extra sweetener should be added to compensate for the bitter flavour of cocoa, the usual recommendation being the same weight of sucrose, or equivalent thereof, as of cocoa.
The simplest way of adding cocoa powder to a mix is to blend it together with other dry ingredients, especially granulated sugar, and incorporate them together with the liquid ingredients in the blending device or with a powder funnel.
As with other mixes, chocolate-flavoured ice creams that are low in fat require higher homogenization pressures than those containing high fat. In general, homogenization pressures should be 3.45 Mpa higher for chocolate mixes than for plain mixes.
Chocolate ice cream is one of the most difficult to freeze, because the mix whips comparatively slowly. The viscosity may be reduced and whipping time decreased by adding 0.1% of citrates or phosphates to the mix. Air in the ice cream dilutes the colour so that in ice cream with high overrun it may be necessary to use dark cocoa to provide the intensity of colour desired. However, very dark cocoa powders often do not impart the pleasing flavour to ice cream that less highly alkalinized powders provide.
Chocolate is used in ice cream in many forms. Included among these are chocolate chips, swirl or variegate, chocolate-covered nuts, chocolate-coated baked goods and fruits, and miniature low-melt filled chocolates. Because of their low melting point several of these products need to be refrigerated until the time they are placed in the ingredient feeder. Furthermore, the quantity added to the hopper of the feeder should be managed so that the pieces do not melt and stick together.
FRUITS IN FROZEN DESSERTS
The ice cream industry is a major market for fruits. Today's manufacturers of fruits and flavourings have taken advantage of new technologies to provide aseptically processed fruits that keep for months at room temperature with little change in quality. These fruits are processed in swept-surface heat exchangers that take the temperature of the fruit/sugar/acid/stabilizer mixture to 88-121Â°C. After heating, the mixture is held in a holding tube for about 3 min. then is cooled in a series of swept-surface heat exchangers (fig 5.1) to about 27Â°C. Movement from the cooling cylinders is direct to the filling machines where the product is metered into sterile containers inside a sterile filling chamber that is bathed in sterile air.
In a second system (Fig.5.2) fruit is pumped through heating, holding, and cooling coils that create secondary flow effects to evenly heat and cool the fruit. The integrity of fruit particles reportedly is better maintained than in swept-surface heat exchangers, because no scrapers are used to renew surface films. The containers are usually multi-layered bags made of polyethylene and foil. Bags are placed in cardboard or rigid plastic containers. Large refillable totes are used where there is a high demand for fruit. In small operations containers of unused opened fruit must be refrigerated and can usually be held for a few weeks with no evidence of spoilage. Because the pH of fruits prepared aseptically must be 4.5 or below, the microorganisms most likely to grow are molds and yeasts.
Aseptic processing provides several advantages in the use of most fruits. Quality is usually much improved over that of kettle-type (Fig.5.3) heat processing. Kettles, being open to the atmosphere, permit heating to only about 100Â°C. This usually takes a minimum of 20 min, and about 20 more min are required at this temperature to destroy molds, yeasts, and acid-tolerant bacteria. Heat transfer is relatively inefficient in kettles, and volatiles are able to escape to the atmosphere causing a loss in natural flavour characteristics. Colour usually darkens. Shelf life is often short, and refrigeration or preservatives may be needed to prevent spoilage. Scrape surface heat exchangers provide more uniform heating to fruits than do kettles. Furthermore, cooling is much faster than in kettle processing. Overall, quality, convenience, and economy are maximized by aseptic processing of fruits.
Fig.5.2. System for aseptically processing fruits by heating and cooling in tubular heat exchangers.
Frozen fruits are quite suitable for use in frozen desserts. However, the cost of holding them frozen can be significant; furthermore, they must be thawed slowly in refrigerated space to maintain their quality. Once thawed, frozen fruits should be used within a few hours. Heating enhances the flavour of some fruits, whereas it degrades the quality and appeal of others. Those fruits whose flavours may be enhanced by heating include cherries and pineapple. Heating can lower the quality of strawberries, peaches, and , to a lesser extent, raspberries. Aseptically processed fruits suffer comparatively small changes in flavour and texture compared to kettle processed fruits.
Fig. 5.3. System for conventional kettle type processing of fruits.
Fruit flavours are available as (1) extracts from the prepared fruit, (2) artificial compounds, and (3) true extracts fortified artificially. These flavours supplement fruits in cases in which it is necessary to limit the amount of fruit, but they are often inferior in flavour to fruits and do not provide the desired fruit pulp. Defects in fruit-flavoured frozen desserts may result from improperly handled fruits, use of insufficient fruit, poor incorporation of fruit, excessive fortified or artificial flavour, and/or poor quality base mix.
In formulating a mix for fruit ice creams, total solids should be made higher than for vanilla ice creams of the same relative quality. This is necessary to offset the effoct of dilution by the fruit pack.
Because fruit ice creams have a higher sugar content than plain ice creams they should be drawn from the freezer about 1Â°F colder. A drawing temperature of 23Â°F for the batch freezer and of 20Â°F for the continuous freezer is generally satisfactory.
Fresh or fresh-frozen fruit is often the best source of flavour. Fresh fruit ice creams have a special sales appeal.
The amount of fruit required to impart the desired flavour depends on the characteristic of the flavour and varies from 10-25% of the weight of the finished product. In any case, the minimum content is 3% by weight of clean, mature, should fruits or their juice. It is also desirable to have pieces of fruit or pulp large enough for easy recognition in the finished product. Suggested level are given in Table 5.1.
When fruits are high in price, less fruit may be used and enough fruit extract added to impart approximately the same flavour intensity as would be provided by the natural fruit. The usual high price of raspberries often dictates the use of raspberry extract to enhance flavour. Another approach is to add fruit concentrates and essences. Popular ones are peach, blueberry, apple, grape, red raspberry, and strawberry. Supplemental use of 3.5-10% fruit equivalent of concentrates improves flavour. Adjustment of the acidity and sugar content of mixes may contribute to improved flavor when concentrates and essences are used. For example, when the base mix contained 20% fruit pack, the most favourable pH, sugar concentration, and supplementation rate in experimental trials for selected fruits were found to be as follows, respectively :
Blueberry ice cream - pH 5.7, 15% sugar and 5% blueberry juice concentrate
Peach ice cream - pH 5.7, 16% sugar and 7.5% peach juice concentrate
Cherry ice cream - pH 5.2, 15% sugar and 15% cherry juice concentrate
Apple ice cream - pH 6.2, 15% sugar and 20% apple juice concentrate
Candied and Glaced Fruits
Candied or glaced fruits such as cherries, pineapple, and citron and such candied fruit peels as orange, lemon, and grapefruit are used chiefly in rich ice creams, puddings, aufaits, and mousses. They make excellent decorative materials on fancy molded ice creams, sherbets, and ices.
Dried fruits, especially apricots, figs, raisins, and prunes make tasty ice creams They are continuously available, are shelf stable, and can be obtained in places where other types of fruit are expensive or unavailable. Dates, figs, and raisins have long been used in frozen puddings.
PROCEDURES AND RECIPES
Strawberry Ice Cream
Variety is the primary determinant of flavour imparted by strawberries. Maturity and promptness of processing are secondary factors. It was found that the amount of fruit used affects the body, texture, flavour, and appearance of strawberry ice cream. In making the ice cream using 6-20% fruit, they noted progressively better flavour upto 15% fruit. Higher amounts increased the flavour only slightly while decreasing the desirability of body and texture.
In adding strawberries for continuous freezing, the juice should be drained from the berries and added to the mix prior to freezing. The berries, which should be chilled to -1 to 0Â°C, should be added with an ingredient feeder. If iciness of the berries in ice cream is a problem, the percentage sugar in the pack can be increased, thus lowering the freezing point. The consequences of adding excess amounts of sugar to fruit are increases in the amount of juice exuding from the berries and the necessity to reduce the sugar content of the mix to avoid excess sweetness. 3:1 berry: sugar ratio most favourable. For the fruit to have the same consistency as the ice cream at the serving temperature, the fruit should contain at least 21% sugar, and such is provided by a 3:1 to 4:1 ratio.
Raspberry Ice Cream
As with strawberries, variety is a major determinant of suitability of raspberries for flavouring frozen desserts. Purveyors of fruits select fruit from desirable cultivators and demand harvest at the peak of flavour.
12-15% of raspberry puree from which 75% of the seeds had been removed, gave a superior product in flavour, but the texture was coarse due to seeds. After reducing the puree content to 10% and adding raspberry extract, an equally fine flavour was observed and the texture was improved significantly. A current recommendation of a major fruit supply house is to add 10% of their aseptically processed raspberry puree.
Peach Ice cream
Mack and Fellers reported that a good flavour of peach ice cream was obtained with the cultivators, Hiley, J.H. Hale, Elberta, Champion, and Crawford. They suggested using 15-20% peaches in a 3:1 pack. Furthermore, yellow-fleshed peaches gave a greater concentration of f lavour than white, and larger shreds of the peach were evident in the ice cream when puree was made from yellow peaches. No flavour was sufficient without the addition of flavouring extract along with the fruit. A current recommendation from industry call for adding 7% nectarine puree and 13.6% peach cubes along with 0.095% peach flavour.
Cherry Ice Cream
Sour cultivars of cherries were found superior to sweet cherries for ice cream.
The addition of a small amount of cherry extract, oil of bitter almonds, or benzaldehyde can enhance cherry flavour. Additives should be well mixed with the fruit before adding to the ice cream.
Ice Cream with Complex Flavours
As technologies have developed, it has become possible to produce many multi-flavoured frozen desserts. One favourite known for many years is butter pecan. Such products are commonly made by adding the background flavouring to the mix then adding the nuts or fruits to the frozen product with an ingredient.
English Caramels and Pecans : Vanilla ice cream with caramel cube pieces and chocolate pecans.
Raspberry Moussecake : Plain ice cream mix flavoured with raspberry and mousse-mix bases to which a mixture of brownie fudge pieces and chocolate-coated chocolate cookies is added.
Banana Split : Contains hot-packed banana fruit base with almond crunch candy pieces and chocolate fudge variegate.
New York Style Strawberry Cheesecake : Cheesecake background complemented with strawberry variegate.
Triple Play Chocolate : A combination of chocolate background flavour with chocolate fudge chips and chocolate variegate.
Apple Appeal : A fresh, juicy, natural apple flavour that combines ideally with cinnamon, rum raisin, or cream.
Blueberry Bonanza : A true-to-nature blueberry flavour that goes great with cheesecake, banana, and cream notes.
Citrus Cream : A natural tangerine emulsion and three-fold vanilla extract.
With the marketing of sugar-free soft drinks and the emphasis among nutritionists on reduction of caloric intake has come increasing demand for sugar free frozen desserts. Suppliers continue to make available increasing numbers and varieties of sugar-free fruits and chocolate products. A sample of ingredients sweetened with aspartame follows: blueberry chunks, raspberry and strawberry revels and purees, chocolate flakes and chunks, chocolate revel, and chocolate-coated peanuts and almonds.
Nutmeats and nut extracts are used extensively in frozen desserts. Among the most popular are pecans, walnuts (English and black), almonds, pistachios, filberts, and peanuts. Nutmeats should be sound, clean, free from rancid flavour, low in count of microorganisms and free of pathogenic bacteria. Methods of eliminating microorganisms other than careful hygienic control during processing include application of dry heat, dipping in a boiling, slightly salty sugar solution for a few seconds or treating with ethylene oxide. The latter is highly effective, but the gas is toxic and must be used under carefully controlled conditions. To prevent sogginess, nuts treated in boiling water should be dried for 3-4 min at 121-149Â°C.
Nuts should be stored in a cool dry place until used. Almonds, filberts, and pistachios should be blanched to remove their skin prior to use. Specifications of permitted pieces of shell should be checked carefully by manufacturers in purchasing prepared nutmeats.
Use concentrations of nuts range from 1-5% depending on the nut and the accompanying flavour(s). The following are flavours and recommended amounts of nuts (calculated as percentage of the unflavoured mix): banana nut, 2.2%; caramel praline; 5%; chocolate caramel nut, 1.7%; maple nut, 2.2%; mud nut; 1.7%, butter pecan, 3.3%' pecan pie, 2.2%, and black walnut, 2.8%.
SPICES AND SALT
Spices such as cinnamon, cloves, nutmeg, allspice, and ginger are used sparingly as favourite in frozen desserts. Ginger ice cream is a favourite in some localities. Cinnamon Nutmeg, and cloves are often used to enhance or modify the flavour of chocolate products, and they complement puddings, eggnog, and certain flavours of punch.
Spices may be purchased in either the finely ground dry form or as extracts. Their flavours are strong so that only small amounts are needed to produce the desired effect.
Salt, although not a spice, is often used in small quantities to enhance certain flavours of ice cream, especially those containing eggs-custards and rich puddings- and in nut ice creams. The recent tendencies of Americans to reduce their intake of sodium coupled with the requirement to indicate sodium on the nutrition label has caused many manufacturers to minimize the amount of salt added to frozen desserts.
COLOUR IN FROZEN DESSERTS
Ice cream should have a delicate, attractive colour that readily suggests to the consumer what the flavour is. Both "certified" and "exempt from certification" (natural) colours are approved for use in ice cream.
Most flavours of ice cream require the addition of at least a small amount of colour. Enough yellow colour is usually added to vanilla ice cream to give it the golden shade of cream produced by the "coloured" breeds of cattle. Fruit ice creams need to be coloured, because the usual amounts of fruit added are insufficient to impart adequate colour. Chocolate ice cream, on the contrary, seldom needs added colour.
Certified colours are most often used in colouring ice cream. Annatto colour is about the only excempt colour used in ice cream. However, annatto tends toward the pink rather than the desired "egg-shade" yellow, especially in mixes that have a high free calcium level.
If colours are purchased in the powder form and made up by the ice cream processor., they should be dissolved in boiling water and stored refrigerated for short times. Longer times of holding are possible when 0.1% sodium benzoate is added to limit the growth of microorganisms. Although solution of colours contain little nutrient material, it is possible for some microorganisms to grow in them. Solutions of colours of the strength is normally used.
FLAVOURING LOWFAT AND NONFAT ICE CREAM
Milkfat alters the maximum intensity of flovours and modifies the timing and sales of dust and diminution of a flavour experience. Flavours soluble in fat, such as vanella, are carried with the fat to the tongue of the consumer. There as fat melts, such flavours are seleased to the oijactory senses. If there is insufficient fat to carry these flavours, they are perceived quickly and tend to disappear selatively quickly from the flavour profile. In addition, fat provides vichness of flavour and lubricity of mouthfeel.
Fat replacers used in nonfat and lowfat ice creams usually consists of modified whey proteins or starch hydrolysales. Both tend to kind and to mask delicate flavours. As little as 1% milkfat can reduce the vapour pressure of flavourable substances.
Some flavours are quite compatible with fat replacers. Among them are butter-scotch, butter pecan, and cheesecake. These "heavier" flavours tend to cover flavours contributed by the fat replacers while providing flavour notes that blend well with those of the typical nonfat product.
THE FREEZING PROCESS
Freezing the mix is one of the most important operations in making ice cream, for upon it depend the quality, palatbility, and yield of the finished product. Freezing consists of two parts: (1) the mix is frozen quickly while being agitated to incorporte air and to limit the size to ice crystals formed, and (2) the partially frozen product is hardened without agitation in a special low- temperature environment designed to remove heat rapidly.
The general prcedure of the freezing process is easily learned since it involves only accurate measurement of the ingredients, movement of the mix into the freezer, operation of the freezer, and removal of frozen product form the freezer. However, mastering the details of freezer operation to produce a uniformly high- quality product requires considerable practice. If all operations are done manually, the several variables are not easily controlled. It is unusual that two persons will execute the details of freezing in exactly the same manner. Therefore, different people obtain different ice creams even when they use the same ingredients, formulas, and equipment. This is a major reason why programmed freezers are becoming widely used.
In the making of ice cream we are concerned with the freezing of the mix. The freezing produces not only ice crystals, but also other crystals under certain conditions. It is essential, therefore, to know some general facts about freezing and crystalization.
The Freezing Point of Solutions
The freezing point of pure water is 0Â°C. If we dissolve anything inthe water, the solution formed will require a lower temperature to freeze, the more concentrated the solutions is, the more the freezing point will be depressed.
If we compare the effect of various substances on the freezing point of water, we find that their depressing effect varies. Solutions of the same concertration do not freeze at the same temperature. There is a very definite relationship that holds for substances that do not dissociate or associate, this relationship is expressed by the formula.
D = K G/M
in which D is the depression of the freezing point in degrees centigrade, K is a constant characteristic for each solvent, G represent the weight of the dissolved substance in 1000 g of the solvent, and M is the molecular wweight of the dissolved substancee.For water the value of K is 1.86. Therefore, the formula in cases where water is the solvent, is
D = 1.86 G/M
Since the fraction G/M represents the numer of moles of the dissolved substance in 1000 g of the solvent, it follows that the freezing point depression is not dependent upon the nature of the dissolved substance, but is dependent upon the number of moles of the dissolved substances. According to the laws of chemistry, mole of various substance( the molecular weight in grams= 1mole) have equal number of molecules.Thus, solutions that contain the same number of molecules in a given amount of solvent, will have the same freezing point.
The above does not hold true for electrolytes because the electrolytes dissociate, and on dissociation, each molecule yields two or more ions. For example,
NaCI on dissociating (NaCI ] Na + CI) yields two ions;
NaSo4 on dissociation (Na2SO4 ] 2Na+ + SO4) yields three ions.
The effect of electrolytes on the freezing point is greater than expected on the basis of the above rule. In the case of common salt, NaCI, the effect approaches twice than expected: the more dilute the solution, the more nearly it approaches this limit, because the dissociation becomes more complete as the dilution is increased. Ions affect the freezing point in the same manner as molecules, therefore, we must restate the above rule as follows.
Solutions, that contain the same number of particles (ions and molecules) in a given amount of the solvent, will have the same freezing point. They will also have the same boiling point, and the same osmotic pressure.
The Freezing Point Of Ice Cream Mixes
The freezing point of ice cream mixes varies appreciably with the composition. The mix constituents, which affect the freezing point directly, are sugar, milk sugar (loctose), milk salts, and any other substance that is added and that is in true solution. The mix constituents which afffect the freezing point indirectly , are the fat content and the protein content or any other constituent not in true solution. When these constituents are increased, there is less water in the mix, and the resulting higher water concentration of the truly soluble substance causes the freezing point to be lowered. Leighton (1927) developed a method for the calculation of the freezing point of ice cream which has been found to be reasonably accurate. It is based on the percentage of lactose in MSNF, the percentage of sucrose and milk salts. Leighton assumes that 54.5 per cent of the serum solids is lactose , and that the mixtures of sucrose and lactose follow the freezing point curve of sucrose.
The methods of calculating the freezing point of an ice cream mix makes use of the sum of the freezing point depression values for the sugars and the calculated values for the salts. For the sugars, lactose and sucrose, the following equation is utilized to determine the parts of disaccharide sugar (expressed as sucrose equivalent) to 100 parts of water in the mix.
(MSNF Ã- 0.545)+ SÃ-100/W = parts of sucrose equivalent /100 parts of water.
Percentage of milk solids not fat is the amount of lactose in the MSNF.
S is the percentage of sucrose in the mix, W is the percentage of water in the mix. The lowering of the freezing point due to the sugars (sucrose equivalent) is obtained by interpolation from data tables.
The freezing point depressing due to milk salts is calculated from the equation
MSNF Ã- 2.37/W = Freezing point depression due to milk salts. 2.37 is a constant based on the apparent molecular weight of the salts.
The lowering of freezing point due to the sucrose equivalent plus the lowering caused by the milk salts gives the freezing point of the mix.
Example: The freezing point of a mix having 12% fat, 11% MSNF, 15% SUGAR, And 0.3% stabilizer can be calculated as follows:
- [(11 Ã- 0.545) + 15] Ã- 100/61.7 = 34.03 parts of sucrose per 100 parts of water will lower the freezing point by 2.09ÂºC
- 11 Ã- 2.37/61.7 = 0.42ÂºC is the lowering of freezing point due to salts.
Total lowering = 2.09 + 0.42ÂºC
The freezing point is = 2.51ÂºC
The above calculation involves assumptions, and therefore, may not give accurate results, if we deviate from these assumptions. For instance, if the mix is made from diary products that have soured slightly, the lowering effect due to the serum solids will be greater than the above, this is also true for neutralization.
Beside the substance discussed above, glucose (corn sugar), invert sugar, honey, fruits and extracts should also be considered. A given weight of glucose depresses the freezing point almost twice as much as the same weight of sucrose. This can be predicted on the basis of the fact that solutions containing the same number of particles in a given amount of solvent have the same freezing point. Since the molecular weight of glucose is 180.1, and that sucrose is 342.17, it follows that when we take equal weight of the two, we will have 342.17/180 or 1.9 times as many glucose molecules as we have of sucrose molecules. Therefore, the freezing point depression due to a given amount of glucose should be approximately 1.9 times as great as caused by the same weight of surcrose. Thus, when glucose is used as a sweetening agent, the lowering effect due to glucose may be calculated as follows:
Percent glucose Ã- 100/Percent water in the mix = Parts of glucose per 100 parts of water
The effects of the solids in invert sugar and honey are the same as that of glucose. Invert sugar, the hydrolysis product of sucrose, consists of equal parts of glucose and fructose. Honey solids contain mainly invert sugar. Corn syrup (liquid or dried) contains dextrins, maltose and dextrose, in proportions that vary according to the stage to which hydrolysis of corn starch was carried. The average molecular weight of this mixture of the hydrdysed products is some what higher than the molecular weight of sucrose. Therefore, corn syrup solids depress the freezing point less than sucrose.
Fruits contain sugar- fructose, glucose and sucrose, fruit acids and some salts. Besides, fruit preparations contain large amounts of added sugar. The sugar that is used in the fruit preparations is glucose (corn sugar) or sucrose(cane or beet sugar). In the preparation of fruit ice cream, the addition of fruit preparations made with glucose cause appreciable lowering of the freezing point. When sucrose is used in the fruit preparations, a similar low freezing point is found on account of the fact the fruit acids cause extensive hydrolsis of sucrose into invert sugar, espsecially in preparations made by heating. Where this low freezing point is troublesome, fruit preparations made with glucose or preparations in which sucrose has been extensively hydrolysed, should be avoided.
Before mixes are frozen they should be tested to determine whether the composition meets the specifications of the formula. Recommended methods of testing are found in Standard Methods for the Examination of Diary Products. They are the ether extraction method for fat and the vacuum of forced draft oven methods for total solids.
Cold, flavoured ice cream mix is pumped into the continuous freezer barrel that is under pressure and is chilled with a liquid refrigerant. The mix is whipped with a dasher. Onto it are attached sharp scraper blades that contact the very smooth surface of the freezing cylinder. Thier function is to scrape minute ice crystals form the cylinder wall. Removal of these crystals immediately upon their formation ensure that the product will have a smooth texture and that an ice layer will not build on the cylinderwall. Dull scraper blades increase the load on the drive motor, lengthen the time to freeze the mix, and produce a coarse texture.
Ice does not conduct heat as fast as does steel. Therefore, if ice is permitted to form a layer inside the cylinder, it acts as an insulator, slowing release of heat form the mix tothe refrigerant. Furthermore,buildup of ice increase the distance heat must be transferred before it enters the liquid refrigerant and transforms the refrigerant into the gaseous state. The rate of heat transfer is a function of the difference in temperture, the thermal conductivity of the heat transfer surface, and the rate of renewal surface films.
The temperature of the mix drops quite rapidly in the freezer cylinder. Removal of sufficient sensible heat (heat that thermometers measure) to start the mixture of freeze should take less than 1-2 min. Meanwhile viscosity decreases as rapid agitation disrupts gel structure and breaks clusters of fat gloules. Rapid agitation also causes air to be incorporated. The air cells that form differ remarkably in size depending on whether the product is being made in a batch or continuous type freezer. The batch type freezer operates at atmospheric pressure, so air that is incorporated exists at the same pressure both inside and outside the freezer. However, the freezing cylinder of the continous freezer is held under pressures up to about 100 psi (7kg/cm2, 690kpa). Pressures of about 75 psi (5kg/cm2, 520kpa) produce about 100% overrun with the normal mix, and the air cells in the freezer make up 15-20% of the volume of the mix. They will constitute 50% of the product volume when the pressures equilibrate to atmospheric pressure (14.7 psi, 1.03 kg/cm2, 1 atmosphere, 101.4 KPa) outside the freezer.
When the freezing point is reached inside the freezer, liquid water begins to change to ice crystals. These crystals are practically pure water, and removal of this solvent from the mix causes dissloved materials to become more concentrated. Consequently, the freezing point of the unfrozen mix is lowered. Crystallization occurs at the expense of removal of latent heat of fusion (heat not measurable with a thermometor)as well as sensible heat. When pure water is being frozen, the temperature will not change noticeably as ice crystals are forming. However, in freezing ice cream the freezing point is continually being lowered by crystallization of water, so the temperature continues to drop but at a slow rate since both sensible heat and latent heat are being removed. As concentration of dissolved substances increase, the freezing point finally drops to a temperature at which no more ice is formed. Therefore, never is all of the water in ice cream frozen, even at tempeprature of hardening. Depending on the drawing temperature, 33 -67 %of the water is crystallized in the freezer, and the hardening process then may account for freezing of an additional 23-57%.
Fig. 7.1 Flow diagram of ice cream manufacture
1. Storage and metering powdered constituents 2. storage and metering of liquid constituents 3. mixing vessel and preheater 4. pasteurizer 5. homogenizer 6. cooler 7. holding tank (at 2Â°C) 8. freezer 9. gear pump 10. filler 11. filling (at -4Â°C) into cooled moulds 12. insertion of sticks 13. mould release by slightly meeting the outer in a warm bath (at 25Â°C) 14. bath for chocolate enrobing (at 25Â°C) 15. packaging machine 16. deep freezing for final hardening (at 40Â°C) 17. fruits, flavours, colour and aroma substances.
Another factor that can slow the drop in temperature in ice cream being frozen is the crystallization of substances dissolved in the aqueous phase. Not only do these substances lower the freezing point & below that of water, but they have unique solubilities and cryohydric points. The latter is the temperature at which a dissolved substance separates out of solution.On seperation the crystals release heat, causing a slight rise in temperature unless more heat is removed than is released. Each of the dissolved substances of ice cream has its own cryohydric point. Representative ones include: lactose -0.28Â°C -4.1Â°C depending on theisomer, glucose at -8.5Â°C to -14.5Â°C, disodium phasphate at -0.9Â°C, potassium chloride at -11.1Â°C and calcium chloride at -55Â°C. The amount of water involved in the change of state increases as the temperature rises. A 1Â°C Change in the hardening room temperature from -20Â°C to -19Â°C involves only about one-fifth as much water as a 1Â°C change in dipping cabinet temperature from -14Â°C to -13Â°C. The latter, in turn involves only about one-fifth as much change in state of water as is involved in a changes at freezer temperature from -6Â°C to -5Â°C. These facts are vital to the understanding of the effects that fluctuating temperatures can have on the quality of frozen desserts.
CHANGES THAT TAKE PLACE DURING THE FREEZING PROCESS
The function of the freezing process is to freeze a portion of the water of the mix, and to incorporate air into the mix. This involves lowering of the temperature of the mix from ageing temperature to the freezing point, freezing a portion of the water of the mix, incorporating air into the mix and cooling the ice cream from the temperature it is drawn from the freezer to the hardening room temperature. When the mix is put in to the freezer its temperature drops very rapidly, while the sensible heat is being removed and before any ice crystals are formed. This process should take less than1-2 min. At the same time, the rapid agitation reduces the viscosity by partly destroying the gel-like structure and by breaking up the fat globule clusters. The gel-like structure may partially reform during the hardening process in the hardening room. Besides, the rapid agitation incorporates air quickly in to the mix.
Fig. 7.5 Explanation of the theory of how destabilization of fat by churning causes coalesence of fat globules during freezing of ice cream
Inside the freezing cylinder the liquid mix,with its suspended fat globules and colloidal proteins, carbohydrates, and salts, is transformed into a highly viscous foam.Ice crystallize from the continuous phase, transforming it into a thick syrup. Air cells form, and hydrophilic colliods adsorb to their surfaces, stabililizing them. Fat globules become increasingly crystalline, and some of them coalesce, forming structure that support the foam, (Fig. 7.5). As the product exits the freezer, it has about one-half of its water frozen and has expanded up to about 100% in volume. The continuous phase is a thick syrup while the disperse phase consists of air cells, ice crystals, fat globules, casein micelles, and other hydrocolloids, This makes ice cream a three-phase system gaseous, solid, and liquid. The product of phase one is soft frozen and flowable, but the degree of stiff ness varies with formula, process, freezer design, overrun,and temperature. Churning of the fat and formation of minute air cells result in a dry- appearing and relatively stiff product as it exits the freezer. Therefore, transport and packaging of the product are affected by each these variables.
Choices of ingredients can remarkably affect the physical properties of the finished frozen dessert.
Table 7.1 : Approximate Percentage of Water Frozen in the Ice Cream Mix at Various Temperatures
(Â°C) (%) (Â°C) (%)
|Temp.||Water frozen||Temp.||Water frozen
REFRIGERATION NEEDED TO FREEZE ICE CREAM
The actual heat extracted in freezing ice cream is dependent upon the composition of the mix and the temperature to which it is frozen. The extraction of heat takes places in three different steps.
- the extraction of the sensible heat of the liquid:
- the extraction of the latent heat of the water frozen; and
- extraction of the sensible heat of the semi-frozen slush. The exact calculation of the amount of heat extracted is difficult to estimate, but a reasonably close value can be obtained.
The objective in freezing ice cream is to produce the maximum number of ice crystals by dropping the temperature of the mix well below its freezing point. The higher the freezing point of the mix the larger the number of ice crystals that will form in the freezer at a given temperature and the smoother the texture of the ice cream. Tiny crystal nuclei that form there can only grow in size after ice cream exits the barrel, because no more nucleation occurs after agitation and scraping cease. Furthermore, a low freezing point (melting point) results in a relatively high amount of melting of ice as the temperature rises. When the temperature is again lowered, the crystals that form are larger than the ones that melted, giving rise to coarse texture.
Although the amount of heat to be removed from a frozen dessert mix is a function of several variables, mix composition is the major one.
Mixes vary in composition, so they vary in freezing point. Furthermore, composition affects thermal capacity or specific heat in calories' per gram that a mix must release for the temperature to be lowered by 1Â°C.These variables, i.e. freezing point and specific heat,are determinants of the amount of ice that will be formed with any given amount of heat energy removed
The freezing point of an ice cream mix is lowered by lactose, sugars salts, and other substances in true solution. Fat and protein have no direct effect on freezing point because fat is immiscible with the aqueous, phase and protiens are very large molecules. However, as these substances are increased in concentration, there is less water in which solutes can dissolve, so the freezing point will be depressed. Freezing point depression is the difference between 0Â°C and the temperature at which the mix first begins to freeze.
There is a formula by which the freezing point of a simple mix of known composition can be estimated. The formula asumes that lactose and sucrose depress freezing point equally and glucose1.9 times as much. Lactose is assumed to constitute 54.5% of NMS and 76.5% of whey solids. The salt effect is assumed to be 4.26 tunes: NMS. The important compositional data, therefore, are % sugar % NMS and % water. This simplified formula assumes negligible effect of fat, proteins, stabilizers, and emulsifiers except as they displace water. Application of this formula by reference to the freezing points of solution of sucrose (table 7.2).
Table 7.2 : Freezing Points of Solutions of Sucrose
1A calorie is the amount of heat required to raise the temperature of 1g of water 1Â°C at 15Â°C (kcal = heat required to raise the temperature of 1kg of water 1Â°C.
Example : Calculate the approximate temperature at which 50% of the water is frozen in mix containing 12% fat, 11% sucrose, and 0.3% stabilizer.
- Find the % of water unfrozen in the ice cream when 50% of the water is frozen (water in mix -solids in mix)/2
% unfrozen water = 100 - (12 + 11 + 15 + 0.3)/2 = 61.7/2 = 30.85
- calculate the sucrose equivalents supplied by sugar and NMS:
sucrose equiv = (%NMS Ã- 0.545) + 15 = 21.00
- Calculate the % sugars in the unfrozen water:
% sugar in unfrozen water = %sugar Ã- 100/%sugar + unfrozen water
= 21.00 Ã- 100/21.00 + 30.85 = 40.50
- Interpolation from table 10.2 indicates the freezing point of 40.50 % solution of sucrose is -4.52Â°C.
- Calculate the freezing point depression by milk salts.
- Calculate the total point depression as contributed by sugars plus salts.
4.52Â°C + 1.52Â° = 6.04Â°C
Therefore, the temperature of the ice cream is -6.04Â°C when 50% of the water in it is frozen.
By adaptation, this process can be used to calculate temperatures when other percentage of water are frozen or when other sweenteners are used. For example, if one knows the dextrose equivalent of corn sweeteners, a close approximination of the effect of the corn sweetener on th freezing point depression is dextrose equivalent times the freezing point depression of glucose (1.9 times that of sucrose).
Thus, corn syrup solids with a dextrose equivalent (DE) of 50 would depress the freezing point times as much as sucrose. The latter number is called the sucrose equivalence factor and would be used in the equation of 2. above. For example, assume the sweetener in the above - stated problem is 12% sucrose and 7% corn syrup soilds testing 52 DE. Then :
One -half the water content = 29.85%
Sucrose equivalents = (11 Ã- 0.545) + 12 + (7 Ã- 0.52 Ã- 1.9) = 24.91
Now, the temperature at which 50% of the water in the new mix is frozen is determined by first calculating the % sugars in the unfrozen water:
24.91 Ã- 100/24.91 + 29.85 = 45.49ÂºC
and this is translated in to total freezing point depression:
- 5.75ÂºC + 1.52ÂºC = -7.27ÂºC
Thus, Changing the sweetener Increased the total soluble solids, decreased the free water, and dropped the temperature at which one-half of the water is frozen by 7.27ÂºC - 6.04ÂºC=1.23ÂºC.
TYPES OF FREEZERS
The Freezing process is divided into two parts,
- the mix, with an appropriate amount of colour and flavour, generally added at the freezer, is quickly frozen while being agitated to incorporate air in such a way as to produce and control formation of ice crystals to give smoothness in body and texture, satisfactory overrun and palatability in the finished ice cream ,and
- when ice cream is partially frozen to a certain consistency, it is drawn from the freezer into package and quickly transfer to hardening tunnels or cold storage where the freezing and hardening process is completed without agitation.
Freezing is characterised by short production runs, necessitated by frequent changes in flavours and container size. Freezer may be classified as follows.
- Batch Freezers
- (a) Salts and ice type (obsolete)
- (b) Brine freezers (obsolete)
- (c) Direct expansion (ammonia of freon refrigerant)
- (i) Vertical -used mainly in some counter freezers
- (ii) Horizontal - mostly replaced by continuous freezer
- (iii) Single -tube freezers
- (iv) Triple -tube freezers
- (v) Four -tube freezer.
- Continuous freezers : Horizontal, direct expansion; used by commercial plants.
Soft serve freezers :
Batch and automatic continuous freezers of the direct expansion type.
Freezers for frozen desserts are designed to perform specific tasks under a variety of conditions and at varying costs. The soft-serve freezer must continue to deliver frozen product intermittently over several hours of operation. Batch freezers are designed to freeze a quantity of mix for delivery in a short time period. Continuos freezers receive mix continuosly from positive displacement pumps and discharge the partially frozen product continuosly.
Freezing times are affected by mechanical and physical factors and the properties of th mix. The mechanical and physical factors are
- type and construction of freezer
- condition of cylinder walls and blades
- speed of dasher
- temperature of refrigerant
- velocity of refrigerant as it passes around freezing chamber
- quantity of oil deposited on the outside of freezing cylinder
- overrun desired
- temperature at which the ice cream is drawn
- rate of unloading freezer (batch type).
The mix characteristics that affect the freezing time are
- freezing point
- methods of processing
- kind and amount of flavouring materials.
The energy required to operate a freezer varies with mix formula, probably as it affects viscosity.
The Continuous Freezer
The continuous freezer process was first patented in 1913 but did not become widely used until the 1930s. The process consists of continuously feeding a metered amount of mix and air in to one end of the freezing chamber. As the mix passes through this chamber, it is agitated and partially frozen then discharged in a continuous stream at the other end of the chamber. This product is dispensed into packages that are placed in a hardening unit to complete the freezing process.
Capacities of continuous freezers range from about 100 to ]3,800 L/hr per freezer barrel. Some freezers have two or three freezer cylinders mounted on a single frame and operated by the same controller. Rating of capacities are generally based on nominal conditions such as:
- Machine is in new or excellent condition
- Refrigerant is clean, free of oil and noncondensable gases
- Full fat ice cream mix is used with approximately 38 % TS
- Temperature of mix entering the freezer is 4.4Â°C, and it is drawn at -5.6Â°C
- Evaporating temperature of the ammonia refrigerant (saturated conditions) is -30.6Â°C or ammonia back pressure at the evaporator is 2 psi
- Rating is stated in terms of L/hr at 100% overrun.
Because the rating is done under optimal conditions, the user cannot be assured that operations can be maintained at that capacity. Furthermore, characteristics of mixes are important determinants of freezer capacities.
The first continuous freezers had a positive type pump metering a constant supply of mix to a second pump that displaced two to three times the volume of the first pump. An air inlet valve was positioned upstream of the second pump so a desired amount of air could be admitted to the cylinder for whipping in to the ice cream. A hold-back valve located at the distal end of the cylinder was adjusted to keep pressure on the cylinder. Instead of depending on air drawn in by vaccum, today's freezers are supplied compressed air through a regulator or a mass flow meter.
Approaches to controlling overrun, stiffness, and drawing temperature of frozen product vary among freezer manufactures and among models with in manufactures. One method is to control the speed of the mix and air pumps with electronic mass flow meters. The controller adjusts the ratio of the quantities of air and mix. A variable frequency drive, employing a frequency inverter, adjusts the speed of the mix pump motor. Product stiffness is determined by monitoring wattage required to operate the dasher shows such a freezer. Variable speed drives on some ice cream freezer pumps are controlled mechanically or hydraulically.
A major variable affecting the stiffness of ice cream exiting a freezer is temperature. However, using temperature to measure stiffness is subject to error since mix composition and overrun are also determinants of stiffness. Back pressure on the freezing cylinder and heat transfer efficiency must be optimal for control of freezing. Back pressure can be monitored and controlled by a programmable controller (PC) that signals for changes in pressure on a product outlet valve.
Another freezer design has both a mix pump and an ice cream pump. The semi-frozen ice cream discharges from the front of the freezing cylinder through the product pump, the speed of which is controlled to keep a constant pressure inside the freezing cylinder.
Change in product viscosity can affect significantly the pressures against which pumps must work in transporting ice cream from the freezer to the filler. These changes in viscosity can arise from changes in extrusion temperature and from heat conducted through pipelines covered with varying amounts of frost. The two- pump system pipelines covered varying amounts of frost. The two-pump system isolates the freezing cylinder from external pressure changes and tends to yield more constant overrun.
In freezers operated manually air flow into the cylinder can be monitored as a function of pressure and flow through a meter that can be observed visually.
Some manufactures offer freezers designed to deliver ice cream at temperatures about 3Â°C (5Â°F) lower than the normal drawing temperature. These low-temperature continuous freezers have two freezing chambers; the first being essentially the same as a conventional freezer and the second having a dasher especially designed to subject the mix to vigorous agitation. The freezer chamber is maintained under 4 to 6 times the pressure of the conventional barrel (14-321kg/cm2, 200-300 psi, 1,400-2,100 kPa) Ice crystal size is reduced as much as 40% , e.g., from 45-55 mm to 18-22 mm Slight decreases are observed in air cell size while the number of cells increases slightly. Air cell lamellae decrease in thickness. The product is more resistant to adverse handling than is regular ice cream.
Another approach to decreasing air cell sizes is to install a pre-emulsifying device ahead of the freezing cylinder. The dasher in the barrel cannot spin fast enough to divide the mass of air injected with the mix until the mix viscosity increases during freezing. Since viscosity increase due to freezing typically starts at about one-third of the distance from the entrance end of chamber, most of the air is incorporated in the distal two-thirds of the chamber. By substantially dispersing the air before mix enters the cylinder, air cells are made smaller. Consequently, more and smaller ice crystals are formed, and the existing product appears dryer than when no pre-emulsification is done. Small air cells are more stable than large ones, rendering ice cream made in this way comparatively less susceptible to shrinkage in the container. Some types of novelty products, such as stickless bars, must be extruded from the freezer in very stiff form. In addition, lowfat and nonfat products need to have very small air cells; therefore, pre-emulsification is a desirable treatment for these products.
Dashers function in ice cream freezers to carry the sharp bladesthat scrape ice from cylinder walls, to agitate the mix and air so a finely divided foam will form, and to partially churn the fat to help stabilize the foam. Dasher design is basically of two types, open and closed (solid), but this is an oversimplification. Displacement dashers, those with a solid core, that rotate at a high speed tend to produce a more stiff product than the open dasher, which is driven more slowly.
In early models about 80% of the volume was displaced by the dasher, and speed of rotation was high. Ice cream produced in such a freezer tends to have minute ice crystals but to be highly churned so that its melting rate is slow. This type of dasher action is desirable for producing extruded products such as ice cream bars that are to be enrobed in chocolate. Here product shape must be maintained long enough to affect hardening, and shape must be maintained when the bar is covered with the warm chocolate. However, the combination of solid dasher with a small annular space between the dasher and the freezer cylinder all limit the volume of mix in the chamber. As the surface-to-volume ratio increases, so do chances of freeze-up within the cylinder.
By increasing the diameter of the freezing cylinder and reducing the displacement of the dasher, the freezer becomes much less sensitive to variations in refrigerant supply. Mix tends to act as a buffer against physical changes within the system, and the output is increased in uniformity of temperature and overrun. However, less churning is likely to occur in such freezers so that the ice cream tends toward wetness in appearance and quickness of melt.
Internal structure of frozen desserts is highly dependent on freezer design and operation. The design and state of repair of the cylinder dasher and blades as well as the capacity of the freezer to carry away heat are important determinants of finished product quality. The ice cream manufacture is advised to gain full knowledge of these parameters from the manufacture of the freezer before making a purchase.
The continuous freezing process has the following advantages over the batch process.
- Less stabilizer is needed because a larger amount of ice crystals can be formed in the freezing cylinder instead of in the hardening unit, where slow freezing produces large crystals.
- A shorter ageing time is needed because incorporation of air is less dependent on viscosity of the mix.
- Smoother ice cream is obtained because the ice crystals are uniformly smaller than those obtained with batch freezing.
- A more uniform yield is obtained wit less variation among packages, especially small ones.
- Continuous freezing facilities the making of specialities such as center molds, special shapes, combinations of flavours or colours in one package, variegated products, or individual serving- sized pakages.
- Thoughput and quantity of product per worker can be greatly increased over that of the batch process.
- The probability of contamination of product during filling is reduced as hand filling is seldom used.
The continuous process has a few disadvantages when compared with the batch process.
- Less tolerance is available for variance in fit of many parts that must fit with minute clearances. This means that parts ,must be manufactured within small tolerances and handled with extreme care during cleaning and assemly of the freezer.
- Greater training is required of operators and maintenance personnel.
- It is easier to obtain exessive overrun.
- Initial cost of the equipment is relatively higher.
- An ingredient feeder is usually required for adding fruits nuts, and other solid flavourings, whereas the batch permits addition of these materials to the barrel.
The Refrigeration System
Commercial continuous ice cream freezers are supplied with liquid refrigerant, usually ammonia, from the in-plant refrigeration system. This refrigerant enters the chamber surrounding the freezing cylinder (fig. 7.6) through an electrically controlled soleniod valve and a float valve that maintains the proper refrigerant level around the freezing cylinder. This cylinder is flooded with liquid refrigerant, which on absorbing heat from the ice cream mix, boils and vapourizes. Vapourized refrigerant flows to the accumulator located above the freezing cylinder. There entrained liquid is separated form the gas and prevented from entering the suction line that carries the gas away from the freezer. The refrigerant is then reliquefied by compression and cooling so it can be again circulated to the freezer.
Fig. 7.6. Schematic diagram showing the refrigeration system for an ice cream freezer in the "on" mode.
For protection against freeze- up, most modern continuous freezers have a hot gas line, equipped with a soleniod valve, to carry hot gas into the chamber surrounding the freezing cylinder. This unit can be activated manually or automatically. For example, if the ammeter,because of high demand of current, were to indicate the torque on the dasher motor to be exceeding a set point, the solenoid valve could be automatically opened so hot gas would flow into the chamber with the liquid refrigerant. The temperature would immediately rise, and defrosting of the freezer cylinder would occur.
Operating the Continuous Freezer
The principal responsibilities of the freezer operator are(1) to regulate the amount of air being introduced into the mix to produce the desired overrun, and ( 2) to control the temperature of the refrigerant on the freezing chamber to give the desired stiffness to the product as it leaves the machine. These two to give the desired stiffness to the product as it leaves the machine. These two variables need consistant monitoruing by manural or microprocessor means, but changes are usually minimal once the system has been brought to a stable condition. Stability is achieved when temperature of the equipment have been lowered to a steady state by removal of the heat stored in them and the rate of flow of mix and air have been stabilized. A source of error in overrun control is entrained air in the mix. This can result from adding incompletely mellted rerun (mix that previously went through the freezer) to the mix tanks, air leaks on the suction side of the mix pump, or air left over from blending operations.
To the achieve optimal freezer operation mixes must consist of the intended composition and be minimised, and air removal may be necessary. Mix to be refrozen should be reprocessed both to optimize freezing and to assure microbioligical safety. Mix should be supplied to the freezer pump at a low and constant temperature and a constant pressure.
Care and maintenance of the freezer and refrigeration system must be given priority if freezing is to be optimal on a daily basis. The follwoing are the chief requisites for keeping the system operating properly.
- Keep the ammonia jacket clean and free form oil, water, and nonvolatile ammonia fractions. Routinely check and drain water, oils, etc. from trap.
- Keep the scraper blades clean and straight. Utmost care should be exercised in handling the blades to avoid bending damaging.
- Keep mix pumps in proper working condition. Especially check lubrication and tightness of bells and chains.
- Make certain there is an adequate supply of refrigerant at the freezer. This requires that the entire refrigerant system be maintained.
- Provide steady suction pressure at about 1lb. lower than the pressure at which the freezer was designed to operate. The continuous freezer requires a steady supply of liquid refrigerant. An insufficient supply or a significant rise in the suction pressure will soon show up as softness of the discharged product.
The freezing cylinder of the batch freezer is made of a liner, usually of nickel silver or stainless steel, pressed inside a steel or copper tube which forms the inside wall of the cooling jacket. This jacket, if the machine is for brine cooling, is constructed of copper with narrow passageways, to make the brine travel a long distance around the cylinder so that it willhave a high velocity and provide good heat transfer. The outside is then insulated with cork and covered with an air tight metal housing. If the machine is to be cooled by direct expansion of a refrigerant such as ammonia, the outer jacket is usually built of heavy steel, properly insulated, and with suitable connections. A connection has to be made at the bottom for liquid ammonia inlet and for oil draining, and a connection at the top carries off the vapour from the refrigerant. Many of the modern direct expansion freezers use the flooded principle on account of its greater efficiency and ease of control. This necessitates the so-called ammonia control with liquid accumulator.
Fig. 7.7 Diagram of the refrigeration system of a continuous freezer.
In one system, the cold liquid ammonia is dropped from the accumulator in to the freezer jacket by opening a valve when the freezing is to start; when the freezing is finished and it is desired to whip, then the valve is closed, and the gas forming in the jacket collects at the top of the ammonia space and blows the liquid refrigereant out at the bottom and up in to the accumulator, thus stopping the freezing, as the liquid is no longer in cotact with the freezing cylinder. The freezing stops very quickly after the closing of the valve. The valve must be gas tight when closed, otherwise the freezer will not empty itself of ammonia when the valve is closed, and the freezer will continue to freeze, causing poor whipping and lack of overrun.
The Batch Freezer
The batch freezer is mostly used in small plants and in large plants for specials, sherbets and ices. The dasher is an important part of every batch freezer. It fits into the freezing chamber and can be easily removed for cleaning. The dasher consists of two parts, the scrapers blades and the beater to perform the following functions :
- aids in the transmission of refrigeration by keeping the mix in continuous contact with freezer walls;
- scrapes the freezer walls free from ice crystals;
- beats in air, and
- pushes the mix continuously forward which is essential in unloading the freezer. It is important to have the dasher in proper alignment with the sharp blades.
The temperature of the refrigerant varies from -23 to -29Â°C in order to obtain a rapid formation of ice crystals. This rapid formation results in small ice crystals and a smoother ice cream. The freezing should be slow enough to permit the whipping of air since this influences the body and texture of the ice cream. The most desirable refrigerant temperature to use depends upon :
- the freezing point of the mix which is dependent upon the amount of sugar, MSNF and the kind flavouring material used;
- the sharpness of the scraper blades,
- the amount of refrigerant available, and
- the efficiency of the freezer, i.e, the rate of heat transfer of the walls.
In the freezers having direct expension system, the freezing chamber is surrounded by liquid ammonia or other refrigerant. Ammonia is the most common refrigerant. It is cheaper than the other refrigerants as it enables to save power and equipment, and increases refrigeration capacity.
Freezing Procedure for Batch Freezers
The first step in the freezing procedure is to prepare the freezer. Its parts should be inspected to be sure they are clean and dry, and then assembled in accordance with the manufacturer's instructions. The operator's hands should be clean, and care taken to avoid contact of the freezer with human hands. When the freezer is assembled, the machine is sanitized by running hot water or a cold solution of a chemical sanitizing agent through the machine. This also serves the purpose of testing leaks and faulty operating condition. While sanitizing, the dasher should not be turned more than a few revolutions, thus avoiding excessive wear. If hot water is used, it must be at least 85Â°C to obtain the desired sanitizing effect. This should be followed by a cold water rinse to cool the freezer. When a chemical samitizing agent is used, complete draining is essential. After the freezer has been properly prepared, the measured amount of mix, flavouring and colour are added. It is always desirable to have the temperature of the mix below 5Â°C when it goes into the freezer. The total volume of the mix, flavour and colour should be about half the total capacity of the freezing chamber.
Fig. 7.8 Batch type ice cream freezer
The flavouring and colouring materials must be added so as to be uniformly distributed, but the actual moment or order of adding them can be varied. Special precautions should be observed while adding nuts, acid fuits, etc. can be said that up to a certain points air is being continually whipped into the ice cream mix. However, after that point is reached, it loses air faster that it gains it, if the whipping is carried on ncessantly. Therefore, the operator must adjust the refrigerant so that the ice cream can be drawn the moment he has obtained the desired overrun and consistency. When the ice cream is drawn from the freezer, it should have a ribbon-like consistency and stiff enough to hold its shape, and yet soft enough to "settle" or loss its shape within a minute or two.