Ice cream mix contains a minimum of 10% milkfat and 20% total milk solids, except when fruit, nuts, chocolate, or other bulky flavors are added. If bulky flavors are added, the minimum milkfat is 8.8% and 16.6% milk solids. According to U.S. Federal Standards, ice cream must weigh at least 4.5 lb/gal. The minimum gallon weight permits the incorporation of air sufficient to roughly double the volume (or 100% overrun). Percent overrun is defined as:
weight of 1 gal mix - weight of 1 gal ice cream
weight of 1 gal ice cream
Superpremium ice creams typically have a range of 10 to 40% overrun (premium, 60—75%; economy, 75—90%).
Ice cream mix normally contains 10 to 15% sucrose, 5 to 7% corn sweetener, 0.2 to 0.3% stabilizer, 0.1% emulsi-fier, and natural or artificial flavors and colors. According to U.S. Federal Standards, ice cream may not contain more than 0.5% stabilizer or 0.3% emulsifier.
Physically, ice cream is an emulsion, a dispersion of ice crystals and a partly frozen foam. The ice cream mix is an emulsion in which the aqueous phases contain solutions of soluble proteins, lactose, mineral salts, and added sugars. The nonaqueous phases consist of dispersed solids such as stabilizers, proteins, and fat globules. During the freezing process and foam formation, the emulsion is partially destabilized. This emulsion destabilization allows the air cells to be stabilized by clusters of intact fat globules, which agglomerate by free oil or protein/protein interactions (4-7). The percentage of water changed into ice is dependent on the production temperature and concentrations of sugars and milk salts in solution.
Butterfat contributes to the rich and creamy flavor and texture of ice cream. Fat particles, which impart a smooth mouth-feel, are dispersed throughout the ice cream mix. Ice cream with high fat content tends to have smaller ice crystals and a slower rate of air incorporation. Butterfat can be obtained from a variety of sources, but fresh sweet cream is by far the most popular and desirable source. Sweet cream is about 40% butterfat. Because of its perishability, for best flavor, cream must be refrigerated and promptly used. Cream can be frozen to preserve the quality; however, extra care must be taken during handling to protect the flavors. Another source of butterfat is unsalted butter, which is about 82.5% fat and can replace 50 to 75% of the sweet cream fat. Other special fat sources sometimes used in ice cream are plastic cream (80% fat), anhydrous butter oil (99% fat), concentrated sweetened cream, and dried cream (65% fat) (3).
Milk proteins are important to the formation of ice cream. Their role in the overall structure is to help stabilize the foam or air cells and to contribute to emulsification of the fat. This is due to hydration during mixing, destabilization and complexing during heating, and attraction to the fat globules surface after homogenization. As a result, the finished ice cream is more stable and smooth.
Two main types of milk proteins are found in ice cream. These are whey proteins and caseins. Casein comprises ap proximately 80% of the proteins found in milk. Numerous sources of milk proteins are available for use in ice cream. Condensed skim milk at about 25 to 35% solids is widely used throughout the industry. However, it is perishable and must be used fresh. Dried skim milk, or nonfat dried milk, is also widely used and has a much longer storage life. Many types of powders (low, medium, and high heat-treated) are available, each with specific functional properties (3). Superheated condensed skim milk is sometimes used as a protein source. Heating the condensed skim milk denatures more protein, thereby imparting increased stability and whipping ability when used in ice cream. However, cooked off-flavors may result. Lactose reduced (LR) skim milk may be used to decrease the lactose concentration in ice cream to discourage the formation of lactose crystals that impart an undesirable grainy, gritty, or sandy ice cream texture.
Specialty products are also available as additional protein sources. These include sodium caseinate and dry whey, which both improve mix-whipping properties by increasing overrun and dryness. Dry whey contains high portions of both lactose and mineral salts and can replace up to 25% of the milk solids nonfat (MSNF). At higher levels, saltiness and sandiness may result. Milk protein concentrate (ultrafiltered skim milk, MPC) has potential use in the ice cream industry, having a higher protein concentration and lower lactose and soluble mineral content than NFDM. MPC is most beneficial due to its clean, milklike flavor (8).
Although excellent ice cream products can be made with only the natural stabilizing and emulsifying materials present in milk (milk protein and phosphates), additional stabilizers and emulsifiers also have potential benefits.
Fluctuations in temperature during normal distribution cause ice crystals to melt and refreeze into larger ice crystals, resulting in negative textural changes in the ice cream. This phenomenon, exposure to either large temperature swings or high temperature for a period of time, is referred to as heat shock. Hydrocolloidal stabilizers function to physically bind the water formed by melting and, therefore, to help prevent the formation of large ice crystals. The amount and type of stabilizer needed is governed by the composition of the ice cream mix, processing condition, processing temperatures, storage times and conditions, and other factors (3,9-11). Most ice cream mixes are made with 0.2 to 0.5% added stabilizer. A stabilizer is capable of binding a large amount of water. A small amount is effective in producing a product with good texture and more resistance to temperature abuse (heat shock). Too much stabilizer will result in product that is gummy.
Many stabilizers used in ice cream are natural. Sodium alginates are widely used as ice cream stabilizers. Algins improve whipping qualities of the ice cream, help prevent heat shock, and give the product good eating qualities. Gelatins, which are derived from animal sources, are also used to stabilize ice cream products. Their structure and affinity for water help prevent the formation of large ice crystals and contribute to texture. Locust bean gum (carob) is normally used in conjunction with carrageenan (Irish moss) as a stabilizer. Locust bean gum has excellent waterbinding properties and imparts superior heat shock resistance. Its tendency to cause wheying off, curdling of milk proteins, is prevented when it is used in combination with carrageenan. Guar gum is a complex carbohydrate that functions as a stabilizer and is usually used in conjunction with carrageenan. Guar has water-binding properties similar to that of locust bean gum and imparts similar heat-shock resistance. It solubilizes easily in cold mix, making it a good choice for mixes undergoing high-temperature short-time (HTST) pasteurization. A newer gum, similar to guar and locust bean, is tara gum, which is cold water soluble and protective to heat shock. Tara gum imparts a buttery mouthfeel and can be used at levels 20 to 25% less than locust bean gum. Sodium carboxy-methyl cellulose (CMC) is also used as a stabilizer. It has excellent water-holding properties and dissolves easily in mix. It possesses some emulsifying properties. CMC provides the best results when used in conjunction with another stabilizer (4). Milk proteins also have stabilizing properties, depending on the heat treatment they have received.
The primary effect of emulsifiers in ice cream is related to their ability to de-emulsify the fat globule membrane formed during homogenization. This de-emulsification, arising from the disruption of the fat globule membranes during freezing, facilitates the agglomeration and coalescence of the fat globules, leading to partial churning out of the fat phase. The agglomerated fat globules stabilize air cells (3). Thus, emulsifiers are used to improve the whipping qualities of ice cream mix by producing smaller ice crystals and smaller air cells, resulting in a smoother ice cream texture and a drier, stiffer ice cream. Generally, a mixture of high and low hydrophile-lipophile balance (HLB) emulsifiers, such as mono- and diglycerides, and polysorbate 80 is used. Milk contains some naturally emulsifying compounds that aid in the manufacturing of ice cream. These include milk proteins, phosphates, and citrates. Egg yolks may also be used as an emulsifier as they are high in lecithin.
To achieve fat reduction in frozen desserts, one needs to replace both the functionality of the fat and the actual quantity of fat removed (formula percentage). Reducingfat in a frozen dairy product generally requires multiple approaches. Solids replacement is critical for stability (ice crystal growth) and the perception of warmth/coldness in the mouth. For example, in nonfat products there is proportionally more ice that must melt at least partially in the mouth so the product will seem "colder" than a high fat ice cream when both products are served at the same temperature. Various fat substitutes may be chosen to provide "body" or a mouth-feel that simulates the lubricity of fat. Flavor levels or flavor character may be changed to modify the flavor impact and release.
Dairy processors worldwide are continuing to adjust their fat-reduced ice cream formulas to duplicate the qual ities of full-fat products. Ingredient suppliers are broadening the range and quality of fat substitutes, which significantly help ice cream manufacturers in their quest for the perfect low-fat and nonfat desserts. A recent market survey of low-fat and fat-free frozen desserts in the United States showed that 38% contained microcrystalline cellulose, 31% contained polydextrose, 31% contained malto-dextrin, 25% contained modified food starch, and 50% contained added protein, for example, whey protein or egg white.
Fat substitutes can be classified as proteins, carbohydrates, or lipids. In addition to fat substitutes, a low-fat or nonfat frozen dessert may require flavor addition to compensate for the lack of fat and its effect on flavor release. When butterfat is reduced in a product with a standard of identity such as ice cream ("low-fat ice cream"), a lost nutrient, such as fat-soluble vitamin A, must also be replaced.
Protein-Based Fat Substitutes. Most of the protein-based fat mimetics function by emulating fat globules. Sim-plesse® is a microparticulated egg white and/or milk protein from The NutraSweet Kelco Company, which is said to mimic fat by simulating the particle size of fat globules. Dairy-Lo® or Dairy-Lite®, a heat-treated whey protein concentrate from Cultor Food Science, is another available protein-based fat substitute.
Carbohydrate-Based Fat Substitutes. Carbohydrates are generally used in ice creams to replace fat solids. Higher molecular weight carbohydrates with low sweetness are usually chosen since they will be used in addition to the carbohydrates (sweeteners) already included in the formula. One such carbohydrate is Oatrim®, a fat substitute that is produced by partially hydrolyzing the starch in oat bran or oat flour into maltodextrins. Modified starches, polydextrose, polyfructose, and pectin, such as Slendid from Hercules, are also commonly used as fat replacers. Microcrystalline cellulose is used to enhance the mouth-feel of frozen dairy products. Gelled carbohydrate solutions or suspensions provide some viscosity and lubricity but do not have the globular mouth-feel characteristics of fat. Carbohydrate usage levels may be limited by increased mix viscosity.
Lipid-Based Fat Substitutes. Lipid-based fat substitutes are the so-called designer fats that can fully replace butterfat functionality at a lower calorie level because they are only partially digested. They generally don't require other formulation changes, while protein and carbohydrates behave as fat mimetics and require formulation changes. These fat "substitutes" are chemically fats, but rendering the fat partially indigestible reduces the caloric content. Several lower-calorie fats are commercially available. Caprennin® from Procter & Gamble is an interester-ified triglyceride with long-chain fatty acids, which contribute to its lower degree of digestibility. Salatrim®, a 5 calories per gram triglyceride from Cultor Food Science marketed as Benefat®, is another interesterification product between glycerol esters of acetic, propionic, and butyric acids and regular fats. FDA has approved Olestra®, a su crose polyester with very low HLB from Procter & Gamble, for salty snacks, but not yet for frozen desserts. Emulsifiers can augment the functionality of fat.
Flavor Delivery Systems. There are two potential flavor issues associated with low-fat products: the fat replacer itself might have an off-flavor, and the loss of fat and replacement by the fat substitute might result in an unbalanced flavor release. Flavor delivery issues exist because fat replacers do not dissolve flavor compounds in the same way as fat. When fat melts, flavors are released in a characteristic sequence. In low-fat and fat-free formulations the flavor release can be faster, stronger, and less pleasurable. Some advances have been made by flavor encapsulation and controlled-release mechanisms. One example is Singer's (12) patented method for the delivery of fat-soluble flavor compounds into nonfat and low-fat food products in which fat components have been replaced by nonlipid fat substitutes. Hatchwell (13) discussed a flavor delivery system composed of fat globules into which elevated levels of fat-soluble flavor compounds have been loaded or incorporated into nonfat and low-fat food products, so that fat-soluble flavor compounds will be released in a more natural and familiar sequence.
Cholesterol Reduction. As consumer demand for low-fat cholesterol-free products has increased innovative technologies to remove cholesterol from butterfat or egg yolk have been developed to different scales of commercialization. The techniques include vacuum steam distillation, short path distillation, melt crystallization, supercritical fluid extraction, and chelation with yS-cyclodextrin or quil-laja saponins (14,15).
Sugars contribute to the sweet flavor of ice cream. The concentration of sugars in the mix will also determine the freezing point of ice cream. The greater the amount of sweetener contained in an ice cream mix, the lower the freezing point. Many kinds of sweeteners can be used in ice cream. They include cane and beet sugar, corn sweeteners, honey, invert sugar, lactose, fructose, and refiner's syrup.
Sucrose, the most widely used sugar, is processed from cane sugar and beet sugar. Most ice creams are made with a combination of corn syrup and sucrose. Corn syrup solids are made from a hydrolysis of cornstarch. Corn syrups are classified by dextrose equivalence (DE), which indicates its degree of hydrolysis. The higher the DE, the sweeter the corn syrup will be. The intermediate range (48-68 DE) and high conversion (58-68 DE) corn syrups are used to produce a product that will be very close in texture to that of a product made entirely with sucrose. Corn syrup imparts a denser body to the finished product and may improve its shelf life (3,9,11). Honey is used in ice cream primarily to make honey-flavored ice cream. Honey usually does not blend well with other ice cream flavors and therefore is rarely used as a sweetener.
A significant part of the MSNF is lactose, which also imparts sweetness. Lactose is a sugar found only in milk.
It is less sweet and less soluble than sucrose. The concentration of lactose in mix is limited because it may separate out of solution in large crystals and produce an undesirable sandy feeling in the mouth. The maximum amount of lactose to avoid sandiness may vary depending on the freezing and storage conditions of the product.
For some diabetics, no-sugar-added products provide opportunities to consume frozen dairy desserts. However, it is the total amount of carbohydrate, not just the simple sugars, that affects the blood glucose response. The technologies for sucrose replacement have been reviewed by Deis (16). Traditionally, polyols such as glycerol, xylitol, sorbitol, lactitol, maltitol, palatinit, and hydrogenated starch hydrolysates are used. Polydextrose, with 1 calorie per gram, is another commonly used bulking agent with high intensity artificial sweeteners. Buzzanell et al. (17) reviewed the status and application of various high-intensity sweeteners including aspartame, saccharin, Acesulfame-K, cyclamate, sucralose, and Alitame®. Sucra-lose® has recently been approved in the United States in April of 1998. A current U.S. market survey of no-sugar-added products shows that the majority contain malto-dextrin, polydextrose, sorbitol, and a combination of Acesulfame-K and aspartame.
A novel process for a no-sugar-added product uses lactase to hydrolyze the lactose in condensed skim to increase the mix sweetness and permit use of a higher concentration of milk without risking lactose recrystallization (sandiness) (18). This allows the complete replacement of sugar with a high potency sweetener (aspartame) without the use of bulking agents.
Hundreds of varieties of flavoring substances are used to flavor ice creams. There are two main characteristics to each flavoring, type and intensity. Both influence how well consumers like the finished products. Serving temperature and overrun also influence flavor.
The U.S. Federal standards of identity place flavors into the following three categories (2):
Category I. Pure extracts (no artificial flavor) Category II. Pure extracts with synthetic or artificial components (natural flavor dominates) Category III. Artificial flavors (or natural and artificial with artificial flavor predominate)
U.S. Federal laws also regulate how these various flavoring categories are to be labeled on the cartons. The majority of the ice creams are made with category II flavors, whereas most premium and superpremium brands use category I or pure flavors.
Vanilla is by far the most widely used flavor. Vanilla flavors are available from each of the three categories I, II, or III; pure vanilla flavor is a product of vanilla bean fermentation. Most vanilla flavors used are of the category II, pure vanilla plus some artificial components. Chocolate products are the second most popular flavorings. They come in many forms; cocoa powders, chocolate syrups, and chocolate liquor are the most predominant. Flavor intensity, color, and texture can be manipulated by using different types of cocoa products. Fruits and fruit extracts are also very popular as flavorings. They can be added fresh, dried, candied, as concentrated juices, or as fruit essences. Nuts, spices, candies, cookies, and sugars such as honey are also added to ice cream to provide a wide variety of flavors. Other substances are swirled throughout the ice cream to produce a rippled effect, or a variegated product. These include flavors such as fudge, butterscotch, marsh-mallow, caramel, and fruits. Liqueur flavorings such as fruit brandy distillates or fruit liqueurs are also used in some ice creams.
Any FDA-certified natural and artificial coloring may be added to ice cream. These must be declared on the label. Annatto is a very popular yellow vegetable color used to color ice cream and to add visual richness.
Egg yolk solids are an optional ingredient. They are added for their emulsifying properties. They also impart a characteristic subtle flavor. Custard-type ice creams, by definition, must include egg yolks.
ICE CREAM PROCESSING—MIX MAKING, FREEZING, AND HARDENING
After the processor has selected the formula, the ingredients are blended together to produce the ice cream mix. The basic steps inherent in almost all processing of this mix into a frozen product are blending ingredients, pasteurization, homogenization, cooling, mix storage, flavoring, freezing, packaging, hardening, and finished product storage and distribution.
To begin mix processing, the fluid ingredients are pumped from their storage vessel into a blending vat. The amount of these ingredients, as determined by the formula, may be either metered or weighed into the blending vat with the use of load cells. Some blend systems contain both meters and load cells to use one to check against the other in an effort to improve the accuracy of ingredient addition.
Dry ingredients such as cocoa powders, nonfat dry milk, corn syrup solids, and stabilizers are added into a blend vat, using a high-shear mixer. A portion of the bulk fluid ingredients is circulated through by the mixer to aid incorporation into the blend vat.
Pasteurization is the process of heating the ice cream mix to a required time and temperature for the primary purpose of destroying pathogenic microorganisms. Two primary methods are recognized for achieving the time and temperature pasteurization requirements, batch also known as low-temperature long-time (LTLT) or continuous. Continuous pasteurization can be accomplished at different combinations of temperature and time: high-temperature short-time (HTST), higher-heat shorter-time (HHST), and ultra high temperature (UHT) methods. The U.S. Public Health Service (USPHS) standards for the pasteurization of mix are (19):
To achieve a higher rate of microbe destruction and to impart the characteristic flavors, most processors will pasteurize at slightly higher temperatures and/or longer times.
Today, the most popular method is the continuous HTST procedure using plate heat exchangers. Using HTST, the processor benefits from (1) improved process control, (2) less stabilizer requirements, (3) savings of time and space, and (4) increased capacity. Other continuous heat exchanger designs include steam injection and infusion, tubular, and swept surface types.
Batch pasteurization may be accomplished by two methods. The classic method is when the batch pasteurizer vessel also acts as the blending vat. After the required ingredients per formulation have been added to the blending vat, hot water is circulated through the jacket of the vat. Depending on the temperature of the hot water and the flow rate through the jacket, the mix arrives at the required batch pasteurization temperature. After the legal hold period, cool water is circulated through the jacket and, when sufficiently cooled, is pumped to a cooling system similar to that for the HTST system. The second method used for batch pasteurization is a modified system employing the regeneration section and the heater section of the HTST system; however, the temperature is raised only to the required batch pasteurization temperatures. The mix is then held in holding tanks for 30 min and then consequently pumped to the regenerator and cooling system.
The purpose of homogenization is to reduce the size of the fat globules so that 90% of the globules are less than 2 fim. Decreasing the fat globule size to smaller globules increases the surface area. This allows for a more uniform and consistent product, smoother in texture, which resists churn out in the freezer.
Acceptable regulatory placements of homogenizers are (1) in front of the heater section, (2) after the heater section and in front of the holding tube, and (3) after the flow diversion device and before the regeneration section. The ho-mogenizer is a positive displacement pump that may act as the timing pump in many HTST systems.
After pasteurization and homogenization, the mix is passed through a cooling unit. The cooling medium may be a refrigerated glycol solution or brine solution (a concentrated food-grade salt solution). The method employed for cooling is a plate heat exchange system similar to the HTST or a surface cooler. The heat exchange system is sufficiently sized to decrease the temperature of the mix to 45°F or less. The cold mix then flows to mix holding tanks for storage.
Although legal standards only require cooling mix to 45°F or less, it is advantageous for product safety and quality to cool the mix to 34 to 36°F. With respect to aging of the mix, it was believed that 24 h was necessary for the physical changes in protein structure and fat crystallization to create more consistent mix that can be processed. However, recent studies do indicate that as little as 4 h accomplish this goal.
During the freezing process, refrigerated ice cream mix is introduced into the ice cream barrel. The barrel is a cylindrical scraped-surface heat exchanger. Ammonia or freon is used as the cooling medium. Part of the water content of the mix is frozen into small ice crystals. At the same time, air is incorporated into the emulsion with agitation. The mix is transformed into a 20 to 25°F flowable mixture that can be filled/shaped into its final form. In commercial freezers, stainless-steel blades attached to the dasher (or mutator) scrape the crystals as they are being formed on the wall surface. The dasher controls the flow dynamics of the mix. Dashers come in many designs that determine the texture of the product. Closed or partial open dashers are used for ice creams. Many freezer manufacturers have specially designed their own dashers to meet their customers needs (20-22).
There are two main types of freezers, batch freezers and continuous freezers. Originally all freezing was done with batch freezers. In the batch process, a certain volume of mix is placed into the freezing barrel. The mix is frozen as the dasher is whipping in air. Batch freezing relies heavily on operator experience.
Currently, most manufacturing plants use continuous freezers. Continuous freezers provide greater process efficiency and product consistency. In continuous freezing, ice cream mix is metered into the freezing barrel at a constant rate. As the mix passes through the cylinder, air is whipped in by the dasher. Particulates (fruits, nuts, cookies) are added through a fruit and ingredient feeder after freezing. Continuous freezers are now available fully automated. Computers check the main control points of the operation. These control points may include overrun, dasher speed, flow rates, viscosity, pressure, and temperature control. These freezers also include automatic start-up, CIP pumps, and freeze-up prevention.
Soft-serve freezers are a scaled-down version of the batch and continuous freezers (23). Mix is added to a refrigerated hopper and flows by gravity into the freezing barrel. Mix is frozen, and air is incorporated in the barrel. The finished product is drawn out at 18 to 20°F onto a cup or cone. Because the product may have to stay in the freezing barrel for hours, the compressor and dasher are cycled on/off to maintain a consistent texture (20-22).
Modern ice cream plants include as much CIP cleaning as possible. CIP refers to the cleaning and sanitizing of process equipment in its assembled condition. The detergent solutions and sanitizers are recirculated through the process pipelines (pasteurizing system, mix tanks, freezer, etc). For effective soil removal, the detergent solution must have sufficient concentration, temperature, velocity (5 ft/s), and time. Manual or clean-out-of-place (COP) cleaning is significantly more time-consuming, more expensive, and generally less effective.
The four primary steps of an effective cleaning and sanitizing program for the CIP circuit are:
1. Prerinse: Flushing with water until the water runs clear.
2. Recirculation of detergent solution at 150 to 180°F for 15 to 30 min, depending on the type of equipment to be cleaned.
3. Postrinse for 10 to 18 min to remove all traces of detergent and soil.
4. Recirculation of sanitizers using chlorine or acid sanitizing agents, or heat sanitization. Heat sanitization is the circulating of hot water, 180°F minimum, for 15 min.
Most ice creams, with the exception of quiescently frozen novelties, must be passed through a final freezing process after packaging called hardening. Hardening completes the ice crystallization that began in the continuous or batch ice cream freezer, enables the ice cream to withstand storage and transportation, and aids in final textural development.
Ice cream hardening must be accomplished through the rapid extraction of heat to ensure that the small ice crystals previously formed in the ice cream freezers are kept intact, giving the ice cream its familiar cool and creamy mouth-feel. Formation of large ice crystals would lead to an undesirable defect of iciness or a coarse mouth-feel. Efficient heat extraction is achieved through the use of hardening tunnels (insulated cold boxes in which the product is continuously conveyed through an extremely cold environment).
The actual heat transfer occurs either by a cold air blast (- 30°F or lower) circulated around the ice cream packages or through hollow stainless steel plates that act as both the conveyor and the heat transfer medium. Brine, ammonia, or glycol chilled to — 20°F flows in the inside of the hollow plates, discharging the heat gained from the ice cream to the plant refrigeration system.
Hardening tunnels are designed to bring the center of a package down to 0°F. The tunnel conveyor system has adjustable speeds to accommodate the various packages that a normal plant will produce. Hardening times for a — 40°F blast tunnel are typically 90 min for pint packages to 4 h or more for 2-1/2-ga bulk packages.
Automatic or manual unloading of the hardening tunnel into a palletizing area completes the plants responsibilities for the product. The product will typically be warehoused at - 20°F. It is then distributed to the retailers where the product should be held at 0°F or colder to survive its trip to the consumer's home.
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