Ice cream is more than a sweet treat. Technically, it is a combination of ingredients that provide thickening, soft gelling, and thin film formation to produce a frozen product with both fat dispersed in water and air dispersed in water emulsions, and a long shelf life, says Donna Klockeman, senior principal food scientist at TIC Gums, White Marsh, Md. (ticgums.com). But you can also think of ice cream as a blank canvas from which manufacturers can formulate products with creative flavors and interesting textures.

Consumers will find such varieties as Green Tea, Toasted Sesame Brittle, and Pineapple Coconut from Häagen-Dazs and Pumpkin Cheesecake, Salted Caramel Brown-ie Ale, and Peach Melba from Ben & Jerry’s. Breyers offers several ice cream varieties made with favorite branded confections like Snickers, Oreo cookies, Heath English toffee, and Girl Scout Cookies Samosas. There’s ice cream, frozen yogurt, and gelato, and products that are lactose-free, gluten-free, reduced-fat, fat-free, and sugar-free. Frozen ice cream novelties come in sandwich and bar forms, on a stick, or in different shapes (i.e., Mickey Mouse–shaped novelties available at Disney Parks), and topped with flavored and colored coatings. Another opportunity for manufacturers to get creative with their ice cream formulations is with limited-time offerings and product tie-ins. Ample Hills Creamery, a privately owned ice cream shop in Brooklyn, in collaboration with Disney Consumer Products, gives a Jedi the chance to choose between the light side and dark side of the force with two limited-edition offerings aligned with the Star Wars: The Force Awakens movie. The Light Side is a marshmallow ice cream with crispie clusters and cocoa crispies while The Dark Side is a dark chocolate ice cream with espresso fudge brownies, cocoa crispies, and white chocolate pearls. Manufacturers can also adjust the texture of the finished product to differentiate the product, creating ice cream that ranges from dense to fluffy to soft (as in soft-serve ice cream). Using fats and protein (from dairy), sweeteners, and stabilizers from various sources, along with any number of colored and flavored inclusions, creates ice cream products in a range of styles and flavors, from nostalgic favorites to new sensory experiences.

Functions of Dairy Ingredients

A wide variety of dairy ingredients, including whole and skim milk, cream, whey protein concentrate, and milk powder, provide the all-important emulsification and texturizing functions to ice cream. Scan the labels of ice cream products and you may even find dairy ingredients like lactose, demineralized whey, and butterfat listed. The first two provide sweet flavor and milk solids-not-fat while the butterfat contributes flavor and mouthfeel and thickening enhancements, explains Susan C.G. Larson, associate researcher at the Center for Dairy Research at the University of Wisconsin–Madison.

The whole milk and heavy cream used in ice cream formulations contribute fat, which is responsible for improving density and developing a smooth texture, along with giving the finished product overall richness. The milk fat content in ice cream can range from 10% to up to 16%. Premium ice creams, which have creamier textures and a very rich taste, have milk fat contents in the upper range. The fat molecules in whole milk are tiny globules suspended in a liquid. On the surface of each globule are the milk proteins casein and whey that have emulsification properties to keep the fat suspended in the liquid. Nonfat dry milk solids also contribute to the overall body and smoothness of ice cream.

In addition to providing functional benefits, dairy ingredients can offer cost benefits to manufacturers. “There is a cost opportunity to using dairy ingredients that is two-fold,” says Larson. “Not only can cost savings occur when formulating is done correctly, but there is an opportunity for improvement in yields through higher overruns.”

The U.S. Dairy Export Council, Arlington, Va. (thinkusadairy.org), offers formulas of popular ice cream and dairy-based frozen products to highlight the functionalities of dairy ingredients. Cream (40% fat) and skim milk powder are each used in both hard-packed ice cream and hard-packed premium ice cream. Cream is used to develop mouthfeel and thicken the ice cream as well as contribute to the color and flavor in these two types of ice cream products, says Larson. Skim milk powder lends emulsifying, water binding, aeration, and foaming functions not only to the two hard-packed ice cream formulas, but to formulas for soft-serve yogurt mix, nonfat ice cream, and soft-serve ice cream dry mix.

In formulas for frozen dairy-based treats made with less fat like soft-serve frozen yogurt or nonfat ice cream, consumers expect the quality and flavor to be as close to traditional ice cream as possible. By using whey ingredients, manufacturers can achieve some of the same functionalities that dairy ingredients with higher amounts of fat would provide (USDEC 2007). In both of the sample formulas provided by the USDEC, whey protein concentrate forms stable emulsions, binds water, aids in the thickening of the product, provides milk solids-not-fat, and contributes to the overall stability of the system, says Larson. In addition to these, whey protein concentrate has whipping/aeration functions that help stabilize and strengthen air cells, leading to a smooth and creamy texture, according to the USDEC. Whey protein concentrate has protein levels ranging from 34% to 89%. One with 34% protein is used in the soft-serve frozen yogurt formula while another with 80% protein is used in the nonfat ice cream formula. Ingredients like these, especially whey protein concentrates with higher protein levels, can be used to boost the protein content of the finished product, says Larson. (The nonfat ice cream formulated with whey protein concentrate 80% has 9 g of protein/100 g serving.)

Ice cream manufacturers can choose from a number of whey protein ingredients and more to bring the desired creaminess and stability to their products. Within its comprehensive range of whey protein ingredients, Hilmar Ingredients, Hilmar, Calif. (hilmaringredients.com), offers several whey protein concentrate and whey protein isolate ingredients that bring emulsification and improved mouthfeel to ice cream and frozen dairy products. Milk proteins sold under the IdaPro brand from Idaho Milk Products, Jerome, Idaho (idahomilkproducts.com), are used in hard-pack ice cream formulations, where they stabilize the ice cream structure and provide a creamy texture. The milk protein ingredients also give body and mouthfeel to low-fat and nonfat hard-pack ice cream. Finally, Agropur Ingredients, La Crosse, Wis. (agropuringredients.com), manufactures dairy ingredients for ice cream applications under it Cornerstone and Darigen lines. The Cornerstone ingredients are dairy proteins that can replace nonfat dry milk solids, improve resistance to thermal shock and meltdown, and offer emulsion stability that is comparable to skim milk. Darigen is a cost-effective nonfat milk solid ingredient for use in soft-serve ice cream, where it functions as a 1:1 nonfat dry milk replacer.

Stabilizers for Quality and Consistency

It is common practice for food manufacturers to add stabilizers to ice cream formulations, as these ingredients offer several functional benefits to the stability and texture of the product and help ensure quality through the entire processing and manufacturing stream. “One of the biggest challenges is maintaining the quality of the ice cream through shipping and storage,” says Ann Tigges, technical service, dairy, at Cargill, Minneapolis, Minn. (cargill.com). “The frozen transportation system from producer to distribution center to supermarket to consumer freezer can have many points where the ice cream might be defrosted slightly. Every time that happens, the eating quality diminishes a little. The use of quality gums and hydrocolloids helps prevent that decline.” Some of the commonly used stabilizers in ice cream include agar-agar, gums like guar, xanthan, gellan, and locust bean, carrageenan, carboxymethyl cellulose, sodium alginate, and gelatin. Other ingredients like egg yolk, monoglycerides and diglycerides, and polysorbate 80 are emulsifiers, but can be thought of as stabilizers because they stabilize emulsions in ice cream.

Stabilizers contribute to the overall consistency and improve the shelf life of ice cream by helping the product survive heat shock. The ingredients also help control ice crystal growth and improve the texture of ice cream. Small ice crystals result in a smooth and creamy ice cream whereas large crystal sizes give ice cream a coarse and grainy texture. Product formulations, processing, packaging, and distribution channels can influence the texture and stability of ice cream, particularly the consistency in size and number of ice crystals that start off processing with the initial freezing of ice cream, says Klockeman. After the ice cream has fully hardened, changes in temperature affect the size of crystals, she adds. “Even as the product is stored at very low temperatures, there is an equilibrium balance between water and ice.” Knowing that ice crystal formation affects the appearance, texture, and shelf life of ice cream, and that it is important to control the size of ice crystal, ingredient manufacturers have developed gums, fibers, and other similar ingredients to address this. For example, Ingredion’s HYDRO-FI TGC-1100 stabilizer blend helps reduce ice crystallization, improve melting properties, and produce a smooth, creamy texture with improved overrun, explains Ivan Gonzales, marketing director, dairy, at Ingredion, Westchester, Ill. (ingredion.us). This ingredient is composed of tara gum, citrus fiber (dried orange pulp or citrus flour), and guar gum.

Reducing fat or sugar in an ice cream formulation affects several qualities of the product, and this is where stabilizers can contribute functionalities. The texture of ice cream changes, particularly the creaminess, smoothness, and meltdown properties, and these changes affect how different flavors are perceived in the final product, explains Gonzales. When the sugar content is reduced, it changes sweetness perception and affects crystal formation and stability over shelf life, he adds. Ingredion offers a broad range of ingredients suited for formulation in ice cream products with reduced fat or sugar contents. Two that Gonzales notes are N-DULGE CA1 tapioca maltodextrin that provides creaminess and enhanced mouthfeel for low-fat dairy products, and NOVATION 8300 functional native rice starch that provides creaminess and mouthfeel with excellent freeze-thaw stability.

Klockeman explains that TIC Gums’ Nutriloid 7000 boosts soluble dietary fiber levels in ice cream and improves mouthfeel without changing the viscosity of the finished product, while soluble fibers can be added to a formulation to replace total solids in reduced-sugar products. “The balance of thickening, soft gelling, and thin film–forming components in stabilizer blends is often adjusted when either fat, sugar, or both are reduced in frozen desserts,” she says.

Fiberstar’s Citri-Fi functional citrus fiber ingredient provides hydrocolloidal properties in dairy applications like frozen dairy, including ice cream products formulated with reduced amounts of fat. Citri-Fi is produced using a patented, “clean” process that opens up the fiber to create more surface area, and thus, more contact points for water and/or oil, explains Amanda Wagner, food technologist at Fiberstar, River Falls, Wis. (fiberstar.net). “The composition contains intact insoluble and soluble fiber and protein. These unique attributes provide the exceptional water binding and emulsifying stabilization when used in frozen dairy-based desserts. As a result, this functional fiber can be used to reduce the fat content in low-fat ice cream while maintaining the full-fat creamy mouthfeel and retardation in melt time that are often lost when reducing the fat content.” Wagner adds that since Citri-Fi is naturally derived, it can be used in so-called clean label formulations to replace certain hydrocolloids (in the United States, Citri-Fi can be labeled as citrus fiber, dried citrus pulp, or citrus flour).

Clean label in food production is an important concern of consumers as they continue to pressure food manufacturers to shorten ingredient statements and formulate with ingredients that are naturally derived and have easy-to-understand names. “For those who are trying to streamline an ingredient legend, Cargill offers several ingredients that may offer ice cream product developers the right combination of functional and ‘clean,’” says Tigges. “While ‘clean’ is a subjective descriptor, ingredients like locust bean gum, guar gum, and lecithin have enjoyed a very clean connotation for years.”

Locust bean gum, a galactomannan derived from the bean of the locust or carob tree, has been used in foods for thousands of years, says Tigges. “It is one of the premium gums for use in ice cream.” Its branching molecular structure traps and holds water, which helps prevent heat shock. Tigges adds that locust bean gum is typically used in combination with guar gum and carrageenan but can function as an effective texturant on its own by controlling free water in a system and contributing to mouthfeel. Emulsifiers like lecithin help to hold the fat and water portions of a system in place, which is useful for processing efficiencies. “It is very helpful in achieving or controlling overrun, which is a big component in the manufacture of ice cream,” says Tigges. Lecithin is derived from many sources, including sunflower, which is not genetically modified and not considered an allergen. “This is useful in formulas where mono- and diglycerides might not be welcome on the label,” says Tigges.

How Sweeteners Affect Ice Cream

Ice cream is sweet, and we have sugar to thank for that. Sugar also plays another important role in ice cream: to lower the freezing point and prevent the formation of large ice crystals. In ice cream, the smaller the ice crystals, the smoother the ice cream. Granulated sugar is most often used, but dextrose and powdered glucose can be used in combination with granulated sugar. Corn syrup, honey, and maple syrup are used to sweeten ice cream, too. Each sweetener contributes a different intensity of sweet taste and quality of sweetness.

Manufacturers of no-sugar-added and reduced-sugar ice cream require different types of sweeteners that provide sweet taste levels close to sugar, similar functionalities to those of sugar, and the ability to work along with other types of ingredients. In no-sugar-added and reduced-sugar ice cream products, high-intensity sweeteners and sugar alcohols are used. Keep in mind that a challenge in using these sweeteners is their effect on the overall texture of the ice cream product.

“Beyond sweetness, freezing point depression of sugars is essential in manufacturing ice creams,” says Wade Schmelzer, principal scientist at Cargill, Minneapolis, Minn. (cargill.com). He explains that alternative sweeteners to sugar can be used, often with other ingredients, to reduce sugar in ice cream and provide some of the sensory characteristics that sugar supplies. For instance, he says that Zerose erythritol can be used to provide the freezing point depression function of sugar in reduced-sugar ice creams. “Since overall sweetness intensity is an integral part of consumers’ taste expectations, it is frequently coupled with stevia leaf extracts,” he explains. “Adjustments to the texturant system may also be necessary to deliver the desired sensory experience. Using this approach, sugar reductions of up to 25% have been possible, but next-generation stevia sweeteners with improved sweetness dynamics, such as Cargill’s ViaTech products, are creating new opportunities to improve taste and achieve deeper reductions.”

Other sweetener manufacturers feature ingredients for sugar reduction in ice cream. Steviva Ingredients, Portland, Ore. (stevivaingredients.com), promotes one of its stevia-based sweeteners as an option for use in reducing sugar in ice cream and other frozen dairy products. Fructevia is a proprietary blend of SteviaSweet 95/60 stevia extract, which itself is a blend of 95% steviosides and 60% rebaudioside A, fructose, inulin, and magnesium carbonate. Ingredion developed MALTISWEET IC maltitol syrup as a replacement for sucrose and corn syrup solids in no-sugar- added and reduced-sugar ice cream products and ENLITEN Reb A stevia sweetener as a naturally derived high-potency sweetener option, says Gonzales. From Tate & Lyle, Hoffman Estates, Ill. (tateandlyle.com), comes an allulose ingredient called DOLCIA PRIMA. This ingredient, which is made of the monosaccharide allulose, is said to depress the freezing point of ice cream, soft-serve ice cream, and frozen yogurt while providing a smooth texture and balanced sweetness and flavor profile. It also works synergistically with other sweeteners. And because it has 90% fewer calories than sugar and is absorbed by the body but not metabolized, the ingredient is well-suited for formulating ice cream with reduced calories, according to the company.

From Nostalgic Favorites to Flavor Adventure

Ice cream may never fall out of favor as a go-to dessert or special treat, especially as product developers continue to innovate and experiment with a number of ingredients to give consumers new flavor and texture experiences and new ice cream novelties to enjoy. Ice cream flavor will continue to be an area of exploration and development, as manufacturers take inspiration from such diverse areas as ingredients and global cuisine, indulgence, and consumers’ interest in nostalgia as ways to drive innovation in the category. Thick and creamy textures along with inclusions like pieces of brownies, fudge, nuts, seeds, and more up the indulgence factor, while continuing to offer favorite ice cream novelties of consumers’ childhood as a way to connect with them. Incorporating ingredients from global cuisines or those traditionally not used in ice cream has really taken off with specialty ice cream producers and independently owned ice cream shops. Tahini, miso, tea, goat cheese, beer, cereal, tropical fruits, vegetables, floral, and chili peppers are some of the ingredients these ice cream makers have used. Whether you like traditional vanilla or stout-spiked ice cream varieties, or novelties in cones, on sticks, or in sandwich form, there is something for everyone.

Next month’s Ingredients section will feature ingredients for fermented and distilled beverages.

Members Only: Read more about ingredients that are used to formulate ice cream at ift.org. Type the keywords into the search box at the upper right side of the home page.

REFERENCES
USDEC. 2007. Whey Products in Ice Cream and Frozen Dairy Desserts Applications Monograph—Ice Cream. U.S. Dairy Export Council, Arlington, Va. thinkusadairy.org.

Introduction

Ice cream is a frozen dairy product made by freezing the ice cream mix with agitation. It is composed of a mixture of food ingredients like milk products, sweetening materials, stabilizers, colors, flavors, and egg products. Ice cream had its origins in Europe and was introduced later to the United States where it developed into an industry. It is widely believed that ice cream evolved from iced beverages and water ices. Ice cream probably came to the United States with the early English settlers. In 1851, the first wholesale ice cream industry in the United States was established in Baltimore, Maryland. Ice cream plants were then established in New York, St. Louis, Chicago, Washington, and Cincinnati. The development of condensed and dried milks and the introduction of the pasteurizer and homogenizer, improved freezers, and other preserving equipments accompanied the growth of this industry after 1900. The ice cream soda was introduced in 1879, and ice cream cone and Eskimo Pie were introduced in 1904 and 1921, respectively.

Global per capita consumption of ice cream and related products is presented in Table 1. The US ice cream market in 2007 was $23 billion. The global value of the ice cream industry is US$59 billion, growing on an average by 4% per annum with a growth rate of 7–9%. More than 90% of American households purchase ice cream. Global market share, according to point of manufacture/consumption, take-home, impulse, or artisanal products, is shown in Table 2. The take-home category is defined as grocery store purchases and home consumption, the impulse category is categorized by hand-held, single-serving eat-on-the-spot products (sometimes referred to as novelty products), and the artisanal category is defined as being manufactured at the site of purchase and consumption or sold directly by the manufacturer to the consumer (e.g., ice cream parlors and street vendors). There are very large differences globally, impacted in large part by the presence of home freezers.

Ice creams are unique frozen food because they are consumed in the frozen state, usually as a scooped product or as a single-serving item, sometimes on a stick and often with other confectionery items. These products rely on a concomitant freezing and whipping process to establish the desired structure and texture. The manufacturing process for most of these products is similar. It involves the preparation of a liquid mix, which is prepared by blending the desired ingredients, followed by pasteurization, homogenization, and aging. It is followed by whipping and freezing this mix dynamically under high shear to soft, semi-frozen slurry. The flavoring ingredients are then incorporated in this partially frozen mix. It is then placed in molds for desired shaping and packaging the product. It is further subject to freezing (hardening) under static, quiescent conditions. Swept-surface freezers are used for the first freezing step, while forced convection freezers, such as air blast tunnels or rooms, or plate-type conduction freezers are used for the second freezing step. A wide variety of ingredients are used to produce ice creams. Even any one kind of ice cream can be made by combining the ingredients in any of several different combinations of ingredients. Ice cream mix is the unfrozen mixture of the ingredients, consisting of all the components of ice cream with the exception of air and flavoring materials. The composition of ice cream is usually expressed as a percentage of its constituents, for example, percentage of milk fat, milk solids-not-fat, sugar, egg solids, stabilizers, and the total solids.

Ice Cream Mix

Ice cream mix comprises milk fat, milk solids-not-fat (MSNF), sweeteners, stabilizers, colors, emulsifiers, and water. Dairy and other ingredients used to supply these components are chosen on the basis of their cost, availability, and quality of the finished product. An account of composition of various frozen dairy desserts is presented in Table 3. The source of fat that is derived from milk ingredients (e.g., cream and butter) is common in North America and many other parts of the world, while fat derived from nondairy sources (e.g., coconut oil and palm kernel oil) is more common in parts of Europe and Asia. The triglycerides in milk fat have a wide melting range of 40 to - 40 °C. Consequently, at refrigeration temperatures, there is always a combination of liquid and crystalline fat within the globules. The resulting solid–liquid ratio at freezer barrel temperatures is important for formation of ice cream structure, as crystalline fat is required for partial coalescence. Nondairy fat sources must also be chosen to provide a suitable solid fat content.

The Milk Solids-Not-Fat

It comprises lactose, casein, whey proteins, minerals, vitamins, and other minor components of the milk or milk products from which they were derived (e.g., skim milk powder, condensed milk, and whey protein concentrate ingredients). Proteins contribute much to the development of the structure of ice cream, including emulsification, whipping, and water-holding capacity. Whipping of proteins in ice cream contributes to the formation of the initial air bubbles in the mix. Emulsification of proteins in the mix arises from their adsorption to fat globules due to homogenization. Viscosity of mix is enhanced due to water holding capacity of proteins, which imparts a desired body to the ice cream, increases the meltdown time of ice cream, and contributes to reduced iciness.

Sweeteners

Sweeteners improve the texture and palatability of the ice cream, impart sweet taste, and enhance flavors. They also lower the freezing point of the mixture, which imparts a measure of control over the temperature–hardness relationship. In determining the proper blend of sweeteners for an ice cream mix, the total solids required from the sweeteners, the sweetness factor of each sugar, and the combined freezing point depression of all sugars in solution (including lactose from the dairy ingredients) must be calculated to achieve the proper solids content, the appropriate sweetness level, and a satisfactory degree of hardness. The most common sweetening agent used is sucrose. It provides sweetness, depresses freezing point, affects freezing performance, affects body and texture, enhances flavor, and contributes bulk or total solids and impacts on economics. Generally, the equivalent of 15% sucrose is considered optimal sweetness in ice cream. Substitute sweeteners derived from starch hydrolysate syrup (glucose syrup) for all or a portion of the sucrose can also be used as it is relatively cheaper. Glucose syrups impart greater smoothness by contributing to a firmer and chewy body, provide better meltdown characteristics, bring out enhanced fruit flavors, and reduce heat shock potential that improves the shelf life of the finished product. Nonnutritive sweeteners, which do not provide any significant calories can also be used for diabetics. They include sucralose, aspartame, saccharin, cyclamates, acesulfame-K, and many others. As little as 0.07% aspartame can provide the sweetness equivalent to 15% sucrose.

Stabilizers

Apart from sweeteners, stabilizers are also important nondairy ingredients. The term stabilizer is used for a group of substances that help stabilize the structure of ice cream. The commonly used stabilizers in ice cream mix are a group of ingredients (polysaccharides) such as guar, locust bean gum, carboxymethylcellulose, and xanthan. Stabilizers increase the viscosity of the mix; produce smoothness in body and texture; retard or reduce ice and lactose crystal growth during storage, especially during periods of temperature fluctuation, known as heat shock; and hold flavorings in uniform suspension. The mechanism of action of stabilizers in enhancing frozen stability is related primarily to its effect on the ice and unfrozen serum phases. Stabilized ice cream has smaller ice crystals than unstabilized ice cream after storage at fluctuating temperatures. Usually, stabilizers are used at 0.1–0.5% levels in the mix, but the actual amount depends on the type and strength of the stabilizer, total solids and fat level of the mix, duration and temperature of storage of ice cream, and the method of pasteurization. High-fat and high-total solid mixes require lower levels of stabilizers. More stabilizer is needed for ice cream that is stored for a long period of time or if the temperature fluctuation during storage is frequent.

Emulsifiers

Emulsifiers are sometimes integrated with the stabilizers in proprietary blends, but their function and action are very different from the stabilizers. They exert their action on the fat phase of ice cream. These are surface-active agents. Emulsifiers facilitate the mixing of fat and water because these molecules have two domains, one that likes water (hydrophilic) and another that likes fat (hydrophobic). When the hydrophobic part of a surfactant interacts with the fat, the water-loving part of the molecule can interact with water, thus facilitating the suspension of fat in water. Generally, mono- and diglycerides and ethoxylated esters of sorbitol (polysorbates) are the com monlyused emulsifiers. Mono- and diglycerides are derived from fatty acids and glycerol. Therefore, emulsifiers are fatty substances. They also show fatlike properties such as melting point and crystallinity, and they can be composed of saturated or unsaturated fatty acids. The presence of emulsifiers in ice cream leads to smoother texture and better shape retention while improving the ability of the mix to incorporate air. The mode of action of emulsifiers is related to their activity at the air–serum and fat–serum interfaces. At the fat–serum interface, they displace proteins from the surface of the fat globules, rendering the fat globules more susceptible to partial coalescence and structure formation during the freezing and whipping process.

Figuring the Mix

Ice cream processing operations can be divided into two distinct stages: figuring the mix and freezing operations. Once a formula has been finalized, a recipe has to be created. Formulas specify composition of the desired mix in terms of percentages of fat, MSNF, sweeteners, stabilizers, and emulsifiers. A recipe calculates the weight and/or volumes of ingredients needed to meet the formula requirements. These calculations are called mix calculations. Mix calculations are essential for manufacturing consistent quality finished products. When composition of raw materials varies or the economics of ingredients changes, the recipe for making an ice cream mix has to change. Further, this change has to occur in a manner that the finished product composition is not altered. In some instances, changing regulatory definitions and health claims may necessitate manufacture of products to carefully defined specifications.

Mix calculations are important in standardizing mixes prior to freezing. Ice cream plants now use computer software programs to calculate the amounts of various ingredients to conform to required specifications of composition of ice cream mix. However, for basic understanding, the fundamentals of the mix calculations are discussed here. Mix calculations can be performed by Pearson’s square, algebraic methods, and arithmetic methods. Pearson’s square is of limited utility; algebraic methods are complicated and involve the use of simultaneous equations and matrices. Arithmetic calculations are simpler and require fewer computations than algebraic methods.

Figuring the mix consists of the following unit operations: combination and blending of ingredients, batch or continuous pasteurization, homogenization, and mix aging. Ingredients are usually preblended prior to pasteurization, regardless of the type of pasteurization system used. Blending of ingredients is relatively simple if all ingredients are in the liquid form, as automated metering pumps or tanks on load cells can be used. When dry ingredients are used, powders are added either through a pumping system under high velocity or through a liquefier that chops all ingredients as they are mixed with the liquid.

Pasteurization

It is designed to destruct pathogenic bacteria, if any are present. In addition, it serves a useful role in reducing the total bacterial load and in solubilizing some of the components (proteins and stabilizers). Both batch and continuous (high-temperature short-time (HTST)) systems are in common use. In a batch pasteurization system, blending of the ingredients is done in large jacketed vats equipped with some means of heating, usually saturated steam or hot water. The product is then heated in the vat to at least 69 °C and held for 30 min to satisfy legal requirements for pasteurization. This is necessary for the destruction of pathogenic bacteria. Various time–temperature combinations can be used, depending on the legal jurisdiction. Continuous pasteurization is usually per formed in an HTST heat exchanger following the blending of ingredients in a large, insulated feed tank. Regulations concerning time–temperature combinations for continuous pasteurization usually specify a minimum temperature of 80 °C for at least 25 s.

Homogenization

Following pasteurization, the mix is homogenized using high pressures. It is responsible for the formation of the fat emulsion by forcing the hot mix through a small orifice under pressures of 15.5–18.9 MPa, depending on the mix composition. The resulting eight- to tenfold increase in the surface area of the fat globules is responsible in part for the formation of the fat globule membrane, composed of adsorbing materials attempting to lower the interfacial free energy of the fat globules. With single-stage homogenizers, fat globules tend to cluster as bare fat surfaces come together or adsorbed molecules are shared, especially in ice cream mix. Therefore, a second homogenizing valve is frequently placed immediately after the first with applied back pressures of 3.4 MPa, allowing more time for surface adsorption to occur. The net effects of homogenization are in the production of smaller, more uniform fat droplet size, resulting in a greater stability of fat droplets during aging, a better whipping ability, and a smoother, more uniform final product with a greater apparent richness.

Aging of the Mix

The time of 4 h or greater at 2–4 °C is recommended for aging following mix processing prior to freezing. Aging is performed in insulated or refrigerated storage tanks, silos, etc. or in single walled tanks in chilled rooms, where valves and pipeline can also be kept cold. Aging allows hydration of milk proteins and stabilizers (some viscosity increase occurs during the aging period), crystallization of the fat globules, and a membrane rearrangement to produce a smoother texture and better quality product. Nonaged mix exhibits a low viscosity, is very wet at extrusion from the dynamic freezer, and exhibits variable whipping abilities. The appropriate ratio of solid–liquid fat must be attained at this stage, a function of temperature and the triglyceride composition of the fat used, as a partially crystalline emulsion is needed for partial coalescence in the whipping and freezing step. Emulsifiers generally displace milk proteins from the fat surface during the aging period. The whipping qualities of the mix are usually improved with aging. Mix temperatures should be maintained as low as possible without freezing.

Flavorings

Flavor is an important attribute of a food. It is a sensory response that has three components, olfactory (odor/smell), gustatory (taste), and tactual (mouthfeel). Ice cream is cold, creamy, refreshing, and sweet and releases aroma upon melting in the mouth. When the word ‘flavor’ is used in everyday parlance, we imply taste and olfaction. Taste compounds are sweet, salty, bitter, and sour. Generally, compounds imparting these tastes can be detected at levels of 0.01–0.5. Olfactory compounds (smelly stuff) are volatile and have thresholds in the parts per million to parts per trillion range. Threshold is defined as the minimum concentration that at least 50% of the population can detect. Threshold of aroma compounds are 10–10 million times less than taste compounds.

Perception of aroma is affected by the composition, physical structure, and temperature of the food. Undesirable flavors are called off-flavors. Off-flavors affect the overall flavors qualities of the food. Deteriorative reactions are time-dependent and cumulative. Therefore, the length and conditions of storage have a profound influence on the perception of overall flavors. These deteriorative reactions occur in ingredients used in ice cream manufacture. Therefore, careful attention should be paid to the quality of ingredients used in ice cream manufacture.

Glass Transition Temperature

Temperature stability of ice cream has focused on carbohydrate glass formation as a function of temperature. A glass is characterized as a metastable solid with a high viscosity. At the glass transition temperature, polymeric materials change from a viscoelastic liquid to an amorphous solid (glass) with an associated increase in viscosity. For ice creams, this temperature is defined as the glass transition temperature of a maximally freeze-concentrated solution. The interest in glassy state in ice creams has increased steadily since the last 15 years. It is now well established that the glassy-state temperature influences the storage stability as above these temperatures, the solutions are unstable and reactive. Ice crystallization can also occur in this phase. Below the glass transient temperature, the materials become glossy and no molecular motion or chemical reaction occurs. The ice crystallization is prevented and the shelf life of the product is prolonged. Glass transition temperature for ice cream is -32 °C. Storage of ice cream at this low temperature to maintain the glassy state for enhanced stability is not practical.

Freezing Process

The process of freezing of ice cream in simple words is the creation of ice from water in the mix. Therefore, the only constituent of the mix being frozen is water. During the freezing process, the equilibrium between water and ice is altered. Freezing is facilitated by the removal of heat from a substance. In the old salt and ice machine, used prior to mechanical refrigeration, ice served as the refrigerant and addition of salt lowered the freezing point of water. The brine extracts heat from the mix. The mix temperature is lowered and the brine temperature increases. Brine is not a good refrigerant. With the advent of mechanical refrigeration, the use of ice and salt for freezing ice cream was relegated to a hobby status.

Modern ice cream freezing consists of two distinct stages: (1) passing the mix through a swept-surface heat exchanger under high-shear conditions to promote extensive ice crystal nucleation and air incorporation and (2) freezing the pack aged ice cream under conditions that promote rapid freezing and formation of small ice crystals. The freezing and whipping process is one of the most important unit operations for the development of quality, palatability, and yield of finished product. Due to the incorporation of air, foam is created. It leads to the formation of the ice phase and the partial destabilization of the fat emulsion. The objective of ice cream manufacturers is to produce ice crystals that are below the threshold of sensory detection at the time of consumption. This threshold has been suggested to be between 40 and 50 mm. Therefore, the freezing steps of the manufacturing process and the temperature profile throughout the distribution system are critical factors in meeting this objective. Flavoring and colorants are added as desired to the mix prior to passing through the barrel freezer, and particulate flavoring ingredients, such as nuts, fruits, candy pieces, and ripple sauces, can be added to the semifrozen product at the exit from the barrel freezer prior to packaging and hardening.

Continuous Freezers

Continuous freezers are commonly used in larger ice cream manufacturing plants where more than 500 gal (1875 l) of ice cream per day may be manufactured. These freezers have larger capacities and can be operated continuously, ingredients can be added in-line, and packaging can be also automated. Also, continuous freezers make it possible to produce ice cream of different shapes through extrusion devices. Novelty extrusions, such as sandwiches, prefilled cones and cups, and cakes, are possible through the use of continuous freezers. The ice cream from a continuous freezer is smoother and creamier than a product from a batch freezer. This is because the ice crystals formed in a continuous freezer are smaller and the air cells may also be more uniform. The ice cream exiting a continuous freezer is also generally colder than that coming out of a batch freezer. There are a number of different types of continuous ice cream freezers; some are vertical freezers, especially for smaller-scale operations; others are horizontal ice cream freezers. Regardless of whether the freezing cylinder is horizontal or vertical, all continuous freezers have a set of blades for scrapping the walls of the freezers. In a continuous freezer, a mixture of air and the mix is introduced at one end and is progressively frozen until ice cream is discharged at the other end. The conveyance of the mixture of air and the mix and the discharge of the ice cream may be facilitated by coordinated pumps in some models. Also, the newer models of freezers are equipped with microprocessor controls that monitor and control the discharge temperature of the ice cream, the viscosity of the ice cream, and the overrun of the ice cream. Further, these microprocessors can work in tandem with other downstream equipment such as ingredient feeders and packaging lines.

These freezers dominate the commercial ice cream industry. In this type of process, ice cream mix is drawn from the flavoring tank into a scraped-surface heat exchanger, which is jacketed with a liquid, boiling refrigerant (usually ammonia; a chlorinated, fluorinated hydrocarbon (CFC) such as R-12, R-22, or R-502; or one of the newly developed CFC substitutes).

A continuous freezer is usually operated under pressure, to minimize the volume of air in the ice cream and thus maximize heat transfer. Air bubbles expand rapidly when the ice cream is drawn through the outlet valve to atmospheric pres sure. The introduction of low-temperature extrusion equipment has been a recent development in ice cream freezing technology. This is a continuous-flow heat exchanger in the form of a screw extruder that reduces the temperature of ice cream from the draw temperature of the barrel freezer, typically -5 °C, to a range of -10 to -14 °C. The ice cream passes at low shear through a twin-screw or single-screw design with a residence time of 1–2 min. Ice cream that results from such processing shows much smaller ice cream and air bubble sizes, and the fat is agglomerated into a structure that provides a creamy texture.

When the product emerges from the low-temperature extruder, it is still pliable, so some particulates can be added to it, and it can be cut and packaged, but it sets up very quickly thereafter to a very firm consistency. Hence, the claim is made that low-temperature extrusion can replace the need for conventional hardening of ice cream.

Batch Freezers

Batch freezers are commonly used by small ice cream units that make ice cream on the premises. In batch freezers, a predetermined amount of mix is charged into the freezing chamber; refrigeration is turned on as is the agitation. Generally, the mix will occupy half of the barrel. The mix is agitated and whipped while being cooled. After some time, the mix begins to freeze, and when it achieves a certain consistency, it begins to incorporate air. Incorporation of air in conjunction with the freezing stiffens the ice cream. At this point, the refrigeration should be turned off and agitation continued for some additional period of time. When the desired overrun is achieved, the ice cream is discharged from the barrel with the agitator mechanism still on.

Just prior to discharge of the ice cream, fruits and nuts can be added to the barrel, but the preferred method of addition of particulate inclusions is to fold them into the ice cream as they are being discharged from the barrel. Once this process is complete, the next batch of mix can be charged into the freezer barrel and the process repeated.

The batch freezer differs considerably from the continuous systems described in the preceding text. The barrel of a batch swept-surface heat exchanger is jacketed with refrigerant and contains a set of dashers and scraper blades inside the barrel. It is filled to about one-half volume with the liquid mix. Barrel volumes usually range from 2 to 12 l. The freezing unit and agitators are then activated, and the product remains in the barrel for sufficient time to achieve the desired degree of over run and stiffness. Whipping increases with time and cannot exceed the amount that will fill the barrel with the product (i.e., 100% overrun when starting half full). Batch freezers are used in smaller operations where it is desirable to run individual flavored mixes on a small scale or to retain an element of the ‘homemade’-style manufacturing process.

The formation of the ice phase during the continuous freezing stage is responsible in part for the structure and texture of the final product. The crystallization of water to ice involves two major steps: nucleation and crystal growth. Nucleation occurs at the wall of the heat exchanger during start-up. After start-up, the continual scraping action of the blades acts as a seeding mechanism, by providing a source of tiny crystals into the bulk where they grow.

Overrun in Ice Cream

Incorporation of air into ice cream, termed the overrun, occurs at this stage. Some overrun is necessary to produce desirable body and texture. Two main types of air incorporation systems are used in continuous freezers. In older systems, the pump configuration resulted in a vacuum either at the pump itself or on the mix line entering the pump. Air was then incorporated through a spring-loaded, controllable needle valve. Newer types of freezers use compressed air, which is injected into the mix. This type of air handling system allows for air filtration (0.65 μm micropore filter) prior to admission into the mix.

Following aeration, the water in the mix is partially frozen as the mix and air combination passes through the barrel of the heat exchanger. Ice forms on the inside wall of the heat exchanger from the water in the mix, resulting in a freeze concentration of the dissolved solutes. Rotating knife blades are responsible for continually scraping this frozen mix off the surface of the heat exchanger wall and mixing it into the bulk flow of freeze-concentrated liquid mix, where these tiny ice crystals grow. The dasher keeps the product agitated inside the barrel and provides a more homogeneous mixture of ice and freeze-concentrated liquid. Residence time for mix through the annulus of the freezer varies from 0.4 to 2 min, although some products may remain for much longer times; especially with open cage dashers, freezing rates can vary from 5 to 27 °C min^-1, and a draw temperature of -6 °C can easily be achieved.

Hardening of Ice Cream

After freezing, the flavors, nuts, chocolates, etc. are added, and the product is packed and immediately transferred to a hardening chamber (-30 °C or colder, either forced convection or plate-type conduction freezers) where the majority of the remaining water freezes. Rapid initial freezing is essential to set up as many crystal nuclei as possible so that during the maturation or growth stage, their size stays small. In the same context, rapid hardening is also necessary to keep ice crystal sizes small.

Further temperature reduction during hardening accounts for continued growth of the preformed crystals. As water freezes out of solution in a relatively pure form, the formation of ice acts to concentrate the dissolved sugars, lactose, milk proteins, salts, and hydrocolloids in an ever-decreasing amount of water. Water and its dissolved components are referred to as the serum or matrix of the mix. Because the freezing point of the serum is a function of the concentration of dissolved solids, formation of more ice concentrates the serum and results in an ever decreasing freezing temperature for the remaining serum. Therefore, at temperatures several degrees below the initial freezing temperature, there is always an unfrozen phase present. Ice cream hardness is a function of temperature due to its effect on this conversion from unfrozen water to ice and further concentration of the serum phase surrounding the ice crystals, which helps to give ice cream its ability to be scooped and chewed at freezer temperatures.

Structure Formation During Manufacture

The texture of ice cream is perhaps one of its most important quality attributes. It is the sensory manifestation of structure. Hence, establishment of optimal ice cream structure is critical to maximal textural quality. The structure of ice cream begins with the mix as a simple emulsion, with a discrete phase of partially crystalline fat globules surrounded by an interfacial layer composed of proteins and surfactants. Ice cream is a complex food colloid in that the mix emulsion is subsequently foamed, creating a dispersed phase of air bubbles, and is frozen, forming another dispersed phase of ice crystals. Air bubbles and ice crystals are usually in the range of 20–50 μm. The serum phase is freeze-concentrated. In addition, the partially crystalline fat phase at refrigerated temperatures undergoes partial coalescence during the concomitant whipping and freezing process, resulting in a network of agglomerated fat,

which partially surrounds the air bubbles and gives rise to a solid-like structure.

Packaging of Ice Cream

A good package must contain the product, protect it, provide convenience, and provide information on the product to the consumer. Food packages provide protection against physical, chemical, and biological damages. It also provides information useful to the consumer, for example, ingredient label, nutritional label, net contents, serving suggestion, and methods of preparing the product. Besides these attributes, a good food package keeps the food at nearly the same quality as when it was manufactured. During distribution, packages are subjected to physical abuses such as shocks, vibrations, compression, and, in the case of ice cream and frozen desserts, heat shock.

For frozen dessert packaging, three main factors have to be considered. First, the package has to protect the product against temperature fluctuations, photooxidation, dehydration, and odor transmittance. Second, it has to take into consideration distribution-related factors such as package integrity, thermal shock, and cube efficiency. Third, municipal solid waste management factors have also to be considered.

Storage and Distribution

Frozen and hardened product is stored and often distributed prior to the enjoyment by the end consumer. The intermediate steps involved in storage vary depending upon the scale of manufacture, market share, point of sale, and consumer preferences. In the simplest case of a retail ice cream manufacturer, the product is made fresh in the store and sold very soon after manufacture, and this requires relatively few controls. Ice cream is unique in that it is the only product that is consumed in the frozen state. Therefore, once it is manufactured, it has to be stored, transported, distributed, and sold in the frozen state. In the United States, frozen foods are distributed in a separate chain than ice cream is because the cold chain for frozen foods is -18 °C (0 °F) and is inadequate for ice cream. Ice cream cold chain maintains -23 °C (-10 °F). The distribution chain is called the cold chain and varies from manufacturer to manufacturer. Regardless of variations, the cold chain is imperfect. This imperfection affects the quality of the product at the point of purchase and impacts consumer satisfaction. Factors affecting the shelf life of ice cream are manufacturing procedures, warehouse equipment, warehouse handling practices, transportation, storage at retail premises, retail display equipment, and retail handling practices.

Uses of Ice Cream

Ice cream is such a frozen dessert that is liked by all age groups. It is an energy-rich product and may be a part of countless recipes and gastronomic specialties. It can be used as a most promising carrier medium for probiotic organisms.

See also: Ice Cream: Composition and Health Effects.

Further Reading

Berger KG (1997) Ice cream. In: Friberg SE and Larsson K (eds.) Food emulsions, 3rd ed., pp. 413–490. New York: Marcel Dekker Inc.
Gaff HO (2002) Formation and stabilization of structure in ice cream and related products. Current Opinion in Colloid and Interface Science 7: 432–437. Goff HD (1997) Colloidal aspects of ice cream – a review. International Dairy Journal 7: 363–373.
Goff HD (2003) Ice cream. In: Fox PF and McSweeney PLH (eds.) Proteins. Advanced dairy chemistry, vol. 1, pp. 1063–1082. New York: Kluwer Academic, 3rd ed.
Goff HD (2006) Ice cream. In: Fox PF and McSweeney PLH (eds.) Lipids. Advanced dairy chemistry, vol. 2, pp. 441–450. New York: Kluwer Academic, 3rd ed. Goff HD (2009) Ice cream. In: Fox PF and McSweeney PLH (eds.) Lactose, water, salts and minor constituents. Advanced dairy chemistry, vol. 3, pp. 69–79. New York: Springer, 3rd ed.
Goff HD and Tharp BW (eds.) (2004) Ice cream II. Brussels, Belgium: International Dairy Federation, Special Issue 401.
Hartel RW (1996) Ice crystallization during the manufacture of ice cream. Trends in Food Science and Technology 7: 315–321.
Kyle SB and Stice E (2013) Elevated energy intake is correlated with hyper responsivity in attentional, gustatory, and reward brain regions while anticipating palatable food receipt. American Journal of Clinical Nutrition 97: 1188–1194.
Marshall RT, Goff HD, and Hartel RW (2003) Ice cream, 6th ed. New York: Kluwer Academic.
Walstra P, Wouters JTM, and Guerts TJ (2006) Dairy science and technology, 2nd ed. New York: CRC Taylor and Francis.

Relevant Websites

http://ajcn.nutrition.org/content/early/2012/02/14/ajcn.111.027003.abstract – American Journal of Clinical Nutrition.
http://www.berryondairy.com/Milk.html – Berry on dairy.
http://www.biomedcentral.com/1471-5945/12/13 – British Medical Council.

There is perhaps no fonder childhood memory than the local ice cream truck driving through the neighborhood, music blaring from its tinny speakers, beckoning all to partake of its frosty delights. But ice cream is not just for kids. U.S. residents consume 1.5 billion gallons of ice cream each year; that’s roughly 5 gallons (19 liters) per person! The ice cream we all enjoy is the result of years of experimentation involving—you guessed it—chemistry!

Air is Important!

If you have ever made ice cream, you already know what goes into it, ingredients such as milk, cream, and sugar. But there is one main ingredient that you may not have thought about, probably because you can’t see it—air.

Why is air so important? If you have ever had a bowl of ice cream melt, and then refroze it and tried to eat it later, it probably did not taste very good. If you set a whole carton of ice cream on the table and let it melt, the volume of the ice cream would simply go down. Air makes up anywhere from 30% to 50% of the total volume of ice cream.

To get an idea of the effect of air on ice cream, think of whipped cream. If you whip air into cream, you get whipped cream. Whipped cream has a different texture and taste than plain cream. Plain cream tastes sweeter than whipped cream. Just like ice cream without air, pure cream has a sickly, overly sweet taste. This is because the structure of a substance can have a big effect on how it tastes, and that the structure often controls the rate at which flavor molecules are released into the mouth. The larger the structure (ice cream, in this case), the longer it takes for the flavor molecules to be released. Flavor molecules that trigger receptors on the mouth and tongue.

The amount of air added to ice cream is known as overrun. If the volume of ice cream is doubled by adding air, then the overrun is 100%, which is the maximum allowable amount of air that can be added to commercial ice cream. The less expensive brands usually contain more air than the premium brands. One side effect of adding a lot of air to ice cream is that it tends to melt more quickly than ice cream with less air.

The amount of air also has a huge effect on the density of ice cream. A gallon (3.8 liters) of ice cream must weigh at least 4.5 pounds, making the minimum density 0.54 gram per milliliter. Better brands have higher densities—up to 0.9 grams per milliliter. The next time you visit a grocery store, compare cheaper and more expensive brands by holding a carton in each hand—you should be able to notice a difference. Then read the net weight on the label to confirm your observation. Due to the high fat content of ice cream, however, and because fat is less dense than water, any ice cream will always be less dense than any aqueous solution, otherwise you would not be able to make root beer floats!

Ice cream is an emulsion—a combination of two liquids that don't normally mix together. Instead, one of the liquids is dispersed throughout the other. In ice cream, liquid particles of fat—called fat globules—are spread throughout a mixture of water, sugar, and ice, along with air bubbles (Fig. 1). If you examine ice cream closely, you can see that the structure is porous. A typical air pocket in ice cream will be about one-tenth of a millimeter across. The presence of air means that ice cream is also a foam. Other examples of foams are whipped cream, marshmallows, and meringue (as in lemon meringue pie).

Sugar and Fat

Milk naturally contains lactose, or milk sugar, which is not very sweet. Ice cream makers need to add a lot more sugar than you probably realize—usually sucrose or glucose. Cold tends to numb the taste buds, making them less sensitive. So more sugar needs to be added to produce the desired effect at the low temperatures in which ice cream is usually served. If you taste ice cream at room temperature it will taste overly sweet. You may have noticed this same effect with carbonated soft drinks. If consumed warm, they taste sickly sweet. In parts of the world where soft drinks are normally consumed warm, there is less added sugar. If these same soft drinks were served cold, they would not taste sweet enough.

A big reason why ice cream tastes so good is because of its high fat content. Unless it is labeled as light, low-fat or non-fat, ice cream must contain at least 10% fat, and this fat must come from milk. (You cannot use lard when making ice cream!) Before milk is homogenized, a thick layer of cream rises to the top. This cream has a high fat concentration—up to 50%—and supplies most of the fat in ice cream.

Premium ice creams may have up to 20% fat, which gives it a velvety, rich texture. Reduced fat ice cream does not taste as good as the real thing, and tends to lack the creamy texture. Although fat is frequently vilified, it has its purpose. Most foods that taste delicious probably contain fat. Fat fills you up, so you don’t have to eat as much to feel full.

The problem with using fat as an ingredient in any food is that it doesn’t mix well with a lot of other substances. Fat is nonpolar, meaning positive and negative charges within the fat molecule are equally dispersed. A polar substance, such as water, has separate regions of positive and negative charge—one end of a polar molecule has a partial positive charge, and the other end has a partial negative charge. Polar and nonpolar substances do not mix. Just like oil floats to the top of water, the fat content in ice cream has a tendency to separate out, as well.

Keeping It All Together

Because ice cream is an emulsion, you would expect that the fat droplets that are present in the mixture would separate after some time, similar to a bottle of salad dressing in which the oil separates from the rest of the dressing. When you shake up a bottle of salad dressing, the two parts come together. But after a few minutes, they begin to separate. That’s because the oil droplets interact with one another, a process called coalescence.

In the case of milk, each fat droplet is coated with a layer of milk proteins that prevents the fat droplets from interacting with one another. These milk proteins act as “emulsifiers”— substances that stabilize emulsions and allow the liquid droplets present in the emulsion to remain dispersed, instead of clumping together. Because these milk proteins have a nonpolar side, and because like dissolves like, the nonpolar sides of the proteins are attracted to the nonpolar fat globules. This is good in milk, but not so good in ice cream, in which the fat droplets should coalesce to trap air.

So another emulsifier is added to allow the fat droplets to coalesce. This emulsifier replaces milk proteins on the surface of the fat droplets, leading to a thinner membrane, which is more likely to coalesce during whipping. A common emulsifier is lecithin, found in egg yolks. Lecithin is a generic term that refers to a group of molecules that consist of long chains of fatty acids linked to a glycerol molecule, along with choline and a phosphate group (Fig. 2).

Lecithin inserts itself between the fat globules, which helps the fat globules to clump together and, as a result, the air bubbles that are present in the mix are trapped by this partially coalesced fat. This adds firmness and texture to the ice cream, enabling it to retain its shape.

Closely related to emulsifiers are stabilizers, which make the texture creamy. Stabilizers have two roles: First, they prevent large crystal formation. In the presence of stabilizers, ice cream contains small ice crystals that are easier to disperse and, therefore, they melt more slowly than larger ice crystals would. Second, emulsifiers act like a sponge by absorbing and then locking into place, any liquid in the ice cream.

Common stabilizers are proteins such as gelatin and egg whites. Guar gum, locust bean gum, and xanthan gum can also be used. Look for carrageenan and sodium alginate on the ingredient label of your ice cream container. Both are derived from seaweed! Without these stabilizers, ice cream might look like a milkshake.

Once you get all of the ingredients together in a mixture, you need to freeze the mixture to form ice cream. The dissolved solutes (mostly sugar) in the liquid portion of the mixture lower its freezing point. A freezing point depression of 1.86 °C occurs for every mole of solute added to 1 kilogram (kg) of water. In other words, if you dissolve one mole of sugar in 1 kg of water, water will no longer freeze at 0 °C, but rather will freeze at –1.86 °C.

Freezing point depression is a colligative property, meaning that the effect is observed regardless of the specific identity of the solute—all that matters is how many moles are dissolved. A typical batch of ice cream will freeze at -3 °C (27 °F), due to the presence of all the dissolved solutes.

A recent trend is ice cream made with liquid nitrogen. One shop in San Francisco, Calif., aptly named Smitten Ice Cream, has a viewing area where customers can watch ice cream being made with liquid nitrogen, accompanied by the impressive plume of fog that is released. Liquid nitrogen, which boils at –196 °C, will freeze ice cream almost instantly. Because the ice cream freezes so quickly, the size of the crystals is small, resulting in a creamy texture. And because it boils when it hits the mixture, the ice cream is aerated during the process. The popular Dippin’ Dots are also made using liquid nitrogen. It is no exaggeration to say that ice cream made with liquid nitrogen is the coolest ice cream around!

Types of Ice Cream

Soft-serve ice cream, frozen custard, and frozen yogurt. What is the difference?

Regular ice cream is typically served at –12 °C, while soft-serve ice cream is served at –6 °C. This higher temperature is responsible for a softer product. Soft-serve ice cream, or soft serve, for short, contains less fat and more air than regular ice cream. Soft serve with insufficient air will have a yellowish color. The whiter the soft serve, the better the quality. As ice cream melts, you may have noticed this yellow color, which is simply the actual color of the ingredients used to make it. By adding air and fluffing it up, ice cream is better able to reflect white light, producing the white color. This is because the molecules in ice cream are large enough to reflect visible light (whereas, for example, water molecules are too small to reflect visible light, because the size of a water molecule is smaller than the wavelengths of visible light).

Frozen custard differs from ice cream in that it contains at least 1.4% egg yolks. The egg yolks are made of lecithin, an excellent emulsifier. The result is a product with a smoother, creamier texture. Another difference is that custard contains much less air than ice cream. No air is mixed during its manufacture; instead, air is introduced during mechanical agitation as the frozen custard is being made. It is churned more slowly during its manufacture to minimize the amount of introduced air. Less air leads to a thicker, denser product. Frozen custard is typically made fresh each day in the store. It is frozen quickly to prevent large crystals—of water, lactose, or any added sugar—from forming.

Frozen yogurt is making a huge comeback these days, with self-serve frozen yogurt shops offering a plethora of toppings popping up seemingly on every corner. Frozen yogurt is viewed as a healthier alternative to ice cream, unless you top it off with a generous helping of gummy bears! It does contain less fat, but that means you can eat more without feeling full. And to compensate for less fat, often a lot of sugar is added. The biggest difference is that instead of cream, yogurt is added as the primary dairy product. From there, the process is similar to making regular ice cream.

Selected References

• Gooch, A. “The Chemistry behind Ice Cream.” Chicago Tribune, June 30, 2004: http://articles.chicagotribune.com/2004-06-30/entertainment/0406300068_1_ice-cream-homemade-ice-ice-crystals-form [accessed Dec 2013].
• Halford, B. “Ice Cream: The Finer Points of Physical Chemistry and Flavor Release Make this Favorite Treat so Sweet.” Chemical & Engineering News, Nov 28, 2004: http://pubs.acs.org/cen/whatstuff/stuff/8245icecream.html [accessed Dec 2013].
• Kilara, A.; Chandan, R. C.; Hui, Y. H. “Ice Cream and Frozen Desserts.” Handbook of Food Products Manufacturing, John Wiley Online Library, Chapter 74, pp 593–633, Aug 1, 2006: http://www.researchgate.net/publication/227580162_Ice_Cream_and_Frozen_Desserts/file/9c9605151b162a696c.pdf [accessed Dec 2013].

Brian Rohrig teaches chemistry at Metro Early College High School in Columbus, Ohio. His most recent ChemMatters article, “Hot Peppers: Muy Caliente!” appeared in the December 2013 issue.