| Ice Cream:|
August 1997 -- Applications
By: Suanne J. Klahorst
The complex physical structure of ice cream presents a challenge for food chemists, who readily concede it's not fully understood. Despite this, food product designers know how to manipulate these structures, creating a wide variety of products packing consumer appeal. When discussing ice cream stabilization, it's necessary to specify the product's composition, since its composition and ratios are critical to stabilizing the various physical phases. A basic understanding of standard ice cream's structure forms a foundation on which product developers create new architecture for its construction.
Water, ice, air, sugar, milk fat and milk protein can be assembled into innumerable combinations, each with unique physical chemistry. Ice cream's sensory attributes, particularly mouthfeel, dictate that ingredient and processing variables in its production strive for as much homogeneity as possible, even though ice cream is far from homogeneous. Simply stated, ice cream design's overall goal is: incorporating several different insolubles (air bubbles, ice crystals and fat globules) into an aqueous phase at the smallest sizes, and in the greatest numbers, possible.
Ice cream stabilizers provide several functions. They maintain homogeneity and control ice-crystal growth during the freezing/aeration process. During storage, stabilizers play a role in resisting structural changes during "heat shock," the inevitable temperature-cycling during storage and distribution that creates ice-crystal growth and other types of deterioration, due to structural changes. During serving and consumption, stabilizers contribute to uniform meltdown, mouthfeel and texture. A stabilized ice cream is one that resists or retards structural changes in a dynamic environment. Although a group of viscosity-producing gums and hydrocolloids exists (generally referred to as "the stabilizers"), it's important to note that other elements also contribute to ice cream stabilization: naturally occurring milk proteins; emulsifiers; and heat and mechanical processing (pasteurization, homogenization and freezing).
Taking many forms
A survey of 69 vanilla ice creams and frozen yogurts available in California, conducted by Christine Bruhn, Ph.D., and John Bruhn, Ph.D., of the University of California, indicates that, per serving, fat content ranges from 0 to 18 g; protein content ranges from 1 to 11 g; and sugar varies from 3 to 24 g. The ice cream varieties that have replaced the now-defunct "ice milks" of yesteryear are subject to a new taxonomy of fat offerings per serving: low fat (3g or less); light (one-third less than standard); reduced fat (25% less than standard); and nonfat (0.5g or less). "Standard" ice cream is minimally 10% milkfat and 10% milk solids. Ice cream composition is described for several standard product offerings in the accompanying table. Ice cream often is described in terms of two phases: continuous and dispersed. The continuous phase is a combination of an unfrozen solution, an emulsion, and a suspension of solids in liquid. Water, sugar, hydrocolloids, milk proteins and other solubles make up the unfrozen solution. Suspended in the aqueous phase are insoluble solids, including ice crystals, lactose crystals and milk solids. The aqueous phase also forms an emulsion with dispersed milkfat globules.
The dispersed phase is a foam, consisting of air bubbles dispersed in liquid and emulsified fat. In a product with 100% overrun (percent volume of air added in relation to the original volume of mix), air accounts for 50% of the product volume. A thin layer of adsorbed milk proteins -- caseins, casein submicelles, whey proteins -- on which partially aggregated milkfat globules are embedded, surround the individual air cells. This layer forms an air-water interface called a "lamella," which possesses mechanical properties defining air-cell stability and size. Fat agglomerates enhance whippability and foam structure, because they strengthen the lamella. Most of the fat and water are in the crystalline state. The air cells and ice crystals form a coarser dispersion than the fat globules. The colloidal nature of ice cream components introduces an inherent instability into ice cream that must be controlled during processing, storage and delivery.
Properties valued Quality ice cream possesses a smooth and creamy mouthfeel, influenced by size, distribution, shape and number of ice crystals. Several types of natural gums can control ice-crystal growth. However, these have limited effects, because the level of addition to effectively retard crystal growth results in an unacceptable gummy texture. To avoid this, several gums with different properties are used together, or in combination, including: alginates, guar, locust bean, xanthan and carrageenan. Alternatively, chemically modified cellulose gums, such as carboxymethylcellulose (CMC), increase viscosity to a lesser extent than natural gums. Insoluble microcrystalline cellulose (MCC) disperses to form a cellulose gel.
Most polysaccharide ice cream stabilizers influence the rheological properties of the continuous phase. They either increase viscosity or form gels during aging, freezing and storage. Some stabilizers form a complex with ice cream constituents. For example, carrageenan complexes with casein and prevents whey separation during mixing. However, when used alone, some stabilizers -- including locust bean gum (LBG), guar, CMC and xanthan gum -- promote whey separation, due to their tendency to precipitate proteins during heating at neutral pH. Ice cream that is stabilized with LBG and carrageenan contains significantly smaller ice crystals than ice cream made under identical conditions without stabilizers. Microcrystalline cellulose also facilitates ice-crystal growth -- one theory credits MCC with providing nucleation sites, resulting in smaller, more uniform crystals in larger numbers. Combining cellulose gums with natural gums can control ice-crystal growth, without imparting excessive viscosity. During storage, stabilizers may slow down ice-crystal growth during heat shock by limiting water migration. This is attributed to their water-holding capacity and formation of a three-dimensional network between stabilizers and other components, especially sugars and proteins.
Stabilizers afford other functionalities, particularly in lower-fat products. They increase stiffness; provide a slower, more uniform meltdown; enhance whippability during aeration; prevent lactose crystallization; prevent shrinkage during storage; stabilize the emulsion; and contribute to body, texture and creaminess.
Maintaining emulsion control Emulsifiers create and maintain ice cream emulsions by controlling the extent of fat agglomeration at the lamella interfaces. These emulsifiers include mono- and diglycerides , ethoxylated esters of sorbitol (polysorbates), polyglycerols, and lecithin (or egg yolk).
Emulsifiers are described by their hydrophobicity-to-hydrophilicity ratio (HLB). Those with a high HLB are hydrophilic; those with low HLB are lipophilic. The fact that fat sticks together is commonly known to anyone who has churned butter from cream. By lowering the interfacial tension between the oil/water interface, emulsifiers prevent coalescence of fat globules during processing. They establish and maintain a more stable structure around the air-cell walls. Emulsifiers stabilize air cells through their effect on milkfat aggregation, thus preventing air cell growth. Since air cell size is related to mouthfeel, emulsifiers enhance the sensory perception of smoothness.
Process relations In order to function as a water binder, stabilizers require time and specific conditions for proper hydration. For gums that interact with each other, the shear forces created during pasteurizing and homogenizing can supply the necessary mechanical energy to promote binding. For gel-forming stabilizers, the term "activation" often is used to describe gel-structure initiation.
Shear forces and heat created during mixing, pasteurization, homogenization and the hold times during aging, all potentially affect stabilizer functionality. Shear during homogenization can break down hydrocolloid molecules, reducing viscosity. Or, it may enhance dispersion and maximize interactions with other ingredients. Optimizing a stabilizer system begins with choosing the right blend for the process. Florian Ward, Ph.D., vice president and director of R&D for TIC Gums, Inc., Belcamp, MD, encourages product developers to share processing information. "New customers are reticent to share information about their process in the beginning," Ward says. "But as mutual trust is established, the customer can benefit from continuous technical service during product development, or in the event of processing changes."
Stabilizers and emulsifiers usually are added after the other ingredients to the blending tank during batching and mixing. In general, dry ingredients are easier to blend when the liquid portion of the mix is warm. Some stabilizers -- such as gelatin, CMC, carrageenan, LBG -- can be dispersed into a cold mix and hydrated during the subsequent processing. Stabilizers can be prehydrated for greater convenience. One prehydration method is achieved by an agglomeration process. A single stabilizer or blend of stabilizers is aggregated, resulting in a slightly higher moisture than the original materials. The added cost for this process is considered a reasonable tradeoff for the reduction in equipment time and increased throughput.
For nonprehydrated stabilizers, dry ingredients generally are preliquefied in part of the mix before addition. Precise timing often is required, since premature hydration can lead to fouled equipment or foam that interferes with the pasteurization and homogenization steps.
•Pasteurization -- either HTST or UHT -- affects the incorporation of microingredients. Some stabilizers inhibit pasteurization by slowing the flow rate over the heat exchanger. Guar gum often is implicated in "burn-on" in UHT pasteurization. Higher temperatures or longer hold times can denature milk proteins. This improves their water-binding properties, and reduces the levels of stabilizer required. It also can help dissolve gums that need heat to properly hydrate, such as LBG.
Homogenization reduces fat-globule size. This, in turn, increases the number of globules up to eight times, and increases the fat's surface area. This process must occur when the fat is liquid, at 50°C or higher. Emulsifiers are critical to maintaining small fat globules during subsequent processing.
As new equipment technologies are developed, new stabilizing systems will be developed for enhanced compatibility with new processes.
"High pressure homogenization technologies are now being utilized in the ice cream industry," says Bruce Tharp, Ph.D., owner of Tharp's Food Technology, Wayne, PA. "Homogenization at four to five times the normal pressure (12,000 rather than the usual 2,500 PSI) can decrease the size, and increase the number of fat globules in the ice cream product, providing better distribution of the available milkfat in a fat-reduced product."
Following pasteurization and homogenization, the mix is cooled and sometimes held for aging before freezing. This provides additional hydration time for stabilizers. Carrageenan prevents serum separation (wheying-off), especially during hold times of several hours.
Tharp notes that new process changes enhance air-cell stability: "A process innovation in ice cream aeration is an added step, called 'preaeration,' in which air cells are incorporated into the mix before the freezing step, rather than the conventional method of aerating and freezing simultaneously. By separating the processes into a sequence, a smaller, more stable air-cell structure can be achieved."
Stabilizer repertoire The list of ice cream stabilizers includes the following ingredients: Gelatin. The first commercial ice cream stabilizer, gelatin forms a gel in the mix as well as during aging, preventing large ice crystals from forming during freezing. Levels for a 250 Bloom gelatin are between 0.25% and 0.50% of the mix. Gelatin requires two to four hours of aging to fully develop its gelling capabilities. Its use in stabilizer blends has decreased due to availability of alternatives. Alginates. Alginates add a type of body and texture to ice cream other gums don't easily duplicate. Extracted from ocean kelp, this natural gum dissolves best at 155° to 160°F. Combined with sodium or phosphate salts, it forms gels at levels from 0.18% to 0.25%. Because it binds calcium, it will reduce the amount of free calcium in the mix. Precipitation of sodium alginate will occur at high calcium levels, making it difficult to hydrate without adding sequesterant.
U.S. alginate usage has declined due to high prices, but it's still popular in Europe, where cellulose-gum use is restricted. A blend of sodium alginate and MCC sometimes is used in cultured, low-pH frozen yogurt to eliminate cellulose gum from the ingredient label. Agar. Extracted from red algae, this gum swells and absorbs large quantities of water. Not easily dispersed, it may require boiling before forming a gel. It's not often used in ice cream because it forms a hard gel. A newer product, called "chelated agar," can be activated in HTST conditions. Carrageenan. Extracted from Irish moss and other species of red seaweed of the class rhodophyceae, carrageenan is a sulfated linear polysaccharide, which reacts with casein by forming ionic linkages between sulfate groups and charged amino acids.
Carrageenan is available in several types, the most common of which are kappa, iota and lambda. For lowfat and soft-serve ice cream compositions, kappa often is used for its gel-forming functionality and its reactivity with casein, which prevents whey separation. Almost always, it's added if an aging step exists in the manufacturer's process. A kappa-iota blend is sometimes preferred, to keep kappa from forming a brittle gel. Lambda blends can be used for ice creams with sufficient fat to stabilize without gelling. Kappa- and iota- solutions require heating for proper hydration. Carrageenan levels typically are in the range of 0.01% to 0.04%. Guar. This widely used ice cream stabilizer is isolated from the seeds of a shrub grown in India. Guar is preferred for its relatively low cost and the body it contributes to the product. It hydrates well in cold water, and often is used in combination with carrageenan and LBG. But, unlike LBG, it does not interact strongly with carrageenan. It is not as effective as LBG in preventing deterioration due to heat shock, so LBG and carrageenan are often used together. Levels used are 0.07% to 0.20%, and depend on the use levels of other added gums. Variability in price and quality pose disadvantages. Locust bean gum. LBG does not form a gel, and creates a less gummy texture than guar. It requires heating to 170°F for full hydration, usually achieved during pasteurization. It also is inert to acid and calcium. This gum enhances aeration, and imparts good body to ice cream. Used alone, it can cause whey-off during processing, so it usually is used in combination with carrageenan. LBG can act synergistically with kappa-carrageenan and xanthan gums. LBG can vary in cost and functionality. Usage levels are similar to guar at 0.07% to 0.20%, again depending on which other gums are used in conjunction with it. Xanthan gum. A product of bacterial fermentation, this giant glucomannan polymer (2 million daltons) makes an excellent emulsion stabilizer because its suspending properties keep emulsions dispersed. It's a popular ingredient in lowfat compositions.
It can be dispersed by blending with skim milk, corn syrups or nonfat milk solids. Xanthan is cold-water soluble, hydrates quickly once dispersed, and provides water-binding. It almost always is used in combination with other gums. It is synergistic with LBG, which reduces the levels of LBG and guar required.
Heat- and pH-resistant, it also has a cleaner flavor than guar's sometimes beany character. It possesses pseudoplastic properties, exhibiting shear-thinning, a useful property for pumping and extrusion in soft-serve ice creams. Usage levels range from 0.015 to 0.040%. However, its cost limits its usage. Overuse can cause excessive gelation, an overly viscous mix, and a chewy ice cream. Modified cellulose. Available in several viscosity ranges, MCC and cellulose gum (CMC) are used together for reducing ice-crystal growth. MCC is a highly refined, insoluble microcrystalline cellulose (0.1 to 0.2 microns), derived from wood pulp. Avicel(r), a widely used MCC from Philadelphia-based FMC Corporation, is coated with CMC to aid in dispersion. Additional CMC must be added for functionality in the ice cream.
MCC has an advantage over many gums in that it is possible to obtain a high level of functionality without encountering a gummy texture in the product. Resistant to pH or temperature changes, MCC has become a ubiquitous ingredient in ice cream formulations for its contribution to product stability during heat shock, increasing stiffness and whippability while controlling meltdown. It also allows the formation of smaller air cells, which improves texture and mouthfeel. It also reportedly structures the water phase around fat globules.
MCC requires shear for activation to make it completely functional. The shear created by homogenization is usually adequate. If not fully activated early in processing, it can continue to activate during aging or subsequent processing, and result in unanticipated viscosity. Other disadvantages are the cost and its inability to carry a "natural" label. MCC is used at levels of 0.30% to 0.45%.
Cutting-edge Newer stabilizers are hitting the market. A cellulose-based thickener and stabilizer, PrimaCel™, produced by the fermentation of Acetobacter xylinum, was commercially introduced in June 1997 by the NutraSweet Kelco Company, San Diego. It is an insoluble polymer that expands to form a network that fills the dispersing medium after shear has been applied. Fiber strands are extremely fine, only about 0.1 micron in diameter. In some applications, it may act as a substitute for MCC, and it is processed in combination with CMC and sucrose to promote dispersion and activation. Like MCC, it requires shear in the homogenizer for complete dispersion. In addition to the conventional benefits of MCC, PrimaCel offers improved flavor due to lower use levels -- from 0.10% to 0.3%, compared to 0.8% for colloidal-grade MCC. In soft-serve products, it's reported to improve the extrusion characteristics and enhance whippability at lower levels of use than MCC. The functionality is not impacted by pH or temperature changes.
"This product provides another tool for ice cream producers to formulate better-tasting, more cost-effective products," says Steve Snyder, director of commercial development, NutraSweet Kelco.
Several companies are planning to incorporate this gum into new stabilizing systems.
A new type of guar, Sherex QSG Stabilizer System, is offered by Quest International, Hoffman Estates, IL, as a price-stable alternative to LBG. Since it meets the specifications for guar, it can be listed on the ingredient label as guar. However, its functionality comes closer to LBG. The flavor is cleaner, with less beany notes than guar or LBG. Hydration requirements also are more similar to LBG. Future products are expected to offer cold-solubility. The proprietary manufacturing technology allows the manufacturer to custom tailor the stabilizer system to meet specific customer requirements.
In addition, FMC is developing a new line of Avicel-derived products, called Avicel-plus!™. Specially designed to meet the ice cream industry's needs, Avicel-plus! IC cellulose gel will be introduced later this year.
Preblend positives Most ice cream manufacturers require preblended emulsifier systems. "It can be a lengthy process to establish specifications for the raw materials used in stabilizer blends," says Linda Dunning, product manager, frozen desserts, Germantown International, Broomhall, PA. "Natural gums are highly variable, both within, and between, suppliers. Through years of experience and expertise, we established very specific raw material requirements to meet defined ranges of functionality." In addition, preblending reduces warehousing of multiple ingredients and streamlines the manufacturing operation.
Available preblended systems can be evaluated based on functional criteria, or they may be targeted to meet the ingredient declaration goals for a given product. Several suppliers possess technologies that can produce custom blends of stabilizer/emulsifier systems in beaded or agglomerated forms, with improved dispersion properties for ice cream producers, provided the order meets the minimum quantity requirements.
Natural concerns The market for "natural" stabilizers is growing. Some consumers consider certain ingredients, such as the mono- and diglycerides and polysorbate emulsifiers, to be undesirable additions to an ingredient label. In response, interest in lecithin is increasing. However, commercial use of lecithin is currently limited by its flavor and cost.
Tharp presented an overview of new product ideas at Inter-Eis 96, Solingen, Germany, late last year. Following are some innovations that may find markets in the 21st Century:
Fat-free, cholesterol-free egg products could benefit the development of natural ice creams if they were commercially available.
Mono- and diglycerides, which utilize milk-fat fractions as the fat source, also could offer enhanced consumer acceptance, if they could be legally labeled as "modified milk fat."
Further investigation into the use of modified milk protein for control of fat behavior could lead to a more acceptable replacement for conventional emulsifiers.
Gelatin could be improved by fractionating it, based on molecular weight, leading to better functionality for controlling ice-crystal development.
With the trend toward diminishing sugar in ice cream, starch-based products -- namely hydrogenated starch hydrolysates -- are useful alternatives to maltodextrin and polydextrose in reduced-sugar products sweetened with lactitol.
Protease enzymes, with activities resulting in protein modification, are being investigated for use in frozen desserts as stabilizer alternatives. New enzymes -- including transglutaminase, which cross-links proteins -- possess the potential for increasing water immobilization properties by forming a gel from milk proteins.
Lactose has a tendency to crystallize at levels higher than 15% to 17% of the weight of water, limiting the level of MSNF (milk solids, non fat) that can be added to lowfat products. The crystallization causes an ice cream defect called "sandiness." This problem can be eliminated through the removal of lactose by filtration, or by hydrolysis of lactose to glucose and galactose with a lactase enzyme, a process used for lactose-free milk.
New standards of identity for ice cream; new technologies; and customer preferences for lower-fat, better-tasting and more natural products require the precise use of combinations of stabilizing microingredients during ice cream product design. Understanding the impact each stabilizer has on the physical structure of the product, and how to establish stabilizer synergies, provides the product designer with greater flexibility for finding the winning "combinatorial chemistry" for their compositional, functional and labeling requirements.
Ice Cream Composition Examples of the approximate composition of some typical ice creams. As the fat level decreases, the amount of stabilizer required increases.
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% Nonfat Milk Solids
% Stabilizers & Emulsifiers
Approximate Total % Solids