Ice Cream: Combination Chemistry

 Ice Cream:
Combination Chemistry
August 1997 -- Applications

By: Suanne J. Klahorst
Contributing Editor

  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.

  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

  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.

  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:

% Milkfat

% Nonfat Milk Solids

% Sweeteners

% Stabilizers & Emulsifiers

Approximate Total % Solids

Nonfat ice cream (hard)

< 0.8

12-14

18-22

1.0

35-37

Low-fat ice cream (hard)

2-4

12-14

18-21

0.8

35-38

Light ice cream (hard)

5-6

11-12

18-20

0.5

35-38

Reduced-fat ice cream

7-9

10-11

18-19

0.4

36-39

Economy ice cream

10-12

10-11

15

0.3

35-37

Premium ice cream

18-20

6.0-7.5

16-17

0.0-0.2

42-45

Super-premium ice cream

20

5-6

14-17

0.25

46

Frozen Yogurt

3-6

8-13

15-17

0.5

30-33

Low-fat Frozen Yogurt

0.5-2.0

8-13

15-17

0.6

29-32

Nonfat Frozen Yogurt

<0.5

8-14

15-17

0.6

28-31

Soft-serve ice cream

3-4

12-14

13-16

0.4

29-31