Food Product Design: Applications - April 2004 - To Foam or Not to Foam

April 2004

To Foam or Not to Foam

By Donna BerryContributing Editor

In high school chemistry, we learn that all matter exists in one of three states -- solid, liquid or gas -- and in food chemistry, we discover that foods and beverages often consist of a mixture of two or three of these forms. The most frequent combination mixes solid and liquid; however, gas plays an important role in numerous foods and beverages, including beer, breads, cake, ice cream, marshmallows, meringues, mousse, salad dressings, sodas and soufflés. Two common processes, carbonation and aeration, help formulators add gas to foods or beverages.

Most people are familiar with carbonation, the addition of pure carbon dioxide to a liquid, a process that gives soda its "sparkle" and can be used to augment or replace natural carbon dioxide bubbles from fermentation in some beer and sparkling wines. Carbon dioxide, a colorless, odorless and slightly acid-tasting gas, consists of one atom of carbon joined to two atoms of oxygen, is about 1.5 times as dense as air and is soluble in water. Carbonation is measured in volumes of CO2, where one volume of CO2 is equal to one liter of carbon dioxide dissolved in one liter of liquid. Temperature and pressure influence the equilibrium concentration of carbon dioxide in solution: Increasing the pressure increases the amount of carbon dioxide and decreasing the temperature increases carbon dioxide solubility. As impatient (or British) beer drinkers may have noted, warm beer tends to foam more easily. Also, once the pressure is released in a can of soda, the carbon dioxide disappears and the soda goes "flat."

In a container, beverage bubbles are formed at the walls where dust or other deposits provide nucleation sites, which cause the CO2 to come out of solution. A bubble's size increases as the gas diffuses from the liquid into the bubble because of the higher pressure of the gas dissolved in the liquid compared with that inside the bubble. Once big enough, the bubble becomes buoyant, detaches from the nucleation site and rises to the top, gathering additional gas as it rises.

While less visually dramatic than carbonation, aeration, the process of adding air into a food or beverage system, covers a wide range of applications. Air, a complex mixture of gases, is not soluble in water. Unlike carbon dioxide, which is purchased in tanks, air is free, making it a highly desirable ingredient to add to food formulas. However, the challenge is keeping air dispersed in a food system.

Solids help both carbon dioxide and air stabilize in liquid. This unique combination of all three forms of matter results in a foam, which simply is a stable mass of bubbles. True foams, such as meringue, break down and dissolve in water. Other foams, such as those that make bread and cake rise, are sometimes referred to as sponges. These foams leaven dough when yeast or chemical leaveners create gas bubbles, increasing the height of the baked product. However, unlike true foams, a sponge derives its name from its ability to soak up large quantities of water without dissolving. Nevertheless, in this discussion, true foams and sponges will both be referred to as foams.

Foams consist of tiny pockets of gas (air or carbon dioxide) surrounded by a thin film of water that has various substances dissolved in it. These dissolved substances are the key to foam, as pure liquid cannot foam, no matter how much it is agitated. This is because all pure liquids have a significant surface tension, a force that makes a liquid assume a compact shape. All of the molecules of a liquid exert an attractive force on each other. This force makes them a liquid -- without it they would become a gas.

Molecules at the interface of liquid and gas are more attracted to each other than to the gas and therefore are pulled back into the liquid. This imbalance of forces minimizes the number, and thus the area, of liquid molecules exposed to the gas. The geometrical shape with the least surface area for a given mass of liquid is a perfect sphere.

When it comes to bubbles -- which remember, consist of tiny pockets of gas surrounded by a thin film of water containing various dissolved substances -- that liquid wall is only a few molecules thick. Therefore, bubbles expose a large surface area of liquid to air. However, bubbles will not form nor remain unless the liquid's surface tension is greatly reduced. At this point, those dissolved solids come into play by interrupting the liquid's matrix of forces just enough to stabilize the bubbles. The most common dissolved molecules are protein.

Foams form as a result of rapid diffusion of protein to the air-water interface, which reduces the surface tension, necessary for foam formation. Then the protein partially unfolds, which encapsulates air bubbles and creates the association of protein molecules, leading to an intermolecular cohesive film with a certain degree of elasticity.

Brewing a good head Most people don't think of beer as containing protein. However, if it weren't for that single gram of protein in a 12-oz. beer, it would not develop a head, which is a brewer's term for foam.

A suitable head on beer is one of the first characteristics by which consumers judge beer quality. Brewing scientists have studied beer foam quality, which is distinguished by its stability, or head retention. This stability is the length of time that foam lasts on a poured glass of beer, along with adherence to the glass and the foam's texture. Beer foam affects mouthfeel, increasing the creamy sensation associated with a fine brew. The quality of the barley and hop raw materials, as well as production, packaging and dispensing processes, all influence these characteristics.

Elevated levels of key malt proteins and hop acids generally have a positive influence on foam quality. In fact, beer proteins derived from malt are considered to be very important factors for foam and colloidal stability of the beverage. Foam proteins have to be retained as much as possible to realize good foam stability, but at the same time, other proteins can produce a haze defect, and these proteins need to be kept below identified levels for specific beer varieties.

TNO-Nutrition and Food Research, Zeist, the Netherlands, has developed analytical tools for measuring beer foam quality, including the industry forerunner -- Foam Analyzer. Recent research has made new methods available for quantifying individual foam-positive and foam-negative proteins. Such tools can determine beer-foam protein quality, as well as monitor foam proteins throughout the malting and brewing process. Brewers can study the effect of changes and adjust the malting or brewing process to maximize the amount of foam-positive proteins in beer.

Factors that negatively affect beer foam include low protein quantity, insufficient protein quality and the presence of impurities, such as detergents, lipids or fatty acids. That is why a thorough rinsing can rid beer glasses of these contaminants to secure a good head.

Surrey, England-based Brewing Research International recently investigated the mechanisms of fatty acids that destabilize beer foam, with results published in the Journal of the American Society of Brewing Chemists (2003, pages 196 to 202).   Researchers measured foam stability of a pilot-brewed beer in the presence of a range of concentrations of fatty acids, similar to those found in a range of commercial beers.

The study showed that fatty acids with either six or 10 carbons had no impact on foam stability, but longer-chain fatty acids were destructive. Data suggest that the fatty acids adsorbed onto the protein-stabilized surface of the bubbles weaken the protective film, causing the bubbles to coalesce and the foam to dissipate.

Light-scattering experiments showed an increased number of aggregates in samples, which suggests that the large-chain fatty acids destabilized beer foam through a mechanical film-bridging mechanism, similar to that used in antifoam systems (which will be addressed later in this article).

Whipping egg whites Eggs and egg products, such as liquid whole eggs, liquid yolks or liquid whites, are frequently used to produce foams, which, when heated, can provide a leavening effect in baked-goods applications. Basically, beating eggs entraps air bubbles in the liquid, which produces foam.

"As beating increases, the foam changes. Trapped air bubbles decrease in size and increase in number, making the foam thick and stiff. It loses its flow properties as increased amounts of air are incorporated," says Glenn Froning, technical advisor for the American Egg Board, Park Ridge, IL. "Where the egg and air interface, the egg coagulates and gives rigidity to the structure created.

"Eggs and egg products are often used for aeration, particularly in baked goods," says Froning. When they are properly added to a batter, he notes, they give the baked product a good cellular structure, an integrated texture and an increased volume, as compared to a batter without eggs.

"Whole eggs can be used to create foams; however, if a lot of volume is desired, it is best to use egg whites only, as egg whites produce foam that is a six- to eight-times increase in volume compared to unbeaten liquid egg white," says Froning. "Yolks may double or triple in volume, at most. Whole eggs produce foam with less volume than either yolks or whites beaten separately, and are less thick than yolks alone."

The remarkable foaming power of egg whites results from the combined activities of the various proteins found in the white, namely the globulins ovomucin and ovalbumin. Globulins promote whipping by contributing to the initial viscosity of the white, thus allowing air to become trapped; their lower surface tension contributes to the small-bubble formation and smooth texture. Ovomucin forms an insoluble film around the air bubbles and stabilizes foam. Ovalbumin, a heat-coagulable protein, imparts permanence to foam when exposed to heat.

"Thin white gives greater volume than thick, but volume decreases with continued beating," says Froning. "Maximum stability is attained at the beating point where volume is greatest in egg-white foam. Egg whites beaten beyond this point lose their elasticity and become stiff and brittle, resulting in clumping and separation of egg-white foam structure." A finished product made with over-beaten egg whites may be dry and of poor volume. It may even collapse.

Many factors can affect egg foams. Temperature is one -- room temperature eggs foam quicker than refrigerated eggs because heat reduces the surface tension, which allows whites to foam more easily.

When the eggs contact other ingredients in a formulation can also affect foam volume and stability. For example, "Salt decreases foam stability by weakening the lattice structure of the protein bonds and causing it to lose moisture, thus it is best that salt is added to other ingredients in a formula," says Froning. "Also, a single drop of fat can reduce foaming by joining with some of the protein and preventing protein molecules from bonding to create foam."

Thus, if it appears that liquid egg white is not reaching its potential in foam volume, it may be because it is contaminated by egg yolk, which is about one-third lipid. A single drop of yolk in egg white can reduce the foam's maximum volume by as much as two-thirds. In general, egg product suppliers strive to keep yolk contamination of egg white below 0.05%.

Egg foams are often used in sweet applications such as meringues, cream fillings, fondants and cakes. Sugar not only sweetens the application, but also increases the foam's stability. "However, it can also retard the foaming of whites, so it is important to add it slowly during the beating process in order to not decrease the volume," Froning says.

Adjusting egg white pH to 6.5 by adding an acid ingredient increases foam stability to heat, because the acid makes foam less prone to over-coagulation. Egg whites naturally have a pH of about 8.7 to 9.0. Increasing the acidity of egg whites makes it more difficult for egg-white protein molecules to bond too tightly. This keeps the foam loose and elastic, and stable enough to hold air cells, allowing the air cells to expand well when heated. This results in better volume. Acids also can enhance the whiteness of egg-white foam.

Cream of tartar (acid potassium tartarate or dipotassium L-tartarate) is the most commonly used acid ingredient in the baking industry. Usage level is about one teaspoon for every eight to 12 egg whites. Combined with baking soda, cream of tartar makes baking powder, acting as the acidulant. Other weak acids, such as lemon juice and vinegar, can also stabilize egg-white foams by lowering pH.

Egg-white foams should be mixed with other ingredients immediately after the foam is formed so the foam does not have time to drain or shrink. For example, when making angel food cake, egg whites are whipped into foam and then gently and quickly incorporated with a minimum of stirring into the rest of the blended ingredients. It is critical that as much of the foam as possible be preserved during the mixing process. As the cake bakes, the air cells expand to raise the batter, and the flour starch gelatinizes, helping the heat-coagulable proteins reinforce the bubble walls.

A similar approach is necessary when making a soufflé. The word soufflé is French for "to blow," "to breathe" and "to whisper," meanings that suggest the fragile nature of soufflés. As with angel food cake, beaten egg whites are gently, yet quickly folded into the soufflé base, which is more like a sauce than a batter. The dish is then baked at a temperature high enough to set the proteins before the foam has reached its maximum expansion and begins to fall, but low enough to heat the interior without first burning the outside. A 325?F temperature gives a uniform solid result.

If the oven door is opened as the air cells are expanding and the soufflé is rising, the change in air pressure and temperature can cause the soufflé structure to collapse. Even the undisturbed soufflé drops a bit when it is removed from the oven as a result of the trapped air and steam cooling and contracting.

A dairy froth Ice cream is one of the dairy foods that is most commonly aerated; the added air is referred to as overrun. This is defined as the percent increase in volume of ice cream greater than the amount of ice cream mix used. Overrun can make up as much as 50% of the product's total volume. At the maximum allowed by the FDA Standards of Identity (SOI), the air content is stated to be 100% overrun.

As with other aerated foods, the bubbles, or air cells, make dairy foods lighter and softer. Ice cream without air cells is very dense and extremely rich tasting. During commercial ice cream manufacturing, compressed air is pumped into the mix and quickly frozen. Freezing stabilizes the foam by solidifying much of the liquid. Ice cream is actually both an emulsion and a foam. However, emulsifiers added to ice cream actually reduce the stability of the fat emulsion by replacing proteins on the fat surface, destabilizing the fat emulsion. This partially coalesced fat stabilizes the air bubbles that the mixing process creates.

Many refrigerated dairy foods can be aerated. The most common is cream, which can be whipped to make desserts or dessert toppings. Unlike egg whites, where a minute amount of fat reduces foaming, whipping creams -- which start at about 20% fat and can go as high as 40% -- readily foam when properly beaten. In fact, in the case of cream, the large concentration of fat actually assists with stabilizing the foam. The secret is the fat must be in a solid state; with cream foams, a minute amount of liquid fat also will interfere with foam development.

The proteins dissolved in the liquid phase stabilize whipped cream. However, unlike egg-white foams, which can increase as much as eight times in volume, cream only doubles in volume. Unfortunately, depending on other conditions, particularly temperature, cream can stubbornly stay a liquid, or at best, whip into a mixture of butter and buttermilk.

When milk is beaten, an extremely delicate foam forms. However, this foam collapses almost immediately after it is formed. Cream foams survive because of the concentration of fat globules, which has a noticeable effect on the viscosity of the liquid; cream is thicker than milk and has a slower flow. The fat globules also cluster together in the bubble walls, where surface forces rupture some of their membranes. The exposed spheres of soft fat then stick to each other and form a rigid but delicate network that the milk proteins alone cannot provide. In milk, with its lower fat content, the fat globules are too few and far between to have an effect, but in cream, their number is adequate to support foam development.

Cream should be whipped at refrigerated temperature. In fact, bakers are advised to use a metal bowl and beaters, and cool both before whipping any cream. This is due to the presence of milk fat, which changes its state over a narrow range of temperatures. During whipping, if the fat globules lining the bubble walls are too soft, the weight of the foam deforms them and the structure weakens. And, if a small amount of fat escapes as liquid from the globule, it will disrupt the ordered system, preventing foam from forming

In general, cream's fat globules cluster more readily at low temperatures. Lower temperatures also make the fluid more viscous and slower to drain from the foam. Cream must not be frozen prior to whipping, as freezing causes water to form ice, and this segregation of phases makes it difficult to redisperse the fat globules evenly during whipping. A temperature of about 35?F to 45?F is recommended. Above 70?F, even heavy creams (40% fat) are too thin and their globules too soft to make stable foams.

Large fat globules produce stiffer foams than small fat globules, which is why cream intended for whipping is never homogenized. Jersey and Guernsey cows give milk with larger fat-globule size than the more common dairy-cow breed -- the Holstein. Dairies sometimes add stabilizers, such as gums or gelatin, to improve cream's foaming properties by making the cream more viscous and helping stabilize bubble walls.

In recent years, it also has become common to aerate fermented dairy foods, such as yogurt. Cream is typically aerated right before consumption; whipped yogurt, on the other hand, is aerated during manufacturing and requires stabilization to prevent the foam from collapsing during distribution and merchandising, especially at varying altitudes.

"A stabilizer system should include an emulsifier from the group classified as mono- and diglycerides and their esters, with experience indicating that one of the most effective emulsifiers is lactylated mono- and diglyceride," says Klass Kammesheidt, vice president of marketing, sales and service, G.C. Hahn & Co. USA, Erie, IL. "Other components of the stabilizer system include starch, gelatin, carageenan, guar, xanthan gum and locust bean gum."

The dry stabilizer blend is typically dispensed directly in the milk mixture prior to pasteurization and before further processing, such as acidification. When the mixture is aerated, the stabilized mixture resists collapsing.

During aeration, the processor utilizes high turbulences to unite the gas with the aqueous phase and the dissolved stabilizers. Emulsifiers reduce surface tension, while the stabilizers increase viscosity. The mixing head mechanically crushes large gas bubbles into very small ones, and the gel network formed by the emulsifiers and stabilizers provides homogeneous gas-bubble distribution at low whipping temperature.

Another aerated dairy food is called mousse, which is a French term that means "foam." Sweet mousses tend to be based on whipping cream and are served either frozen or from the refrigerator. Mousses also can include egg-white foam and are often stabilized by gelatin or a similar texturant. The dessert chocolate mousse is well-known internationally, whereas savory mousses, such as mousse de jambon (ham), are associated with French cuisine.

Indeed, savory mousses are often culinary masterpieces. In fine-dining establishments across the United States, instead of drizzling a sauce or gravy onto an entrée, chefs can add it in the form of foam, which fills the mouth with a mousse-like fullness that dissolves on the tongue, exploding with flavor. Such foams have been described as edible fireworks that ignite taste buds.

The undesired foam Not all foams are desirable in food processing. In fact, foams can cause many problems, including vessels overflowing, processing and packaging interference, damaging materials and housekeeping issues.

Process foams must be controlled to maintain an efficient operation and produce consistent, high-quality product. Manufacturers can control undesirable foam in two ways. The first is to prevent it from ever forming by using an antifoaming agent. The other is to destroy foam when it is formed by applying a defoamer. Sometimes the terms defoamer and antifoam are interchanged, but in fact they are very different in their approach to controlling process foam.

Many chemicals and formulated products behave as either or both a defoamer or an antifoam. Title 21 of the Code of Federal Regulations (CFR) 173.340 identifies such approved chemicals and their approved usage levels in specific applications. Some of the more common approved additives are dimethylpolysiloxane, polyethylene glycol, polysorbate and silicon dioxide.

Formulators should ensure that defoamers and antifoams have minimal impact on a food system. Their sole purpose should be to destroy or suppress foam. In general, the smaller amount of defoamer or antifoam required, the less impact on the end product.

Silicon dioxide is likely the most common chemical used in defoamers and antifoams, as it can destroy and suppress processing foams. It penetrates a bubble wall when it forms, spreading the liquid-gas interface. This causes the bubble wall to become unstable and collapse immediately. Silicon-based defoamers and antifoams have low surface tension for effective foam control in a variety of media.

Any product developer recognizes that foam is necessary to make many foods. Without malt proteins, beer would not have a head. Without egg-white foam, there would be no angel food cake. And without overrun pumped into ice cream mix, all ice creams would be very dense. Foam is an important part of many formulations ... but equally a nemesis in some processes. Knowing when to produce foam and when to get rid of it are one of the many keys to product success.

Donna Berry, president of Chicago-based Dairy & Food Communications, Inc., a network of professionals in business-to-business technical and trade communications, has been writing about product development and marketing for 9 years. Prior to that, she worked for Kraft Foods in the natural-cheese division. She has a B.S. in food science from the University of Illinois in Urbana-Champaign. She can be reached at [email protected] .

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