MILK AND ICE CREAM
3.3 ICE CREAM
“cisalpine,” meaning “on the near side of.” This bond creates a bend or kink in the carbon chain and alters the physical character and melting point of the fat. The high temperatures used in hydrogenation create a handful of fats that are in the trans configuration. Here the carbons are still in a double bond, but instead of the bend, the trans double bond containing carbon chains is straighter—more like a saturated fatty acid. Trans fats are potent antimicrobials and, unlike cis fatty acids, they resist reac-tion with oxygen, which results in rancid fats and spoiled food. However, an increase in trans fatty acid consumption has been associated with a number of health risks. In particular, a diet high in trans fat increases the LDL or “bad cholesterol” responsible for forming plaques and heart disease. Modern hydrogenation processes use different pressures, times of reaction, and temperatures to give rise to fats free of trans fats.
Artificial or low‐fat butter is another butter substitute that works well for spreading on food but not for cooking. These spreads are a mixture of vegetable oil emulsified with whey protein, buttermilk, and water. Depending on the product, many are very high in starches, gums, and milk proteins, all of which burn easily, making cooking with these butter substitutes difficult.
water), and pockets of air trapped in the freezing mixture by mixing. These three phases are scattered among each other forming a colloid. A colloid is a mixture with properties of homogeneous and heterogeneous mixtures and is formally defined as a microscopically dispersed mixture in which dispersed particles do not settle out. Ice cream can be described as an emulsion and a foam, both of which are examples of colloids. The formation of an emulsion (the solid phase of frozen fat globules and ice water distributed through the liquid phase of sugar water and cream) is typically unstable, but proteins and lipids coat the fats and stabilize the mixture from collaps-ing into two separate fat and water phases. These mediators between fat and water phases are called emulsifiers. The foam nature of ice cream is due to the trapped pockets of air created as the freezing cream is mixed. Overrun is the increase in volume of the ice cream before and after mixing due to this trapped air. Some ice cream can have an overrun of nearly twice the volume of the ingredients before freezing and mixing.
The ratio of each phase of the colloid is critical for the mouthfeel and creaminess of the ice cream. Too much fat and the ice cream will have the consistency of butter, too much sugar or milk solids creates a weak ice cream, while the emulsifiers limit the amount of crystals keeping the ice cream from becoming crunchy.
Federal standards (21 CFR § 135.110) require that ice cream contain a minimum of 10% milk fat and 20% milk solids—the solids refer to proteins and sugars like lactose or sucrose. Most ice creams include stabilizing emulsifiers to minimize the formation of ice and fat crystals that decrease the taste of ice cream. Fat is important for taste, providing both a creamy feel to the tongue and sweetness. The proteins and sugar add body or chewiness to the ice cream. A number of different stabilizers or emulsifiers can be found in ice cream. Added whey protein or gelatin protein from muscle tissue is used to coat the fat and provide the body. Custards use egg yolks, FIgURE 3.24 Ice cream. A careful control of chemistry (freezing point depression) and biology fatty globule composition can help make a better ice cream.
which have the phospholipid lecithin as an emulsifier. Another commonly used emulsifier is Polysorbate 80. This is a complex carbohydrate with a long unsaturated fatty acid bonded to it. As an emulsifier in ice cream, Polysorbate 80 can be found in fairly high concentration where it keeps the ice cream scoopable. The carbohydrate portion of the molecule interacts with water and protein, while the fatty acid tail of Polysorbate 80 hydrophobically interacts with the fat globules. This coating keeps the fat and water phases together. Stabilizers include complex carbohydrates (starches and gums) and are commonly found in the ingredient list of commercial ice cream.
A common additive used to reduce the formation of ice crystals is alginate. Also a complex carbohydrate, alginate is isolated from the cell walls of algae. Alginate contains many ─OH functional groups and readily binds water through hydrogen bonding. The extensive hydrogen bonding of alginate limits the flow of water and forms a gel that acts as thickener. The organization of the water–carbohydrate com-plex also defeats the formation of ice crystals. The cell wall carbohydrate from red algae (seaweed), carrageenan, is used in place of alginate in many foods (Fig. 3.25).
Ice cream can come in many confusing grades and styles. Superpremium and premium ice cream has low overrun and high fat content with the best quality ingre-dients. Standard ice cream has more overrun (air) than superpremium or premium ice cream and meets the minimum requirements of 21 CFR § 135.110. Fat‐free ice cream has less fat than the CFR standard and must have less than 0.5% fat per serving. In contrast, light ice cream is a description of the amount of calories coming from fat.
Light ice creams must have less than half of its total calories per serving from fat.
Low and reduced fat ice creams fall somewhere between light and fat‐free in their fat composition. Standard vanilla ice creams, also called Philadelphia‐style ice cream, differ from French vanilla in that French‐style ice creams, like custards, use egg yolks as an emulsifier, while standard or Philadelphia‐style ice creams (also called New York) use no egg or just the egg whites. Gelato is a frozen ice cream‐like dessert that has higher fat and almost no overrun. Sherbet stretches the ice cream‐like properties with fruit juice and some milk fat, whereas sorbet is not an ice cream at all! Sorbet contains no milk or cream and is instead a frozen puree of fruit with added alcohol or wine to reduce freezing temperature. Soft serve ice cream is low fat (3–6%) with up to 60% air overrun.
O
OH HO OH
z O y
x w
w+x+y+z = 20 O
O O
O
O
FIgURE 3.25 Polysorbate 80. An emulsifier often used in ice cream and other food and cosmetic compounds is made from fatty acids and other organic compounds.
Making ice cream is pretty straightforward and while an ice cream maker helps, it can be done without a machine. A simple base recipe is a combination of milk, heavy cream, sugar, and salt. From this base, flavorings including vanilla and chocolate can be added and are as diverse as there are ice cream creations. Richer custard or French‐style ice creams include adding egg yolks as emulsifiers followed by heating and cooling the mixture. Cream and milk are added to a mixture of egg yolk and sugar, which is then cooled before freezing. With your ice cream mixture complete, you are ready to freeze it, but now comes the work! Air must be introduced, crystallization must be limited, and the fat and liquid phases must be kept together while freezing. This is all accomplished by mixing. Mixing can be accomplished by hand by placing the liquid ice cream into a larger container of ice, water, and salt. The salted ice bath will have a lower temperature than ice water alone (see Box 3.3) allowing the sugar and fat water “ice cream” to freeze. Ice cream makers maintain a constant mixing as the liquid ice cream mixture begins to freeze. Once frozen, the ice cream can be eaten or left in the freezer to “harden.” At freezer temperature (−4°F/−20°C) about only 75% of the water is frozen, and the rest is a liquid sugar–
water mixture. Rapid and deep freezing causes most of the liquid water to freeze without forming unwanted crystals. Partial thaw and refreeze cycles will increase the amount of the liquid phase, and larger crystals will form, giving the ice cream an off‐taste and crunchy tooth feel (texture).
bOX 3.3 COLLIgATIvE PROPERTIES: FREEzINg POINT DEPRESSION. OR WHY THE ICE AND SALT bATH?
A very practical requirement for making ice cream is to freeze the solution of fat, water, and sugar. Ice cream mixtures have been immersed in ice and salt baths since the first ice cream makers in 1843. Under normal conditions, pure water freezes at 32°F/0°C, and a simple ice bath would not be cold enough to freeze the liquid ice cream. Milk freezes at about 31.1°F/0.5°C, and with the added sugar and other components, a typical ice cream solution won’t begin to freeze until five or so degrees colder. Therefore a colder‐than‐ice temperature is needed to make ice cream, and a mixture of salt and ice will do the trick. Temperatures of
−4°F/20°C can be reached with enough table salt (sodium chloride) and −40°C with calcium chloride hexahydrate salt.
bOX 3.2 bAggIE ICE CREAM
One fun way to make ice cream at home is to use two strong sealable baggies. The inner bag is mostly filled with the liquid ice cream, sealed with strong tape, and placed inside a larger one gallon‐sized plastic bag filled with crushed ice and a cup of salt. Roll or shake the bags for 10 or 15 min, and remove the inner bag and enjoy.
REFERENCES
[1] Title 21, Vol. 8, Ch. 1, Pt 1240, subpart A, Section 1240.3(j), Release 13.
[2] Bloom, G. and Sherman, P.W. (2005) Dairying barriers and the distribution of lactose malabsorption. Evol Hum Behav. 26: 301–312.
[3] Andrew, C. (2013) Milk revolution. Nature. 500: 20–22.
[4] Boyer, R.F. (2005) Concepts in Biochemistry. 3rd edn, p. 110. Wiley, Hoboken.
[5] Meyer, K. (2009) Factor‐analytic models for genotype × environment type problems and structured covariance matrices. Genet Sel Evol. 41: 24. doi:10.1186/1297‐9686‐41‐24 Mixtures of ice and salt do not “melt” the salt, but instead the combination of the two lowers or depresses the melting point of the ice water. This is called a col-ligative property. These are characteristics or properties of a solution that depend on the number of particles of solute (in this case salt) in a solvent (water or ice).
These properties are not impacted by the chemistry of the compounds dissolved in the water or the size, just the number of particles. For freezing, the more particles dissolved in the solvent, the greater the impact on freezing point depression. Salts are ionic molecules comprised of a cation and an anion attracted to one another by opposite charges. The ions will separate in water as each is solvated by water molecules. Sodium chloride will dissolve into the sodium cation (Na+) and chloride anion (Cl−). Thus, for each molecule of NaCl dissolved in water, both sodium and chloride particles are available to impact the freezing point depression. This can be measured using the following equation:
Tf iK cf m
where ΔTf is the change in freezing point, i is the number of ions present, Kf is a constant for the solvent, and cm is the concentration of particles dissolved in the water.
From this equation, you can see that more particles mean a higher total concentration, which creates a greater change in freezing point. Sucrose, lactose, and other sugars do not dissociate into ions when dissolved in water, so one mol-ecule of NaCl will have twice the impact on freezing point than will sucrose. This is how an ice–salt bath can achieve a temperature low enough to absorb heat from the ice cream.
The Science of Cooking: Understanding the Biology and Chemistry Behind Food and Cooking, First Edition. Joseph J. Provost, Keri L. Colabroy, Brenda S. Kelly, and Mark A.Wallert.
© 2016 John Wiley & Sons, Inc. Published 2016 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/provost/science_of_cooking