by Clarissien Ramongolalaina
While the true origins of ice cream are somewhat unclear, there are a number of stories describing how ice cream was first created in ancient China via Marco Polo and other tales of the Roman Emperor Nero sending slaves to the mountains to collect snow for an ice cream‐like treat. The rise in popularity and availability of ice cream certainly correlates to key scientific advances, particularly the concept of freezing point depression. The principle of freezing point depression states that adding a salt (solute) to an ice water mixture (solvent) reduces the temperature at which the mixture freezes.
Ice cream is far from being a solid frozen bowl of cream. In fact, ice cream is a mixture of solids (ice and partially frozen milk fat), liquid (unfrozen cream and sugar 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 collapsing 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, 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 complex 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.
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 ingredients. 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.
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 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).
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