"There are no secrets in ice cream formulations, there is a variety of formulas which are used to derive recipes," says Dr. Robert Roberts, associate professor of food science and director of the Penn State Ice Cream Short Course, the nation's oldest and best-known educational program on ice cream manufacturing.
Legally, ice cream must contain no less than 10 percent milk fat, and no less than 20 percent milk solids. In general, most ice creams contain 10 to 16 percent fat and 9 to 12 percent non-fat milk solids with 11 to 15 percent sucrose or equivalent for sweetness. Then, of course there are the flavorings and the emulsifiers and the most important, and often forgotten component, air. Choices within these ranges produce economy, premium and super-premium ice cream. "Many people think that the higher the quality of the ice cream, the higher the fat content since fat makes the ice cream feel unctuous and creamy," Roberts told attendees today (Feb.17) at the annual meeting of the American Association for the Advancement of Science in Boston. "Fat is also a cold insulator and is involved in trapping air and perhaps most importantly, it tastes good," he notes.
In essence, ice cream is a frozen foam. During the freezing and whipping process, proteins in the ice cream mix encircle the air bubbles incorporated in the liquid and then the fat stabilizes the bubbles.
"Protein traps the air, but cannot hold it, much like skim milk foams," says Roberts. "The fat in ice cream partially destabilizes and traps the air. In ice cream, in contrast to other products, emulsifiers are added to destabilized the fat, allowing partial agglomeration and air cell stabilization."
During ice cream mix manufacture, the ingredients are measured by weight, then mixed, pasteurized and homogenized.
"The pasteurization process is required by law to destroy any potential pathogens and make the product safe for consumption," says Roberts "Homogenization, a high pressure process designed to reduce the size of the fat globules and increase whipability is very important. Without homogenization, the mix might over destabilize during the freezing process leading to a defect know as buttery, which is definitely not what people want in ice cream."
The pasteurized, homogenized mix is cooled and allowed to age for at least four hours to create some fat crystals. Allowing time for the fat to assume the appropriate form is a critical step. "The surface area of the fat in a quart of mix is equal to about 1,200 square yards," says the Penn State scientist. After aging, the mix is ready to be frozen.
Commercial ice cream freezers, though much larger, operate on the same principle as hand crank machines. The outside wall of the freezer gets cold and a series of blades removes the ice crystals from the wall and moves them toward the center, also incorporating air. Recently, Roberts and others have looked at the speed at which the dasher moves to determine if an optimum setting exists. Contrary to previous understanding, about 50 percent of the energy removed by the refrigeration process is due to the frictional heat created by the dasher scraping the freezer wall.
"Some researchers have suggested slower dasher rates, which use less energy, will produce ice cream of the same quality," says Roberts. "Our research has shown that the slower beater rates produce an ice cream that has the same physical structure and sensory attributes as ice cream made at higher rates."
While freezing under agitation, only about half of the water in the ice cream mix freezes, leaving the other half liquid. The proteins, salts and sugars in the mix lower the freezing point enough to require further freezing. The hardening stage, when the rest of the mix solidifies, must be done rapidly to avoid the formation of large ice crystals.
"Fond memories of hand cranked ice cream are probably based on eating the product fresh and on the novelty of the process because when the product is frozen and stored it tends to become very coarse and icy," says Roberts. Anyone who has eaten frozen hand-cranked ice cream the next day understands how large ice crystals can form if the mix is not rapidly frozen.