Total Pageviews

Sunday, November 23, 2008

The Structure of Ice Cream - Emulsions and Foams

An emulsion is defined as liquid droplets dispersed in another immiscible liquid. The dispersed phase droplet size ranges from 0.1-10 µm. Important oil-in-water food emulsions, ones in which oil or fat is the dispersed phase and water is the continuous phase, include milk, cream, ice cream, salad dressings, cake batters, flavour emulsions, meat emulsions, and cream liqueurs. Examples of food water-in-oil emulsions are butter or margarine. Emulsions are inherently unstable because free energy is associated with the interface between the two phases. As the interfacial area increases, either through a decrease in particle size or the addition of more dispersed phase material, i.e. higher fat, more energy is needed to keep the emulsion from coalescing. Some molecules act as surface active agents (called surfactants or emulsifiers) and can reduce this energy needed to keep these phases apart.

A foam is defined as a gas dispersed in a liquid where the gas bubbles are the discrete phase. There are many food foams including whipped creams, ice cream, carbonated soft drinks, mousses, meringues, and the head of a beer. A foam is likewise unstable and needs a stabilizing agent to form the gas bubble membrane.

Ice cream is both an emulsion and a foam. The milkfat exists in tiny globules that have been formed by the homogenizer. There are many proteins which act as emulsifiers and give the fat emulsion its needed stability. The emulsifiers discussed above in the Ingredients section which are added to ice cream actually reduce the stability of this fat emulsion because they replace proteins on the fat surface. When the mix is subjected to the whipping action of the barrel freezer, the fat emulsion begins to partially break down and the fat globules begin to flocculate. The air bubbles which are being beaten into the mix are stabilized by this partially coalesced fat. If emulsifiers were not added, the fat globules would have so much ability to resist this coalescing due to the proteins being adsorbed to the fat globule that the air bubbles would not be properly stabilized and the ice cream would not have the same smooth texture (due to this fat structure) that it has.

This fat structure which exists in ice cream is the same type of structure which exists in whipped cream. When you whip a bowl of heavy cream, it soon starts to become stiff and dry appearing and takes on a smooth texture. This results from the formation of this partially coalesced fat structure stabilizing the air bubbles. If it is whipped too far, the fat will begin to churn and butter particles will form. The same thing will happen in ice cream which has been whipped too much.

Also adding structure to the ice cream is the formation of the ice crystals. Water freezes out of a solution in its pure form as ice. In a sugar solution such as ice cream, the initial freezing point of the solution is lower than 0oC due to these dissolved sugars. As ice crystallization begins and water freezes out in its pure form, the concentration of the remaining solution of sugar is increased due to water removal and hence the freezing point is further lowered. This process, known as freeze concentration, continues to very low temperatures. Even at the typical ice cream serving temperature of -16oC, only about 72% of the water is frozen. The rest remains as a very concentrated sugar solution. This helps to give ice cream its ability to be scooped and chewed at freezer temperatures. The air content also contributes to this ability as mentioned above in discussing freezing.

Thus the structure of ice cream can be described as a partly frozen foam with ice crystals and air bubbles occupying a majority of the space. The tiny fat globules, some of them flocculated and surrounding the air bubbles also form a dispersed phase. Proteins and emulsifiers are in turn surrounding the fat globules. The continuous phase consists of a very concentrated, unfrozen solution of sugars.

One gram of ice cream of typical composition contains 1.5 x 1012 fat globules of average diameter 1 µm that have a surface area of greater than 1 square meter (in a gram!), 8 x 106 air bubbles of average diameter 70 µm with a surface area of 0.1 sq. m., and 8 x 106 ice crystals of average diameter 50 µm with a surface area of another 0.1 sq. m. The importance of surface chemistry becomes obvious!

Prof. Douglas Goff, Ph.D.