Structure of Ice Cream


Ice cream structure is both fascinating and confusing. The way we perceive the texture of ice cream when we consume it (smooth, coarse, etc.) is based on its structure, and thus structure is probably one of its most important attributes.

Colloidal aspects of structure

Please look at

this diagram of the fat structure in ice cream

when reading the following description, and try to put the two together in your mind. Also, please look at the last paragraph of this page for links to electron micrographic images of the structure of ice cream.

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 that act as emulsifiers and give the fat emulsion its needed stability. The emulsifiers are added to ice cream to actually reduce the stability of this fat emulsion by replacing proteins on the fat surface, leading to a thinner membrane more prone to coalescence during whipping. 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 or destabilize. 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.


[image] Effect of emulsifier on fat destabilization in ice cream 17 KB

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.


Ice Cream Meltdown

One of the important manifestations of ice cream structure is its melt-down. When you put ice cream in an ambient environment to melt (as in a scoop on a plate), two events occur; the melting of the ice and the collapse of the fat-stabilized foam structure. The melting of the ice is controlled by the outside temperature (fast on a hot day) and the rate of heat transfer (faster on a hot, windy day). However, even after the ice crystals melt, the ice cream does not "melt" (collapse) until the fat-stabilized foam structure collapses, and that is a function of the extent of fat destabilization/partial coalescence, which is controlled mostly by the emulsifier concentration, for reasons we have just described above.

This is shown in the diagram below, which shows ice cream sitting on a mesh screen at ambient temperature:

[image]

You can see above the increased amount of shape retention and slowness of melt that comes from the added emulsifiers, particularly polysorbate 80.



Structure from the Ice crystals

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 0° C due to these dissolved sugars (freezing point depression), which is mostly a function of the sugar content of the mix. 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 is shown here, schematically.

[image]

This process of freeze concentration continues to very low temperatures. Even at the typical ice cream serving temperature of -16° C, only about 72% of the water is frozen. The rest remains as a very concentrated sugar solution. Thus when temperature is plotted against % water frozen, you get the phase diagram shown below. 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 in discussing overrun.

[image]

The effect of sweeteners on freezing characteristics of ice cream mixes is demonstrated by the plot shown on the ice cream freezing curve.

Also critical to ice cream structure is ice crystal size, and the effect of recrystallization (heat shock, temperature fluctuations) on ice crystal size and texture. A primer on the theoretical aspects of freezing will help you to fully understand the freezing process. Please see the discussion and diagram on ice crystallization rate, as shown on that page, to fully understand this process. Recrystallization (growth) of ice is discussed elsewhere in the context of shelf life.

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 10exp12 fat globules of average diameter 1µ m that have a surface area of greater than 1 square meter (in a gram!), 8 x 10exp6 air bubbles of average diameter 70 µ m with a surface area of 0.1 sq. m., and 8 x 10exp6 ice crystals of average diameter 50 µ m with a surface area of another 0.1 sq. m. The importance of surface chemistry becomes obvious!

Microscopy

Before we leave ice cream structure, I want to draw your attention to the following address: "Foods Under the Microscope". This is a link to an absolutely marvelous website developed by my good friend Dr. Milos Kalab, with many high-quality images of the structure of milk and dairy products obtained during Dr. Kalab's long and outstanding career as a food microscopist with Agriculture and Agri-Food Canada in Ottawa. Dr. Kalab asked me to contribute microscopic images of ice cream structure as a guest microscopist. You can find my (Doug Goff) first contribution under "Guest microscopists", and I have also copied it here. Subsequent to that submission, I have prepared another one for D. Kalab that focuses on the use of cryo-fixation and TEM for visualization of fat and air structures in ice cream. One of my graduate students, Alejandra Regand, also made a contribution, based on her M.Sc. thesis work, focusing on the structure of polysaccharides in frozen solutions.

Ice cream structure is an active area of our research here at the University of Guelph. Please see my publications for more details of our research.

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