Foods Under the Microscope _ Ice Cream Structure
Professor of Food Science Dr. Doug Goff has studied a great variety of dairy foods at the Department of Food Science, University of Guelph in Ontario, Canada. One of the foods that he examined by microscopy is ice cream. It is very popular with consumers but has received little attention from the microscopists. Dr. Goff used cryo-SEM and TEM to produce the micrographs below, and he provides us with the following description of what he sees.
Ice Cream Structure
by H. Douglas Goff
TEM of 3 fat globules (orange) showing only little crystallinity (lighter lines) inside the globules. Proteins can be seen as black spheres.
TEM of 1 large and 1 small fat globules (orange) showing their contents almost completely crystallized (lighter lines inside the globules).
Ice cream structure under the microscope is a marvelous thing to see. While most consumers see it as a cold, creamy, smooth, delicious dessert, it is no easy task to produce and maintain a structure that will deliver these attributes! Information on the manufacture of ice cream can be found at the University of Guelph's Dairy Science and Technology Education website. Briefly, the first step of ice cream manufacture is to combine the ingredients (cream, milk, milk solids, sugars, and <0.5% of stabilizers and emulsifiers) into a mix, which is pasteurized and homogenized. This creates a milkfat emulsion, comprised of millions of tiny droplets of fat dispersed in the water phase, each surrounded by a membrane of proteins and emulsifiers. The proteins can be seen in the top image as the black spheres adsorbed onto the fat globule surface. The sugars, also added to the mix during processing, are dissolved in the water phase. After the mix is cooled, the milkfat partially solidifies (as does butter when you cool it), so that each droplet consists of solid fat crystals cemented together by liquid fat. The two images at left, produced by TEM, show low crystallinity (top image) and a high crystallinity (lower image) of fat droplets in an ice cream mix.
The ice cream mix is then whipped and frozen, a process that creates two more discrete structural phases, millions of tiny ice crystals and air bubbles dispersed in the concentrated unfrozen mix. The water, which comes from the milk or cream, freezes into ice, and the dissolved sugars become increasingly concentrated in the unfrozen phase as more ice forms. Ice crystals should be 30-50 µm in diameter - the larger they are from manufacture or become due to temperature fluctuations in storage, the more coarse/icy the ice cream will taste. The whipping processs helps keep the ice crystals small and discrete. The colour image below, produced by cryo-SEM, shows a cross section of frozen ice cream, illustrating the four microscopic phases of frozen ice cream: ice crystals (blue - 'C'), air bubbles ('A'), fat droplets ('F' - for details see the micrograph at right), and the unfrozen phase ('S' - yellow).
The whipping process also helps to incorporate air in the form of tiny bubbles 50-80 µm in diameter. Approximately one half of the volume of ice cream is air (without it, ice cream could not be scooped or chewed in the mouth), but the fact that it is dispersed in tiny bubbles means that the ice cream tastes smooth and the air is not noticeable. All of the fat droplets play an important role at the air interface, helping provide that smoothness. The process of freezing and aeration of the mix causes the milkfat emulsion to undergo a process called partial coalescence, in which the fat droplets form clusters and aggregates of fat that surround and stabilize the air bubbles. This same process is what creates structure in whipped cream (the structure of the fat and the air in whipped cream and ice cream are very similar). The colour image at right, also produced by cryo-SEM, shows another cross section of frozen ice cream, illustrating an air bubble lined with the agglomerated fat and individual droplets (yellow).
I have prepared another narrative of our visualization of ice cream structure by freeze-substitution (cryo-fixation) transmission electron microscopy (TEM) for Dr. Miloslav Kalab.
The next time you enjoy a cone of ice cream, pause for a moment and marvel at its structure!