This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Globules of butter fat are suspended in the milk. They are surrounded by films of adsorbed caseinates, albuminates, and globulinates. The fat globules of milk are too large to form a permanent emulsion, so they gradually rise to the top of the milk in the form of cream. If the milk or cream is put through a machine called a homogenizer, the fat globules are reduced in size. This is accomplished by using pressure and forcing the milk or cream through small openings. Homogenized milk or cream may form a stable emulsion if the fat globules are reduced enough in size. Hence, when the fat is broken into fine enough globules the cream will not rise to the top of the homogenized milk.
The size of the fat globules after homogenization depends upon the temperature of the milk during homogenization and the pressure used. With increase in temperature the degree of dispersion increases rapidly from 40° to 65°C, so that the smallest fat particles are obtained at 65°. Ordinarily temperatures above 65° are not used for homogenization. The size of the fat particles also decreases with increased pressure.
Whipped cream is stabilized by proteins. The fat globules in cream are surrounded by films of protein substances. Homogenized cream also has the film of adsorbed proteins around the fat particles. Clayton states the fat particles in homogenized cream may be 1000 times greater in number than before homogenization. Since the number of fat particles is increased, the amount of globulinates, caseinates, and albuminates used in forming films is very much greater, for the surface area of the fat globules has increased enormously.
Whipped cream is both an emulsion and a foam. The fat particles must be surrounded by a film of protein in order to be stabilized, and the air globules must be surrounded by a film of protein to stabilize them. In homogenized cream most of the protein is used in surrounding the fat globules, on account of the increased surface area of the smaller and increased number of fat globules, and thus there is not enough left to surround the air bubbles. Hence, homogenized cream seldom whips unless protein is added for film forming.
Factors that affect the whipping quality of cream. In addition to the protein or film forming in whipping cream, the fat content, the size of the fat particles, the temperature of whipping, and the viscosity are important factors. Dahlberg and Hening have studied the relation of viscosity, surface tension, and whipping properties of milk and cream. They have found that increased viscosity increases the whipping properties of cream, but the lowering of the surface tension does not improve the whipping qualities. They have reported two changes taking place during whipping. The incorporation of air depends upon the milk proteins forming the film around the air globules, and the rigidity or stiffness of the whipped cream depends upon the clumping together of the fat particles. The best whipping cream did not give as large a volume as some other creams, but it had less liquid drain out of it after whipping.
As the fat particles clump at the liquid/air interface, or within the liquid, the increased rigidity they give the foam permits inclusion of more air bubbles and extension of the films with the result that the dryness of the foam is increased. See Fig. 31. Larger fat particles clump more readily and thus form the structural support offered by the fat more easily. This offers an explanation of why cream from milk containing larger fat particles, milk from Jersey and Guernsey breeds, whips more quickly than cream containing smaller fat particles, milk from other breeds.
Aging improves the whipping qualities of cream. The viscosity increases with aging. As a general rule, treatment that increases the viscosity increases the whipping properties. Pasteurization tends to reduce the whipping quality of cream.
Babcock has found that the best whipping is obtained at a temperature of 45°F. or lower. At this temperature agitation favors the clumping together of the fat particles. At high temperatures, both the higher temperature and the agitation increase the dispersion of the fat. Above 50°F. the decrease in stiffness of whipped cream is in direct ratio to increase in temperature, so that 30-per cent cream will not whip at 72°F.
Babcock found acidity up to 0.3 per cent, at which sour taste is evident, had no effect on whipping quality. If acid was added in excess of 0.3 per cent, whipping quality improved, whether added to fresh or aged cream, when the amount added began to curdle the cream.
The addition of sugar to cream either before or after whipping was found by Babcock to decrease the stiffness of the cream. For each 2 tea-spoons added to 100 cc. (about 1/2 cup) the stiffness decreased four points on the stiffness scale. Adding the sugar before whipping the cream decreased the volume obtained and increased the whipping time.
The denaturation or coagulation of the protein film at the air/cream interface, increases the stiffness of the whipped cream. Since colloidal reactions require time, it is a better practise to add sugar to whipped cream after, rather than before or during the whipping process. By this procedure denaturation is more complete, which offers an explanation of why the volume of the cream is less affected by the addition of the sugar last. The addition of sugar lessens the stiffness and decreases the volume of the whipped cream because it either prevents denaturation and/or peptizes the protein film.
Fig. 31. - Diagram of a cross section through whipped cream. Fat globules of the cream are shown as small black spots. Magnification about 180 (Rahn).