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Factors Influencing the Viscosity of Sweetened Condensed Milk

Research Laboratories of the Dairy Division, United States Department of Agriculture

ABSTRACT

Acidity within reasonable limits, contained air, and the salts of copper have no significant influence on the increase of viscosity of sweetened milk. Cane sugar increases the initial viscosity slightly, but affects the increase in viscosity only as it inhibits the growth of bacteria.

The increase in viscosity is more, rapid at higher storage temperatures, but this increase is not in direct relation to the temperature. The effect of the temperature is very slight at 20°C. (68° F.) or lower, but is marked at 30°C. (86° F.) or higher.

The tendency to thicken on standing increases with, but not in direct relation to, the concentration of solids not fat.

The casein is the constituent principally concerned in the production of the viscosity. The albumen is a factor when the forewarming temperature is much above its coagulating point.

The phosphates, probably through their combination with the casein, may have influence on the change in the viscosity. An increase in the total phosphates of the raw milk by as much as 7 per cent will cause a distinct acceleration in the progressive increase in viscosity.

The tendency of the milk to thicken is greatly increased by the high temperature ordinarily used in the forewarming. Satisfactory results from a bacteriological standpoint may be obtained to about 63°C. (145.4° F.) for twenty to thirty minutes. The viscosity of the product made in this way is low, and the tendency for the viscosity to increase in storage is much reduced. When the viscosity is very low in condensed milk made with low forewarming temperature, there is a tendency for the fat to separate. It is not probable that this condition would occur in large batches of milk made under commercial conditions.