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The Viscosity of Evaporated Milks of Different Solids Concentration

Division of Dairy Research Laboratories, Bureau of Dairy Industry, Agricultural Research Administration, U. S. Department of Agriculture

ABSTRACT

1. The viscosities of evaporated milks of 26 to 36 per cent solids concentrations followed a well-defined pattern during processing and storage. Thickening occurred during the sterilization process; this was followed by a loss of body and a thinning early in the storage period. The low storage viscosity was maintained for various and unpredictable lengths of time, during which fat-separation occurred. Late in the storage period a final thickening, which often proceeded to gelation, sometimes set in.
   
2. The rate of thickening of evaporated milk during sterilization at 115° C. (239° F.), although variable, becomes greatest in the 10-minute period immediately preceding coagulation. A heavy, creamy body could be developed during this pre-coagulation stage. In general, the lower the heat stability of a milk, the higher the viscosity it would develop during the 10-minute period preceding coagulation. Concentrated milks of high heat stability which reached the end of the sterilization period before entering the thickening phase did not develop, during processing, the high viscosity shown by milks of lower heat stability.
   
3. The body formed during the sterilization of milks of different concentrations became greater as the solids content of the milk was increased. The heaviest viscosities were produced in milks of high concentration and low heat stability.
   
4. Variations in milk quality and in forewarming procedures affected the viscosity of evaporated milk indirectly through their effect upon the heat stability of the milk.
   
5. The magnitude of the decrease in the viscosity of evaporated milk during early storage was dependent upon the storage temperature. At temperatures below 16° C. (60.8° P.) the loss of body was small. At temperatures above 30° C. (86° P.) evaporated milk sometimes lost 40 per cent of its original viscosity during the first 10 days of storage.
   
6. After the initial thinning in the viscosity of evaporated milk held in storage at constant temperature, a basic storage viscosity level was reached beyond which the rate of viscosity loss during prolonged storage was small.
   
7. Some evaporated milks, especially those receiving light heat treatments and those with a high concentration of solids, began to show increases in viscosity even to the point of gelation, late in the storage period.
   
8. The results indicate that the procedure for developing and maintaining a satisfactory body in evaporated milk which is used by many manufacturers conforms to the best practice that can be devised from our present state of knowledge. Assuming efficient homogenization and uniform handling of the canned product, important steps in body control are: (a) adjust the heat stability to a value of a few minutes greater but not more than twice the cooking time, bearing in mind that viscosity increases rapidly during the 10 minutes preceding coagulation; (b) store the finished milk at a temperature below 21° C. (70° P.); (c) turn the cases approximately every six weeks; (d) the viscosity may be increased by raising the milk solids content of the product.