Food Science and Processing

FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 25 : Processing and Preparation of Milk and Milk products

Effects of heat on milk constituents

1. Effect on milk proteins
   When milk is sterilized, the albumin is completely coagulated. Even in the milk which has been boiled for 10 minutes, a greater part of the albumin is coagulated. When milk is boiled in an open pan and allowed to cool, the ‘skin’ formed at the surface contains about 13 per cent of the solids of milk. This includes a greater part of the albumin and about ¼ of the fat present in milk. Casein is relatively stable to heat. It does not coagulate at pasteurization temperature or when milk is boiled for 5 minutes. Casein undergoes coagulation when milk is heated at 100 0 C for 12 hours or 1 hour at 135 0 C or 3 minutes at 150 0 C.


2. Effect on fat
   The layer of fat that may form on milk that has been boiled results from the breaking of the films of proteins that surround the fat globules in the unheated milk. The breaking of films of emulsifying agents permits the coalescence of fat globules.


3. Effect on milk salts and pH
   The pH of fresh milk ranges from 6.5 - 6.7. When milk is heated at 80 0 C for 30 minutes there is loss of CO2, which causes an increase in pH. At the same time, some of the soluble calcium and phosphate are converted into insoluble calcium phosphate with the liberation of acid phosphates. These two opposing factors counter balance each other and there is no net change in pH. Heat treatment at high temperature (110-120 0 C) leads to the production of lactic and formic acids.
   
   Iodine is a volatile substance and when heated tends to be lost from milk. The dispersion of calcium phosphate in milk is decreased by heating and part of it is precipitated. Some of it collects on the bottom of the pan with coagulum of albumin and some is probably entangled in the scum on the top surface of the milk.