The heat stability of milk is of tremendous importance in the successful processing of milk and the manufacture of most dairy products. Appropriate application of heat becomes necessary not just for destruction of spore-resistant micro-organisms but also for the preservation of the most desirable product characteristics. However, milk should not coagulate on heating. Heat coagulation is chiefly caused by de-stabilization of the milk proteins. Sommer and Hart’s Salt Balance Theory (1919) conceives that optimum heat stability is dependent on a certain ratio/balance of calcium plus magnesium-ions to phosphate plus citrate-ions. Any disturbance in the salt balance, due to excess or deficiency of either group, accelerates heat coagulation.
(Calcium + Magnesium) / (Citrate + Phosphate)
Acidity combined with heat is mainly responsible for coagulation of milk. When milk is concentrated, the total solid content increases due to a decrease in the water content. As a result, the total acidity of the milk also increases. If this milk, with a decreased pH, is subjected to heat treatment, it might result in coagulation of milk. Hence to prevent the concentrated, homogenized milk from coagulating while sterilizing, stabilising salts are added. The stabilisation salts, such as calcium chloride, citric acid and sodium citrate, sodium salts of orthophosphoric and polyphosphoric acid, are alkaline and upon addition raise the pH of the concentrated milk. The salt also binds a part of the present Ca2+ causing the casein protein to get a more negative, deflecting charge. Both these changes increase the heat stability.
As each lot of the milk can differ, small amounts are generally first used for a number of test sterilisations, to determine the right amount of stabilisation salt to be added in order to keep their quantity minimum. These tests when conducted while producing...