Heat stability of reconstituted, protein-standardized skim milk powders.

Discipline: milk powder; Keywords: heat-treatment, shelf life, pH , protein content, standardization material.

Milk and milk products are heat-treated to increase their shelf life and make them safe for consumption. Therefore, it is important that milk and milk powders which are used in various food applications should be heat stable. The study reported here by Dr Sikand and co-workers was designed to investigate the influence of a number of factors on the heat stability of skim milk powders. The work was published in the Journal of Dairy Science, No 12 of Volume 93 of 2010, page 5561 to 5571, under the title: Heat stability of reconstituted, protein-standardized skim milk powders.

The authors studied the effects of standardization material, protein content and pH on the heat stability of reconstituted milk made from low-heat and medium-heat non-fat dry milk. The two heat treated products were standardized downward from a protein content of 35.5% to respectively 34, 32 and 30% protein, by adding either edible lactose powder or permeate powder from skim milk ultra-filtration. These powders were called standardized skim milk powders. The low-heat and medium-heat non-fat dry milk and the standardized skim milk powders were subsequently reconstituted to 9% total solids, where after samples were taken to measure heat stability at respectively unadjusted pH, and pH adjusted to between 6.3 and 7.0. Heat stability in the industry is defined as the ability of milk to withstand high temperatures without flocculation, gelation or protein separation, and for this purpose the authors considered heat stability as heat coagulation time at 140^oC of the products tested.

The results showed that at the unadjusted pH, all of powder type, standardization material and protein content influenced heat stability. The heat stability of the reconstituted low-heat non-fat dry milk and standardized skim milk powder was higher than the corresponding products treated with medium-heat. Heat stability in both categories generally decreased as protein content was decreased by standardization. When edible lactose powder was added to medium-heat standardized skim milk powder, heat stability was not affected. However, when pH was adjusted to values between 6.3 and 7.0, powder type, standardization material and pH itself had a substantial effect on heat stability, whereas protein content did not. Maximum heat stability was obtained at pH 6.7 for both reconstituted low-heat non-fat dry milk and standardized skim milk powder and at pH 6.6 for the corresponding medium-heat products, the difference which in practice may be wrongly considered of no significance. In the pH adjusted treatments, higher heat stability was obtained for reconstituted low-heat standardized skim milk powder containing permeate powder compared with reconstituted milk from low-heat standardized skim milk powder containing edible lactose powder, whereas the corresponding medium-heat treatments did not differ in this context.

The authors concluded that heat treatment and standardization material can be factors explaining differences in heat stability found in industry. Differences may also be associated with differences in the pH of maximum heat stability.