Kinetics of lysine loss in an infant formula model system at conditions applicable to spray drying

The kinetics of lysine loss at conditions applicable to spray drying were established for an infant formula model system. The composition of the model system was derived from the typical composition of milk-based infant formulas. The model system contained skim milk powder, lactose, and whey protein isolate as reactive compounds. The impacts of the water activity, temperature, time, and physical state on lysine loss were evaluated. The samples were equilibrated at water activities of 0.06-0.75 and then heated at 60-90° C for 30 s to 30 min. A maximum in lysine loss of 81% was observed at a water activity of 0.17 at 90° C and 30 min of heating. The water activity range of maximal lysine loss was shifted to higher water activities for lower heating temperatures. In order to study the impact of the physical state on lysine losses, the physical state of the model system was determined across the whole parameter range. For a water activity a_w < 0.17 the lactose contained in the model system did not crystallize at any experimental condition. Between water activities of 0.23 and 0.43 the lactose crystallized in the course of heating and the delay of crystallization was influenced by the temperature and the water activity. At water activities of 0.53 and 0.75 the lactose crystallized during the equilibration of the model system, i.e., before heating. The highest lysine losses were determined in the transition zone between the rubbery and the crystalline state.