Influence of temperature and the physical state on available lysine in powdered infant formula

Abstract

In this study the kinetics of lysine loss in an infant formula model system were studied at conditions relevant for spray drying. The model system was composed of skim milk powder, lactose and whey protein isolate as reactive compounds. This composition was derived from the typical composition of milk-based infant formulas. Lysine blockage was studied under stationary conditions, i.e. the samples were adjusted to discrete water activities (0.11 – 0.43) and the impact of temperature (60 – 90°C) was analyzed at a heating time of 30min. Lysine losses reached up to 13.8 ± 3.5% at 60°C, 29.2 ± 5.7% at 70°C, 53.8 ± 6.1% at 80°C and 78.5 ± 3.5% at 90°C. The obtained lysine losses were correlated with the physical state of the system and the phase transitions taking place during heating. The smallest lysine losses were detected in the glassy state whereas the highest losses were found in the transition zone from the rubbery to the crystalline state. To highlight the impact of the state of the system further, the molecular mobility was measured by means of low-resolution 1H-NMR. T2-relaxation times showed that the mobility continuously increased from the glassy to the crystalline state. This is in contrast to the kinetics of lysine blockage which show a maximum in the transition zone from rubbery to crystalline state. So it can be concluded that crystalline lactose is less reactive with regard to lysine blockage.