Solubility and digestibility of milk proteins in infant formulas exposed to different heat treatments.

There are presently several fundamentally different technologies to produce infant formulas (IF), such as sterilization, spray-drying, and treatment at ultrahigh temperature (UHT). The effects of heat treatment on milk proteins in IF were analyzed by column chromatography, gel electrophoresis, and Kjeldahl analysis, revealing strong protein-protein and protein-lipid interactions in processed milk. These interactions were more pronounced in conventionally (in-can) sterilized than in spray-dried (powdered) and UHT products confirming their temperature dependency. Analysis of raw materials, intermediate and end products of IF processing revealed that after homogenization the first indications of protein denaturation occurred, but that in-can sterilization as the final heat treatment caused irreversible denaturation of proteins and strong protein-lipid interactions. Lowering of the pH to 4-5, which is physiological for the stomach of young infants, enhanced the interactions. Support for an impairment of protein digestibility was given by in vitro analysis of protein digestibility, demonstrating significantly lower digestibility of in-can sterilized IF compared to their spray-dried and UHT counterparts. To investigate the effect of heat treatment on chemical reactions, i.e., occurrence of Maillard products, we showed by a fluorimetric assay that the amount of "available" lysine is lower in sterilized than in powdered IF. Our findings suggest that a more differentiated view regarding the protein quality of IF is needed.