Effects of Industrial Heating Processes of Milk-Based Enteral Formulas on Site-Specific Protein Modifications and Their Relationship to in Vitro and in Vivo Protein Digestibility

Heat treatments are applied to milk and dairy products to ensure their microbiological safety and shelf lives. Types of heating processes may have different effects on protein modifications, leading to different protein digestibility. In this study, milk-based liquid nutritional formulas (simulating enteral formulas) were subjected to steam injection ultra-high-temperature treatment or in-can sterilization, and the formulas were investigated by proteomic methods and in vitro and in vivo digestion assays. Proteomic analyses revealed that in-can sterilization resulted in higher signals for N ε-carboxymethyllysine and dephosphorylation of Ser residues in major milk proteins than in steam-injected formula, reflecting the more severe thermal process of in-can sterilization. In vitro and in vivo digestion assays indicated that steam injection improved protein digestibility, supposedly by denaturation, while the improvement seemed to be overwhelmed by formation of aggregates that showed resistance to digestion in in-can sterilized formula. Adverse effects of heat treatment on protein digestibility are more likely to be manifested in milk-based formulas than in cow’s milk. Although the differences might be of limited significance in terms of amino acid bioavailability, these results emphasize the importance of protein quality of raw materials and selection of heating processes.