Effects of radio frequency heating on the glass transition, protein structure, and volatile compounds profile of commercial powdered infant formula milk

Radio frequency (RF) has found its potential in ensuring the safety of packaged powdered infant formula milk (PIFM). PIFM was processed by RF at different temperature-holding time treatments (70 degrees C-115 min, 75 degrees C-50 min, 80 degrees C-25 min, 85 degrees C-10 min, and 90 degrees C-5 min) to obtain 5-log reduction of Cronobacter sakazakii, and its physicochemical characteristics were investigated. Various RF treatments possessed insignificant effects on color difference (Delta E < 1.5), solubility (>99%), crystallinity (similar to 5%), surface chemical composition (>30% fat, 15%-20% protein, and 45%-50% lactose), and microstructure of PIFM. Lower temperature-longer holding time treatment as 70 degrees C-115 min caused 51% reduction in water activity and 71% reduction in moisture content, thereby considerably increasing the glass transition temperature of PIFM from 60.3 degrees C to 78.4 degrees C. Protein secondary structures were preserved; however, 75 degrees C-50 min and 80 degrees C-25 min treatments significantly (P < 0.05) decreased the alpha-helix structure. All RF heating conditions significantly (P < 0.05) altered the concentration of volatile organic compounds in PIFM and increased the relative abundance of lipid oxidation substances such as (E)-2-heptenal, 2-heptanone, and n-nonanal. PIFM can be treated at 90 degrees C for 5 min for better quality and processing efficiency.