Effect of β-Casein Fortification in Milk Protein on Digestion Properties and Release of Bioactive Peptides in a Suckling Rat Pup Model

β-casein (β-CN) is the predominant casein fraction in breast milk, while current infant milk formula (IMF) contains substantially lower β-CN levels than breast milk. The impact of β-CN fortification on neonatal digestive characteristic and bioactive peptide release remains an understudied area in vivo. This study investigated the effect of β-CN fortification in milk protein on digestion properties and release of bioactive peptides using a suckling rat pup model. Rat pups were, respectively, gavaged with two milk protein solutions: one with ordinary β-CN content (OBCN) and the other with fortified β-CN content (FBCN). The gastric emptying rate, proteolytic efficiency, and peptidomic profiles of intestinal digesta were evaluated. Results indicated that the FBCN group exhibited accelerated gastric emptying into the intestinal phase and enhanced proteolytic efficiency compared to OBCN group. Furthermore, the FBCN group generated greater peptide diversity in the small intestine, with significantly elevated abundance of bioactive peptide candidates exhibiting broader functional spectra. These findings provide additional evidence for the health effects of β-CN fortification in IMF.