Impact of Gastric pH on Milk Protein Hydrolysis: A Pilot In Vitro Study Using Pediatric Human Gastric Juice in the Context of Infant Digestive Physiology

Background/Objectives: Gastroesophageal reflux disease (GERD) is prevalent in infants and frequently managed with acid-suppressive medications that elevate gastric pH. This pilot study aimed to evaluate how varying gastric pH levels (2.5, 4.0 and 6.0) influence the hydrolysis of milk proteins in human milk (HM), cow’s milk-based infant formula (CMF), and goat milk-based infant formula (GMF). Methods: Samples were subjected to a 30 min in vitro gastric digestion using pediatric human gastric juice obtained from clinical donors. Protein degradation was analyzed via SDS-PAGE densitometry, comparing digested aliquots to undigested controls. Results: At pH 2.5, caseins were highly digested in all samples, especially in HM and GMF. At pH 4.0, GMF displayed an apparent 51% greater casein degradation relative to CMF and HM in this pilot analysis. α-lactalbumin degradation was markedly higher in GMF at all pH levels; notably, at pH 4.0 and 6.0, only GMF exhibited digestion of this protein. Albumin showed almost complete degradation in HM and GMF at pH 2.5, and GMF maintained greater degradation at higher pH levels. β-lactoglobulin (absent in HM) was better digested in GMF at pH 2.5, whereas CMF showed higher hydrolysis observed at pH 4.0 and 6.0. Lactoferrin digestion was most efficient in HM and GMF at pH 2.5, with no differences observed at higher pH levels. Conclusions: These preliminary findings suggest that GMF may offer digestive advantages for infants with GERD under pharmacological acid suppression, particularly regarding casein and α-lactalbumin breakdown at higher pH. The distinct digestion kinetics of CMF and GMF at different pH levels provide a physiological basis for targeted infant feeding strategies. Further large-scale studies are required to validate these exploratory observations.