Abstract:
The aims of this study were to investigate the digestive characteristics of cow milk infant formula (CIF) and soy-based infant formula (SIF) during in vitro gastric digestion and to conduct mathematical modelling on the degradation kinetics of the major proteins present. Both CIF and SIF were digested in simulated infant gastric fluid in the presence of pepsin at pH 2.0, 3.5, 4.5, and 5.5. The effect of pH on the digestion behaviour of the respective infant formula was determined by sodium dodecyl sulphide-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, particle size distribution measurement, and first-order kinetics modeling. It was found that different pH values modulated the digestive behaviors and degradation kinetics in the gastric environment. Both infant formulas showed higher protein degradation at lower pH values, probably due to the pH-dependent pepsin activity. Varied susceptibility to pepsin digestion was observed amongst the different protein fractions, resulting in the differences in protein degradation kinetics under gastric conditions. Similar patterns for protein degradation at different pH values were observed in the CIF and SIF, respectively. In the CIF, caseins were the most susceptible to pepsin digestion, whereas whey proteins including α-lactalbumin, and β-lactoglobulin survived the 120 min gastric digestion under all pH conditions, suggesting their resistance to gastric digestion. A delayed hydrolysis of caseins was observed at pH 4.5, probably due to the casein coagulation at the vicinity of the isoelectric point (pI 4.6). Apparent flocculation of oil droplets and/or aggregation of proteins occurred in the CIF during in vitro gastric digestion. Similar digestion patterns for SIF were observed at all pH values, with rapid hydrolysis of soy protein within the first 5 min of digestion, after which the hydrolysis rate slowed down, before reaching a plateau. Digestibility for specific soy protein were shown as below: peptide 2 (MW 23.5 kDa) > acidic polypeptide > peptide 1 (MW 19.8 kDa) > basic polypeptide. The dissociation/aggregation behavior of the particles in SIF varied depending on the pH of the digestion system. Experimental evidence suggested that protein aggregation occurred after the first few minutes of digestion and this hindered susceptibility of proteins to digestion in the SIF. Digestion kinetics reflective of the aforementioned phenomena were obtained from the experimental data using the Logarithm of Slope (LOS) method. The acquired LOS plots indicated a single phase first-order kinetics for the digestion of cow milk proteins and soy proteins. In agreement with the experimental observations, the first-order digestion kinetics for the majority of the protein fraction was able to illustrate how pH affect the enzymatic activity and its subsequent effect on the gastric digestion process. The new empirical model developed capturing this phenomenon will be useful, and can be implemented to give insight into the local digestion kinetic in the gastric environment. This will contribute to the understanding of the more complex and non-constant pH modelling of the human digestion system.