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pH-triggered intragastric gelation of whey protein/alginate and its effect on sucrose release
Protein digestion is highly influenced by gastric conditions, protein structures, and the presence of other food components in the gastrointestinal tract. Protein and dietary fibers are common food ingredients; however, the effect of dietary fiber on protein digestion is not fully understood. Our previous study showed that whey protein/pectin mixture formed intragastric gel under simulated gastric conditions, which slowed the degradation of protein and could potentially affect the digestion and release of other nutrients. The objective of this study was to investigate the in vitrogastric behavior of mixed whey protein and alginate, and its effect on the digestion pattern of protein and sucrose release.
Mixed solutions of 5% whey protein isolate (WPI), alginate (0.01 to 0.05 alginate to WPI wt. ratio) and 10% sucrose were prepared by heating them together at 85 oC for 30 min. Simulated gastric fluid (SGF) consisted of 0.034 M NaCl, 3.2 mg/g pepsin, and pH was adjusted to 1.2, 2, 3 and 4. The in vitrodigestion was carried out using reciprocating cylinder dissolution apparatus, with 10 g sample added to 78 g SGF (pepsin : protein = 1: 2). Rheological properties and electrophoresis were performed to evaluate the gastric behavior of the mixture, and HPLC was used to measure sucrose release during digestion.
At low alginate to WPI ratios, alginate did not significantly affect the degradation of whey protein and the bioavailability of sucrose, as shown by SDS-PAGE and HPLC, respectively. Increasing biopolymer ratio to 0.05 led to extensive intragastric gelation immediately when samples were mixed with SGF at pH 1.2. The mechanism behind intragastric gelation is believed to be the cross-linking between oppositely charged protein and alginate molecules when pH was reduced to lower than the pI of protein. Sucrose was entrapped in the gel network since no sucrose was detected in the digestion media once the intragastric gel was formed. During dissolution, physical movement and proteolysis by pepsin led to slow degradation of the gel, which also resulted in the slow release of sucrose from the matrix in 20 min. Intragastric gelation was only observed in SGF at pH 1.2 and 2.0.
This study indicated that at certain conditions whey protein and alginate mixtures could form intragastric gel, which delayed protein digestion and sucrose release from the matrix. These results can potentially lead to formulation of whey protein beverage having lowered postprandial glycemic response.
Keywords:
intragastric gelation, digestion, sucrose