Effect of in vitro gastric digestion conditions on physicochemical properties of raw apple fruit cell wall polysaccharides

Abstract

The effects of in vitro gastric digestion conditions on the properties of the soluble (SC) and insoluble (IC) components of apple cell walls were investigated. Apple fruits were homogenized and successively treated in natural saliva (3 min, 37 °C) and in simulated gastric fluid (SGF, 2 h, 37 °C). The incubation pH (5.0, 3.0, 1.8) and the ratio of apple mixture to SGF (1:0.05, 1:0.5, 1:1, v/v) were varied. Pectin (GalA 74 wt%, Mw 1800 × 103 g/mol, yield 1.9–2.4%) was a major high-molecular-weight component of SC obtained at pH 1.8, whereas at pH 3.0 and 5.0, proteins (41–55 wt%) and phenolic compounds (2.7–3.1 wt%) were dissolved together with pectins (15–24 wt% GalA, Mw 28-33 × 103 g/mol, yield 0.6–0.8%). Rhamnogalacturonan-I (RG-I) and homogalacturonan (HG) were identified in all pectins. HG was the major component of pectins extracted at pH 1.8; RG-I was the major component of pectins extracted at pH 3.0 and 5.0. The dissolution of cell wall pectins of apple fruits in SGF increased the viscosity of the SC of the apple mixture, decreased the hardness of the apple mixture, increased the water retention and water swelling capacity of the IC, and improved the capacity of high-molecular-weight soluble and insoluble components to retard glucose diffusion. The difference in surface morphology of insoluble components obtained at different values of pH incubation was observed under a SEM.