The Properties of Pectin Extracted from the Residues of Vinegar-Fermented Apple and Apple Pomace

In this study, both apple slices and apple pomace, the by-product of apple juice processing, were subjected to vinegar fermentation. The pectins extracted from the solid residue of vinegarfermented apple slices (FAP) and apple pomaces (FAPP) were compared to the pectin extracted from non-fermented apple pomace (AP). All samples were classified as high-methoxyl pectins, and vinegar fermentation increased pectin extraction yield. FAP, which has a lower degree of methylation, also exhibited lower thermal stability. The changes in the pectin structure were dependent on both fermentation and the state of the raw material used to produce the vinegar. Compared to AP, the proportion of homogalacturonans (HGs) increased, and the proportion of rhamnogalacturonan I (RG-I) decreased in FAP, providing higher linearity, whereas in FAPP, the RG-I region became more dominant with reduced linearity. The molecular weight distribution of samples showed that pectin from vinegar-fermented sources changed the average molecular weights and mass fraction ratios of two peaks (1255 and 340 kDa) eluted from AP. In FAP, the mass fraction of the first peak (1294 kDa) increased from 35 to 89%, whereas in FAPP, the mass fraction of the second peak (478 kDa) increased to 91%. FAPP showed greater viscosity and a more noticeable shear-thinning behavior. G′ and G′′ in FAPP were also higher than those of AP and FAP at the same concentrations (5%, 7%, and 10% w/v). This study found that applying vinegar fermentation to apple slices and apple pomaces altered the structural and rheological properties of the extracted pectins. FAP and FAPP could be suitable food additives when certain properties are required.