The effect of three pectin fractions variation on the browning of different dried apple products

Pectin is an important active polysaccharide in the matrix of dried fruit slices. Its components and structural character not only affect the texture but also impact the color of the dried fruit products. Present work studied the variation of pectin fractions of water-soluble (WSP), trans-cyclohexane-1,2-diaminetetraacetic acid-soluble (CSP), and sodium carbonate-soluble (NSP) from apple slices dried by using freeze-drying (FD), 60 °C hot air drying (HAD60), and 90 °C hot air drying (HAD90), to investigate the correlation of their monosaccharide composition, molecular weight distribution, Fourier transform infrared (FTIR) spectrum and small-angle X-ray scattering (SAXS) intensity plot of pectin fractions with the appearance browning of the apple slices. It was found that thermally process ruptures the cell wall structure and pectin component, leading to 12.57% less yield of WSP, and a maximum of 6.35% more yield of NSP pectin fractions. In FD, HAD60, and HAD90 apple slices, the linearity of WSP decreased but the branching extent of the rhamnogalacturonan I (RG-I) domain of WSP and NSP increased successively. The weight average molecular mass (Mw) and number average molar mass (Mn) of WSP and NSP increased as drying temperature increased, whereas they decreased in CSP. The thermal treatment promoted the dehydration of the pectin and caused higher methyl esterification, inducing the structure of pectin more compact and tighter. Moreover, the pectin structural characteristics of the degree of esterification (DE), the radius of gyration (Rg) value, conformational index (ν), and galactose content of three soluble pectin fractions were positively correlated with the a* and b* value of different dried apple slices.