Profiling the water soluble pectin in clear red raspberry (Rubus idaeus L. cv. Heritage) juice: Impact of high hydrostatic pressure and high-temperature short-time processing on the pectin properties

Abstract

Water soluble pectin (WSP), inherently present in the particle and serum phase of processed red raspberries, largely contributes to their overall structure organization. Nevertheless, pectin losses were preliminarily observed in hydrostatic pressure (HHP)-, and high-temperature short-time (HTST)-processed clear red raspberry juice. This study therefore aims to elucidate the process-induced pectin loss and conversion by profiling the WSP and its subdomains (homogalacturonan, HG; rhamnogalacturonan I, RG I) from differently processed red raspberry juice. First, pectin content and pectin-related enzymes (polygalacturonase [PG], pectin methyl esterase [PME], pectate lyase [PL], and β-galactosidase) in fresh, HHP-, and HTST-processed juice were analyzed. Results suggested that the pectin loss in HHP-processed juice was attributed to the residual activities of endogenous HG-modifying enzymes (PG, PME, and PL), whereas that in HTST-processed juice was due to the nonenzymatic depolymerization. Subsequently, WSP, RG I -, and HG-rich fractions were selectively isolated from differently processed juice. These fractions ranged in molecular weight (Mw, 34.03–83.69 kDa), D-(+)-galacturonic acid (GalA) content (32.22%–88.32%), and degree of esterification (3.10%–38.71%). Fourier transform infrared spectroscopy and nuclear magnetic resonance spectra confirmed the low esterified GalA-rich backbone. X-ray diffraction (XRD) patterns indicated an amorphous structure. Notably, monosaccharides composition and microscopic observations demonstrated the presence of side branches in HG-rich fractions. Differences in Mw, monosaccharides composition, and XRD revealed process-induced depolymerization occurred in the HG subdomain of WSP. However, the HG-rich fractions from processed juice showed higher thermostability. These basic findings provided novel insights into the pectin loss and conversion in processed red raspberries.