Production of pea hull soluble fiber-derived oligosaccharides using subcritical water with carboxylic acids

This study aimed to evaluate a two-stage sequential approach based on subcritical water technology and the use of citric and malic acids to first extract soluble polysaccharides from pea hull fiber and then hydrolyze them into oligosaccharides. Subcritical water extraction with aqueous citric acid (120 °C/50 bar/30 min) yielded 11.8% pea soluble polysaccharides comprised of pectic fragments (17.4% galacturonic acid) with an esterification degree of 50.9%. Pectic fragments were estimated as rhamnogalacturonan-I, xylogalacturonan and xyloglucan. The HPSEC/RID and HILIC-ELSD analyses showed that subcritical water hydrolysis of pectic fragments into oligosaccharides was favored by aqueous citric and malic acids (0.2%) at 100 bar between 125 °C and 155 °C. The yields of released gluco-oligosaccharides (2–6 DP) were 6.1% and 20.4% at 125 °C and 135 °C/100 bar/120 min, respectively. The main hydrolysis mechanism exhibited a stepwise pattern, where rhamnogalacturonan-I side chains were cleaved first followed by their breakdown into reduced molecular weight compounds and oligosaccharides.