New findings on acid-extractable pectins from soy hull

Abstract

Soy hull has been considered a potential source of commercial pectin. The aim of the present study was to investigate its real potential as a source of pectin. Soy hull (sample 1) was extracted with 0.1 M HCl, for 45 min, at 90 °C (fraction A), conditions previously reported to result in yields and GalA in the range of commercial pectins. The extraction resulted in low uronic acid content (UA 39 %) and lower yield. Similar values were obtained using harsher conditions (boiling 0.14 M HNO3 for 30 min and 60 min – Fractions B and C, respectively). HSQC-NMR confirmed the coextraction of galactomannans. Considering the unexpected results, three other soy hull samples (2, 3 and 4) were used for extraction. The yields and UA were in the range of 10–13 % and 26–48 %, respectively, also below published data. Prior removal of galactomannan by water extraction increased the UA content to 62 % and gave rise to a pectin with a degree of methyl-esterification (DM) of 29 %. The pectin had remarkable amount of rhamnogalacturonan I and xylogalacturonan and did not form gel with calcium. The findings using four different commercial samples did not support previously published data and demonstrated that soy hull is not suitable as a raw material for production of food grade pectins by conventional extraction.