Galacturonic Acid Content and Degree of Esterification of Pectin from Sweet Potato Starch Residue Detected Using 13C CP/MAS Solid State NMR

Starch residue samples from two Australian sweet potato varieties (Beauregard and Northern Star) and two Indonesian sweet potato varieties (Bis192 and Bis183), and a commercial sample of sweet potato starch residue, were studied for their pectins. Pectins were extracted using 0.1M HCl, 0.05M NaOH, 0.1M HCl/0.75% SHMP and 0.05M NaOH/0.75% SHMP. Hydrolysis of residual starch in the cell wall of sweet potato using heat stable α-amylase and amyloglucosidase was employed prior to pectin extraction to eliminate starch contamination. Pectins were characterised for yield, galacturonic acid content (GA), and the degree of esterification (DE). Conventionally, pectin is characterized by titration, photometry and HPLC. However these methods are cumbersome and time consuming. On the other hand, 13C CP/MAS solid-state NMR, a non-destructive, efficient and direct method, has been found to be well-suited for these purposes since pectin has well-defined 13C NMR spectra. Therefore 13C CP/MAS solid state NMR was used for pectin determination. The pectin characteristics are dependent on variety and extraction process; however, the extraction methods gave variable results. Yields were between 7 and 30% of the cell wall. GA varied from 27 to 80% with the highest found in Bis192 extracted using NaOH/SHMP. DE varied between traceable and 57%. HCl extraction gave higher DE, while NaOH/SHMP caused demethylation. Overall, this study demonstrated that pectin from sweet potato starch residue is mainly low in methoxyl groups.