Starch residue samples from two Australian sweet potato varieties (Beauregard and Northern Star) and two Indonesian sweet potato varieties (Bis192 and Bis183), and 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, moisture, ash, galacturonic acid content (GA), degree of esterification (DE), neutral sugars content and molecular weight (MW). Selected pectins were also characterised for their dispersion and gel properties. Pectin with the highest yield was compared to commercial low methoxyl pectin (GRINSTED??YF 450) for its application in milk pudding. The pectin characteristics are dependent on variety and extraction process; however, the extraction methods were dominant. Yields were between 7 and 30 % of the cell wall. Ash varied from 2.9 to 6.8%. NaOH/SHMP extraction gave higher yields and ash contents. Moisture was between 5.5 and 6.7. 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. Neutral sugars varied from 10 to 25% where galactose was the predominant sugar, followed by arabinose, rhamnose, glucose and xylose. Pectin MW was very heterogeneous. HCl extraction gave higher MW than the others. Dispersion of sweet potato pectins displayed shear-thinning. Consistency coefficient (K) increased, while flow behaviour index (n) decreased with increasing pectin concentration. HCl-extracted pectins formed stiff sugar-gels at concentrations higher than 2%. The optimum Ca2+ concentration for NaOH/SHMP-extracted pectin gels was 20 mg/g pectin. Pectin with the highest yield and stiffness (Norther Star extracted using NaOH containing SHMP) was used for milk pudding preparation. This pudding had comparable texture to pudding prepared from commercial low methoxyl citrus pectin (GRINSTED??YF 450). Overall, this study demonstrated that pectin from sweet potato starch residue is mainly low methoxyl pectin with possible chemical structure of rhamnogalacturonan I (RGI) that has potential for food applications.
| Identifer | oai:union.ndltd.org:ADTP/257567 |
| Date | January 2008 |
| Creators | Nurdjanah, Siti, Chemical Sciences & Engineering, Faculty of Engineering, UNSW |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://unsworks.unsw.edu.au/copyright, http://unsworks.unsw.edu.au/copyright |
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