| Sweet potatoes are the fourth large crops planted in china, and the major commercial utilization of sweet potatoes in china includes starch and starchy food production, which generates a large quantity of sweet potato residues. Normally, the residues are used as animal feed or discarded as watste, which in turn causes environmental pollution and waste of resources. Sweet potato residues contains considerable amount of pectin, but few researches have been made to sweet potato pectin. In this study, the preparation technology of sweet potato pectin by extracts of disodium hydrogen phosphate solution was optimized, then the pectin extracted in the optimized condition was purified by ion exchange resin method. Finally, pectin was extracted from sweet potato residues by extracts of acid solution and disodium hydrogen phosphate solution in the optimized condition, then ultrafiltration, alcohol-precipitation and ion exchange ion methods were adopted to purify pectin, so as to compare the composition and gelling characteristics of pectin by different extraction and purification methods. The result is as below.Pectin was extracted from sweet potato residues by extracts of disodium hydrogen phosphate solution. Based on the results of one-factor experiment, the effects of liquid: solid ratio, extraction time, extraction temperature and solution pH on pectin yield were investigated by quadratic regression orthogonal rotation combination desigen experiment, the selected optimal extraction conditions were: liquid: solid ratio 20:1, extraction time 3.3 h, extraction temperature 66℃and solution pH 7.9. Under this extraction condition, the yield of pectin was 10.24%.The galacturonic acid content with disodium phosphate solution in the optimized condition was 60.02±0.03%,and the starch and ash contents were 10.42% and 10.58%, respectively.Ion exchange resin method was adopted to purify the pectin crude extract by disodium phosphate solution in the optimized condition. On the base of yield, ash content of pectin and relative colour of effluent, the optimum ion exchange resin (D001 and D201) were chosen from four ion exchange resin (001×7, 201×7, D001 and D201) and then form ion exchange column (D001- D002). D001- D002 ion exchange resin column was used in different rate of on column with yield, ash content of pectin and relative colour of effluent as value indexes. And the optimal rate of 1 BV/h for the column process was chosen. Under the optimum purification condition, the galacturonic acid content of pectin increased from 60.02% to 65.02%, and the relative colour of effluent significantly decreased from 0.384 to 0.124. The ash content of pectin significantly decreased from 10.58% to 3.2%, especially the P content of pectin, which significantly decreased from 19,000ug/g to 0.11ug/g. The results show the ash content of pectin was decreased obviously, and the pectin purity was improved significantly by using ion exchange resin method.The salt extraction by extracts of disodium hydrogen phosphate solution gave a higher yield (7.2-9.7%) than that of acid extraction (4.6-7.0%). And it had a higher molecular weight (480.6-1201.0 kDa) and a higher gel strength (61.9-278.1 g/cm~2) than those of acid extraction (250.8-771.1kDa and 149.4-534.0g/cm~2, respectively). What's more, the pectins extracted by disodium hydrogen phosphate solution gave a lower degree of esterification(9.08-10.4%) than that of acid extraction (28.7-31.11%).For salt extraction by extracts of disodium hydrogen phosphate solution, ion exchange ion method gave the highest galacturonic acid content (65.0%), which was significantly higher than that of two other purification methods (45.0-51.5%). What's more, the ash content was significantly lower (3.2%) than those of two other purification methods (9.6-10.6%), and the neutral sugar content was the loweset (22.6%), with a highest gel strength (534.0 g/cm~2). However, the pectins purified by ion exchange ion method gave a lower molecular weight(1098.0 kDa)than ultrafiltration(1201.0 kDa).For the acid extraction, ultrafiltration showed a higher galacturonic acid content (63.1%) than that of two other purification methods (57.1-58.5%), and the neutral sugar content (16.3%) was also significantly lower than that of two other purification methods (20.2-22.6%). The pectins purified by ultrafiltration method gave the highest molecular weight and gel strength, which were 771.1kDa and 278.1 g/cm~2, respectively. |
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