The Studies On Protein Extraction From Waste-water Of Sweet Potato Starch Production And Its Antioxidant Activity

Sweet potato (Ipomoea batatas L.) is an important food crops, cash crops and industrialraw materials. The total acreage and yield of sweet potato rank the first in the world. In thepresent study, four sweet potato varieties (Xushu18, Yushu13, Zhengshu04-4-2, and SL-19)which are rich in starch and widely cultivated in Henan province were chosen as theexperimental material, and several methods to extract protein from the sweet potato starchproduction waste water were compared. Then, sweet potato protein structure and antioxidantactivity were analyzed.In laboratory, we simulated the industrial process of sweet potato starch production,collected its waste water. Single factor experiments showd that the optimum conditions forprotein extraction were as follows: pH8.0, extraction temperature60℃, extraction time1.5hours, isoelectric point of sweet potato protein pH4.5. Based on the result of single factorexperiments, we optimized the method to extract alkali-soluble and acid-soluble proteins fromsweet potato starch production waste water. The total extraction ratio reached72.66%and thehighest protein purity was73.68%.Colour chromatism analysis shows that acid-soluble protein was lighter in colour thanalkali-soluble protein from the same cultivar. Among all the proteins extracted from starchproduction waste water, those from Yushu13and SL-19had lighter colour, so they are moresuitable to be used as food additives and nutrition fortifiers. Scanning Electron Microscopy(SEM) result showed that surface of sweet potato protein had smooth sheet structure, exceptthat the surface of SL-19alkali-soluble protein had holes with varying size and depth, and itsacid-soluble protein had honey-comb structure. SDS-PAGE result displayed the molecularweight of sweet potato protein was25.0kDa. Infrared Spectroscopy analysis showed thatthere are obviously characteristic absorption spectral bands of amideⅠ a rea, amideⅡ area,amide Ⅲ area in Sweet potato protein.Fitting analysis indicated that spectra ofamideⅠ showd that structure of β-sheet and β-turn accounted for a larger proportion, bothabout40%; And structure of α-helix and no rules curly took a smaller proportion, both about10%. Amino acid analysis results showed that both of alkali-soluble and acid-soluble proteinof SL-19contained17amino acids species. The content of essential amino acids ofalkali-soluble proteins in SL-19was very close to amino acid pattern recommended by WHO. Determination of antioxidant activity showed that reducing power of sweet potatoprotein increased with the increase of protein concentration. The reducing power of bothalkali-soluble and acid-soluble protein from Xushu-18was significantly higher than those ofthe proteins from the other two sweet potato cultivars. DPPH radical scavenging rate ofalkali-soluble protein fell at first and then rose with the increase of protein concentration;DPPH radical scavenging rate of the protein from SL-19was the highest among all kinds ofprotein. On the other hand, DPPH radical scavenging rate of acid-soluble protein increasedwith the increase of protein concentration. And Xushu-18protein had the maximum DPPHradical scavenging rate. The capacity of hydrogen peroxide scavenging and anti-linoleic acidperoxidation increased with increase of protein concentration, and alkali-soluble protein issuperior to acid soluble protein; And Xushu-18protein had the highest hydrogen peroxidescavenging ability and anti-linoleic acid peroxidation capacity.