The Properties Of Sweet Potato Protein Concentrate And The Antioxidant Properties Of Its Hydrolysates

Sweet potato is one of the major rich sources of starch which attracts most starch industries. It is grown in more than 100 countries and mainly used for food and starch production. Besides starch sweet potato contains a little amount of proteins which can be recovered and used as ingredients in different food products. However, the properties of sweet potato protein and its potential application area have not been completely elucidated.The objective of this study was to examine the properties of protein concentrates from the fresh sweet potato and the sweet potato starch residue, the factors affecting the solubility and other properties of sweet potato protein concentrates, the antioxidant properties of the sweet potato protein hydrolysate and structure of peptide responsible for its antioxidant activity.Protein concentrates were extracted and isolated from fresh sweet potato and the commercial sweet potato starch residue with isoelectric precipitation technique. The protein content for the sweet potato protein isolate from industrial residue (SPPr) was 73% and that from fresh sweet potato (SPPC) was 76%. Both sweet potato protein concentrates (SPPr and SPPC) showed a good nitrogen solubility (>80%) compared to commercial potato protein isolate (PPI) (p<0.05), and SPPr had a relatively higher nitrogen solubility. SPPr and SPPC were able to foam in both high acidic (pH 2) and high alkaline condition (pH 12). The highest foaming capacity (FC) of SPPr and SPPC was nearly 170%. While FC of PPI was increasing as pH was increased except at pH 4. Generally, foaming stability (FS) for SPPr and SPPC was quite stable in acidic condition (≥50% for SPPC), while PPI was relatively stable in alkaline condition (p<0.05). SPPr showed maximum emulsification activity index (EAI) at pH 10, which was same as PPI, and the highest EAI for SPPC was at pH 4. The emulsification stability index (ESI) of SPPC and SPPr was highest at pH4 (≥85%), while which of PPI was at pH 10-12 (p<0.05).SPPC was further separated to determine the isoelectric point (pI) of protein constituents in the sweet potato. Ultra filtration (UF) with 100 kDa Millipore cutoff membrane was used, the fraction with molecular weight (MW) lower than 100 kDa was further separated with Superdex 75 and then DEAE fast flow anion exchanger. One protein constituent was partially purified and its pI was pH 4, in which the zeta potential was near the net charge, it was further proved as sporamin with Native-PAGE, which is a major soluble protein in sweet potato.Sweet potato protein concentrate (SPPC) was hydrolyzed for 6 hours by Alcalase (Alc) and Flavourzyme (Flv) proteases (SPPH) and its antioxidant activities were studied in detailed. 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS+), and 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity, reducing power, Fe2+ chelating activity, and Linoleic oxidation inhibition were tested to determine the antioxidant efficacy of Sweet potato protein hydrolysates (SPPH). The results showed that SPPH from Flavourzyme (SPPH (Flv)) had higher Fe2+ chelating activity, while SPPH from Alcalase (SPPH (Alc)) had higher reducing power. The DPPH'radical scavenging activity of SPPH (Flv) was higher than that of SPPH (Alc) instead had higher ABTS+ radical scavenging activity. Linoleic oxidation inhibition of SPPH (Alc) is higher than that of SPPC and SPPH (Flv).The SPPH was further separated with an ODS-2 C18 HPLC and more than 20 components were obtained. The active peptide sequences with a prominent ABTS radical scavenging activity identified by LC/QTOF-MS were STY, DPMLR, VIKPTDV for SPPH (Alc) and GVGKGGGL, TPRSAGGGV, and SLPFGGAV for SPPH (Flv). The results suggested that free radical scavenging activity of SPPH was related to its molecular weight and sequence of the active peptidesThe thermal stability for antioxidant activity of SPPH was assayed in pure water system. The results showed for antioxidant activity of SPPH was not affected (p>0.05) by heat at 80℃compared to the control or unheated hydrolysates. The antioxidant activity tended to increase as temperature increased to 100 and 120℃(p<0.05) but was decreased at 150℃in all assays. Hydrolysates obtained from industrial residue SPPr (Alc) and SPPr (Flv) were having a high antioxidant activity and were quite stable than hydrolysates from fresh sweet potato SPPH (Alc) and SPPH (Flv).The results suggested SPPr and SPPC could be explicated in the food industry as attractive ingredients, not only for their functionalities and thermal behavior but also for their availability, and thus can be applied for a wide range of food products.