Physicochemical Properties Of Starches From Different Sweet Potato Varieties

Sweet potato（Ipomoea batatas[L.]Lam）is an important starchy crop,and has hundreds of varieties or lines.The contents and properties of starches are significantly different among different sweet potato varieties,resulting in different applications.Starch is semi-crystalline granules,and the crystalline structure is one of the main factors affecting the properties and uses of starch.The formation of crystalline structure is greatly affected by the growth environment in sweet potato.Therefore,it is very important to study the physicochemical properties of starches from different sweet potato varieties for the protection and utilization of sweet potato variety resources.Previous studies were mainly focused on the comparison of physicochemical properties of starches from a few potato varieties in the same country or region.It is lack of the comparison of starch physicochemical properties of sweet potato varieties from different countries and regions,especially the effect of varieties and growing locations on the physicochemical properties of root starch and flour in sweet potato,which limits the utilization and promotion of sweet potato variety resources.In this study,44 sweet potato varieties from different countries and regions were selected from the sweet potato germplasm resource bank of the National Sweetpotato Genebank.The physicochemical properties of their root starches were studied and compared.Some starches with different crystalline structures were selected as materials,and their molecular structures were analyzed to reveal the relationships between starch molecular structure and physicochemical properties.On the above basis,three sweet potatoes with different color root tubers were selected and planted in four growing locations,and the physicochemical properties of their root starches and flours were investigated.The effects of sweet potato variety,growing location and their interaction on the physicochemical properties of root starches and flours were analyzed using two-way ANOVA.The main results were as follows.1.Physicochemical properties of root tuber starches from different sweet potato varieties.The 44 sweet potato varieties from 15 countries and regions were selected as materials,and planted under the same growing location.Starches were separated from their root tubers.Their physicochemical properties including morphology,particle size,composition,crystalline structure and thermal properties were measured and compared using morphological observation,physicochemical measurement and spectral analysis methods and technologies.The correlations between starch physicochemical properties and sweet potato varieties were analyzed.The results showed that starch granules had round,polygonal,oval and semi-oval shapes,and contained small and large granules with the hila in the center of the granules.There were no significant differences in starch morphologies among different sweet potato varieties.The starch exhibited a bimodal particle size distribution,and the starch particle size（D[4,3]）ranged from 8.0 to 15.3 μm among 44 sweet potato varieties.The 44 sweet potato starches had different iodine absorption characteristics with OD680 from 0.259 to 0.382 and OD620/550 from 1.142 to 1.237.The 44 sweet potato starches contained apparent amylose content from 19.2%to 29.2%and true amylose content from 14.2%to 20.2%.The X-ray diffraction patterns showed that sweet potato had A-,CA-,Cc-and CB-type starches.The thermograms of 44 starches exhibited one-,two-or three-peak curves,leading to significantly different gelatinization temperature ranges from 13.1℃ to 29.2℃.The correlation analysis showed that the crystalline structure and amylose content of sweet potato starch significantly affected the thermal properties of starch.The 44 sweet potato varieties could be clustered into different groups based on their starch physicochemical properties.2.Molecular structure of starches with different crystalline types in sweet potato.One A-type starch,five CA-type starches,three Cc-type starches and one CB-type starch were used as materials.Their thermograms were further analyzed,and molecular weight distribution and amylopectin chain length distribution were investigated.The relationships between molecular structure,crystalline structure and thermal properties of starch were analyzed.The results showed that the CN03,JP06,MA01,US03 and US04 contained B-type and C-type starch,the CN02,ML01 and PE01 contained C-type and A-type starch,the TZ01 only contained B-type starch,and the JP14 only contained A-type starch according to the fitted peaks of thermograms.The 10 starches had amylopectin short branch-chains from 55.1%to 59.7%,amylopectin long branch-chains from 21.3%to 25.3%,and amylose content from 18.7%to 20.7%.The amylopectins of 10 starches contained A,B1,B2 and B3+branch-chains from 20.2%to 25.1%,from 44.0%to 49.6%,from 12.6%to 14.4%and from 14.3%to 18.8%,respectively,and had average branch-chain length from 22.3 to 24.6 DP.The partial least squares regression（PLSR）showed that the molecular structure and thermal properties were similar between A-type and CA-type starch,the molecular structure and thermal properties were significantly different between Cc-type and CB-type starch,the amylopectin short branch-chains were high in CB-type starch,and the resistance to gelatinization was strong in A-type starch.3.Effects of variety and growing location on physicochemical properties of starch in sweet potato.The Ningzishu 1 with purple-fleshed root tuber,Sushu 16 with yellow-fleshed root tuber and Sushu 28 with white-fleshed root tuber were planted in four growing locations including Linquan,Rugao,Yangzhou,and Zunyi.Starches were isolated from fresh root tubers,and their physicochemical properties were determined.The effects of variety,growing location and their interaction on starch properties were analyzed.Th e results showed that the particle size（D[4,3]）varied from 12.1 to 18.2 μm,the iodine absorption parameters varied from 0.260 to 0.361 for OD620,from 0.243 to 0.326 for OD680 and from 1.128 to 1.252 for OD620/550,and the amylose content varied from 16.4%to 21.2%among starches from three varieties and four growing locations.Starches from three varieties and four growing locations all exhibited C-type,and had ordered degree from 0.634 to 0.726 and lamellar thickness from 9.72 to 10.21 nm.Starches had significantly different swelling powers,water solubilities,thermal properties and pasting viscosities among three varieties and four growing locations.The native starches had rapidly digestible starch（RDS）from 2.2%to 10.9%and resistant starch（RS）from 58.2%to 89.1%,and the gelatinized starches had RDS from 70.5%to 81.4%and RS from 10.8%to 23.3%among three varieties and four growing locations.The two-way ANOVA analysis showed that starch physicochemical properties were affected significantly by variety,growing location,and their interaction in sweet potato.4.Effects of variety and growing Location on physicochemical properties of root tuber flour in sweet potato.The flours were further prepared from the root tubers of three sweet potato varieties planted in four growing locations,and their physicochemical properties were determined.The effects of variety,growing location and their interaction on physicochemical properties of root tuber flours were analyzed in sweet potato.The results showed that the total starch,soluble sugar and protein contents varied from 50.3%to 65.7%,from 3.5%to 12.7%and from 1.98%to 7.59%,respectively,among root tuber flours from three varieties and four growing locations.The swelling power and water solubility of root tuber flours varied from 20.1 to 27.1 g/g and from 27.6%to 72.7%,respectively,and their thermal and pasting properties showed significant differences among three varieties planted in four growing locations.The two-way ANOVA analysis showed that the physicochemical properties of root tuber flours were significantly affected by variety,growing location and their interaction in sweet potato.It was noteworthy that the starch content,soluble sugar content,swelling potential,water solubility and pasting properties were most affected by growing location,the thermal property was most affected by the variety,and the protein content was most affected by the interaction between the variety and growing location.The above research results showed that the genotype was the mainly reason for the differences in the physicochemical properties of sweet potato starch,and the molecular structure of starch determined the crystalline structure of starch and influenced the thermal property.The physicochemical properties of starch and root tuber flour were affected significantly by variety,growing location and their interaction in sweet potato.This study not only enriches the basic knowledge of sweet potato variety resources and their starch structure and physicochemical properties,but also provides reference data for the applications of sweet potato root tuber starch and flour in food and non-food industries and the development and planting of sweet potato variety resources.