Structural Characterization Of C-type Starch In Sweet Potato Root Tuber And Its Response To Soil Temperature Change

Starch is a semi-crystalline granule and composed of amylose and amylopectin.Starches are divided into A-,B-and C-type according to their crystal compositions.The C-type starch contains A-and B-type crystals,and their contents and distribution affect structural characterization of C-type starch.Therefore,the structural characterization of C-type starch is more complex than those of A-and B-type starches.The root tubers of sweet potatoes are divided into purple,yellow and white according to their colors.Starch is the main component of different colored root tubers.At present,most reports show that sweet potato root tuber starch is C-type.However,the detailed structural characterization of starch is lack in different colored root tubers of sweet potatoes,especially for allomorph distribution and development regulation of C-type starch.In this research,purple,yellow and white root tuber sweet potatoes were chosen,the isolation method of starches from different colored root tubers was optimized,their structural characterizations were analyzed,the allomorph distribution of C-type starch was revealed,and the responses of structural characterization of C-type starch to soil temperature change were mainly studied.The main results were following:1.The isolation method of starches from different colored root tubers was optimized,and the effects of separation medium on structural properties of starch were investigated.Sweet potato root tubers exhibit different colors due to their different contents of pigments and phenolic compounds.Therefore,it is very important to remove the pigments and phenolic compounds effectively during starch isolation.In this study,starches were isolated from root tubers of 1 purple,1 yellow and 1 white sweet potato using four different extraction media,including H2O,0.5%Na2S2O5,0.2%NaOH,and both 0.5%Na2S2O5 and 0.2%NaOH,in combination with ethanol cleaning.The NaOH and Na2S2O5 treatments were able to increase the whiteness of starches from purple and yellow root tubers.The four extraction methods had no effect on starch morphology,but had a slight effect on starch granule size.The protein content,amylose content,crystalline structure and ordered degree of isolated starch were not affected by extraction media,but the starch functional properties such as thermal properties,swelling power,water solubility and pasting properties were affected slightly.The high-quality starch can be quickly and efficiently prepared from different colored root tuber of sweet potatoes using water extraction in combination with adequate ethanol cleaning.2.The structural characterizations of starches had no relationships with the root tuber colors of sweet potatoes.In this study,3 white,3 yellow,and 3 purple root tuber sweet potatoes were chosen and planted in the same environment.Their starch structural properties were measured,and the correlation between starch structural characterization and root tuber color was analyzed.The white root tubers had the highest starch content(61.5%—6 7.5%)and the lowest soluble sugar content(13.1%-18.0%),and the yellow root tubers had the lowest starch content(45.8%-53.1%)and the highest soluble sugar content(28.9%31.5%).Starches from different varieties had different granule sizes(12.3-18.1 μm)and amylose contents(24.1%-27.2%),but all exhibited CA-type crystalline structure.The starches from the same or different colored root tubers exhibited the differences in relative crystallinity,ordered degree,swelling power,water solubility,gelatinization temperature,pasting viscosity and digestion properties.The cluster analysis based on starch property parameters indicated that starch structural characterization had no significant relationship with root tuber color.3.C-type starch of sweet potato root tuber contained A-,B-and C-type granules.Allomorph distribution of C-type starch in different colored root tubers of sweet potatoes was investigated using differential scanning calorimetry,X-ray diffractometry,and hot stage microscopy.Starches from different colored root tubers all exhibited CA-type XRD patterns and wide DSC thermal curves.The thermogram curves could be fitted into three peaks with different gelatinization temperature using PeakFit software.The hot stage microscopy analysis also showed that starch granules could be divided into three groups according to their gelatinization temperatures,corresponding to three-peaks fitting of thermal curves.The ungelatinized starch granules at different gelatinization temperatures were isolated.Their crystalline structure changed gradually from CA-to A-type with the increase of gelatinization temperature,and thermogram curves changed gradually from three-peak to a typical single-peak pattern.Therefore,C-type starch of sweet potato root tuber contained A-,B-and C-type granules.The A-type granules were only composed of A-type crystals with high gelatinization temperature,B-type granules were only composed of B-type crystals with the lowest gelatinization temperature,and C-type granules contained both A-and B-type crystals with the gelatinization temperature between A-and B-type granules.4.Soil temperature regulated the amylopectin chain length distribution and the formation of A-,B-and C-type granules.In this study,1 purple,1 yellow and 1 white sweet potato variety were planted in May in the field and their root tubers were harvested at different developing stages.The content of B-type crystal was gradually increased with development of root tuber.When the sweet potatoes were planted and harvested under high and low soil temperature in the field,the content of A-type crystals and amylopectin long branch-chains were significantly higher under high soil temperature than under low soil temperature,but amylose content was similar,In order to further reveal the regulation of soil temperature on starch structural properties,one yellow sweet potato variety was planted in a controlled environment growth chamber at three levels of soil and air temperatures(21,25 and 28℃)with the same humidity,light time/intensity,soil composition,and organic matters,which can exclude the influence of other growth environment.The soil temperature did not change the morphology and amylose content of starch,but markedly increased the size of starch from 12.2 to 17.0 μm with increasing soil temperature.Starch exhibited less A branch-chains and lower branching degree of amylopectin and more B2 and B3+branch-chains of amylopectin under high soil temperature than under low soil temperature.With increasing soil temperature,starch changed from CC-to CA-type with increasing relative crystallinity,and its gelatinization temperature increased.The lamellae thickness and gelatinization enthalpy of starch did not significantly change under different soil temperatures.The expression of isoamylase1(ISA1),soluble starch synthase Ⅲ(SSSⅢ)and starch branching enzyme 1(SBEI)in root tuber under high soil temperature increased significantly,resulting in changes of amylopectin structure.This research not only enriches the basic theoretical knowledge of the development,structural properties and allomorph distribution of C-type starch,but also provides some important references for the cultivation and starch utilization of sweet potato varieties with different colored root tubers.