Qitang ZHANG,Qiheng CHEN,Jiansheng CAO,Zhiming ZENG

1.Chongqing Qunying Agricultural Investment Group Co.,Ltd.,Hechuan 401520,China;2.Pengshui Qunying Engineering and Technology Research Center for Sweetpotato,Pengshui 400800,China

Sweet potato(Ipomoea batatas(L.)Lam.)is one of the major crops in the world.There were total113 countriesplanting sweet potato with acreage of 9×106hm2in 2005[1].China is the world’s largest producer of sweet potato,and the cultivation area and output all rank first in the world.Sweet potato is a high-and stable-yielding and stress-resistant crop,and it tubers are rich in starch.Therefore,sweet potato is a kind of industrial raw material with higher added value[2].

Yanget al.[3]studied the contents ofnutritionalingredients in purple sweet potato residue after pigment extraction.Hanet al.[4]analyzed the nutritional composition of 10 kinds of sweet potato starch residue and their prepared dietary fiber.Sweet potato tubers contain fat,protein,VA,VC,Zn,Fe,Se and Ca,as well as a large amount of starch,which are benefi cial to human health[2].In this study,the sweet potato tubers were crushed first and then used for starch extraction,and the remaining residues were used as test material.The nutritional composition of sweet potato starch residue was studied.In addition,the reutilizing value of sweet potato starch residue was evaluated.

Materials and Methods

Sweet potato cultivar

In this study,the used sweet potato cultivar was Shangshu 19,which was identified by the National Sweet Potato Variety Appraisal Committee of China in 2003[5].The cultivation test of Shangshu 19 was conducted in Wanzhou, Hechuan and Youyang of Chongqing in 2012.The average yield of sweet potato was 40 191.0 kg/hm2;and in the sweet potato,the average starch yield and content were 7 957.5 kg/hm2and 19.94%,respectively[6].

Overview on sampling

The sampling was conducted simultaneously in the Xiushan,Youyang,Pengshui and Bishan branches of the Chongqing Qunying Agricultural Investment Group Co.,Ltd.on December 11,2012.Total 3 kg of sweet potato starch residue was sampled from each processing site.The sweet potato starch residues were preserved in plastic bags at 4℃.The contents of relevant nutritional components in the sweet potato starch residues were determined and analyzed by the Agricultural and Livestock Products Quality Supervision and Inspection Station of Sichuan Province the next day.

Determination of nutritional composition

The moisture content in the sweet potato starch residue was determined according to GB 5009.3-2010;the starch content was determined according to GB/T 5009.9-2008;the fat content was determined according to GB/T 6433-2006;the protein content was determined according to GB 5009.5-2010;the VAcontent was determined according to GB 5413.9-2010;the VCcontent was determined according to GB/T 5009.86-2003;the Zn content was determined according to GB/T 5009.14-2003;the Fe content was determined according to GB/T 5009.90-2003;the Se content was determined according to GB 5009.93-2010;the Ca content was determined according to GB/T 5009.92-2003;the chlorogenic acid content was determined according to TGB 22250.93-2008.

Data processing and statistics

The means,standard deviations and variation coefficients of contents of nutritional ingredients in the sweet potato starch residues collected from four different processing sites were calculated[7].

Results and Analysis

Nutritional composition of sweet potato starch residue

The contents of moisture,starch,fat,protein,VA,VC,Zn,Fe,Se,Ca and chlorogenic acid in the sweet potato starch residues sampled from Xiushan,Youyang,Pengshui and Bishan were shown in Table 1.

As shown in Table 1,in the fresh sweet potato starch residue,the average contents of moisture,starch,fat,protein,Zn,Fe,Se and Ca were 83.75%,101.15g/kg,0.255%,0.4973%,11.18 mg/kg,1 219.00 mg/kg,0.032 3 mg/kg and 6 460.00 mg/kg,respectively.In all the sweet potato starch residues,none of VA,VCor chlorogenic acid was detected.

Reutilizing value of sweet potato starch residue

Edible alcoholBased on the average starch yields and contents of Shangshu 19 in Wanzhou,Hechuan and Youyang of Chongqing in 2012[6],the starch contents in sweet potato starch residues were calculated,and the corresponding alcohol yields produced by starch in starch residues were also calculated(Table 2).

As shown in Table 2,there were significant differences in starch content among sweet potato starch residues sampled from four different processing sites.The average starch content was 101.2 kg/t.If the starch remained in the sweet potato starch residue was all used for producing edible alcohol,1 t of fresh sweet potato starch residue could produce 53.6 kg of edible alcohol(common type)meeting national standards,and the output was 321.6 yuan.Per unit area of fresh sweet potato starch residue,the average starch content was 4 036.7 kg/hm2;the average edible alcohol yield was 2 139.5kg/hm2;the output was 12836.6 yuan/hm2.

Dietary fiber health foodTable 1 showed that fresh sweet potato starch residues contained fat,protein,Zn,Fe,Se and Ca,which are beneficial to human health.With purple sweet potato residue after pigment extraction as test material,the energy yield,carbohy-drate,crude fiber,ash,P,K,Cu and Mn contents and amino acid composition,as well as fat,protein,Zn,Fe,Se and Ca contents,were determined by Yanget al[3].The results showed that the contents of nutritional components mentioned above were all relatively high in sweet potato pigment residue after water extraction.The properties of sweet potato starch residue used in this study are similar to those of purple sweet potato pigment residue.Hanet al.[4]analyzed the contents of dietary fiber in residues of 10 sweet potato cultivars.They found that there were significant differences in dietary fiber content among different sweet potato residues,and the dry matter content ranged from 143.3 to 245.5 g/kg.According to the findings above,it is concluded that sweet potato starch residue has great development advantage and broad development prospects when it is used as raw material for producing dietary fiber health foods.

Table 1 Contents of nutritional ingredients in sweet potato starch residues from four different processing sites

Table 2 Starch contents and edible alcohol yields and outputs of sweet potato starch residues from four different processing sites

Discussion

After starch extraction,sweet potato residue still contains a large amount of starch.This study found thatin fresh sweetpotato starch residues collected from four different processing sites,the starch contents ranged from 9.4% to 11.0% (equivalent to 57.32% -67.53% of dry weight),which is consistent with the study results(41.45% -64.89% of dry weight)of Hanet al[4].When fresh sweet potato starch residue is used as raw material for producing edible alcohol,the added value of sweet potato processing is increased.In addition,the problem of environmental pollution will also be solved.Furthermore,if a small proportion of sorghum and corn is added to sweet potato starch residue,the yield and quality of edible alcohol will all be improved,thereby increasing economic benefit.

In addition,since sweet potato starch residue is rich in dietary fiber,as well as fat,protein,Zn,Fe,Se and Ca,which is beneficial to human health,it can be used for developing dietary fiber health foods with broad market prospects.When sweet potato starch residue is used as raw material for developing dietary fiber health foods,the production process is relatively simple,the equipment investment is less,but the product yield is higher,improving the economic efficiency.

VAwas not detected in sweet potato starch residue in this study,which might be because that Shangshu 19 is a white-flesh cultivar,and its tubers do not contain VA.Previous studieshave shown thatred-flesh sweet potato cultivars contain carotene,which is the precursor for biosynthesis of VA[3].VCwas also not detected in sweetpotato starch residue,which is inconsistent with the study result of Yanget al[5],who found that the content of VCin purple sweet potato pigment residue was 0.63 mg/g after water extraction.This difference might be caused by different test materials (pigment residue and starch residue)or VCloss or degradation during starch processing.Previous studies have shown that VCis water soluble;the nature of VCis unstable,so VCis easy to be decomposed.

The starch contentsin sweet potato starch residues collected from four different processing sites ranged from 9.4% to 11.0%,and the average content was 10.115%.The differences in starch content might be caused by the differences in production and operation among differentprocessing sites.Therefore,during the processing and production,the technology management and training all should be strengthened,and the technical regulations for production operation should be fully obeyed;on the other hand,the processing machinery and production process all should be improved to improve starch yield.

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