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Abstract Using ??Luotian chestnut?? as the raw material， the content changes of resistant starch and main nutrients in the processed fruit were studied after taken common processing of high??temperature steaming of canned food， sand??mixed frying and cooking without shells， with the aim to find out the effects of different processing methods on the content changes of nutrients in chestnuts. The result showed that there were significant differences in the resistant starch contents by different processing methods. After processing， the retention degree of resistant starch was the lowest using the methods of sand??mixed frying， of 42.42%， while the highest was found in canned chestnut products， of 93.16%. Among the main nutrients， adding water and sugar would reduce content of resistant starch， while increasing the protein content would promote the retention of resistant starch retention. The changes in lipid content had no effect or not a single promoting or weakening effects on the content of resistant starch.
Key words Chestnut； Resistant starch； Nutrients
Chestnut （Castanea mollissima BL.） is a deciduous tree of the family Fagaceae， and a species of chestnut. It is a traditional agricultural and sideline product in China， well??known as the "king of dried fruit". Its fruit， nuts， are mainly composed of starch， and contain many nutrients such as protein and other nutrients. Resistant starch （RS） refers to "starch， including its degradation products， that escapes from digestion in the small intestine of healthy individuals in foods"[1]. It is divided into 4 types： RS1??physically embedded starch， RS2??resistant starch granules， RS3??retrograded starch， and RS4 chemically modified starch. Resistant starch accounts for 68.93% of the total starch content in chestnut[2]， and resistant starch is fermented in the large intestine， producing short??chain fatty acids， which acts as a dietary fiber that can benefit human health by lowering blood sugar， reducing intestinal diseases[3]. However， eating too much will increase the capacity of chyme， increase the proportion of inflated substances in chyme， and cause stickiness， which makes it difficult to digest[4]. It is more obvious in the elderly and children.
At present， there are various developments in the processing of Chinese chestnut， and breakthroughs have been made in various researches， developing marrons glaces， canned chestnut， quick??frozen chestnut kernels， preserved chestnut， chestnut powder， chestnut paste， chestnut beverage， chestnut wine， and so on[5]. The contents of resistant starch in them are different， mainly due to different processing methods such as slicing or crushing processing and different environmental nutrient substances. Proteins， fats， small molecule sugars， and inorganic minerals all affect the formation of resistant starches， but studies have focused on the influence of major components such as proteins and fats on the formation of resistant starch[6]. In this study， complete chestnuts or nuts were used as the materials to study the changes of resistant starch and main nutrients under 3 different processing of canned， fried and cooked methods， and primary conclusion was made to the effects of nutrients on the formation of resistant starch， with the aim to provide scientific bases to increase or decrease the contents of resistant starch and nutrients in processed chestnut products.
Materials and Methods
Test materials
The tested chestnut variety was ??Luotian chestnut??.
Reagents included petroleum ether， 2% boric acid， NaOH， HCl， sucrose， glucose standard solution， anthrone， ethyl acetate， ethanol， acetic acid， sodium acetate， 3， 5??dinitrosalicylic acid， potassium sodium tartrate， anhydrous sodium sulfite， and all were analytical pure. ????amylase glucosidase was provided by Beijing Double Helix Microbiology Company.
Instruments and equipment were as follows： analytical balance （accuracy to 0.000 1g）， centrifuge tube （10 ml）， volumetric flask （50， 100 ml）， thermostat shaker （ZHWR??1102C）， constant temperature water bath （HH??6）， spectrophotometer （722）， table??top & high??speed refrigerated centrifuge （HR1716M）， fat analyzer （SZC??C）， and nitrogen analyzer （KDN??08A）.
Test methods
Fresh chestnut processing Chestnut nuts were obtained after peeling off the shells to make it able to test the water content. The fresh nuts were than dried to constant mass in a 50 ?? oven， and then crushed in a grinder before sieving over the 100??mesh sieve， obtaining the dry sample to determine the contents of resistant starch， total sugar， fat and protein.
Canned chestnut processing The processing was based on the method of Zhou et al.[7] with appropriate modifications. After husking and peeling， chestnut nuts were steamed at 100 ?? for 20 min， and canned after added with leaching mixture of sucrose （10%） and citric acid （0.1%）. Some nuts were wiped to measure the water content； some nuts were rinsed with fresh water for several times， and then dried at 50 ?? to constant mass and smashed and sieved to obtain dry sample for the determination of the resistant starch， total sugar， fat and protein contents.
Frying chestnut with shells Chestnuts with shells were put into an iron pan filled with the nuts fully covered. After heating the pan， the chestnuts were fried for 20 min until aroma arose and the samples were well fried. After peeling off the shells， the fried nuts were pout into a 50 ?? oven to determine the water content. After dried to constant mass， the nuts were crushed and sieved， obtaining the dry sample， so as to determine the contents of resistant starch， total sugar， fat and protein. Cooking chestnut without shells After peeling off the shells， the nuts were washed by clean water and then poured into the iron pot for daily cooking added with cooking oil， salt， sugar， monosodium glutamate. After being fully cooked， the nuts were washed with water and then dried to determine the water content. After dried to constant mass， the nuts were crushed and sieved， obtaining the dry sample， so as to determine the contents of resistant starch， total sugar， fat and protein.
Item determination
The content of resistant starch was measured referring to the AOAC method[8] with appropriate improvements. Accurately 100 mg （mass recorded as W） of processed dry sample was taken and put into a 10mL plugged test tube， and the sample was shaken to the tube bottom by knocking the tube. First， soluble sugars were removed with 66% ethanol， followed by adding 4.0 ml of ????pancreatic amylase （10 mg/ml） containing AMG （3 U/ml） to each tube， and then the plugs were covered to achieve fully mixing. The tube was oscillated at 37 ?? water bath for 16 h， and after wiping the outside drops， the tube was added with 4.0 ml of anhydrous ethanol. After mixed thoroughly， the tube was centrifuged at 4 500 r/min for 10 m， obtaining the precipitate. And the tube was put in the ice bath， added with 2 ml of 2 mol/L NaOH solution to the precipitate， and the mixture was stirred on ice for 20 min to dissolve the precipitate and the resistant starch. Then， 8 mL of 1.2 mol/L sodium acetate buffer （pH 3.8） was added and well mixed， followed by immediately adding 0.1mL of AMG （3 300 U/ml）， and then digested in a 50 ?? water bath for 60 min. Then， the sample solution in the tube was transferred to a 100 ml volumetric flask， and then set to the constant volume of 100 ml with distilled water， followed by fully mixing. The glucose content （Gr） was determined using the DNS method.
Resistant starch content = （Gr??N??0.9/W）??100%.
Where， Gr is the mass of reducing sugar； N is diluent volume； W is sample mass.
The total sugar content was determined referring to the anthrone colorimetric method. Water content was determined referring to GB5009.3??2010； fat content referring to GB/T 5009.6??2003 with fat analyzer； protein content referring to GB5009.5??2010 with Kjeldahl nitrogen analyzer[9-10].
Results and Analysis
Changes of resistant starch content in chestnut after processing
As shown in Table 1， after various ways of chestnut processing， the content of resistant starch changed due to processing conditions， but the total amount was still relatively high. Compared with 51.09% of resistant starch content in fresh chestnuts， resistant starch was decomposed to a certain extent after processing. The content of resistant starch was 21.68% in the fried samples under high temperature， and the remaining resistant was only 42.42%， which was the lowest among the several ways of processing. The content was 28.02% in the cooked samples without shells， while the resistant starch content in the canned products was 42.42%， the highest of all with the retention rate reaching up to 93.16%. Water content and its ratio to resistant starch content
As shown in Fig. 1， the water content was 45.84% in fresh chestnut， and its ratio with resistant starch content was 1.46. In the processed samples， the water content increased in the canned products， reaching up to 54.13%， and in the canned samples， the ratio of water content to resistant starch content reached up tot 5.36. After frying and cooking， the water content decreased. The water content of fried chestnut was 34.12%， which showed a more significant decrease than that of cooked chestnut of 41.30%. The ratio of water content to resistant starch content was the smallest of 0.99 in the fried samples， while the ratio was 2.52 for the coked samples， which was higher than that of fresh chestnut but lower than that of canned chestnut.
Fat content and its ratio to RS content
As shown in Fig. 2， the ratio of fat content to dry mass was not high， close to 1%， and the changes of fat content were not significant after processing. In fresh chestnut， the fat content was 0.95%. The fat content decreased in the chestnut of both canned and fried processing， and the decrease was more significant in the chestnut processed with frying with shells than the canned chestnut. However， the fat content increased to 1.09% in home cooking processing， which was due to the fact that oil was added to the cooking process， which caused some penetration into the nuts. The differences in the ratio of fat content to starch resistant content were significant. The ratio of canned products was 3.54%， which was slightly less than 3.89% of the cooking processing which had large fat content， and both were higher than that of the fresh chestnut of 1.84%. The ratio was the smallest of 1.15% in the samples with fried processing.
Total sugar content and its ratio to resistant starch content
As shown in Fig. 3， there were significant differences in the total sugar contents and the ratios of total sugar contents to resistant starch content. Total sugar content was the highest in the canned chestnut of 20.26%， followed by 15.84% for cooking processing， 9.61% for stir??fried and 8.34% for fresh chestnut. After processing， the total sugar contents were all increased. The ratios to resistant starch contents also showed a similar trend. The ratio of canned products was 92%， cooked products of 56%， and stir??fried of 20%， slightly higher than 16% of fresh chestnut.
Total protein content and its ratio to RS content As shown in Fig. 4， the contents of total protein changed after the 3 processing methods， but the changes were not significant. Compared with 8.70% of fresh chestnut， the protein content of canned products decreased by a small amount to 7.36%， and the protein content of fried and cooked chestnuts increased slightly to 9.12% and 9.41%. The reason for this change was that the nuts were well kept during frying and cooking， and the inside fruit was not highly heated， thereby reducing the decomposition of the protein. For the ratio of protein to resistant starch， the ratio of fried processing was 17.1%， which was similar to 16.9% of fresh chestnut， while the ratios were significantly increased in the cooked and canned processing， reaching 30.1% and 42.7%， respectively.
Agricultural Biotechnology 2018Discussion and Conclusion
In all kinds of processing ways， there is a high??temperature treatment process， which can damage RS1 （physically embedded starch） and RS2 （resistant starch granules） to a certain extent， so that the amylase can smoothly enter to proceed digestion. Thus， in each processing method， the resistant starch decreases. Canned chestnut can retain more resistant starch， which may be due to high temperature cooling and low temperature storage in the water， resulting in the formation of a stable structure of RS3 （retrograded starch） from amylase retrogradation. In general， because of the processing conditions， such as moisture content， sugar??adding amounts， the retained content of resistant starch will show a certain degree of difference， so further study is needed to explore the impact of various nutrients on the decrease of resistant starch content in the processing.
No matter under which kind of processing， the overall water content remains high. The increase in water content in the canned products comes from increased water absorption during the water cooking process and during storage in sugar water. The decreases in water content after frying with shells and cooking without shells are supposed to be due to water evaporation caused by high temperature for a certain period of time. However， the decline after cooking is relatively small compared to that after frying， which is related to the presence or absence of water during processing. Since the proportion of water content in the nuts is high in each processing method， the ratio of water content to resistant starch content is greater than 1. Taking any 2 groups for comparison， such as the fresh chestnuts and canned chestnuts， the percentage of water content increase is far smaller than the increase of the ratio of water content to resistant starch content， indicating that the increase in water can reduce the amount of resistant starch， which is not conducive to the retention of resistant starch. McDonald et al.[11] found that under steam processing， continuous and stable high temperature could better destroy the embedded starch. The analysis on the ratio of fat content to resistant starch shows that fat content only accounts for 1%-4% of the content of resistant starch. The comparison on the fresh chestnuts with canned chestnuts shows that the fat content decreases but the ratio increases， indicating that the decrease of fat can decrease the content of resistant starch. The comparison on the canned chestnuts with the fried chestnuts with shells shows that the decrease of fat content is far smaller than the decrease of the ratio， indicating that the decrease of fat can increase resistant starch content. Therefore， there may be no relationship between fat content and the formation of resistant starch， or the relationship between them is not simply the linear increase or decrease， which needs further research. Meng[12] found that the effect of lipids on the formation of resistant starch was not too great， and only slight inhibition was observed through the control of fried potatoes. Yang et al.[13] suggested that endogenous lipids significantly affected the formation of RS3， and a small amount of lipids could increase the RS content， while excessive amount can decrease the RS content. Luo et al.[14] also found that the addition of glyceryl monostearate and sucrose fatty acid esters was not conducive to the formation of RS.
Among the various processing methods， the total sugar contents of all chestnut samples are higher than those of the initial fresh starch contents. The total sugar content is the highest in the canned chestnuts， followed by the cooked chestnuts， and the total sugar content of fried chestnuts is similar to that of fresh chestnut. The addition of sugar during the canned， cooking and frying processes should be related with the fact that the intake of sugar from outside during processing is greater than the decomposition of sucrose itself， and the difference in the amount of increase should be related to the integrity of the chestnut kernels in different processing. By comparing any two groups of processing methods， the increase of total sugar content is slightly less than the change range of the ratio of total sugar content to and resistant starch content， indicating that the increase of total sugar content can reduce the remaining resistant content. The decrease in total sugar content is slightly less than the change in the ratio of total sugar content to resistant starch content， suggesting that the reduction in total sugar content can retain more resistant starch. Therefore， it is inferred that increasing the total sugar content during processing promotes the decomposition of resistant starch. Eerlingen[15] found that sugar content at high concentrations could decrease the content of resistant starch in wheat. Luo et al.[14] suggested that sugar acted as a plasticizer in the formation of resistant starch. Before reaching the optimal amount of sugar， the resistant starch content increased with the increase of sugar content， and after reaching the optimal value， the inverse relationship appeared. Comparison between any 2 of the methods shows that when the protein content increases， the ratio of protein content to resistant starch content decreases， while the decrease of protein content has the increase of the ratio， indicating that the increase of protein content can promote the retention of resistant starch. It has found that protein affects resistant starch by embedding starch granules， which enhances the anti??digestive capacity of starch[16]. Huang et al.[17] found that the effect of protein on the content of resistant starch varied with the protein variety.
The results of the comprehensive experiment show that in the 4 edible ways of chestnuts， the content of resistant starch in fried and cooked processing is lower than that of the other 2 processing， and it is better for the elderly and children. Although canned chestnut contain more resistant starch， it cannot give people the chapped and indigestible feeling due to the sufficient moisture， making it a good dietary fiber food. Fresh chestnut has sufficient nutrient content and is suitable for eating as a snack.
In the processing of chestnut， increasing the water content can reduce the retention of resistant starch； increasing the total sugar content can promote the decomposition of resistant starch； increasing the protein content can promote the retention of resistant starch. However， the effect of lipid may be more complicated， which still needs further discussion. Therefore， in the processing of chestnut， controlling the contents of nutrients can affect the content of resistant starch， which can provide scientific basis for chestnut processing and consumption of chestnut food.
References
[1] LORRAINE L. Resistant starch： A potential functional food ingredient[J]. Nutrition Food Science， 2002， 32：62-67.
[2] QIN HB. On preparation and characteristics of chestnut resistant starch[D]. Enshi： Hubei University for Nationalities， 2010.
[3] YUAN LM. Preparation process and characteristics of Lisbon yam resistant starch[D]. Haikou： Hainan University， 2012.
[4] YI ZH， ZHANG JY. Effects of dietary fiber and inulin， fructooligosaccharides on intestinal physiological function[J]. China Animal Health， 2008， 118（12）： 2-4.
[5] YANG ZB， YANG L， XU XY. Prospect on utilization of current status and residual for Castanea mollissima[J]. Hubei Forestry Science and Technology， 2007（1）：57-59.
[6] ZHANG ZZ， CHEN DP， SHEN XH， et al. Factors influencing the formation of resistant starch[J]. Journal of Nuclear Agricultural Sciences， 2008（4）：483-487，509. [7] ZHOU JH， CHANG H， XIONG R， et al. Research on processing characteristics and quality assessment on chestnut of different species about canned chestnut[J]. Food Industry， 2013（3）：37-40.
[8] CHENG YF， WANG J， LI SX， et al. Comparison of resistant starch of green banana by using different methods[J]. Food and Fermentation Industries， 2007（8）：153-157.
[9] ZHANG SH. Food analysis experiment[M]. Beijing： Chemical Industry Press， 2010.
[10] CHEN J， WANG HM， ZHANG CL， et al. Nutrient components and processing adaptability of Chinese chestnut[J]. Modern Food Science and Technology， 2013（4）：725-728，736.
[11] MCDONALD K， SUN D W， KENNY T. Comparison of the quality of cooked beef product scooped by vacuum cooling and byconventional cooling[J]. LWT??Food Science and Technology， 2000， 33（1）： 21-29.
[12] MENG TZ. Effects of Chinese cooking on potato resistant starch[D]. Hangzhou： Zhejiang University， 2012.
[13] YANG G， DING XL. Effect of endogenous lipids on the formation of resistant starch type III[J]. Journal of Wuxi University of Light Industry， 2000， 19（6）：594-596.
[14] LUO ZG， GAO QY， YANG LS. Influence of salts on the forming of granular resistant starch[J]. Journal of the Chinese of Cereals and Oils Association， 2005（6）：30-33，43.
[15] EERLINGEN RG. Enzyme??resistant starch I： Quantitative and qualitative influence of incubation time and temperature of autoclaved starch on RS formation[J]. Cereal Chemistry， 1993， 70（3）： 339.
[16] CHANDRASHEKAR A， KIDEIS AW. Influence of protein on starch gelatinization in sorghum[J]. Cereal Chin， 1998， 65（6）： 457-462.
[17] HUANG W， LI JN， HU XZ， et al. Effects of oat lipid and protein components on resistant starch formation[J]. Journal of the Chinese of Cereals and Oils Association， 2011（8）：11-16.
Key words Chestnut； Resistant starch； Nutrients
Chestnut （Castanea mollissima BL.） is a deciduous tree of the family Fagaceae， and a species of chestnut. It is a traditional agricultural and sideline product in China， well??known as the "king of dried fruit". Its fruit， nuts， are mainly composed of starch， and contain many nutrients such as protein and other nutrients. Resistant starch （RS） refers to "starch， including its degradation products， that escapes from digestion in the small intestine of healthy individuals in foods"[1]. It is divided into 4 types： RS1??physically embedded starch， RS2??resistant starch granules， RS3??retrograded starch， and RS4 chemically modified starch. Resistant starch accounts for 68.93% of the total starch content in chestnut[2]， and resistant starch is fermented in the large intestine， producing short??chain fatty acids， which acts as a dietary fiber that can benefit human health by lowering blood sugar， reducing intestinal diseases[3]. However， eating too much will increase the capacity of chyme， increase the proportion of inflated substances in chyme， and cause stickiness， which makes it difficult to digest[4]. It is more obvious in the elderly and children.
At present， there are various developments in the processing of Chinese chestnut， and breakthroughs have been made in various researches， developing marrons glaces， canned chestnut， quick??frozen chestnut kernels， preserved chestnut， chestnut powder， chestnut paste， chestnut beverage， chestnut wine， and so on[5]. The contents of resistant starch in them are different， mainly due to different processing methods such as slicing or crushing processing and different environmental nutrient substances. Proteins， fats， small molecule sugars， and inorganic minerals all affect the formation of resistant starches， but studies have focused on the influence of major components such as proteins and fats on the formation of resistant starch[6]. In this study， complete chestnuts or nuts were used as the materials to study the changes of resistant starch and main nutrients under 3 different processing of canned， fried and cooked methods， and primary conclusion was made to the effects of nutrients on the formation of resistant starch， with the aim to provide scientific bases to increase or decrease the contents of resistant starch and nutrients in processed chestnut products.
Materials and Methods
Test materials
The tested chestnut variety was ??Luotian chestnut??.
Reagents included petroleum ether， 2% boric acid， NaOH， HCl， sucrose， glucose standard solution， anthrone， ethyl acetate， ethanol， acetic acid， sodium acetate， 3， 5??dinitrosalicylic acid， potassium sodium tartrate， anhydrous sodium sulfite， and all were analytical pure. ????amylase glucosidase was provided by Beijing Double Helix Microbiology Company.
Instruments and equipment were as follows： analytical balance （accuracy to 0.000 1g）， centrifuge tube （10 ml）， volumetric flask （50， 100 ml）， thermostat shaker （ZHWR??1102C）， constant temperature water bath （HH??6）， spectrophotometer （722）， table??top & high??speed refrigerated centrifuge （HR1716M）， fat analyzer （SZC??C）， and nitrogen analyzer （KDN??08A）.
Test methods
Fresh chestnut processing Chestnut nuts were obtained after peeling off the shells to make it able to test the water content. The fresh nuts were than dried to constant mass in a 50 ?? oven， and then crushed in a grinder before sieving over the 100??mesh sieve， obtaining the dry sample to determine the contents of resistant starch， total sugar， fat and protein.
Canned chestnut processing The processing was based on the method of Zhou et al.[7] with appropriate modifications. After husking and peeling， chestnut nuts were steamed at 100 ?? for 20 min， and canned after added with leaching mixture of sucrose （10%） and citric acid （0.1%）. Some nuts were wiped to measure the water content； some nuts were rinsed with fresh water for several times， and then dried at 50 ?? to constant mass and smashed and sieved to obtain dry sample for the determination of the resistant starch， total sugar， fat and protein contents.
Frying chestnut with shells Chestnuts with shells were put into an iron pan filled with the nuts fully covered. After heating the pan， the chestnuts were fried for 20 min until aroma arose and the samples were well fried. After peeling off the shells， the fried nuts were pout into a 50 ?? oven to determine the water content. After dried to constant mass， the nuts were crushed and sieved， obtaining the dry sample， so as to determine the contents of resistant starch， total sugar， fat and protein. Cooking chestnut without shells After peeling off the shells， the nuts were washed by clean water and then poured into the iron pot for daily cooking added with cooking oil， salt， sugar， monosodium glutamate. After being fully cooked， the nuts were washed with water and then dried to determine the water content. After dried to constant mass， the nuts were crushed and sieved， obtaining the dry sample， so as to determine the contents of resistant starch， total sugar， fat and protein.
Item determination
The content of resistant starch was measured referring to the AOAC method[8] with appropriate improvements. Accurately 100 mg （mass recorded as W） of processed dry sample was taken and put into a 10mL plugged test tube， and the sample was shaken to the tube bottom by knocking the tube. First， soluble sugars were removed with 66% ethanol， followed by adding 4.0 ml of ????pancreatic amylase （10 mg/ml） containing AMG （3 U/ml） to each tube， and then the plugs were covered to achieve fully mixing. The tube was oscillated at 37 ?? water bath for 16 h， and after wiping the outside drops， the tube was added with 4.0 ml of anhydrous ethanol. After mixed thoroughly， the tube was centrifuged at 4 500 r/min for 10 m， obtaining the precipitate. And the tube was put in the ice bath， added with 2 ml of 2 mol/L NaOH solution to the precipitate， and the mixture was stirred on ice for 20 min to dissolve the precipitate and the resistant starch. Then， 8 mL of 1.2 mol/L sodium acetate buffer （pH 3.8） was added and well mixed， followed by immediately adding 0.1mL of AMG （3 300 U/ml）， and then digested in a 50 ?? water bath for 60 min. Then， the sample solution in the tube was transferred to a 100 ml volumetric flask， and then set to the constant volume of 100 ml with distilled water， followed by fully mixing. The glucose content （Gr） was determined using the DNS method.
Resistant starch content = （Gr??N??0.9/W）??100%.
Where， Gr is the mass of reducing sugar； N is diluent volume； W is sample mass.
The total sugar content was determined referring to the anthrone colorimetric method. Water content was determined referring to GB5009.3??2010； fat content referring to GB/T 5009.6??2003 with fat analyzer； protein content referring to GB5009.5??2010 with Kjeldahl nitrogen analyzer[9-10].
Results and Analysis
Changes of resistant starch content in chestnut after processing
As shown in Table 1， after various ways of chestnut processing， the content of resistant starch changed due to processing conditions， but the total amount was still relatively high. Compared with 51.09% of resistant starch content in fresh chestnuts， resistant starch was decomposed to a certain extent after processing. The content of resistant starch was 21.68% in the fried samples under high temperature， and the remaining resistant was only 42.42%， which was the lowest among the several ways of processing. The content was 28.02% in the cooked samples without shells， while the resistant starch content in the canned products was 42.42%， the highest of all with the retention rate reaching up to 93.16%. Water content and its ratio to resistant starch content
As shown in Fig. 1， the water content was 45.84% in fresh chestnut， and its ratio with resistant starch content was 1.46. In the processed samples， the water content increased in the canned products， reaching up to 54.13%， and in the canned samples， the ratio of water content to resistant starch content reached up tot 5.36. After frying and cooking， the water content decreased. The water content of fried chestnut was 34.12%， which showed a more significant decrease than that of cooked chestnut of 41.30%. The ratio of water content to resistant starch content was the smallest of 0.99 in the fried samples， while the ratio was 2.52 for the coked samples， which was higher than that of fresh chestnut but lower than that of canned chestnut.
Fat content and its ratio to RS content
As shown in Fig. 2， the ratio of fat content to dry mass was not high， close to 1%， and the changes of fat content were not significant after processing. In fresh chestnut， the fat content was 0.95%. The fat content decreased in the chestnut of both canned and fried processing， and the decrease was more significant in the chestnut processed with frying with shells than the canned chestnut. However， the fat content increased to 1.09% in home cooking processing， which was due to the fact that oil was added to the cooking process， which caused some penetration into the nuts. The differences in the ratio of fat content to starch resistant content were significant. The ratio of canned products was 3.54%， which was slightly less than 3.89% of the cooking processing which had large fat content， and both were higher than that of the fresh chestnut of 1.84%. The ratio was the smallest of 1.15% in the samples with fried processing.
Total sugar content and its ratio to resistant starch content
As shown in Fig. 3， there were significant differences in the total sugar contents and the ratios of total sugar contents to resistant starch content. Total sugar content was the highest in the canned chestnut of 20.26%， followed by 15.84% for cooking processing， 9.61% for stir??fried and 8.34% for fresh chestnut. After processing， the total sugar contents were all increased. The ratios to resistant starch contents also showed a similar trend. The ratio of canned products was 92%， cooked products of 56%， and stir??fried of 20%， slightly higher than 16% of fresh chestnut.
Total protein content and its ratio to RS content As shown in Fig. 4， the contents of total protein changed after the 3 processing methods， but the changes were not significant. Compared with 8.70% of fresh chestnut， the protein content of canned products decreased by a small amount to 7.36%， and the protein content of fried and cooked chestnuts increased slightly to 9.12% and 9.41%. The reason for this change was that the nuts were well kept during frying and cooking， and the inside fruit was not highly heated， thereby reducing the decomposition of the protein. For the ratio of protein to resistant starch， the ratio of fried processing was 17.1%， which was similar to 16.9% of fresh chestnut， while the ratios were significantly increased in the cooked and canned processing， reaching 30.1% and 42.7%， respectively.
Agricultural Biotechnology 2018Discussion and Conclusion
In all kinds of processing ways， there is a high??temperature treatment process， which can damage RS1 （physically embedded starch） and RS2 （resistant starch granules） to a certain extent， so that the amylase can smoothly enter to proceed digestion. Thus， in each processing method， the resistant starch decreases. Canned chestnut can retain more resistant starch， which may be due to high temperature cooling and low temperature storage in the water， resulting in the formation of a stable structure of RS3 （retrograded starch） from amylase retrogradation. In general， because of the processing conditions， such as moisture content， sugar??adding amounts， the retained content of resistant starch will show a certain degree of difference， so further study is needed to explore the impact of various nutrients on the decrease of resistant starch content in the processing.
No matter under which kind of processing， the overall water content remains high. The increase in water content in the canned products comes from increased water absorption during the water cooking process and during storage in sugar water. The decreases in water content after frying with shells and cooking without shells are supposed to be due to water evaporation caused by high temperature for a certain period of time. However， the decline after cooking is relatively small compared to that after frying， which is related to the presence or absence of water during processing. Since the proportion of water content in the nuts is high in each processing method， the ratio of water content to resistant starch content is greater than 1. Taking any 2 groups for comparison， such as the fresh chestnuts and canned chestnuts， the percentage of water content increase is far smaller than the increase of the ratio of water content to resistant starch content， indicating that the increase in water can reduce the amount of resistant starch， which is not conducive to the retention of resistant starch. McDonald et al.[11] found that under steam processing， continuous and stable high temperature could better destroy the embedded starch. The analysis on the ratio of fat content to resistant starch shows that fat content only accounts for 1%-4% of the content of resistant starch. The comparison on the fresh chestnuts with canned chestnuts shows that the fat content decreases but the ratio increases， indicating that the decrease of fat can decrease the content of resistant starch. The comparison on the canned chestnuts with the fried chestnuts with shells shows that the decrease of fat content is far smaller than the decrease of the ratio， indicating that the decrease of fat can increase resistant starch content. Therefore， there may be no relationship between fat content and the formation of resistant starch， or the relationship between them is not simply the linear increase or decrease， which needs further research. Meng[12] found that the effect of lipids on the formation of resistant starch was not too great， and only slight inhibition was observed through the control of fried potatoes. Yang et al.[13] suggested that endogenous lipids significantly affected the formation of RS3， and a small amount of lipids could increase the RS content， while excessive amount can decrease the RS content. Luo et al.[14] also found that the addition of glyceryl monostearate and sucrose fatty acid esters was not conducive to the formation of RS.
Among the various processing methods， the total sugar contents of all chestnut samples are higher than those of the initial fresh starch contents. The total sugar content is the highest in the canned chestnuts， followed by the cooked chestnuts， and the total sugar content of fried chestnuts is similar to that of fresh chestnut. The addition of sugar during the canned， cooking and frying processes should be related with the fact that the intake of sugar from outside during processing is greater than the decomposition of sucrose itself， and the difference in the amount of increase should be related to the integrity of the chestnut kernels in different processing. By comparing any two groups of processing methods， the increase of total sugar content is slightly less than the change range of the ratio of total sugar content to and resistant starch content， indicating that the increase of total sugar content can reduce the remaining resistant content. The decrease in total sugar content is slightly less than the change in the ratio of total sugar content to resistant starch content， suggesting that the reduction in total sugar content can retain more resistant starch. Therefore， it is inferred that increasing the total sugar content during processing promotes the decomposition of resistant starch. Eerlingen[15] found that sugar content at high concentrations could decrease the content of resistant starch in wheat. Luo et al.[14] suggested that sugar acted as a plasticizer in the formation of resistant starch. Before reaching the optimal amount of sugar， the resistant starch content increased with the increase of sugar content， and after reaching the optimal value， the inverse relationship appeared. Comparison between any 2 of the methods shows that when the protein content increases， the ratio of protein content to resistant starch content decreases， while the decrease of protein content has the increase of the ratio， indicating that the increase of protein content can promote the retention of resistant starch. It has found that protein affects resistant starch by embedding starch granules， which enhances the anti??digestive capacity of starch[16]. Huang et al.[17] found that the effect of protein on the content of resistant starch varied with the protein variety.
The results of the comprehensive experiment show that in the 4 edible ways of chestnuts， the content of resistant starch in fried and cooked processing is lower than that of the other 2 processing， and it is better for the elderly and children. Although canned chestnut contain more resistant starch， it cannot give people the chapped and indigestible feeling due to the sufficient moisture， making it a good dietary fiber food. Fresh chestnut has sufficient nutrient content and is suitable for eating as a snack.
In the processing of chestnut， increasing the water content can reduce the retention of resistant starch； increasing the total sugar content can promote the decomposition of resistant starch； increasing the protein content can promote the retention of resistant starch. However， the effect of lipid may be more complicated， which still needs further discussion. Therefore， in the processing of chestnut， controlling the contents of nutrients can affect the content of resistant starch， which can provide scientific basis for chestnut processing and consumption of chestnut food.
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