Zhiming Wang,Mingwei Zhang,Guang Liu,Yuanyuan Deng,Yan Zhang,Xiaojun Tang,Ping Li,Zhencheng Wei

Sericultural&Agri-Food Research Institute Guangdong Academy of Agricultural Sciences,Key Laboratory of Functional Foods,Ministry of Agriculture and Rural Affairs,Guangdong Key Laboratory of Agricultural Products Processing,Guangzhou 510610,China

ABSTRACT Simiao rice is a variety of long-grain Indica rice popular in South China.However,researches on the milling technology of Simiao rice are limited.The effect of different degrees of milling(DOMs)of Simiao rice(0-12%)were evaluated in terms of the physicochemical properties and in vitro starch digestibility of the rice.The results showed that,as the DOM increased from 0 to 12%,the head rice yield(HRY)and the content of protein,lipid,dietary fiber,vitamin B1,vitamin E and niacin nonlinearly decreased,while the content of total starch and amylose and paste viscosity parameters nonlinearly increased.The differences in chemical constituents between the brown and milled Simiao rice with different DOMs lead to the variation in their pasting properties and in vitro digestibility.Because of lower content of slowly digestible starch(SDS)and resistant starch(RS),milled Simiao rice exhibited greater digestibility and a higher estimated glycaemic index(EGI).The EGI of Simiao rice ranged from 77.98 to 85.55,and remained almost constant(approximately 84)when the DOM was above 4%.Correlation analysis indicated that the EGI had strong negative correlations with lipid,protein,dietary fiber and RS,but a positive correlation with amylose.These results provide theoretical guidance for the production of Simiao rice with desired physicochemical properties and digestibility by adjusting the DOM.

Keywords:Simiao rice Degree of milling In vitro digestibility Estimated glycaemic index

1.Introduction

Simiao rice is a variety of long-grain Indica rice popular in South China.According to the group standard from the Seed Association of Guangdong Provice,Simiao rice should meet the following requirements:thousand grain weight of paddy≤21 g,grain length of brown rice≥6.5 mm,ratio of grain length to width of brown rice≥3.5,and content of amylose within the range of 13% to 19%,among others.Thus,Simiao rice differs greatly in external features from short-grain Japonica rice or even from conventional long-grain Indica rice.It is well known that the external features of rice,such as kernel length and ratio of kernel length to width,greatly affect milling quality[1,2].

Many researchers have studied the effect of milling on the physicochemical properties and in vitro starch digestibility of both Japonica and Indica rice[2-5].Sandhu et al.[2]reported that Indica rice with varied DOMs differed in chemical composition,causing variations in pasting and cooking properties,and the changes in physicochemical properties differed between long and short Indica rice cultivars.It was reported that proteins,minerals and lipids are unevenly distributed in the brown rice kernel(long-grain cultivar),and starch is mainly concentrated in the endosperm core fraction[4].Mohapatra et al.[6]milled three varieties of rice(long slender,medium grain and short-bold grain)to various DOMs,and found that grain size and shape significantly affected the cooking qualities,such as optimum cooking time,hardness,adhesiveness and cohesiveness,as well as texture.Hence,the physicochemical properties especially nutrient components of rice can be optimized by optimizing the DOM.However,to the best of our knowledge,there are no reports investigating the effect of processing on both the physicochemical properties and in vitrodigestibility of Simiao rice.

The head rice yield(HRY)is affected by the grain size and shape:typically,long and slender grains have a lower HRY than short and broad grains.However,in recent years,consumers have tended to favour white rice with better eating quality[7,8],but a lower HRY and less nutrient.The application of an appropriate DOM may be a good strategy to obtain milled rice with better nutrient and sensory qualities.

With the continuous development of living standards,chronic diseases such as type 2 diabetes and obesity are increasingly becoming prevalent.Low glycaemic-index(GI)food has attracted much interest.White rice has a wide range of GIs(48-92)depending on the grain cultivars and the type of cooking method[9],but is generally classified as a high-GI food[10,11].It is well known that the constituents of rice can influence the GI.The higher the RS or amylose content is,the lower the GI is[12,13].Moreover,the non-starch constituents such as proteins and lipids and dietary fiber present in rice might delay the hydrolysis of starch and partly lead to a lower GI[14,15].Hence,we hypothesized that the milling process may affect the GI of rice by altering its internal constituents.

Simiao rice was chosen for our study,because of its superior eating quality and popularity in South China.In the present work,Simiao rice paddies were de-husked to obtain brown rice and subjected to varying milling processes to obtain milled white rice with different DOMs(0-12%).The physicochemical properties,pasting properties and in vitro digestibility of milled Simiao rice with different DOMs were investigated,and the relationship between the estimated glycaemic-index(EGI)and composition of Simiao rice was discussed.

2.Methods

2.1.Samples preparation

The paddies of Simiao rice(Meixiangzhan No.2,Rice Research Institute Guangdong Academy of Agricultural Science,China),Japonica cultivar Suijing18(S18,Cultivated in North of China)and Indica cultivar T-You 15(T15,cultivated in South of China)were de-husked using a laboratory rice huller(J1500-A,Shandong Catheano Machinery Co.,Ltd.,China),and brown rice(0 DOM)was obtained after removal of the impurities and broken rice.The brown rice was milled to varying DOMs(2%,4%,6%,8%,10%and 12%)using a rice polisher(BLH-3500,Zhejiang Bethlehem Apparatus Co.,Ltd.,China)as described earlier[16].The DOM was calculated from the weight of rice before and after milling.Briefly,brown rice(100 g)was placed in the rice polisher at 3,000 r/min and 0.2 MPa,and milled for 12,24,55,85,121,and 164 s to obtain 98,96,94,92,90 and 88 g of milled rice,corresponding to 2%,4%,6%,8%,10% and 12% DOMs respectively.Brown rice and milled rice were ground to rice flour using a BJ-800A grinder(Shanghai BAIJIE Industrial Co.,Ltd.,China)at 3,000 r/min,and the obtained rice flour passed through a 60-mesh sieve and stored in a desiccator at 25°C until analysis.

The head rice yield(HRY)was calculated from the weight of paddy and head rice.For comparison,S18 and T15 was milled to 4%,8%and 12%DOM,and the HRY of S18 and T15 with varying DOM was obtained.

2.2.Physicochemical characteristics

The moisture content was determined from the weight of rice before and after drying at(105±3)°C with natural air circulation for 24 h.The contents of ash,protein(nitrogen×5.95),lipid,dietary fiber(DF)and Vitamins(vitamin B1,vitamin B2,niacin and vitamin E)were determined according to AACC Method 08-01 and Method 86-80,respectively[17].The amylose content(CA)was determined by absorption at 720 nm using an iodine colourimetric reaction[15].The total starch content(CTS)was determined based on the procedure reported by Goñi et al.[18].Starch was measured as glucose using dinitrosalicylic(DNS)method[19].

where CTGand CSWare the content of total glucose in hydrolysate and sample weight.

2.3.Pasting properties

The pasting properties were measured using a Rapid Viscoanalyser(RVA-2,Perten,Sweden).A sample of(3.0±0.1)g of rice flour was transferred into a canister,and approximately(25.0±0.1)mL of ultrapure water(corrected to compensate for the moisture content)was added.The pasting properties were determined based on a procedure reported previously[2,20].The peak viscosity(PV),trough viscosity(TV),breakdown viscosity(BDV),final viscosity(FV),and setback viscosity(SBV),peak temperature(PT)and RVA pasting curves were recorded.

2.4.In vitro starch digestibility

The in vitro starch digestibility of the rice samples was measured based on a previously reported procedure[21].Briefly,0.3 g sample was diluted with 10 mL of acetate buffer(0.2 mol/L,pH 5.2)and boiled for 15 min,and mixed with 10 mL of mixed enzyme solution containing 290 U/mL of porcine pancreatic α-amylase(10,000 U/g,Sigma-Aldrich Chemical Co.,USA)and 15 U/mL of amyloglucosidase(70,000 U/g,Sigma-Aldrich Chemical Co.,USA).The mixture was incubated at 37 °C with the shaking speed of 120 r/min.The reducing sugar(glucose)content of an enzymatic mixture(0,20 and 120 min)was determined by the DNS method.In vitro starch digestibility was characterized by the percentages of rapidly digestible starch(RDS),slowly digestible starch(SDS)and resistant starch(RS),which were calculated as follows:

where wRDS,wSDSand wRSare the percentages of RDS,SDS and RS;G0,G20and G120are the reducing sugar(glucose)contents in the enzymatic solution after digestion for 0,20 and 120 min,respectively,and CTSis the total starch content of the sample.

2.5.Estimated glycaemic-index

Add 100 g wheat starch(12.8% moisture,0.5% ash,14.2% protein),59 g water and 2 g yeast flour in a fullauto bread maker(MM-ESC,Guangdong Midea Household Appliance Manufacturing Co.,Ltd.,China)to make white bread,and place the obtained white bread(11.4% moisture,73.4%starch)in a desiccator at 25°C before use.

The EGI of rice flour was determined according to the method described by Goñi et al.[18].A rice sample(50 mg)was first boiled(15 min)with 2 mL ultrapure water and then used to determine EGI.The reducing sugar(glucose)content of the hydrolysate(0,30,60,90,120 and 180 min)was determined by the DNS method.White bread was used as reference food.The hydrolysis rate(HR)of total starch at the sampling time was determined as follows:

where C1is the content of reducing sugar of the hydrosate at the sampling time,and C0is the total starch content.

The starch hydrolysis curve was plotted using the percentage of HR as the ordinate and time as the abscissa.The area under the curve(AUC)was obtained,and the EGI was calculated as follows:

where EGI represents the estimated glycaemic index,AUC and AUCs represent the area under the hydrolysis curves for the rice samples and white bread,respectively.

2.6.Statistical analysis

All measurements were performed in triplicate.Statistical analysis was performed using SPSS 22 software(IBM Corp.,USA).The obtained data were analyzed for significance using One-Way ANOVA followed by the Duncan's Post Hoc test(P＜0.05).

3.Results and discussions

3.1.Head rice yield(HRY)

The effect of the DOM on the HRY of Simiao rice was shown in Fig.1.The HRY of Simiao rice significantly decreased from 62.13%to 33.16%with the DOM increasing from 0 to 12%.The HRY decreased more dramatically after endosperm core milling,because the removal of the hard endosperm core requires more force than initial milling processes[1].The average HRY of 39 varieties developed by the International Rice Research Institute(IRRI)was approximately 51.27%[1].In addition,milling recoveries ranging from 49.43%-75.94% were reported for long-grain cultivars with 0-8% DOMs[2].The HRY of Simiao rice was lower than that of the control samples(S18 and T15,Fig.1)as well as that in a previous report at the same DOM,and it was below 47% when the DOM was above 8%,which indicated that Simiao rice is prone to breakage and over-milling.Hence,Simiao rice is unique,and investigating the effect of milling on the physicochemical properties of Simiao rice is necessary.

Fig.1.Effect of DOMs on HRY.

3.2.Physicochemical properties

The nutrient contents of Simiao rice with different DOMs were shown in Table 1 and Fig.2.Each milling stage led to an increase in total starch and amylose,and a decrease in ash,protein,lipid and dietary fiber(Table 1).

Table 1 Physicochemical properties of Simiao rice of different DOMs. (unit: %)

Fig.2.Vitamin content of Simiao rice.

The moisture content varied between 10.65% and 11.02% for the studied DOMs,but the differences were not significant,which was consistent with a previous report in which the moisture content of milled rice with various DOMs was not significantly different[22].

As the DOM increased,the ash content decreased from 1.65%to 0.63%,wherein it had the largest drop(27.2%)with the DOM increasing from 6%to 8%,and then kept relative constant within DOM of 10%to 12%.Similar results were reported previously[23].

The protein content decreased gradually with the increase in DOM,and the maximum decrease(3.27%)was observed when DOM increased from 0 to 2%.With further increases in the DOM,the protein content decreased slowly.These results could be attributed to the uneven distribution of protein in Simiao rice.The pericarp layer and embryo possess the highest protein content[4]and are removed first in the early stage of the milling process.Sandhu et al.[2]demonstrated a smaller decrease in the protein content compared to that in our study.Analysis showed that a 1.39% and 1.10% reduction was observed in the protein content of short-and long-grain rice respectively when the DOM increased from 0 to 2%[2].Differences in the reduction in protein content may be a result of the differing pericarp and embryo protein contents in Simiao and the short/long-grain rice.

The lipid content showed a similar trend as the protein content.The lipid content was reduced significantly from 2.90% to 0.79% when DOM increased from 0 to 12%.This significant reduction was consistent with previous literature[3],in which lipid content decreased from 5.49%(0 DOM)to 1.03%(8%DOM).The lipid content declined over 50%when the DOM increased from 0 to 6%,which in line with previous report[5].A study revealed that a thin layer corresponding to the layer removed by a DOM ranging from 0 to approximately 5%contained more than half of the fat in a rice grain,and the fat decreased significantly as bran layers were removed[23].

It was reported that the endosperm core contained the highest concentration of total starch(TS)[5].In this study,the total starch and amylose contents increased gradually with increasing DOM.The total starch contents was 68.18% in brown rice(0 DOM)and increased to 76.02%at 12%DOM.The maximal increase in total starch content of 5.0%was observed when the DOM increased from 0 to 2%,and with further increases in the DOM,the increase in the total starch content was less obvious.However,a linear relationship between the total starch and DOM(r=0.88)was observed in a previous study[5].The difference in the removal of bran during the milling process may be responsible for the difference in correlation observed here.Similarly,the amylose content increased from 16.20%to 18.65%as the DOM increased from 0 to 12%.A possible explanation is the higher amylose content in the endosperm core than in its outer counterpart.In addition,the dietary fiber content(1.64%-0.72%)was found to be negatively correlated with the DOM,and the largest drop(37.7%)in the dietary fiber content was observed when the DOM increased from 2%to 4%.A possible explanation for the largest drop observed here is the removal of pericarp,which is rich in dietary fiber,in the early stage of the milling process.Roy et al.[24]also reported that the dietary fiber content decreased from 1.3% to 0.5%when the DOM increased from 0 to 10%.

To evaluate the effect of the DOM on the nutritive value of Simiao rice,the vitamin E,vitamin B1,vitamin B2and niacin contents were determined,and the results were presented in Fig.2.The contents of these four vitamins decreased gradually with increasing DOM.The vitamin E and niacin contents dropped significantly,while the vitamin B1and vitamin B2content decreased slowly.It could be seen from Fig.2 that the maximum reductions in niacin and vitamin E were observed when the DOM increased from 0 to 2%.However,vitamin B2content remained almost constant with each successive increase in the DOM.The contents of vitamins B1and B2was out of the limit of detection 0.1 mg/100 g and 0.05 mg/100 g,and could not be detected by the method when the DOM was above 8%and 6%.The variation in the vitamin loss after each DOM increase may have been due to the uneven distribution of different vitamins in different layers of bran.The results indicated that the highest concentrations of niacin and vitamin E were present in the pericarp(DOM 0-2%),vitamin B1was concentrated in the testa and aleurone layers,and vitamin B2was evenly distributed in Simiao rice.

The above results indicated that some nutrients in rice,such as protein,lipid,dietary fiber,vitamin E,niacin and vitamin B1were removed with increase in the DOM.Furthermore,some health-promoting minerals(e.g.,calcium,magnesium and zinc)were also reported decrease gradually with an increase in the DOM[2].Therefore,a low DOM could be optimal for nutrient retention in milled Simiao rice.

Other grains(quinoa,barley and wheat,for instance)show similar changes in physicochemical properties as Simiao rice in the milling process.Similar as Simiao rice,protein is concentrated more in the bran of quinoa and is gradually removed with increase in DOM[25].In addition,the contents of protein,lipid,dietary fiber and minerals are the highest in pericarp and testa of barley,the removal ratio of those nutrients shows similar trend as Simiao rice with the increase in DOM,after three times of milling(removed 10% bran each time),the protein,lipid and dietary fiber content of milled quinoa decreased by 23.1%,42.0%and 66.3%,respectively[26].In the milling process of wheat grain,the kernel outer tissues was not removed homogeneously,and the first 3%removed consisted of mainly pericarp,which resulted in obvious drops in the protein and lipid content of milled wheat[27].

3.3.Pasting properties

The effect of the DOMs on the pasting properties of Simiao rice was shown in Table 2 and the RVA pasting curves were shown in Fig.3.Brown rice(0 DOM)had the lower paste viscosity parameters(PV,TV,BDV,FV,and SBV)than the milled rice.Those viscosity parameters gradually increased with increases in the DOM,and the changes in pasting properties of Simiao rice could be observed intuitively in the RVA pasting curves(Fig.3).In general,paste viscosity parameters are largely affected by the amylose content,and lower-amylose rice exhibits lower paste viscosity[28].On the other hand,paste viscosity parameters(PV,TV and FV)were reported to be decreased by adding protein to rice[29].Hence,variations in chemical constituents(protein,amylose and total starch)should be responsible for the changes in paste viscosity of Simiao rice.

Table 2 Pasting properties of Simiao rice of different DOMs.

Fig.3.RVA pasting curves of Simiao rice with different DOMs.

When the DOM increased from 0 to 12%,the BDV of rice increased significantly from 507 cP to 1,491 cP,indicating that the swollen granules of milled rice with a lower DOM were more stable at elevated temperatures,potentially due to the lower protein content in milled rice(Table 1).The FV indicates the degree of interaction between amylose and amylopectin when rice flour forms a gel structure as a result of starch retrogradation.Typically,the higher the FV is,the stronger the interaction between amylose and amylopectin is[30].Milled rice with a 12%DOM thus had a higher amylose content and lower protein content,which resulted in a higher FV(3,017 cP)than that of brown rice(FV 2,372 cP).

Pasting temperature and PV are key parameters for evaluating rice pasting properties.The higher the PV or/and the lower the PT,the easier it is for rice to absorb water for gelatinization during cooking and swelling.It was reported that consumer acceptance was highly associated with the PV[8],which indirectly indicated milled rice is preferred by consumers.With the increase in the DOM,the PT decreased gradually in the range of 88.9-80.1°C.These values were higher than those(65.5-71.7°C)reported previously[2],which may be ascribed to differences in the rice cultivars.Milled rice had a lower PT,further highlighting that proteins and lipids might delay the initiation of gelatinization.It was also reported that proteins and lipids present in rice and other cereals could prevent the starch granules from swelling and forming gels[31].

3.4.In vitro starch digestion characteristics

3.4.1.In vitro digestibility

Fig.4 showed the hydrolysis curves of total starch of Simiao rice flour and wheat bread.For hydrolysis by pepsin and α-amylase,the hydrolysis rate first rapidly increased from 0 to 60 min and then plateaued from 60 to 180 min.The hydrolysis ratio increased at the fastest rate from 0 to 30 min,during which the hydrolysis ratio of starch in white bread increased to 61.2%.The hydrolysis ratio of Simiao rice with different DOMs was lower than that of the white bread throughout the experimental period,indicating that the non-starch constituents in rice delayed the hydrolysis of starch[14,15].Hydrolysis was terminated at 60 min,after that the proportion of hydrolyzed starch remained constant.However,the maximum proportion of the hydrolyzed starch in Simiao rice was only(60.2%±2.01%)for the 12%DOM rice.Therefore,the starch was not hydrolyzed completely in the in vitro experiment.The brown Simiao rice had the lowest hydrolysis ratio(52.1%±1.46%),which may have been attributed to the highest protein,lipid and dietary fiber contents in the brown rice(Table 1).These parameters related to the digestion of milled rice agreed with the data(55.1%±3.0%)reported by Goñi et al.[18]and indicated that Simiao rice flour could be hydrolyzed more easily after removal of protein,lipid and dietary fiber.

Fig.4.Hydrolysis curves of Simiao rice flour with different DOMs and white bread.

To further investigate how the DOM impacts the digestibility of Simiao rice flour,the contents of RDS,SDS and RS were determined,and the results were shown in Table 3.Starch digestibility varied among the rice samples with different DOMs.When the DOM increased from 0 to 12%,the content of RDS increased from 68.69% to 73.51%,while those of SDS and RS decreased from 24.96%and 6.35%to 22.19%and 4.30%,respectively.It was reported that starch digestibility might be affected by the particle size and structure of starch as well as the amylose content[32].Therefore,as the DOM gradually increases,the increase in amylose content and removal of protein,lipid and dietary fiber might be the main reason for the better starch digestibility of Simiao rice.

Table 3 Effects of DOMs on RDS,SDS and RS percentage content and EGI of Simiao rice.

3.4.2.Estimated glycemic index

The EGI of Simiao rice with different DOMs varied from 77.98 to 85.55(Table 3).The EGI increased with an increasing DOM.The greatest increase(3.9%)in EGI was observed when the DOM increased from 0 to 2%,which corresponded to the largest decrease observed in protein and lipid(Table 1),further highlighting the important role of protein and lipid in the EGI.Proteins and lipids have been reported to delay the glycaemic response[14,15].In addition,the higher the content of amylose is,the lower the EGI is,which was reported by Hu et al.[13].Hence,we speculated that the effect of protein and lipid on the EGI was more significant than that of amylose in our work.Furthermore,brown rice had the lowest EGI(77.98±1.84)partially due to its attenuated gastric emptying rate[33,34].

White bread or glucose is commonly used as a reference food to evaluate food EGI[18,35].When white bread is used as the reference food,the EGI for the tested food was multiplied by 0.7 to obtain the EGI with glucose as the reference food[9].Therefore,the EGI of Simiao rice ranged from(54.58±0.83)to(59.03±0.93).Hence,the Simiao rice used in our work can be classified as a medium-GI(56-69)food.In recent years,the GI of rice has been extensively investigated.Kumar et al.[36]reported that brown rice samples of 102 rice genotypes with different ecologies/traits had widely varying GIs ranging from 57.9 to 75.9.Shobana et al.[37]showed that brown rice exhibited a lower glycaemic response(GI=57.6)than under-milled rice(GI=73.0)and white rice(GI=79.6).Wordu et al.[38]reported that the mean GI of cooked white rice,brown rice and parboiled rice were(66.61±9.86),(60.24±8.16)and(55.97±6.01),respectively.Compared to the results reported previously,Simiao rice had a relatively low GI value,even for polished rice.Prasad et al.[33]observed that long and slender rice grains exhibited a lower glycaemic response than short and broad grains,which agreed with our findings.In addition to the grain cultivar,the GI is also affected by the heat,water and duration of cooking[9,38].Hence,further studies on the reason for the lower GI of Simiao rice are needed for clarification.

3.5.Pearson's correlation coefficients

Table 4 showed the correlation analysis between the DOM,EGI and nutrient content.Strong negative correlations were observed between the DOM and lipid(r=-0.946),dietary fiber(r=-0.927)and protein(r=-0.843).The Pearson's correlation coefficients between RDS,SDS,RS and the DOM were 0.998,-0.972 and-0.983,respectively.The digestion properties of starch were determined from starch structures and nonstarch components(e.g.,protein,lipid and dietary fiber)present in the rice.Compared to protein and dietary fiber,lipid exhibited higher Pearson's correlation coefficientswith RDS,SDS and RS,indicating that lipid is a key factor affecting the digestion characteristics of Simiao rice,which was reported precious[36].

Table 4 Pearson's correlation coefficients between EGI and nutrient content.

The EGI displayed a negative correlation with lipid(r=-0.911),protein(r=-0.883),dietary fiber(r=-0.831)and RS(r=-0.819)but a positive correlation with total starch(r=0.955).Amylose exhibited a weak correlation with the EGI.The effect of lipid,out of all the constituents,on EGI was the most significant.It was also reported that rice with a high lipid content or with added lipid may have a lower EGI,due to the formed amylose-lipid complexes reducing the susceptibility of starch to α-amylase digestion[36].Furthermore,RS showed a strong negative correlation with the EGI.Similar findings have been reported by Kumar et al.[36]RS was negatively correlated(r=-0.778)with the GI and was considered to be a main determinant of the GI.Interestingly,amylose showed a weak positive correlation with the EGI(r=0.621)in our work.However,Prasad et al.[33]and Fitzgerald et al.[39]reported that the EGI was inversely correlated with the amylose content.Their studies investigated the effect of amylose on the EGI in different rice cultivars with the same DOM,but the effect of differences in the protein and lipid contents on the EGI was not considered.In our work,the same rice cultivars(Simiao rice)with differences in the DOM and protein/lipid contents were studied to evaluate the correlation between the EGI and amylose content.

4.Conclusion

The purpose of this study was to investigate the effect of DOM on the physicochemical properties and in vitro starch digestibility of Simiao rice.As the research has demonstrated,Simiao rice is prone to breakage and over-milling,investigating the milling technology of Simiao rice is practical.With the increase of DOM,nutrients present in Simiao rice were gradually removed.Compared to brown rice,the milled rice with an up to 12%DOM had lower contents of protein,lipid,dietary fiber,niacin and vitamin E,with a reduction of 7.2%,72.8%,57.3%,33.6%and 34.0%,respectively.The changes of chemical components decrease the content of SDS and RS of milled Simiao rice,and milled Simiao rice exhibited greater digestibility and a higher estimated glycaemic index compared to brown Simiao rice.Furthermore,the DOM and EGI were negatively correlated with lipid,protein,dietary fiber and RS,but positively correlated with amylose.Hence,an appropriate DOM can be applied to obtain white Simiao rice with desired physicochemical properties and digestibility.Furthermore,the effect of DOM on sensory qualities of Simiao rice should also be investigated to obtain milled Simiao rice with pleasant taste.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Author Contributions

Zhiming Wang:Data curation,Writing-review &editing;Mingwei Zhang:Funding acquisition;Guang Liu:Writing-review&editing;Yuanyuan Deng:Data curation,Formal analysis;Yan Zhang:Data curation,Formal analysis;Xiaojun Tang:Investigation;Ping Li:Writing-review &editing;Zhencheng Wei:Conceptualization,Writing-review&editing.

Acknowledgements

This work was supported by the China National Key Research and Development Program(2017YFD0401105 and 2017YFD0400200);The Guangdong Provincial Scientific and Technological Research Program(2020B020225004,2018A050506047 and 2018A050506049);Guangzhou International Collaborative Fund(201807010061);Innovative Research and Promotion Project for Modern Agricultural Industry Technology System of Guangdong Province(2020KJ105);The Application-oriented Projects of Guangdong Province(2017B020232002);The Special Fund for Modern Agricultural Industry Technology System of Guangdong Province(2018LM1084 and 2020KJ117);The Guangzhou Municipal Scientific and Technological Research Program(201803010120);The National Natural Science Foundation of China(31501478),and Guangdong Special Support Program(2019BT02N112).