Physico-chemical properties and structure of rice cultivars grown in Heilongjiang Province of China

Yinyun He,Fenglin Chen,?,Ynguo Shi,Zhexin Gun,N Zhng,?,Osvldo H.CmpnellbCollege of Food Science nd Engineering,Hrbin University of Commerce,Heilongjing,Hrbin,150076,Chin

bDepartment of Agricultural and Biological Engineering,Purdue University,745 Agricultural Mall Drive,West Lafayette,IN,47907,USA

Keywords:

Rice flour

Gelatinization

Thermal

Morphology

Texture

ABSTRACT

The relationships between chemical composition, structure and physicochemical properties such as pasting,thermal,and textural properties of 12 temperate rice cultivars grown in Heilongjiang Province,China were investigated in this study.There were significant differences in the content of fat,protein,ash,amylose,damaged starch among 12 cultivars.Brabender Viscograph results showed that gelatinization time,peak viscosity,pasting and peak temperature ranged from 9 min to 15min,2100 BU to 2500 BU,58.75?C to 72.00 ?C and 63.75 ?C to 84.45 ?C,respectively,except for the waxy one Longjing 9.To,Tp,Tcand ΔH were found in the range of 58.34 ?C to 67.96 ?C,65.39 ?C to 75.38 ?C,78.47 ?C to 90.69 ?C and 2.036 J/g to 3.127J/g respectively.Scanning Electron Micro-photograph results showed that all starch granules were polyhedral with irregular shapes,except Longjing 9 showing signs of the loss of physical integrity.The hardness,adhesiveness,springiness,gumminess,cohesiveness and resilience ranged from 395.87g to 1161.76g,-90.842g·mm to-36.4g·mm,0.498 to 0.747,78.631g to 466.837g,0.199 to 0.569,0.321 to 0.504,respectively.Correlation studies indicated that amylose,damaged starch,protein,fat and ash played a more important role in determining the rice properties of pasting,thermal,crystallinity and texture.

1.Introduction

Rice is the staple food for approximately one-half of the world population[1,2].The main planting areas of rice are India,China,Japan,Bangladesh and Southeast Asia.The cultivation of rice in China can be traced back to 5000 years ago,and it is considered to be the most important cereal grain cultivated in China.Rice can be divided into indica andJaponicarice.Heilongjiang Province is the main producer ofJaponicarice in China[3].

The physicochemical characteristics of rice including composition,gelatinization,texture and so on determine their usage in food and non-food applications,so it is necessary to have them documented.The quality of rice depends on starch,protein and fat contents and the structure of starch.Starch is the major component of rice grain that mainly determines the physico-chemical and cooking properties[4].Pasting and gelatinization characteristics of starch depend on the amylose:amylopectin ratio,chain length,the degree of branching of amylopectin,and the granular structure of statch affect quality of rice flour[5–7].The amylose content is different due to rice variety,which is one of the most important determinants of rice starch quality[8].Rice starch has a bland taste,white color,smooth texture and is hypoallergenic to many people[9,10].Furthermore rice starch possesses superior attributes such as greater acid resistance,spreadability,and relatively good freeze-thaw stability[11,12].In addition,the quality and quantity of protein in rice are also important determinants of rice quality.Lin[13]reported that the differences in rice paste viscosity profiles were attributed to protein network formation during processing.

In this backdrop,the objective of the present study was to compare the physico-chemical characteristics including crystallinity,micromorphology,texture properties,pasting(BV)and thermal properties of 12Japonicarice varieties commonly cultivated in Heilongjiang Province,China.

Table 1Growing area of rice cultivars.

2.Materials and methods

2.1.Materials

Twelve 2016 purebred white rice were chosen from Heilongjiang Province,China.Heilongjiang Province is the main planting area ofJaponicarice.According to the temperature index,the planting area in Heilongjiang province is divided into six accumulated temperature zones:the first(＞ 2700?C),the second(2500–2700?C),the third(2300–2500?C),the fourth(2100–2300?C),the fifth(1900–2100?C),the sixth(1900?C).Rice is mainly grown in the first,the second and the third accumulation zone(Table 1).All of the samples were non-waxyJaponicarice except LD-9(waxy)and all of them were kept below 15?C before analyses.

2.2.Preparation of rice flour

Rice samples from each cultivar were dehulled using a dehusker(THU-35,Satake,Hiroshina,Japan)and milled for 30s using a universal high-speed smashing machine(mill,MF-2200,Guangzhou,China) and then the resulting flour was passed through a 100-mesh sieve to obtain rice flour.

2.3.Preparation of rice starch

Rice starch was isolated using alkaline steeping method.Defatted rice flour(100g,dry basis)was soaked in 200mL 0.1% NaOH for 18h.Then the sample was blended in a Waring blender at high speed for 2 min, passed through a 63-mm screen and centrifuged at 4200r/min for 10min.The yellow top layer was carefully removed,and the underlying starch layer was reslurried with 0.1% NaOH and centrifuged at 4200r/min for 10min.This process was repeated at least three times until there was no yellow layer.The starch layer was then washed with deionized water and centrifuged at 4200r/min for 10min.The starch was reslurried,neutralized to pH 6.5 with 1.0mol/L hydrochloric acid and centrifuged again.Neutralized starch was washed with deionized water three times.After being dried at 40?C for 48h,the starch was passed through a 100-mesh sieve and kept below 15?C before analyses.

2.4.Chemical compositions

The contents of total starch,amylose,damaged starch,moisture,ash,crude protein and fat were determined according to AACC methods 76-13,61-03.01,76-30.02,44-01.01,08-01.01,46-09.01,30-10.01,respectively(2002).

2.5.Determination of the pasting properties(BV)

Pasting properties of rice flour were determined using Brabender Amylograph(BV,Brabender Ltd.,Germany).To get a mixed samples with certain proportion,80g powder was dissolved in 450 mL distilled water and stirred evenly by blade.The temperature change was as set as follows,the suspensions were equilibrated at 30?C for 1min,heated to 95?C at the rate of 1.5?C/min and kept for 5min.Pasting temperature and time,peak temperature and viscosity were recorded.

2.6.Thermal properties

Thermal properties of flour were determined using a differential scanning calorimeter(DSC4000,Perkin Elmer USA)according to the method explained by Kolawole O.Falade andRosanaColussia[14,15].Flour sample(approximately 10–12mg)was weighed into the sample pan,mixed with distilled water( flour:water=1:2).The pan was hermetically sealed,shaken well and kept still for 1h.The samples were heated from 30?C to 110?C at a heating rate of 10?C/min.The onset temperature(To),peak temperature(Tp),conclusion temperature(Tc)and enthalpy change of gelatinization(ΔH)were recorded.

2.7.X-ray Diffractometry

X-ray diffraction analysis was performed using an X-ray diffractometer(X’Pert PRO MPD,Panalytical,Netherlands)operated at voltage of 40 kV and current of 40 mA with Cu.Rice flour was packed tightly into the aluminum sample holder and diffraction data were collected over an angular range from 10?(2θ)to 30?(2θ)with a scanning speed of 2.0?/min.Relative crystallinity degree(%)was estimated from the ratio of the peak area to the total area of a diffractogram.

2.8.Scanning electron microscopy

To verify the structure,rice flour,rice starch or rice grain samples(Truncate the middle of the rice with a blade and obtain the sample)were attached to a Scanning Electron Microscopy(SEM)stubs.Specimens were then coated with gold by a Sputter Coater Baltec(SCD 005 SEM Coating Unit,Liechtenstein,Germany).The crosssection was observed by SEM(Hitachi,S-400N,Tokyo,Japan)at a magnification of 4000 and 5000 times.Images were then recorded visually.

2.9.TPA analysis of cooked rice

Ten grams rice samples from each cultivar were mixed with 1.6 times water.After heated boiling,they were cooked for 40min.Warm-keeping 20min before the measurement of texture.

Texture Profile Analysis(TPA)was performed using a texture analyzer(TA-XT2i;Stable Microsystems,Surrey,United Kingdom)with a 5kg load cell,fitted with a 35mm diameter cylinder aluminum probe.Hardness,adhesiveness,springiness,gumminess,cohesiveness and resilience of the cooked rice samples were measured.Tweezers were used to pick up 10gains of steamed rice from the middle of the container,and evenly placed them individually in the center of the platform in a radial shape.Compression force was measured during the test,the pretest speed was 2.0m/s,test speed 1.0m/s,and after test speed was 1.0m/s,a compression ratio 60% was used and the trigger force was set to 1g.

2.10.Statistical analysis

Data was analyzed by the Statistical Analysis System software(SAS 2000)in order to assess significant differences among samples.Differences were considered significant whenP＜0.05.

Table 2Physicochemical properties of rice endosperms(%).

Table 3Pasting parameters of rice flours.

3.Results and discussion

3.1.Chemical composition

Chemical compositions of different rice flour differ depending on its variety, genetic background, climatic and soil conditions during development[16].The results are presented in Table 2.The starch content of rice ranged from 78.00% to 85.94%,M-31 having the highest starch content of 85.94%,while LD-24 having the lowest(78%).The amylose content varied from 0.56%(LD-9)to19.63%(LD-20).It is one of the most important factors determining the quality of rice products.The protein content was followed by total starch in rice,in which glutelin was the main component[17].The protein content ranged from 4.03% to 6.99%.Highest were LD-20 and LD-23,while the lowest was LD-9.The fat,ash and moisture content involved in our research were in the range of 0.27%(LD-20)–1.53%(LD-9),0.34%(LD-20)– 1.57%(LD-9)and 11.15%(WY-4)–13.40%(LD-20),respectively.During milling or grinding to break cereal grains into flour, force can cause damage to starch granules.Asmeda et al.[18]reported the damaged starch content was 10.7%(dry grinding)followed by 7.82%(semi-wet grinding)and 7.37%(wet grinding).It can be explained that different grinding processes would produce different degrees of damaged starch to the flour depending on the mechanical forces and temperature during grinding process.Dry milling was used in this research,so the damaged starch content of rice flours varied from 6.31% to 9.92% with the lowest in LD-9(6.31%)and the highest in LD-23(9.92%).

3.2.Pasting properties of rice flours

The pasting properties measured by a Brabender Viscograph are presented in Table 3.The temperature of pasting and peak of the flours ranged from 58.5?C to 72?C and 63.75?C to 84.45?C,respectively.The highest and the lowest pasting temperatures were LD-19 and LD-23 respectively,and the highest and the lowest peak temperatures were WY-4 and LD-9 respectively.The gelatinization time varied from 2min for LD-9 to 14min for WY-4.Peak viscosity of 12 Heilongjiang rice flours varied from 278 BU to 2480 BU.Except for LD-9 the peak viscosities of all rice tested were around 2000BU. It was confirmed that the LD-19(2480BU)and LJ-31(2330 BU) flours showed higher viscosity while the LD-9(278BU)showed lower peak viscosity than other cultivars. It has been suggested that amylopectin fractions affect the viscosity more due to their role on the disentanglement in swollen particles[3].Therefore,the amylopectin may play a role on viscosity of LD-19(2480 BU)and LJ-31(2330 BU).Granules undergo further disruption meanwhile amylose molecules leach out into solution,leading to a reduction in the viscosity at the minimum level and coming about peak viscosity(hot paste viscosity)[19].Apparently,the peak viscosity of LD-9 was lower than non-waxyJaponica.

As shown in Table 4,according to Pearson correlation coefficients,amylose showed a strong significant negative relationship with peak viscosity(r=-0.877,P＜0.01)since higher amylose content contributes to lower viscosity[3]and a significantly negative relationship with gelatinization time(r=-0.553,P＜0.05).Protein showed a strong significantly negative correlation with peak viscosity(r=-0.882,P＜0.01)and a significantly negative correlation with pasting temperature(r=-0.66,P＜0.05).Smanalieva et al.[12]reported that peak viscosity increased with the decreasing protein content as well.There was a significantly negative correlation between pasting temperature(r=-0.529,P＜0.05),gelatinization time(r=-0.562,P＜0.05)) and fat content. A strong significantly negative correlation between peak viscosity(r=-0.915,P＜0.01)and fat content was observed.There were significantly negative correlations between moisture and gelatinization time(r=-0.566,P＜0.05),ash and peak viscosity(r=-0.664,P＜0.05).Falade and Christopher[14]also considered that pasting behaviors which resulted from interactions between starch and non-starchcomponents.From the above,it can be concluded that the pasting properties of flour were affected not only by amylose,fat and protein,but also by moisture and ash content.It was also shown that the peak viscosity was strong significant negatively correlated with their damaged starch content(r=-0.859,P＜0.01).It may be due to easier bursting of starch granules under high damaged starch content,where starch crystal zone was vulnerable to damage,which reduces the peak viscosity[20].

Table 4Correlation coefficients between pasting parameters and basic parameter of rice flours.

Fig.1.DSC thermograms for rice flours dispersions prepared from 12 rice varieties.

Table 5Basic statistical parameters of thermal properties of rice flours.

3.3.Thermal properties

The rice flour undergoes physical and chemical transitions during thermal processing.These modifications depend on some characteristics,such as water content,heat flow and environment provided[21].In this study,all kinds of rice showed a distinct single endothermic peak during heating(Fig.1).The DSC data for 12 rice flours were presented in Table 5.In general,onset(To),peak(Tp),conclusion(Tc)temperatures and endothermic enthalpy(ΔH)of gelatinization were significantly different among these samples.Differential scanning calorimetry results suggested that gelatinizedTo,TpandTcof flour samples varied from 58.34?C to 67.96?C,65.39?C to 75.38?C and 78.47?C to 90.69?C respectively,which were similar to the results reported by Asmeda et al.[18].Rice with higherTcshowed higher ΔH,such as WY-4,LD-5,LD-9,LD-19,and LJ-31.LD-19 possessed the highestTo,TpandTc,while LD-23 possessed the lowestTpandTc,and M-31 possessed the lowestTo.Cai et al.[22]reported that the difference in gelatinization temperature may be due to the difference in amylose content,distribution size,formation of starch granules as well as to the internal arrangement of starch fractions within the granules.Heat enthalpy provides an overall measurement of crystallinity and is an indicator of the loss of molecular order(primarily re flects the loss of double helical order)within the granules[23,24].Besides this,the gelatinization enthalpy might also be affected by such factors as the starch granule shape and the percentage of large and small granules[13].LD-19 had the highest ΔHvalues(3.127J/g)which also had the highestTo,Tp,Tcand LD-20 had the lowest(2.036J/g)among 12 samples in our research.

Table 6Correlation coefficients between thermal parameters and basic parameter of rice flours.

Mir and Bosco[16]reported that proteins,lipids and amylose contents and lipid amylose complex significantly influenced thermal properties of rice flour.According to Table 6 Pearson correlation coefficient,amylose showed a strong significantly negative correlation with ΔH(r=-0.877)andTc(r=-0.769,P＜ 0.01),and a significant negative correlation withTp(r=-0.549,P＜0.05).Protein showed strong significant negative relationships among ΔH(r=-0.735),To(r=-0.712),Tp(r=-0.752)andTc(r=-0.87,P＜0.01).A strong significant negative correlation was observed among ΔH(r=-0.832,P＜0.01),Tc(r=-0.831,P＜0.01)and fat,and significant negative correlations were found amongTo(r=-0.577),Tp(r=-0.633,P＜0.05)and fat.Similar results were reported by Lin et al.[13].Whereas ΔHshowed significant negative correlations with moisture(r=-0.533,P＜0.05)and damaged starch(r=-0.669,P＜0.05),while a strong significant negative correlation with ash(r=-0.829,P＜0.01).Tcshowed significant negative relationships with ash(r=-0.604,P＜0.05)and damaged starch(r=-0.599,P＜0.05).

Table 7Correlation coefficients between crystal and basic parameter of rice flours.

Table 8Correlation coefficients between gelatinization,thermal parameters and crystal of rice flours.

Fig.2.X-ray diffraction pattern of 12 rice flours cultivated in Heilongjiang province.

3.4.X-ray Diffraction pattern

The XRD patterns of 12 individual rice flours were very similar as shown in Fig.2,which revealed that grain starch granules contained crystalline,amorphous and semi-crystalline regions.The semi-crystalline granule consists of the aligned branches of amylopectin providing the crystalline region,the low branching regions of amylopectin and amylose comprising the amorphous region[25].All of native rice flours in this study showed major X-ray diffraction peaks at 15?,a doublet at 17?and 18?,and 23?(2θ),indicating A-type crystalline structure[26,27],which was also in agreement with XRD patterns of normal cereal starches[22].Japonicaflours showed the peak at 20?(2θ)indicating the V-type structure and existence of amylose-lipid complex[28].However,this structure in waxy rice flour was not apparent.This result was in line with the results reported by Liu and Cheng[29],who considered waxy rice contained more non starch-fat complex than non-waxy rice.The relative crystallinity of the rice samples,calculated from X-ray diffraction patterns ranged from 19.59% to 59.23%(Fig.2).LD-9 had the highest crystallinity(59.23%)followed by LD-19(51.04%),LJ-31(49.57%)etc,and LD-20 had the lowest one(19.59%).

Coefficients of Pearson’s tests between chemical components,gelatinization,thermal parameters and crystal of all samples are summarized in Table 7 and 8.According to correlation test,there was a drastically significant negative correlation between crystallinity and fat(r=-0.927,P＜0.01),protein(r=-0.798,P＜0.01),amylose(r=-0.961,P＜0.01),damaged starch(r=-0.811,P＜0.01)and ash(r=-0.727,P＜0.01),which approved that the most basic components of rice had an influence on starch crystallization.It has been reported that crystallization area was strengthened gradually and crystallinity increased with the decreasing of amylose content[17].As shown in Table 8,crystal showed significant positive correlations with peak viscosity(r=0.867,P＜0.01),Tc(r=0.715,P＜0.01)and ΔH(r=0.897,P＜ 0.01),and a positive correlation with gelatinization time(r=0.573,P＜0.05).Similar tendency was reported by Huang et al.[23].

3.5.Granule morphology observbed by SEM

Distinct morphological differences upon the truncation surface of rice kernels(Fig.3 3A-3L),rice starches(Fig.3 4A-4L)as well as rice flours(Fig.3 5A-5L)were observed.Minute amounts of proteins and fat droplets were observed on the surface.Meanwhile the typical waxy one from LD-9,whose rice granules surface was rough and its cross section at edges and corners were not obvious as shown in Fig.3 3A-3L.

Generally,starch shows a uniform texture[30].The isolated starch images from 4000magnification SEM in Fig.3 4A-4L showed that starch with high amylose content was more uniform than those with lower ones.All rice starches displayed a smooth surface and plump structure except LD-9.Compared with the non-waxy rice,the structure of LD-9(waxy)starch granules was more assembled,and the surface was more rough and irregular with apparent scallops and apertures.Therefore,we might conclude that the waxy starch showed signs of the loss of physical integrity.A similar tendency was reported by Zhao et al.[31]that surface of starch particles showed less scallops and apertures,and then got much smoother with the increasing amylose content.Moreover most starch granules possessed angular shape and were polyhedral with irregular shapes,while some of them were oval in shape.As shown in Fig.3 4A-4L and Table 5,the size of starch granules was smaller for WY-4,LD-5,LD-9,LD-19 and LJ-31,meanwhileTcand ΔHwere higher than others.This result indicated that the particle size of starch granules had a significant effect on the gelatinization characteristic of rice flour.As discussed by Song and Malshick[32];Wani et al.[10],the size and shape of rice starch were intrinsic properties which influence the composition,swelling capacity,crystallinity,pasting properties,and gelatinization properties of the rice starches.

SEM of rice flours at 5000 magnification indicated that starches were strongly packed in enclosed structure, and non-waxy rice was more robust than waxy one(Fig.3 5A-5L).The caves on waxy flour surface were more than that on non-waxy flour surface.

3.6.Texture properties of cooked rice

The gel texture properties of these 12 rice varieties from Heilongjiang Province rice are shown in Table 9.Range of hardness,adhesiveness,springiness,gumminess,cohesiveness and resilience were from 395.87g to 1161.76g,-36.42 g·mm to-90.84g·mm,0.498 to 0.747,78.631g to 466.837g,0.199 to 0.569,0.321 to 0.504,respectively for 12 samples.

Fig.3.SEM micrographs for section profile of rice granules(3A-3L),rice starche(4A-4L),and rice flours(5A-5L).A.Wuyou rice 4,B.Longdao 5,C.Longdao 9,D.Longdao 19,E.Longdao 20,F.Longdao 23,G.Longdao 24,H.Longdao 25,I.Longjing 31,J.Longjing 46,K.Mudanjiang 29,L.Mudanjiang 31.

Fig.3.(Continued)

Table 9Texture characteristic parameters of rice.

Table 10Correlation analysis between basic parameter and texture characteristic of rice.

As shown in Table 10,amylose showed strong significant positive correlations with hardness(r=0.729),gumminess(r=0.824),cohesiveness(r=0.758)and resilience(r=0.689)(P＜0.01),and the results were in line with the research reported by Shevkani et al.[24]and Jiamjariyatam et al.[28].It was also reported that cooked rice appeared to be rough and low sticky with high amylose content,on the contrary,it showed moist and sticky with low amylose content[33].Most Heilongjiang rice belongs to the latter.Protein content showed strong positive correlation with hardness(r=0.889,P＜0.01)and gumminess(r=0.894,P＜0.01),and a significant positive correlation with cohesiveness(r=0.595,P＜0.05).Higher content of protein would occupy the space among starch granules and compact the structure of rice granules,leading to lower rate and amount of water absorption.Therefore,the rice with high protein content showed longer cooking time,and the starch emerged gelatinization insufficiently.Consequently,the cooked rice showed lower viscosity and had a relatively loose structure.So the lower the amylose and protein content the better the palatability of rice.Fat was strong significantly positive correlated with hardness(r=0.966,P＜0.01)and gumminess(r=0.89,P＜0.01),significantly negative correlated with adhesiveness(r=-0.654,P＜0.05),and significantly positive correlated with springiness(r=0.517,P＜0.05),the same as Lin et al.[13]reported.Meanwhile,there was higher significant positive correlations between ash and hardness(r=0.78)and gumminess(r=0.71)(p＜0.01).A similar report was shown by Saleh and Meullenet et al.[34].There was a strong significant positive correlation among damaged starch,hardness and gumminess,and a negative correlation(r=0.79,P＜0.01)between damaged starch and adhesiveness(r=-0.73,P＜0.01).

4.Conclusions

The 12 purebred rice from Heilongjiang Province,China showed significant differences in physico-chemical properties especially glutinous one.All rice produced in Heilongjiang wereJaponicain which amylose content varied from 14.05% to 19.63% except for LD-9,and protein content ranged from 4.03% to 6.99%.TheJaponicarice with high amylose and protein content showed strong significant positive correlation with hardness,gumminess,cohesiveness and appeared to be rough and low sticky after cooking.Apart from LD-9,the eleven rice samples showed V-type structure.All starch granules possessed angular and were polyhedral with irregular shapes,while the waxy starch LD-9 showed signs of the loss of physical integrity with distension of the granular surface,and its granules surface was not smooth. There was a certain correlation among rice composition,physical characteristics,crystallinity and gelatinization properties.

Acknowledgments

This research was financially supported by Major Science and Technology Program of Heilongjiang Province(No.2019ZX08B02),Harbin Municipal Bureau of Science and Technology in Heilongjiang province-Grant(No.2017RAQXJ030),National Natural Science Foundation(No.31871747),and Innovative Research Fund for Postgraduates of Harbin University of Commerce(YJSCX2019-565HSD).