Effect Of High Temperature During Grain Filling Stage On Physicochemical Properties Of Rice Starch

Six high-yield rice varieties were used as experimental materials in this study.A series of experiments were designed,including natural high temperature in the field,the combined high night-time and day-time temperatures,and high temperature different periods of grain filling stage controlled by artificial climate box.This study aimed to determine the effects of high temperature during grain-filling stage on processing quality,appearance quality,composition,structure and physicochemical properties of rice starch.Furthermore,it aimed to analyze the different responses of starch physicochemical indexes to high temperature,screen the key starch physicochemical indexes,determine the influence mechanism of high temperature at grain filling stage on the physicochemical indexes of rice starch,and establish an evaluation method for high temperature tolerance of different rice qualities.The main results and conclusions are as follows:1.Effects of high temperature during grain filling stage on rice processing and appearance quality.According to the experiment of sowing at different dates in field and temperature control in artificial climate box,it was found that the high temperature,including natural high temperature in the field,combined high night-time and day-time temperatures,and high temperature at different periods of grain filling stage did not have a significant effect on the brown rice rate,polished rice rate and ratio of milled rice length to width,but significant reduction on the head milled rice rate and increased chalkiness degree had been observed.The results showed that milled rice rate and chalkiness were sensitive to high temperature.Specifically,chalkiness was found to be the index of appearance and processing quality that was most sensitive to high temperature during grain filling period.The effects of high temperature on processing and appearance quality of rice at different times during grain-filling stage showed that high temperature at both the early and late stage of grain filling significantly reduced the head rice percentage and increased the chalkiness,but high temperature stress at the early stage of grain filling had a greater effect.Meanwhile,the degree of influence to high-temperature sensitive varieties was greater than that of high-temperature tolerant varieties.2.Effects of high temperature during grain filling stage on starch composition and structure.The results showed that high temperature including natural high temperature in the field,combined high night-time and day-time temperatures,and high temperature different at periods of grain filling stage,decreased amylose content,but the degree of decrease of amylose content was different for the different rice varieties.Results also showed that rice with low amylose type was more sensitive to high temperature at the grain filling stage.High temperature during grain-filling stage would significantly reduce the amylose content,hence the amylose type might be reduced to very low amylose type to make the eating and cooking quality declined.High temperature during grain filling stage significantly decreased amylopectin A chain and B1 chain.Meanwhile,high temperature during grain filling stage significantly increased medium B2 chain and long chain B3 chain.The increase at the early time of grain filling for high-temperature sensitive varieties was significantly higher than that at the late stage of grain filling.The response of amylopectin to high temperature at grain-filling stage mainly increased medium and long chain,and this trend was more obvious for high temperature-sensitive varieties.On the other hand,high temperature significantly increased the proportion of surface area,volume and number of small and medium starch granules,while the proportion of surface area,volume and number of large starch granules significantly decreased.Under high temperature conditions,the surface of starch granules became uneven to make the starch granules more fragmented.The effect in the high temperature sensitive varieties was more serious at the early stage of grain filling stage.3.Effects of high temperature during grain filling stage on gelatinization property and pasting property.High temperature during grain-filling stage significantly increased gelatinization temperature and gelatinization enthalpy,which led to higher cooking temperature and longer cooking time.T₀,T_p,T_c andΔH of the high-temperature treated materials at the early stage of grain filling stage were significantly increased,while the T₀ and T_P were also significantly increased at the late stage,but the increase was lower than that of the high temperature treated materials at the early stage of grain filling stage.PV,HPV and CPV had no significant response to high temperature,while setback（SB）and breakdown（BD）were more sensitive to high temperature during grain-filling stage.The high temperature at the early stage of grain filling stage has the highest SB and the lowest BD,which made the viscosity properties worse.The high temperature at the late stage of grain filling stage had the lowest SB and the highest BD,which made the viscosity properties slightly better.There was no change in crystal type between rice starch matured under high temperatures and control temperature,but significant changes in relative crystallinity were observed under both environmental conditions.4.Correlation analysis among starch physicochemical properties under high temperature stress during grain-filling stage.Correlation analysis showed that the gelatinization property was significantly negatively correlated with the proportion of A chain and positively correlated with the proportion of B3 chain,respectively.The increase of B3chain and decrease of A chain increased the gelatinization temperature and gelatinization enthalpy under high temperature stress during grain-filling stage.The viscosity property was mainly the result of the combined regulation of amylose content,the proportion of B3 chain and starch granule number distribution.Head milled rice rate was negatively correlated with B3 chain,and the increase of B3 chain resulted in the decrease of milled rice rate.Chalkiness was the result of the joint regulation of amylose and starch granule number distribution.There was little correlation between the volume and surface area distribution of starch granule and starch physicochemical properties.The proportion of amylopectin B3 chain had a great influence on the physicochemical properties of starch granules,including gelatinization properties,viscosity properties and milled rice rate.Amylose content was significantly correlated with starch granule number distribution.Amylose content,the fine structure of amylopectin and the number distribution of starch granule were influenced by high temperature stress,which would further change the physicochemical properties of starch.5.Evaluation method for high temperature tolerance of different rice qualities.Combined with the significance analysis and variable coefficient analysis under high temperature,AAC,BD,SB,CHA andΔH were selected as the starch key physicochemical indexes to evaluate the tolerance to high temperature of different rice qualities.Principal component analysis was used to transform these five relevant key indexes into three independent comprehensive indexes with cumulative contribution rate of more than 96%.CHA,SB and BD were the most important key indexes to evaluate the high temperature tolerance of different rice qualities.Further,subordinative function was used to evaluate the high temperature tolerance of six rice varieties from strong to weak,which were Huanghuazhan,Y liangyou 957,Taiyou 390,Tianyouhuazhan,Zhongzao 39 and Y liangyou 1hao.Taking the comprehensive evaluation value as the dependent variable and the character stress indexes of five key physicochemical indexes as the independent variables,the multiple linear regression equation was established.The determining coefficient of the equation was0.99,showing the reliability of the evaluation method used in this study.