| Rice flavor is of great significance to the commercial value and consumer acceptance of rice.Therefore,fragrant rice with high-quality flavor has been commercial promoted and favored by consumers.The profile and content of rice volatiles are affected by many factors including variety and planting area.Meanwhile,cooking process of rice will also change the volatile profile,and the volatile compositions of raw and cooked rice are significantly different.However,most of the current studies focused on the single system of raw or cooked rice ignoring the mutual relation of them,and also did not consider the influence of cooking process.Therefore,in current study,the discriminant volatiles in raw and cooked rice between fragrant and non-fragrant japonica rice and those among japonica rice samples planted in different regions were selected.It can provide theoretical basis for the japonica rice adulteration recognition and origin identification.At the same time,the effect of cooking process on the volatile content of different japonica rice varieties was explored.In addition,the relationship between the important volatiles retention during cooking and the properties of rice was studied,and the relationship between the release of important volatiles in cooked rice and the properties of cooked rice matrix was examined,which aimed to provide theoretical guidance for the improvement of sensory quality of japonica cooked rice.The main research contents and conclusions were summarized as follows:The discriminant volatiles in raw and cooked rice between fragrant and non-fragrant japonica rice samples were quite different and only 2-acetylpyrrole and decanal were common.An untargeted headspace solid phase microextraction coupled with gas chromatography-mass spectrometry-based(HS-SPME/GC-MS-based)metabolomics was employed to investigate the volatile profile of Chinese fragrant(13 samples)and non-fragrant(14 samples)japonica samples before and after cooking.Partial least square discriminative analysis(PLS-DA)along with univariate analysis including t-test and fold change(FC)analysis was taken to seek specific markers in raw and cooked rice distinguishing fragrant and non-fragrant japonica rice samples.Results showed that the volatile profile of raw rice dominated by alkanes was completely different from that of cooked rice predominated by aldehydes.This study applied the variable importance for the projection(VIP)in PLS-DA analysis >1.0 together with P<0.05 and FC>1.5 or <0.6 as the thresholds to recognize the differential metabolites playing important roles in grouping fragrant and non-fragrant rice samples.Thirteen and eleven volatile compounds were selected as the potential discriminative markers in raw and cooked rice,respectively.Fragrant rice varieties contained more 2-acetylpyrrole,(E)-2-octenal,nonanal,1-tetradecene,decanal and(E)-2-decenal in raw rice.While they contained more2-acetyl-1-pyrroline(2-AP),2-acetylpyrrole,1-hexanol,decanal and indole in cooked rice.The discriminatory volatiles in raw and cooked rice among japonica rice samples produced in different regions were also different,and only hexanal,3,5-octadien-2-one and2-butyl-2-octenal were common.An untargeted metabolomics based approach using HS-SPME/GC-MS was applied to distinguish japonica rice samples originated from Southern,Northern and Northeastern China in raw and cooked form,respectively.Orthogonal partial least-squares discriminant analysis(OPLS-DA)models exhibited satisfactory geographic discrimination.Combined S-plot analysis from OPLS-DA and univariate ANOVA,sixteen and twenty-two volatiles were selected as the geographical discriminant markers in raw and cooked rice,respectively.According to the secondary metabolic pathways related to the volatiles production in plants,it suggested that the volatile profile variations of raw and cooked rice from different geographic regions were substantially dependent on the intensity of related secondary metabolic pathways.The geographical discriminatory volatiles in raw rice were mainly involved in terpenes synthesis(palustrol),lipoxygenases(hexanal,1-hexanol,2-butyl-2-octenal and 3,5-octadien-2-one),L-tryptophan synthesis(indole,etc),and benzoic acid and phenylpropanoid synthesis(2-phenylethanol,benzyl alcohol,methyl benzoate,etc)pathways.Markers in cooked rice were mainly derived from lipid oxidation(1-octen-3-ol,2-pentylfuran,(E)-2-heptenal,(E)-2-nonenal,3-octen-2-one,hexanal,2-octen-1-ol,1-octanol,2-butyl-2-octenal,nonanal,3,5-octadien-2-one and octanal).Cooking would resulted in the decrease of 2-AP which is the key aroma volatile in fragrant rice,and cooking also significantly influence the volatiles originated from lipid oxidation or phenolic acid degradation.The semi quantitative analysis of 39 volatile compounds in raw and cooked rice of 26 different japonica rice varieties was carried out using the modified direct solvent extraction GC-MS methodology.2-AP was under the limit of detection in non-fragrant japonica rice and it was only detected in fragrant varieties which decreased after cooking(-81.94%–-43.97%).It seemed that the 2-AP in cooked rice was originated from raw rice and cooking could not produce extra 2-AP.Great losses of 1-hexanol,esters and long-chain ketones were exhibited after cooking.Cooking accelerated lipid oxidation and degradation of phenolic acids,thus volatiles originated from them raised in majority of the samples after cooking.Hexanal(48.01%–306.02%),octanal and nonanal(67.03%–544.15%)were derived from the direct cleavage of lipid hydroperoxides,and their increases upon cooking could be observed in all samples.The concentrations of acetophenone,2-methoxy-4-vinylphenol(11.95%–297.61%)and vanillin(14.29%–319.25%)were enhanced in all japonica varieties upon cooking which might be derived from the degradation of phenolic acids in rice.According to the origin of the volatiles,it seemed that positive correlations existed between volatiles with common precursors or involved in same pathways.The retention of 2-AP during cooking was associated with the starch properties in rice.Based on the result that the 2-AP content was decreased in fragrant japonica rice upon cooking,the composition,crystalline,thermal and pasting properties of 12 fragrant japonica rice varieties were studied,and the relationship between these parameters and the 2-AP retention upon cooking was discussed.Results showed that the relative crystallinity determined with X-ray diffraction(ρ=0.678,P=0.015)was positively correlated with 2-AP retention upon cooking.Meanwhile,the gelatinization enthalpy(ρ=0.685,P=0.014)and gelatinization temperature range(ρ=0.587,P=0.045)analyzed with differential scanning calorimetry(DSC)were also positively correlated with 2-AP retention upon cooking.No significant correlation was observed between pasting properties obtained with Rapid Viscosity Analyzer(RVA)and 2-AP retention,however,the differences between the conclusion gelatinization temperature obtained with DSC and the pasting temperature obtained with RVA was highly correlated with 2-AP retention(ρ=0.709,P=0.010).It indicated that the retention of 2-AP was primarily dependent on starch properties,and 2-AP might mainly existed as starch-bound form in rice.In addition,the starch-bound 2-AP had a greater effect on the 2-AP retention upon cooking.The volatilization rate of 2-AP in cooked rice was related to the adhesiveness of cooked rice matrix.The ratio of the relative peak area of 2-AP in cooked rice obtained by headspace method to the relative peak area of 2-AP in cooked rice obtained by solvent extraction method was defined as the volatilization rate of 2-AP in cooked rice.The texture properties,solid content of coated layer,starch content and composition coated layer and molecular properties of coated layer of cooked rice of 12 different fragrant japonica varieties were studied,and the relationship between these parameters and the volatilization rate of 2-AP in cooked rice was discussed.The results showed that the volatilization rate of 2-AP in cooked rice was mainly affected by the adhesiveness of cooked rice matrix(ρ=-0.89,P<0.001).The rice matrix with high adhesiveness would enhance the interaction between matrix and 2-AP and reduce the mass transfer rate of 2-AP.Amount of the starch content(ρ=-0.69,P<0.05),the amylopectin content(ρ=-0.69,P<0.05)and the fine structure of amylopectin leached during rice cooking could influence the adhesiveness of cooked rice texture,thus affecting the volatilization rate of 2-AP. |
PDF Full Text Request