Using SPME-GC/REMPI-TOFMS to Measure the Volatile Odor-Active Compounds in Freshly Cooked Rice

Analysis of rice characteristic volatiles and their influence on rice aroma

Rice aroma, one of the most important qualities of rice, was the comprehensive result of volatiles in rice and human sense. In this study, the main volatile compounds in rice were analyzed by using gas chromatography-mass spectrometry and gas chromatography-olfactometry, and their correlations with sensory score were investigated. A total of eighty-five volatiles were found in rice samples. By combining odor activity value and correlation analysis, nine volatiles were considered as potential characteristic volatiles in rice aroma, namely hexanal, 2-pentylfuran, octanal, 2-acetyl-1-pyrroline (2-AP), 1-octen-3-ol, trans-2-octenal, decanal, trans-2-nonenal and trans, trans-2,4-decadienal. It was found that the volatiles negatively correlated with sensory scores were positively correlated with hexanal. It indicated that hexanal might be a representative of the negative volatiles of rice aroma. The effects of the nine potential characteristic volatiles on rice aroma were investigated by using sensory analysis. The results showed that the odor intensity and preference level of 2-AP, hexanal, and 1-octen-3-ol were significantly affected by the content. Furthermore, the aroma of cooked rice was significantly different after adding 2-AP, hexanal or trans, trans-2,4-decadienal. Rice aroma was increased by adding 2-AP and deteriorated by adding hexanal or trans, trans-2,4-decadienal, indicating that 2-AP contributed positively to rice aroma while hexanal and trans, trans-2,4-decadienal contributed negatively to rice aroma. Hexanal, 2-AP, and trans, trans-2,4-decadienal were suggested to be the key characteristic volatiles for future aroma evaluation.

Comparative Metabolome Profiling for Revealing the Effects of Different Cooking Methods on Glutinous Rice Longjing57 (Oryza sativa L. var. Glutinosa)

Glutinous rice (GR), an important food crop in Asia, provides prolonged energy for the human body due to its high amylopectin content. The non-volatile metabolites generated by different cooking methods that affect the nutritional value and color of GR are still poorly understood. Herein, a widely targeted metabolomics approach was used to understand the effects of different cooking methods (steaming, baking, and frying) on the metabolite profiles of GR. Compared with other treatments, steamed GR had a brighter color and significantly lower contents of total sugar, starch, amylopectin, and amylose, at 40.74%, 14.13%, 9.78%, and 15.18%, respectively. Additionally, 70, 108, and 115 metabolites were significantly altered in the steaming, baking, and frying groups respectively, and amino acid and carbohydrate metabolism were identified as the representative metabolic pathways based on KEGG annotations. Further evaluation of 14 amino acids and 12 carbohydrates in steamed GR, especially 4-aminobutyric acid, suggested its high nutraceutical value. Additionally, multivariate analysis indicated that total sugar content, amylose content, beta-alanine methyl ester hydrochloride, and 4-aminobutyric acid played a critical role in color formation in raw and cooked GR. Finally, the levels of major amino acids and carbohydrates were quantified by conventional methods to verify the reliability of the metabolome. Consequently, this in-depth understanding of metabolite profiling in normal cooking methods has provided a foundation for the processing of GR products.

Absolute contents of aroma-affecting volatiles in cooked rice determined by one-step rice cooking and volatile extraction coupled with standard-addition calibration using HS-SPME/GC-MS

To quantify volatiles in cooked rice, analysis methods for one-step rice cooking and volatile extraction in a single headspace vial, combined with standard addition calibration using solid-phase microextraction and GC-MS were developed and applied to 41 rice varieties with various fragrances and palatability. The newly developed methods significantly improved the qualitative and quantitative recovery of volatiles compared with conventional methods. Among 29 aroma-affecting volatiles, the highest average contents (ng/g) were observed for nonanal (39.30), octanal (13.29), and 1-octen-3-ol (13.18); the total volatile contents of aldehyde, base, and alcohol groups were 4156, 2481, and 1739 ng/g, respectively. Fifteen rice varieties contained 2-acetyl-1-pyrroline in range of 41.37-421.70 ng/g. Although there were no linear correlations among volatiles and the Toyo taste-score, multivariate PLS-DA analysis of the volatile could discriminate between low- and high-palatability rice varieties. The results indicated the accuracy and practicality of the newly developed methods for quantifying volatiles in cooked rice.

Characterization of Cooked Nonglutinous Rice Cultivars Based on Flavor Volatiles and Their Change during Storage

Characterizing fleshly cooked rice cultivars according to the volatile aroma compounds helps consumers select a favorite and is useful for the development of new cultivars that will have a pleasant aroma. In the present study, six Japanese nonglutinous cultivars, which were freshly harvested in 2021, were characterized based on their flavor volatiles after being freshly cooked. In order to extract the volatile compounds just after cooking, the vaporized compounds were extracted for 5 min using a solid-phase microextraction (SPME) fiber and were measured via gas chromatography/mass spectrometry (GC/MS). Multiple comparison tests statistically detected four volatile aroma compounds: 2-pentylfuran, nonanal, 4-vinylphenol, and indole. From among the six rice cultivars tested, the proportions of the latter two compounds showed significant differences, and in principal component analysis of cooked rice, these two best characterized freshly harvested and freshly cooked Japanese nonglutinous rice cultivars; indole was indicative of Nipponbare, and 4-vinylphenol was indicative of Koshihikari and Ichihomare. In the present study, changes in the volatile aroma compounds of the freshly cooked rice cultivars were found to slightly differentiate according to storage times: 2-pentylfuran tended to increase, nonanal first increased and then decreased, and 4-vinylphenol and indole either remained almost unchanged or were only slightly decreased during storage. Therefore, establishing the differences in rice cultivar types revealed that the characteristics of the flavor volatiles of freshly cooked rice after long-term storage significantly depend on how the rice cultivar is stored.

Identification of volatile organic compounds related to the eating quality of cooked japonica rice

Eating quality (EQ) of rice has a complex nature composed of physicochemical properties. Nevertheless, breeding programs evaluating EQ through sensory test or taste-evaluation instruments have been laborious, time-consuming and inefficient. EQ is affected by both taste and aroma. However, in actual breeding programs, aroma of cooked rice has been considered the least due to lack of information. Here we identified a total of 41 volatile compounds potentially affecting the EQ of non-aromatic, cooked japonica rice, identified by GC-MS, sensory panel test, and Toyo taste-meter analyses. Partial least squares discriminant analysis demonstrated an outstanding classification effect of the identified volatile compounds on eating-quality discrimination. Several volatile compounds related to lipid oxidation and fatty acid degradation were identified to affect the EQ in japonica rice. Of them, 1-octen-3-ol, 1-ethyl-3,5-dimethylbenzene, 2,6,11-trimethyldodecane, 3-ethyloctane, 2,7,10-trimethyldodecane, methyl salicylate, 2-octanone, and heptanal were selected as important compounds. The discriminant model for the classification of the quality of cultivars was robust and accurate, an r-squared value was 0.91, a q squared value was 0.85, and an accuracy was 1.0. Overall, the results of this study characterize EQ of rice cultivars based on volatile compounds, suggesting the application of metabolite profiling data for rice breeding of high eating quality.

Absolute Quantitative Volatile Measurement from Fresh Tea Leaves and the Derived Teas Revealed Contributions of Postharvest Synthesis of Endogenous Volatiles for the Aroma Quality of Made Teas

Characteristic aroma is a well-appreciated feature contributing to tea quality. Although extensive studies have been made to investigate aroma biosynthesis and gene expressions during tea making processes, it remains unclear whether the endogenous volatile biosynthesis during postharvest tea processing contributes to the aroma quality of made tea. To critically evaluate this question, we used the same batch of fresh tea leaves and produced three different types of tea with different degrees of fermentation (green tea, oolong tea, and black tea). Total volatiles were extracted by solvent-assisted-flavor evaporation, then quantified by gas chromatography-flame ionization detector combined with response factor correction for quantitative measurement. Compared with fresh tea leaves, the volatile profiles of the made teas were dramatically altered, with significant loss for the majority of endogenous volatiles and simultaneous gain for non-endogenous volatiles. By calculation of odor-activity values, the potential volatiles contributing to the aroma characteristics of each tea type were identified. Our data suggest that postharvest synthesis of endogenous volatiles did not contribute to the aroma quality of made tea.