Cooking is an important process before rice is consumed and constitutes the key process for rice flavor formation. In this paper, dynamic changes in aroma- and sweetness-related compounds were tracked during the entire cooking process (including washing with water, presoaking, and hydrothermal cooking). The volatiles, fatty acids, and soluble sugars in raw rice, washed rice, presoaked rice, and cooked rice were compared. After being washed with water, the total volatiles decreased while aldehydes and unsaturated fatty acids increased. Meanwhile, oligosaccharides decreased and monosaccharides increased. The changes in fatty acids and soluble sugars caused by the presoaking process were similar to those in the water-washing process. However, different changes were observed for volatiles, especially aldehydes and ketone. After hydrothermal cooking, furans, aldehydes, alcohols, and esters increased while hydrocarbons and aromatics decreased. Moreover, all fatty acids increased; among these, oleic acids and linoleic acid increased most. Unlike with washing and presoaking, all soluble sugars except fructose increased after hydrothermal cooking. Principal component analysis showed that cooked rice possessed a volatile profile that was quite different from that of uncooked rice, while washed rice and presoaked rice possessed similar volatile profiles. These results indicated that hydrothermal cooking is the pivotal process for rice flavor formation.

It is important to clarify the effects of starch fine structure and protein components on the eating quality of indica rice. In this study, seven indica rice varieties with similar apparent amylose content (AAC) and protein content (PC) but different sensory taste values were selected and compared systematically. It was found that except for AAC and PC, these varieties showed significant differences in starch molecular structure and protein components. Compared with rice varieties with a low sensory taste value, varieties with a higher sensory taste value showed significantly lower amylose and higher amylopectin short chains (degree of polymerization 6–12) content; the protein component showed that the varieties with good taste value had higher albumin and lower globulin and glutelin content (p < 0.05). Rice varieties with lower AC, globulin, and glutelin content, as well as a higher content of albumin and amylopectin short chains, resulted in a higher swelling factor, peak viscosity, breakdown value, and ratio of hardness to stickiness, in which condition cooked rice showed a higher sensory taste value. Moreover, this study indicated that rice varieties with a higher content of albumin and amylopectin short chains were conducive to the good appearance of cooked rice. This study lays the foundation for the taste evaluation of good-tasting indica rice.

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.

A method was developed for the determination of 17 cytokinins in rice by solid phase extraction and liquid chromatography-tandem mass spectrometry. The rice samples were cryogenically ground under liquid nitrogen and extracted with methanol-water (80:20, v/v) at 4 °C for 16 h. The supernatant was purified on a column packed with polymer cation exchange resin (PCX). The samples were transported and eluted on an analytical column ZORBAX Extend-C18 (100 mm x 2.1 mm, 1.8 μm) by methanol and 5 mmol/L ammonium formate aqueous solution. All the analytes were detected in selected reaction monitoring (SRM) mode under positive electrospray ionization (ESI+) and quantified by external standard method. The separation conditions were optimized in order to achieve the sufficient separation for the several isomers of cytokinins, such as trans-zeatin-7-glucoside (tZ7G), trans-zeatin-9-glucoside (tZ9G), and trans-zeatin-O-glucoside (tZOG). Moreover, the extraction efficiency of different extraction solvents and clean-up effects of PCX cartridge for each analyte were further investigated. The results showed that the correlation coefficients were not less than 0.9984 in the linear range, and the limits of detection were ranged from 0.01 to 0.05 ng/g. The mean recoveries of the 17 cytokinins at three spiked levels of 0.2, 1 and 5 ng/g were from 60.2% to 125.4% with the relative standard deviations (RSDs) of 5.4%-29.7% (n = 6). Finally, five endogenous cytokinins were successfully quantified in real sample, and their contents were between 0.02 and 0.93 ng/g. It means that this method is reliable for quantitative analysis of cytokinins in rice.

A method was developed for the simultaneous determination of glyphosate (Gly) and aminomethylphosphonic acid (AMPA) residues in rice using hydrophilic interaction chromatography-tandem mass spectrometry (HILIC-MS/MS). The samples were extracted with water, and purified using a C18 solid phase extraction cartridge followed by an ultrafiltration membrane to remove interfering materials. Gly and AMPA were separated on an HILIC column with the mobile phases of 1 mmol/L ammonium acetate (pH 11.0 adjusted with ammonium hydroxide) and acetonitrile, and finally detected with negative electrospray ionization (ESI-) in multiple reaction monitoring (MRM) mode. The matrix-matched external standard calibration curves were used for quantitative analysis. The sample pretreatment of Gly and its metabolite was successfully carried out without any derivatization. Under the optimal analytical conditions, the linearities of Gly and AMPA were in the concentration ranges of 0. 001 to 0. 250 mg/L and 0. 002 5 to 0. 250 mg/L respectively, with the correlation coefficients of 0. 999 5 for the both analytes. The limits of detection (S/N = 3) of the method were 0.010 mg/kg for Gly, and 0.020 mg/kg for AMPA. For all the samples, the mean spiked recoveries of Gly and AMPA were in the range from 96.3% to 107.3% at 3 spiked levels (0.100, 0.500 and 2.500 mg/kg), and the relative standard deviations (RSDs, n = 3) were in the range of 1.3%-9.1%. The method is easy, fast, sensitive and accurate, and can meet the requirements of the determination of Gly and AMPA pesticide residues in rice.

A method was developed for the simultaneous determination of mepanipyrim, silthiofam, boscalid, fluopicolide, mandipropamid, cyflufenamid in vegetables and fruits by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The analytes were extracted from the samples by acetonitrile and purified by Florisil SPE. The six novel amide fungicides were separated on a Poroshell 120 EC-C18 column with the mobile phases of water and acetonitrile, and finally detected by MS/MS with positive electrospray ionization (ESI+) in multiple reaction monitoring (MRM) mode, quantified by external standard method. Under the optimal analytical conditions, the correlation coefficients (r) of the six novel amide fungicides were not lower than 0. 999 0 in the concentration range from 0.5 to 100 microg/L. The limits of detection (S/N > or = 3) of the method were 0.15 microg/kg for boscalid, silthiofam, mandipropamid, cyflufenamid, 0.10 microg/kg for mepanipyrim and 0.17 microg/kg for fluopicolide. The recovery tests were performed for the 7 types of vegetables and the 3 types of fruits at the spiked levels of 0.5, 5 and 50 microg/kg, and the recoveries of the six analytes ranged from 65% to 124% with the relative standard deviations (RSDs, n = 5) of 1%-18%. The matrix effects in vegetables and fruits of the six amide fungicides were significantly reduced by the purification of Florisil SPE compared with the modified QuEChERS. The method is easy, fast, sensitive and accurate, and can meet the requirements of the determination of the six amide fungicide residues in vegetables and fruits.