Evolution of volatiles and quality of Chinese steamed bread during storage at different temperatures.

The aim of this work is to investigate the evolution of volatiles and quality of CSB during 4 d of storage at 4 °C and 25 °C, respectively. Rapidly increasing hardness and decreasing resilience were observed in CSB after 1 d of storage at 4 °C. However, relative soft CSB was found after 1 d of storage at 25 °C as a result of the lower rate of retrogradation. Volatiles were monitored by gas chromatography-mass spectrometer. Significant (P < 0.05) decrease of 4 esters and 2-pentylfuran were observed with prolonged storage time for CSB stored at both 4 °C and 25 °C. PCA analysis indicated that the storage temperature of 4 °C was beneficial to remain CSB volatiles during long storage time (2-4 d). These findings might be beneficial to retain more volatiles and quality and finally extend shelf-life of CSB.

Effect of optimal-water boiling cooking on the volatile compounds in 26 Japonica rice varieties from China.

The aim of this study was to scientifically investigate the impact of optimal-water boiling cooking on the volatile profile of 26 japonica rice varieties. A modified direct solvent extraction combined with gas chromatography-mass spectrometry was applied to analyze the volatiles in raw and cooked samples. 2-Acetyl-1-pyrroline (2AP) was only detected in aromatic varieties and decreased after cooking (-81.94% - -43.97%). Great losses of esters and long-chain ketones were exhibited after cooking. Cooking accelerated lipid oxidation and degradation of phenolic acids, thus volatiles originate from lipid oxidation and several benzenoid compounds showed increase trends in majority of the samples. Increases of 3 saturated aldehydes including hexanal (48.01% - 306.02%), octanal and nonanal (67.03% - 544.15%) could be observed in all samples. Acetophenone, 2-methoxy-4-vinylphenol (11.95% - 297.61%) and vanillin (14.29% - 319.25%) were also enhanced upon cooking. Correlations existed in volatiles with common precursors or involved in same pathways. According to multivariate and univariate statistical analysis, 18 volatiles most greatly influenced by cooking were selected. This study will facilitate the aroma improvement of cooked rice in the future.

A comparative HS-SPME/GC-MS-based metabolomics approach for discriminating selected japonica rice varieties from different regions of China in raw and cooked form.

Japonica rice is widely planted in different regions of China. Rice of different geographical origins may have substantially different economic values. In this study, An untargeted metabolomics based approach using headspace solid-phase microextraction coupled to gas chromatography-mass spectrometry (HS-SPME/GC-MS) was applied to distinguish 27 japonica rice varieties originated from South, Northern and Northeastern China in raw and cooked form, respectively. Orthogonal partial least-squares discriminant analysis (OPLS-DA) models exhibited good geographic discrimination. Sixteen and twenty-two volatiles were selected as the discriminant markers in raw and cooked rice, respectively. However, only hexanal, 3,5-octadien-2-one and 2-butyl-2-octenal were selected both in raw and cooked rice. Markers in raw rice mainly involved in terpenes, lipoxygenases, indole, and shikimate and benzoic acid pathways. Markers in cooked rice were mainly derived from lipid oxidation. The results provided a deeper understanding of volatiles variation of rice in China from different geographic origins.

Effect of Na2CO3 on quality and volatile compounds of steamed bread fermented with yeast or sourdough.

The effects of Na2CO3 on the quality, change of protein subunits and volatile compounds of sourdough leavened Chinese steamed bread (sourdough-CSB) and yeast leavened CSB (yeast-CSB) were investigated. Results suggested that, low Na2CO3 level endowed both CSB with softer crumb and little change of surface color. Besides, Na2CO3 addition improved the overall aroma profile by inhibiting the production of aroma-negative compounds (butanoic acid, 1-octen-3-ol, hexanal and heptanal). Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis showed an obvious increase in intensity of protein bands with low molecular weight, consistent with the result of size-exclusion high-performance liquid chromatography analysis and free sulfhydryl group (SH) content, indicating the hydrolysis of glutenin macropolymer (GMP) under alkaline condition in yeast-CSB. While in sourdough-CSB, GMP and SH content firstly decreased at low Na2CO3 level (0-0.2%) and then increased at high Na2CO3 level (0.3%-0.5%).

Characterization of lipid oxidation process of beef during repeated freeze-thaw by electron spin resonance technology and Raman spectroscopy.

In this study, electron spin resonance (ESR) and Raman spectroscopy were applied to characterize lipid oxidation of beef during repeated freeze-thaw (RFT). Besides the conventional indexes including peroxide values (PV), thiobarbituric acid-reactive substances (TBARS) and acid values (AV) were evaluated, the radical and molecular structure changes were also measured by ESR and Raman spectroscopy. The results showed that PV, TBARS and AV were increased (P<0.05) after RFT. This suggested that lipid oxidation was occurred during RFT. With the increase of radical signal intensity, lower oxidation stability was presented by ESR. Raman intensity of ν(CC) stretching region (1655cm-1) was decreased during RFT. Furthermore, lower Raman intensity ratio of I1655/I1442, I1655/I1745 that determine total unsaturation was also observed. Significant correlations (p<0.01) were obtained among conventional methods, ESR and Raman spectroscopy. Our result has proved that ESR and Raman spectroscopy showed great potential in characterizing lipid oxidation process of beef during RFT.