Analysis of Volatile Compounds' Changes in Rice Grain at Different Ripening Stages via HS-SPME-GC-MS

Abstract

Aroma is a crucial determinant of rice taste quality, with volatile organic compounds (VOCs) playing a key role in defining this characteristic. However, limited research has explored the dynamic changes in these aromatic substances during the ripening stages of rice grains. In this study, we analyzed VOCs in rice grains across four ripening stages post-flowering using headspace solid-phase microextraction combined with gas chromatography-mass spectrometry (HS-SPME-GC-MS). A total of 417 VOCs were identified, among which 65 were determined to be key aroma-active compounds based on relative odor activity value (rOAV) analysis. Most of these aroma-active compounds exhibited an accumulation pattern as the grains matured. Notably, 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone and 2-Methyloxolan-3-one had the largest rOAV values. Additionally, (Z)-6-nonenal, (Z,Z)-3,6-nonadienal, 2-thiophenemethanethiol, 5-methyl-2-furanmethanethiol, 2,2,6-trimethyl-cyclohexanone, and 3-octen-2-one were identified as potential key markers for distinguishing rice-grain maturity stages. Moreover, 2-acetyl-1-pyrroline (2-AP), heptanal, and 1-nonanol were identified as marker metabolites differentiating aromatic from non-aromatic brown rice. These findings contribute to a deeper understanding of the dynamic variation and retention of aroma compounds during rice-grain ripening, and they offer valuable insights into the improvement of fragrant rice varieties.

Keywords: HS-SPME-GC-MS; VOCs; aromatic rice; rOAV.

Figure 1

Volatile compound detection and analysis from all rice samples of YX430, MXZ2, and YJSM2 at four stages. (A) Classification of the 417 VOCs of 12 rice-grain samples. (B) The relative amounts of VOCs from YX430, MXZ2, and YJSM2 of four stages. (C) PCA score plot of different stages using all volatile compounds. Three same-color dots represent three repetitions of each variety in the same period. (D) Cluster heatmap analysis of volatile compounds of all samples. Each sample is represented as a column, and each metabolite is displayed in a row. Red shows relatively high metabolite abundance, while green indicates relatively low abundance. I, II, III and IV refer to the Group I, Group II, Group III, and Group IV, respectively.

Figure 2

OPLS-DA score plots of YX430 (A), MXZ2 (C), and YJSM2 (E). Two hundred permutation tests of YX430 (B), MXZ2 (D), and YJSM2 (F). (G) OPLS-DA score plots of brown rice in 30 d. (H) Two hundred permutation tests of brown rice in 30 d.

Figure 3

(A) Venn diagram of different VOCs among YX430, MXZ2, and YJSM2 during grain ripening. Red means YX430, blue means MXZ2, and green means YJSM2. (B) Scatterplot of rOAV odor activity values. The horizontal coordinate represents different groups, and the vertical coordinate represents the rOAV value of VOCs. These points represent all VOCs.