## 3 Results and Discussion  
### Analysis of VOC Components During the Ripening of Rice Grains  
The VOC profiles of rice-grain samples at different stages of ripening were analyzed using HS-SPME-GC-MS, with the total ion chromatogram presented in Figure S1. Across three rice cultivars at four stages post-flowering, 417 VOCs were identified in brown rice (Table S1). These VOCs were divided into esters (17.51%), heterocyclic compounds (17.03%), terpenoids (14.87%), hydrocarbons (11.99%), alcohols (9.35%), aldehydes (7.76%), ketones (7.19%), aromatics (5.28%), acids (1.92%), amines (1.92%), phenols (1.92%), nitrogen compounds (1.20%), sulfur compounds (0.72%), halogenated hydrocarbons (0.48%), and other compounds (0.96%) (Figure 1A). The total VOC content increased progressively as the rice grains matured, with the relative contents for YX430, MXZ2, and YJSM2 at the four stages being 53.18-62.15 \(\upmu\)g/g (10 DAF), 58.96-72.99 \(\upmu\)g/g (15 DAF), 65.74-71.78 \(\upmu\)g/g (20 DAF), and 76.46-88.60 \(\upmu\)g/g (35 DAF), respectively (Figure 1B). This indicates a general accumulation of VOCs as the rice grains matured. Furthermore, we investigated the content of VOCs in different classes. Among these volatiles, aldehyde generally had high contents in all rice samples. Aldehydes are mainly produced via lipid oxidation and decomposition as the most important volatile components in raw rice because of their relatively low odor threshold [2]. These substances contribute fresh and fruity aromas to rice grains. 2,6-Dimethyl-5-heptenal and hexanal had the highest content of aldehydes and existed in the whole ripening stages. Hexanal, with a green grass-like aroma, can be used as a marker for green/grassy aromatics [33]. The levels of hexanal indicate the freshness of tested rice cultivars [24]. In stored rice, hexanal is formed  
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. through the automatic oxidation of linoleic acid resulting in an off and stale flavor [16]. What followed were heterocyclic compounds, alcohol, terpenoids, ester, and phenol. Heterocyclic compounds were the major volatile flavor compounds produced through Maillard reactions, and they were also the second most abundant substance in the study. Furans were the heterocyclic compounds, produced from both lipid oxidation and the Maillard reaction of amino acids and sugars at high temperatures [2]. The extraction temperature of VOCs from the sample was 60 \({}^{\circ}\)C in this study; the Maillard reaction cannot be avoided during the extraction process. So, part of furans may come from the Maillard reaction. 2-Methylfuran, 2-pentylfuran, etc. play important roles in forming rice aroma [2]. 2-Pentylfuran has a characteristic nutty odor in dilute concentration; it was reported as an odor-active compound in aromatic, non-aromatic, and black rice [4; 34]. Alcohols were formed from the further breakdown of aldehydes; they made some contribution to the overall aroma of rice, and the content was second only to heterocyclic compounds. Terpenoids exist widely in plants. Almost all flower-aroma volatile components contain terpenoids, which are also characteristic aroma components of some fruits [35]. In contrast, hydrocarbons and ketones were present at lower levels, ranging from 1 \(\upmu\)g/g to 3 \(\upmu\)g/g across all samples.