## 3 Results and Discussion
### Effect of Nitrogen Fertilizer Reduction on Volatile Metabolomics of Suijing 18
#### 3.2.9. Flavoromics Analysis of Differential Volatile Metabolites  
We carried out a sensory analysis of differential volatile metabolites to examine the sensory flavor profiles in the samples. We focused on annotating the 10 most dominant sensory flavor features with differential volatile metabolites before identifying those with the highest number of annotations [37]. Our results indicated that the differential volatile metabolites of A1, A2, and A3 largely affected the sweetness of the rice's overall flavor (Figure 10). Various differential volatile metabolites were proven to provide sweetness, including (z)-3,7-dimethylylocta-2,6-dienal, allyl benzoate, 2-methylnaphthalene, 4-methyl-benzeneacetaldehyde, and hexyl acetate. We found sweetness to be the most important flavor characteristic of Suijing 18; the compounds allyl benzoate and 4-methyl-benzeneacetaldehyde have the most significant effect on this quality. The other most influential aromas (in ascending order) on the flavor of Suijing 18 are the grassy aroma, floral aroma, and fruity aroma. The role of 2-methylnaphthalene in the floral aroma cannot be ignored. Grassy aroma also has a significant influence on the flavor of Suijing 18 and is largely provided by trans-pipertol and 1-methoxycyclohexene. The other flavor profiles of Suijing 18 include caramel, camphor, berry, banana, and apple flavors. As shown in Figure 11, we found that hexyl acetate had a significant effect not only on sweetness but also on the apple, banana, and grassy aromas. Allyl benzoate significantly contributed to sweetness, the berry flavor, and floral aroma. Therefore, we concluded that as the amount of nitrogen fertilizer applied was reduced, the levels of allyl benzoate and 4-methyl-phenylacetaldehyde in Suijing 18 gradually increased, and the sweetness of the rice also increased. This may be related to the increase in the starch content of Suijing 18, which was caused by the reduction in the nitrogen fertilizer applied.  
The volatile compounds released from the rice depended not only on the amounts of these molecules in rice, but also on their interaction with other ingredients, which then affected the capability of the release. The effect of nitrogen reduction on other components should be considered in this study, especially as it pertains to proteins. As complex systems with the presence of several components (such as hydrocolloids and active compounds), the aggregation of proteins will always vary between foods. [PERSON] et al. [38] concluded that two pathways largely affect the aggregation of proteins. One is that of protein conformation, which is affected by factors such as temperature, mechanical forces, pressure, etc. [39; 40]. The other is that of driving forces, such as pH, protein source, etc., and their interactions [41]. In the future, our team will conduct in-depth research on the impact of protein changes caused by reduced nitrogen fertilizer on the quality of Suijing 18.  
Figure 9. Effect of reduced nitrogen fertilizer on the differential metabolite pathway of Suijing 18 (A3 compared with A1).  
Figure 11: Plot of the sensory flavor profile and differential metabolite association network of different nitrogen fertilizer treatment groups.  
Figure 10: Effect of reduced nitrogen fertilizer on the differential metabolite flavor of Suijing 18.