## 4 Discussion  
Amino acids are important factors involved in the synthesis of fermentation-derived aroma compounds. Studies have correlated amino acid profiles with grape varieties [26]. In this study, we show differences in amino acid composition and consumption dynamics during fermentation between spine grape and Cabernet Sauvignon. _S. cerevisiae_ has a nitrogen-catabolite repression (NCR) system that suppresses alanine assimilation until ammonium gets exhausted from the media. Therefore, alanine is always consumed late [27]. Here, the start of alanine consumption was delayed by more than 3 days during Cabernet Sauvignon fermentation, while it was consumed rapidly to depletion during spine grape fermentation. Meanwhile, tyrosine was not consumed in large amounts in either system. This observation is consistent with the findings of [PERSON] et al. [28] who reported that tyrosine is not fully consumed during wine alcoholic fermentation due to the presence of aromatic amino acid biosynthetic pathway in certain _S. cerevisiae_[29]. However, [PERSON] et al. [10] showed complete consumption of tyrosine during alcoholic fermentation of four different _V. vinifera_ varieties. These contradictory results indicate that tyrosine evolution might be highly dependent on fermentation conditions. The levels of leucine, tryptophan, and methionine were not different between the two grape varieties. Alanine, isoleucine, and phenylalanine were lowest in spine grape must while their levels were still within the rational concentration range [1]. Moreover, yeast cannot assimilate proline [30], and therefore, its evolution and correlation with esters are not discussed. Fruity esters are the main aroma compounds in wine [31]. Only a few studies have so far focused on aroma compounds in spine grape wine. [PERSON] et al. analyzed the characteristic aroma components in spine grape berries instead of spine grape wine [32]. [PERSON] et al. analyzed ester content in spine grape wine without comparing it with other types of wine and found that the fruity ester content in spine grape wine was significantly  
Figure 3: Redistribution of carbon fluxes in the central metabolic network by different nitrogen treatments. PYR, pyruvate; GA3P, glyceraldehyde 3-phosphate; ACTA, acetaldehyde. Data are mean \(\pm\) standard deviation. Differences are significant at 95% (*), 99% (**)confidence level. AHA, acetate of higher alcohol; MCFA, medium-chain fatty acid; MCFAEE, medium-chain fatty acid ethyl ester; CK-SP, spine grape control; IN, DAP supplement; MAA, mixed nitrogen supplement with alanine, phenylalanine, and isoleucine; IN+MAA, mixed nitrogen supplement with DAP, alanine, phenylalanine, and isoleucine. less than that in Cabernet Sauvignon wine [20]. Fruity esters, which are the fermentative volatiles, contributed significantly to the aroma of spine grape wine. This indicates that a low level of fruity esters is responsible for the weak fruity aroma of spine grape wine. The amino acid profile of grapes and wine varied between varieties [33]. Different methods have been used to establish a model to determine the correlation between amino acids and aroma compounds. [PERSON] et al. [8] established a fingerprint of the amino acid and aroma compound profiles by partial least-squares regression (PLS) analysis, and [PERSON] et al. [7] established a linear model between amino acids and volatile aroma components of synthetic juice. In this study, [PERSON]'s correlation was used to determine the association between amino acid profile and fruity ester content during spine grape and Cabernet Sauvignon grape fermentation. The analysis revealed that alanine, phenylalanine, and isoleucine at low levels and with low correlation coefficients may have led to limited fruity ester production during spine grape fermentation, which is in accordance with our previous study that the low content of specific amino acids could negatively affect ester productions during alcoholic fermentation [22].