## 2 Results and Discussion
### Leu/Ile-Derived VOCs  
The catabolism of Leu and Ile (branched-chain amino acids) is initiated by an aminotransferase enzyme, BcAT, that catalyses the hydrolysis of Leu and Ile to the \(\alpha\)-keto acids 4-methyl-2-oxopentanoic acid (\(\alpha\)-ketoisocaproate) and 3-methyl-2-oxopentanoic acid (\(\alpha\)-keto-\(\beta\)-methylvalerate), respectively. In many LAB, ArAT is also involved in the transamination of Leu. After the transamination reaction, \(\alpha\)-keto acids undergo the following biochemical reactions: 1. Oxidative decarboxylation of 4-methyl-2-oxopentanoic acid and 3-methyl-2-oxopentanoic acid to 3-methyl butanoic acid and 2-methyl butanoic acid, respectively (enzyme: ketoacid dehydrogenase (KaDH)); 2. Decarboxylation of 4-methyl-2-oxopentanoic acid and 3-methyl-2-oxopentanoic acid to 3-methyl butanal and 2-methyl butanal, respectively (enzyme: keto acid decarboxylase (KdcA)); and 3. Reduction of 4-methyl-2-oxopentanoic acid and 3-methyl-2-oxopentanoic acid to 2-hydroxy-4-methylpentanoic acid and 2-hydroxymethylvalerate, respectively (enzyme: hydroxy acid dehydrogenase (HycDH)). Further, 3-methyl butanal and 2-methyl butanal can be either reduced to alcoholsby AlcDH (3-methyl butanol and 2-methyl butanol, respectively) or oxidised to acids by an aldehyde dehydrogenase (AldDH) (3-methyl butanoic acid and 2-methyl butanoic acid, respectively) [34; 35; 36]. In the current study, the highest peak area for 2-methyl butanol was observed in DM supplemented with Ile (Figure 5A, DMI) at 0.1% acetate. Similarly, the highest peak area for 3-methyl butanol was obtained in DM supplemented with Leu (Figure 5B, DML) at 0.1% acetate. The addition of Leu has previously been reported to increase 3-methyl butanol production: in synthetic grape must fermented by commercial wine yeast strains [39], in synthetic cassava medium fermented by _Ceratocystis fimbriata_[43], and in papaya juice fermented by _Willipopsis saturnus_ var. _mrakii_[41]. The addition of an amino acid mixture (Val, Leu, Ile, and Phe) to soy (tofu) whey has also been reported to increase the peak area of 3-methyl butanol fermented by _Toruslospora delbruuckii_ Biodiva [40]. Further, the addition of Ile has also been reported to increase 2-methyl butanol production during the fermentation of papaya juice by _Willipopsis saturnus_ var. _mrakii_[41]. The gene _bcaT_ encodes the BcAT enzyme which initiates the transamination reaction on branched-chain amino acids, and the gene _amT_ encodes the ArAT (putative) enzyme that initiates the transamination reaction on Leu, were both absent in a genome study of _[PERSON]_ ATCC 367 [53]. In the current study, LB672 fermentation resulted in the production of 2- and 3-methyl butanol from Ile and Leu, respectively. This result suggests that LB672 possesses BeAT or ArAT enzymes, which catalysed the transamination of Ile and Leu because 2- and 3-methyl butanol could not have been produced without a transamination step. Figure 4: Normalized peak area of methanethiol (**4A**), methanethiol (**4B**), and dimethyl disulphide (**4C**) across different medium compositions (see Table 1) at 0.1% and 1.2% acetate. Values are presented as mean \(\pm\) standard error (\(n=6\)). Different letters represent significant difference between the different medium compositions according to [PERSON]’s test at \(p<0.05\). Ethanol, which is a marker compound in fermentation studies, can be produced from sugars via an intermediate acetaldehyde using the PK pathway [21], from the degradation of Thr [64], or from acetate through acetyl-CoA [65]. Overall, the ethanol peak area was higher in DM containing 1.2% acetate compared to 0.1% (Figure 6A). This may have been a result of the acetate being converted into ethanol. Interestingly, in DM supplemented with either Leu, Ile, or Phe at 0.1% acetate, the peak area for ethanol was significantly (\(p<0.05\)) lower than in other media with either 0.1 or 1.2% acetate.