## 2 Results and Discussion
### Volatile Organic Compounds
The application of high (10 kHz, 90 dB @ 20 uPa) and low (100 Hz, 90 dB @ 20 uPa) audible sound to yeast significantly decreased the production of ethyl octanoate compared to the silence control [6]. Therefore, it is also possible that the changes in cell numbers in suspension and/or glucose utilization may underlie the differences in the observed levels of esters in our fermentThe three organic acids identified in the current study, 2-methylpropanoic acid, hexanoic acid, and octanoic acid, showed no significant (\(p>0.05\)) effects on their abundance during fermentation (Table A1) as a result of sound treatment. The lack of a significant impact of sound on organic acid production does not appear to result from an inability to detect organic acids, as increases in the abundance of hexanoic acid were detected over the course of the fermentation. Figure 4: Abundance of higher alcohols during fermentation for five sound treatments including a silent control. (**a**) Phenylethyl alcohol (A32); (**b**) 1-heptanol (A15). Low-frequency and low-intensity (LF_LI); low-frequency and high-intensity (LF_HI); high-frequency and low-intensity (HF_LI); high-frequency and high-intensity (HF_HI); silence (S); word (W). The results shown are means \(\pm\) standard deviations of six measurements (3 biological fermentation replicates \(\times\) 2 analytical replicates). * Significant difference observed (Table A1). Citronellol, linalool, 1,2-dihydrolinalool, geraniol, and 2-methylbutyl isobutyrate are hop-derived compounds that were identified in the present study. Citronellol (Figure 6) increased over time in all treatment samples, likely due to yeast biotransformation reactions, as previously reported [32; 33], as did 1,2-dihydrolinalool (Table A1). It has previously been reported that higher audible sound (10 kHz, 90 dB @ 20 uPa) enhanced (by 7.8-fold) the production of limonene by yeast compared to the silence control [6]. In contrast, the abundance of 2-methylbutyl isobutyrate, linalool, and geraniol decreased as fermentation progressed in this study. The decreases in the abundance of these hop-derived compounds may result from stripping effects of CO\({}_{2}\) during fermentation or from biotransformation  
Figure 5: Abundance of esters during fermentation for five sound treatments, including a silent control. (**a**) Isoamyl acetate (A6); (**b**) ethyl hexanoate (A10). Low-frequency and low-intensity (LF\({}_{-}\)LI); low-frequency and high-intensity (LF\({}_{-}\)HI); high-frequency and low-intensity (HF\({}_{-}\)LI); high-frequency and high-intensity (HF\({}_{-}\)HI); silence (S); sort (W). The results shown are means \(\pm\) standard deviations of six measurements (3 biological fermentation replicates \(\times\) 2 analytical replicates). Total Ion Chromatogram (TIC). * Significant difference observed among treatments (Table A1). reactions by yeast [32; 33]. However, no consistent effects of sound treatment were observed for any of these hop-derived compounds. Principal component analysis (PCA) was performed to visualize the relationships between treatments, fermentation time and the VOCs identified (Figure 7). The first two principal components, PC1 and PC2, accounted for 75.68% of the total variability. Fermentation time (h) dominated the explained variance and explained the separation on PC1, where fermions at 24 h (on left) were separated from fermions at 144 and 120 h (on right), specifically S_120, S_144, HF_LI_144, and LF_LI_144. On PC1, the majority of VOCs had high positive loadings and contributed more to the separation of samples on PC1 than the VOC with negative loadings. The compounds that were most positively associated with fermentation time at S_120, S_144, HF_LI_144, and LF_LI_144 on PC1 were 3-methyl butyl octanoate, phenylethyl acetate, ethyl acetate, ethyl octanoate, ethyl 9-decenoate, ethyl, and 1-heptanol. Some yeast metabolites are produced and accumulate during fermentation.