## 2 Results and Discussion
### Volatile Organic Compounds  
A total of 33 volatile organic compounds (VOCs) were identified in the beer samples from the five sound treatments (Table 1). Among the VOCs identified, there were twelve esters, nine alcohols, three acids, three aldehydes, and six were hop-derived compounds. For the higher alcohols (HAs), the application of some of the sound treatments tended to decrease their synthesis, specifically for 2-methyl-1-propanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 1-hexanol, and phenylethyl alcohol relative to the silence condition at each fermentation time point. For example, phenylethyl alcohol had a significantly higher (\(p<0.05\)) abundance (higher peak area) in the silence control relative to treated samples at 48 (HF_LI, LF_HI, LF_LI), 72 (HF_LI, LF_LI), 120 (LF_LI), and 144 h (LF_HI, LF_LI) (Figure 4a; Table 1). These results suggest that sound treatment could potentially be utilized to reduce the concentration of phenylethyl alcohol. In addition, at 48 h, HF_LI had a significantly lower (\(p<0.05\)) abundance of 1-heptanol (Figure 4b), relative to the silence control. Likewise, HF_LI-treated samples had a significant (\(p<0.05\)) reduction in abundance of 1-heptanol at 72 h. At 144 h, HF_LI and the silence control had a higher abundance of 2-methyl-1-butanol compared to the rest of the treatments (i.e., HF_HI, LF_HF, LF_LI). However, the abundance of 2-methyl-1-butanol and 1-hexanol converged at the end of the fermentation (144 h), thus resulting in no differences in their abundance being detected in the final beer (\(p>0.05\)). It has been previously reported that the predominant HAs in beer are 3-methyl-1-butanol (60-80%), 2-methyl-1-propanol (15-25%), and 1-propanol (4-7%), which are formed as byproducts during biosynthesis of amino acids [23; 24; 25]. Amino acid biosynthesis has previously been reported to be upregulated as a result of sound treatments (music and low-frequency sound (100 Hz, 92 dB @ 20 uPa) [3]. The decrease in HAs synthesis at certain sampling times during fermentation observed in this study may be due to inhibition of aminotransferases, pyruvate decarboxylalases (_pdc1, pdc5_, and _pdc6_), and alcohol dehydrogenase (_Adh1, Adh2, Adh3, Adh4_, and _Adh5_ or _Sfa1_) [26; 27]. Despite the yeast growth and fermentation rate not being significantly altered, subtle differences for some yeast-derived esters were observed at certain times during fermentation. The abundance for isoamyl acetate (Figure 5a) at 24 h for HF_LI was significantly higher (\(p<0.05\)) than for silence by 14%. At 144 h, significant differences in abundance between some treatments for ethyl hexanoate, ethyl octanoate, and phenylethyl acetate were observed. Specifically, the abundance for ethyl hexanoate was significantly (\(p=0.001\)) higher for the LF_LI-treated ferment compared to the silence control but not for the rest of the treatments at 144 h (Figure 5b). HF_HI, LF_LI, and HF_LI exhibited a higher abundance of ethyl octanoate compared to LF_HI and silence at 144 h. For phenethyl acetate, its abundance in the HF_LI-treated ferment was significantly higher (\(p<0.05\)) than for LF_HI at 144 h but not HF_HI, LF_LI, or silence. Despite these differences, there were little consistent effects of sound treatment on the abundance of esters in the experimental samples under the current conditions. Volatile esters impart beer with fruits, candy, and perfume-like flavor characters [28; 29]. Acyl-coenzyme A and acetyltransferase catalyze the synthesis of esters. It has previously been reported that ultrasound and cavitational implosion can alter monomeric and polymeric enzymes in yeast [4]. Therefore, the difference observed in the abundance of isoamyl acetate and ethyl hexanoate may be due to differences in acetyltransferase enzymatic activity triggered by the sound treatments. However, it has also been reported that hydrostatic pressure, and the amount of nitrogen and glucose in worn can alter ester synthesis [30; 31].