## 3 Results and Discussion
### Cider VOC Development throughout the Fermentation Process
Seven esters (ethyl-3-methylbutyrate, ethyl octanoate, ethyl decanoate, ethyl dodecanoate, ethyl dodecanoate, ethyl-9-decenoate, 3-methylbutyl acetate, and 3-methylbutyl octanoate) and one higher alcohol (2-phenylethyl alcohol) were identified in the final cider, but they were not detected at all in the headspace of the original apple juice. Additionally, three esters (propyl butyrate, butyl-2-methylbutyrate, and hexyl-2-methylbutanoate) and estragole were present in the apple juice but entirely absent from the final cider headspace. This is because most cider esters are generated through the esterification of alcohols with fatty acids during fermentation [4]. Several species persisted from the original apple juice to the final cider aroma profile but changed in abundance significantly as a direct result of fermentation. Those species that increased in abundance significantly (>50%) throughout fermentation were 2-methyl-1-butanol (+444.3 \(\pm\) 97.8%), ethyl hexanoate (+436.6 \(\pm\) 96.1%), and ethyl acetate (+62.1 \(\pm\) 13.7%), which are well known fermentation by-products. Those species that decreased significantly (>50%) were as follows: 1-hexanol (\(-84.6\pm 18.6\%\)), 2-methylpropyl butanoate (\(-84.5\pm 18.6\%\)), 2-methylbutyl acetate (\(-79.7\pm 17.5\%\)), butyl acetate (\(-77.1\pm 16.9\%\)), ethyl butyrate (\(-77.1\pm 17.0\%\)), ethyl propionate (\(-58.2\pm 12.8\%\)), and ethyl-2-methyl butyrate (\(-56.3\pm 12.4\%\)). Based on these data, the metabolic by-products of yeast fermentation dominated the finished cider  
Figure 4: A hydrometer was used to measure specific gravity throughout a cider fermentation to monitor fermentation progression (**A**). Specific gravity measurements were taken before yeast inoculation (**red**), at various stages during fermentation (**purple, black, green**), and when fermentation was complete (**blue**). Four chromatograms were obtained by headspace solid phase microextraction gas chromatography–mass spectrometry (HS-SPME GC-MS), with a DVB/CAR/PDMS fiber, in order to compare changes in aroma profiles based on volatile constituents (**B**). All chromatographic peaks are numbered in order of their observation throughout the fermentation process. Peaks one through fifteen (**1**: ethyl acetate, **2**: ethyl propionate, **3**: 2-methyl-1-butanol, **4**: ethyl butyrate, **5**: butyl acetate, **6**: ethyl-2-methyl butyrate, **7**: 1-hexanol, **8**: 2-methyl butyl acetate, **9**: propyl butyrate, **10**: 2-methylpropyl butyrate, **11**: ethyl hexanoate, **12**: hexyl acetate, **13**: butyl 2-methylbutyrate, **14**: estragole, and **15**: hexyl 2-methylbutanoate) were detected in the juice sample (**red**). Peaks one through eight, ten through twelve, and fourteen through twenty-two (**16**: ethyl 3-methylbutyrate, **17**: 3-methylbutyl acetate, **18**: 2-phenylethyl alcohol, **19**: ethyl octanoate, **20**: ethyl decanoate, **21**: 3-methylbutyl octanoate, and **22**: ethyl dodecanoate) were identified in samples taken on day three (**purple**), day eight (**green**), and at the completion of fermentation (**blue**). Only one species (**23**: ethyl-9-decenoate) was detected on day 13 (fermentation completion) only. chromatogram when compared to the original apple juice VOCs. Therefore, our next investigation focused specifically on several previously unexplored yeast strains and their fermentation metabolites after a six-month maturation using the same apple juice, under equal conditions.