## 3 Results and Discussion
### Higher Alcohol and Ester Analysis  
Three concurrent trials of Gravenstein apple juice were fermented to a finished cider product and monitored for changes in VOC concentrations over the course of 135 days. The chemical composition of the headspace of these ciders changed dramatically over this time frame as a result of the changing byproducts of yeast metabolism. In total, 89 VOCs were identified by HS-SPME-GC-MS throughout fermentation and maturation: 11 higher alcohols, 7 acetate esters, 10 ethyl esters, 32 other esters, 5 aldehydes, 8 ketones, 5 volatile acids, and 11 terpen. The details of these detections are included in Supplementary Table S1. Five esters and four higher alcohols that were consistently measured above the LOQ throughout fermentation have published odor perception thresholds [14,23] that were used to display the long running trends of ester and higher alcohol development throughout fermentation and maturation in both Table 1 and Figure 4.  
A comparison of these esters and alcohols is provided in Table 1a,b in order of their chromatographic retention times (RT), along with their published odor descriptors and per  
Figure 4: HS-SPME-GCMS measurements of headspace ester (**A**) and higher alcohol (**B**) concentrations that represent the overall trends observed from juice (day 0) to mature cider (day 135). Day 25 (vertical black line) is the date of cold crashing and racking the cider samples. Error bars and are reported as \(\overline{x}\pm 1\) s of triplicate samples. Figure 3: Specific gravity (**blue**) and pH (**green**) are shown for the entire fermentation and maturation process. On the left y-axis, the average specific gravity of three Gravenstein apple cider fermentations is plotted against time. The right y-axis shows pH measurements for the fermentations. Day 0 corresponds to measurements taken of pre-inoculated apple juice in triplicate and Day 135 represents the end of cider maturation (prior to dry hopping). ception thresholds (PT). Concentrations of four alcohols (1-butanol, 1-propanol, 1-hexanol, and 2-phenethylethanol) and five esters (ethyl butanoate, ethyl hexanoate, hexyl acetate, ethyl octanoate, and ethyl decanoate) are shown for the 22-day period before racking (Table 1a) and the period after racking until the end of maturation (Table 1b). Prior to day 12, higher alcohols, primarily originating from the juice, dominated the volatile composition of the ciders. By day 12 of fermentation, esters had risen significantly in concentration. In fact, from juice to cider (135 days post inoculation), total ester concentration increased by a factor of 71, whereas the total higher alcohol concentration increased by only a factor of 3. This agrees with the literature, as the esterification of higher alcohols efficiently occurs in fermentations with _saccharomyces cerevisiae_[24]. A practical insight for cider makers comes from a direct comparison from day 22 (pre-racking) to day 28 (post-racking). The total concentration of higher alcohols increased by 14 \(\pm\) 1%, and the total concentration of esters decreased by 10 \(\pm\) 1% across this 6-day period. Previous research indicates that esters (including ethyl hexanoate and ethyl octanoate) are influenced by the time of contact with lees [25], but this is the first time to our knowledge that a quantifiable decrease in ester concentration has been linked directly to a racking event in cider making. Similarly, the simultaneous increase in higher alcohol concentration after racking highlights the importance of this process in tuning the cider aroma. As Table 1 shows, many of the major higher alcohols contribute floral green notes, whereas the esters are primarily perceived as fruity. These data suggest that in the days immediately following a racking event, the cider aroma may lose fruity notes. If allowed to rest on the lees for longer time periods without racking, the fruity esters have a chance to grow in.