## 4 Conclusions  
Cider quality is closely aligned with the smell of volatile molecules produced during fermentation, but the evolution of cider fragrance throughout the fermentation process and after maturation are not yet well understood. We have optimized an efficient method for the chemical analysis of aroma during cider production and suggest a practical pre-screening technique early in the fermentation process (72 h post inoculation) for off-flavor detection. Our measurements of cider VOCs have resulted in several discoveries of interest to cider makers and chemists alike. Firstly, the SPME fiber type that extracted the highest concentrations and the widest varieties of odor-active species in cider was the DVB/CAR/PDMS fiber, when held at 40 \({}^{\circ}\)C for 30 min in the cider headspace. This fiber consistently outperformed the CAR/PDMS (49% less efficient) and PA fibers (98% less efficient) for esters, higher alcohols, and phenols. Secondly, we established a VOC formation timeline in order to better understand the release of odor-active species during cider preparation. The cider aroma profile developed rapidly after the beginning of fermentation but remained surprisingly consistent after only 72 h at 23 \(\pm\) 1 \({}^{\circ}\)C (with major esters and higher alcohols varying by 18.6% \(\pm\) 4.1% and 12.3% \(\pm\) 2.6% thereafter, respectively). This allows for early pre-screening for off-flavors such as 4-ethylphenol and 4-ethyl-2-methoxyphenol, which are also readily detected by this analytical method. Thirdly, we utilized yeast strains which have been traditionally chosen for beer fermentation (Edinburgh, Belgian II, French Saison, and Abbey) to produce four commercial grade ciders via a six-month maturation process. These ciders yielded significantly variant sensory qualities from identical starting apple juices, providing evidence for the central importance of yeast strain choice in cider making. Twenty-seven major and minor VOCs were identified across these ciders, and only eleven of them were common to all ciders produced, evidencing the unique metabolic processes of each strain. Lastly, four compounds known to define cider aroma (ethyl acetate, ethyl octanoate, 2-phenylethyll alcohol, and 3-methyl-1-butanol) were quantified within desirable concentration ranges in all four ciders. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Analytic (Odor)**} & **Edinburgh** & **Belgian II** & **French Saison** & \multirow{2}{*}{**Abby Yeast**} & **Perception Threshold** \\  & **Yeast** & **Yeast** & **Yeast** & & **(mg\(\cdot\)L\({}^{-1}\))** \\ \hline Ethyl acetate (solvent, fruity, baslamic) [b] & \(1.89\pm 0.42\) 1  & \(33.00\pm 7.26\) & \(2.52\pm 0.55\) & \(41.28\pm 9.08\) & \(12\)2  \\ Ethyl octanoate (fruity, candy, pineapple) [b] & \(0.13\pm 0.03\) & \(0.16\pm 0.04\) & \(0.29\pm 0.08\) & \(0.20\pm 0.05\) & \(0.014\)2  \\
2-Phenylethyl alcohol (Rose, honey) [c] & \(6.85\pm 1.03\) & \(11.95\pm 1.79\) & \(9.59\pm 1.44\) & \(8.98\pm 1.35\) & \(0.390\)3  \\
3-methyl-1-butanol (Alcohol, nail polish, whiskey) [b] & \(13.40\pm 3.58\) & \(32.36\pm 8.64\) & \(44.47\pm 11.87\) & \(38.51\pm 10.28\) & \(30\)2  \\ \hline \hline \end{tabular}  
* Concentrations are expressed in mg\(\cdot\)L\({}^{-1}\), and data are reported as \(\overline{x}\pm 1\) for triplicate SPME experiments.
* [21]\({}^{\circ}\)[29].  
\end{table} 
Table 3: Important aroma compounds quantified in cider fermented (six months) with the same apple juice and different _S. cerevisiae_ yeast strains 1. **Author Contributions:** [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] designed the study, interpreted the results, and drafted the manuscript together. [PERSON], [PERSON] and [PERSON] collected and analyzed all reported experimental data under the mentorship of [PERSON] and [PERSON]. All authors have read and agreed to the published version of the manuscript.