###### Abstract  
Volatile organic compounds (VOCs) play a crucial role in cider quality. Many variables involved in the fermentation process contribute to cider fragrance, but their relative impact on the finished odor remains ambiguous, because there is little consensus on the most efficient method for cider volatile analysis. Herein, we have optimized and applied a headspace solid phase microextraction gas chromatography-mass spectrometry (HS-SPME GC-MS) method for the chemical analysis of cider VOCs. We determined that the 30 min exposure of a divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) solid phase microextraction (SPME) fiber at 40 \({}^{\circ}\)C yielded detection of the widest variety of VOCs at an extraction efficiency >49% higher than comparable fibers. As a proof-of-concept experiment, we utilized this method to profile cider aroma development throughout the fermentation process for the first time. The results yielded a very practical outcome for cider makers: a pre-screening method for determining cider quality through the detection of off-flavors early in the fermentation process. The aroma profile was found to be well established 72 h after fermentation commenced, with major esters varying by \(18.6\%\pm 4.1\%\) thereafter and higher alcohols varying by just \(12.3\%\pm 2.6\%\). Lastly, we analyzed four mature ciders that were identically prepared, save for the yeast strain. Twenty-seven key VOCs were identified, off-flavors (4-ethylphenol and 4-ethyl-2-methoxyphenol) were detected, and odorants were quantified at desirable concentrations when compared to perception thresholds. VOCs varied considerably following fermentation with four novel strains of _S. cerevisiae_, evidencing the central importance of yeast strain to the finished cider aroma. a 2020 6 40 3390 10.3390 10.  
techniques in order to create the highest quality ciders in an efficient manner. The work presented herein is the result of a novel, ongoing collaboration between orchards, cideries, and chemists in Southwest Colorado which aims to better understand the myriad variables that contribute to local cider production. It is well-established that aroma and flavor define cider quality. These variables have been thoroughly studied through sensory means [4; 5], but further research is required to understand the molecular profiles responsible for those organoleptic properties and the methods tied to their production [6]. There remains little consensus on what specific cider-making practices, apple characteristics, or yeast strains lead to the desired aroma-active compounds in the headspace of apple cider or how best to measure those compounds. In this work, we report the optimal analytical practices to address these gaps in knowledge of cider volatile organic compound (VOC) extraction, create a timeline of odorant development during the process of cider fermentation, and analyze the key molecular constituents of cider fragrance produced using four different strains of _Saccharomyces cerevisiae_ (previously unexplored for cider production). The central aim of this work is to optimize a cider analysis method and then apply it in order to better understand the development of VOCs both during cider fermentation and after maturation. Headspace solid phase microextraction gas chromatography-mass spectrometry (HS-SPME GC-MS) is a reliable technique for the pre-concentration and determination of the odorant volatile composition of beer and wine [7; 8; 9]. Recent experiments involving cider headspace analysis have also utilized solid phase microextraction (SPME), incorporating fibers such as polydimethylsiloxane (PDMS) [10; 11], polydimethylsiloxane/divinylbenzene (PDMS/DVB) [12], and divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) [6] over a range of exposure times and temperatures. These publications reveal a current lack of agreement on the optimal analytical approach for cider headspace analysis.