## 4 Conclusions  
Quantitative analysis of triplicate Gravenstein apple cider fermentations has revealed a large array of VOCs. The 89 total VOCs detected by HS-SPME-GC-MS throughout fermentation and dry hopping included higher alcohols (11), acetate esters (7), ethyl esters (10), other esters (32), aldehydes (5), ketones (8), volatile acids (5), and terpenes (11). Specifically, species above the published perception thresholds were monitored carefully, revealing three unique ciders with appealing citrus (Citra hops), fruity (Galaxy hops), and floral (Mosaic hops) aromas. During the fermentation process, esters develop quickly, but drop in concentration (\(-10\%\)) immediately following packing events. This is of practical interest to cider makers when planning the timeline of racking and bottling. By contrast, a single cold crash and racking event increased higher alcohol concentrations (\(+14\%\)) for 3 days afterwards. Although few studies have focused on dry hopping in a cider context, we hypothesized based on previous beer publications that not only the variety of hops, but the timing of hop addition significantly alter the identity and concentration of aromas-important VOCs in cider. This hypothesis was confirmed. Citra hops added the highest concentration of terpen in the cider headspace, and Mosaic hops added the highest concentration of esters. From day 5 to day 8, a significant increase in ester concentration was observed for all hop varieties. Although further research is needed to fully characterize the dry hopping process for cider making through detailed sensory analysis, this work provides novel analytical insight into how cider makers might make informed decisions on the application of hops to diversify the aroma profile of their beverages. Similarly, we have provided useful information on racking events and insight into how a single varietal apple cider's aroma profile changes throughout fermentation, maturation, and dry hopping. Relevant data discussed in this work have been added to the MORP cider chemistry database to inform cider makers about the properties of southwest Colorado cider apples. There are myriad variables involved in cider making, all of which are worthy of further exploration. Important future work should include optimizing the dry hopping process to limit the quantity of hops used while maximizing desirable aroma contributors, thereby decreasing cost for cider makers and decreasing the environmental impact of excessive hop use. The following are available online at [[https://www.mdpi.com/article/10.3390/app12010310/s1](https://www.mdpi.com/article/10.3390/app12010310/s1)]([https://www.mdpi.com/article/10.3390/app12010310/s1](https://www.mdpi.com/article/10.3390/app12010310/s1)), Table S1: Semi-quantification of volatile compounds (mg/L) in juice, cider, and dry-hopped cider and Table S2: Semi-quantified VOCs a (mg L\({}^{-1}\)) added to cider headspace as a result of dry hopping for 5 and 8 days with Citra, Galaxy, and Mosaic hops. [PERSON], undergraduate students [PERSON] and [PERSON], and collaborator [PERSON] implemented the study and interpreted the results together. Undergraduate students [PERSON] and [PERSON] collected and analyzed all reported experimental data under the mentorship of [PERSON] and [PERSON], [PERSON] prepared the manuscript draft and all manuscript editing was performed by C.A.C. C.A.C. acquired all funding that supported this work and led project administration and supervision. All authors have read and agreed to the published version of the manuscript. This work was made possible thanks to the 2019-2020 Undergraduate Analytical Research Program (UARP) grant of the Society for the Analytical Chemists of Pittsburgh (SACP) and the 2019-2020 American Society for Mass Spectrometry Research at PUIs Award funded by Agilent Technologies. We thank these organizations for their financial support of undergraduate research.