## 2 Results  
### VOCs Detected by HS-SPME-GC-TOfms  
A total of 879 VOCs (661 identified VOCs) were detected in 24 distilled liquors (Table S1). Among the distilled liquors analyzed, C1 had the largest number of VOCs, with 466 compounds detected (344 identified VOCs). This was more than four-fold greater than V4, which had the lowest number of VOCs with 114. The amounts of VOCs detected in the whiskies, brandies, and runs were comparable, with more than 300 VOCs (Figure 1A). Among the VOCs detected, 159 were esters, 92 were alcohols, 105 were aldosterone, and 166 were benzene derivatives. A small number (23) of organic acids were also detected (Figure 1A). Esters, as one of the most important classes of flavor compounds in distilled liquors, are commonly generated by esterification reactions between organic acids and alcohols during the fermentation and aging processes [21; 22]. Esters had the highest total concentration among the VOCs in all distilled liquors assessed except for some vodka samples (V2, V3, and V4), in which benzene derivatives were more abundant (Figure 1B). In most Western liquors--such as whisky, brandy, and rum--octanoic acid ethyl ester had the highest ester concentration (ranging from 23 to 405 mg/L, none in R1), followed by decanoic acid ethyl ester (ranging from 21 to 70 mg/L, none in any whisky, R2, or R3). These two compounds constituted 8-48% of the total VOCs (10-48% in whiskies, 19-29% in brandies, and 8-47% in runs). The dominated esters were quite different in Chinese baijius. Lactic acid ethyl ester was the most abundant ester in Chinese soy sauce aroma type liquor (929 mg/L in C1 and 903 mg/L in C2), followed by acetic acid ethyl ester (771 mg/L in C1 and 851 mg/L in C2). Hexanoic acid ethyl ester had the highest concentration in Chinese strong aroma type liquor (772 mg/L in C3 and 643 mg/L in  
Figure 1: Volatile organic compounds (VOCs) in distilled liquors detected by HS-SPME-GC-TOFMS. (**A**) Number of compounds in major classes of VOCs in 24 representative distilled liquors. (**B**) Contents of major classes of VOCs. (**C**) Relative concentration of major classes of non-VOCs. Abbreviations: C1 and C2, Chinese soy sauce aroma type liquor 1 and 2, respectively; C3 and C4, Chinese strong aroma type liquor 3 and 4, respectively; C5 and C6, Chinese light aroma type liquor 5 and 6, respectively; W1–W5, Whisky 1–5; B1–B5, Brandy 1–5; R1–R4, Rum 1–4; and V1–V4, Vodka 1–4. C4), followed by pentanoic acid ethyl ester (317 mg/L in C3 and 224 mg/L in C4). In Chinese light aroma type liquor, acetic acid ethyl ester was the dominant ester (250 mg/L in C5 and 167 mg/L in C6) followed by decanoic acid ethyl ester (125 mg/L in C5 and 165 mg/L in C6). The two of the most abundant esters constituted 23-29% of the total VOCs in Chinese baijius (Tables S1 and S2). Alcohols and benzene derivatives alternated as the second most abundant VOCs in the distilled liquors assessed. Alcohols are produced by yeasts as by-products of reactions involving amino acids and carbohydrates [23]. The concentrations of alcohols differ significantly among distilled liquors ranging from 3 to more than 500 mg/L (account for 3-30%) (Figure 1C). Chinese soy sauce aroma type liquor had the highest concentration of alcohols (506 mg/L in C1 and 514 mg/L in C2), followed by Chinese strong aroma type liquor (417 mg/L in C3 and 390 mg/L in C4) (Figure 1B). However, R4 had the highest relative concentrations of alcohols accounting for 30% of the total VOCs, which was much higher than in Chinese baijius (7-11%), although their contents in R4 (217 mg/L) were lower than in Chinese baijius. Consistent with a previous study [23], 1-butanol, 3-methyl- was the major alcohol in most of the distilled liquors except for Chinese baijius, in which 1-hexanol was more abundant (Tables S1 and S2). Although the quantity of benzene derivatives were comparable with esters, their concentrations (40-372 mg/L) were much lower than esters (22-4044 mg/L) in the distilled liquors assessed (Figure 1A,B).