## 1 Introduction  
_Soju_ is one of the most popular alcoholic beverages in Korea. _Soju_ is a distilled liquor manufactured using a pot still or continuous distillation after saccharification of grains or starchy raw materials [1]. Traditionally, distilled _soju_ is made through a pot still at atmospheric pressure, but currently, modern vacuum distillation is widely applied and gaining more popularity in the industry. The vacuum distillation method, which is a method of distilling by lowering the pressure without applying direct heat by using a stainless-steel concentrator, has the advantage of maintaining high volatile aroma components with less heated and burnt flavors of the distilled liquor [2]. The headspace solid-phase microextraction (HS-SPME) methodology, which has the advantage of shortening the extraction time and increasing the sensitivity by using a variety of fibers, is generally applied to volatile analysis in alcoholic beverages as well as general foods [3]. HS-SPME is widely used to detect volatile compounds in whiskey [4], brandy [5], Chinese _Baiju_[6], and other alcoholic beverages. The distribution and chemical composition of aroma compounds result from the fermentation raw material, fermentation and distillation conditions, aging containers, and durations of distilled liquor [7]. Traditional _soju_ manufactured by atmospheric distillation leaks a lot of middle-high-boiling components compared to vacuum distilled liquor [8]. It was reported that the higher the distillation temperature, the more the esterification reaction occurs during the distillation, resulting in longer-chain ester compounds [9]. _Soju_ distilled under atmospheric pressure has heated, burnt, and grainy flavors with a strong bitter taste. These flavors are gradually decreasing as atmospheric distillation is being replaced by vacuum distillation [8]. In the case of _soju_ manufactured from mashed products using _nuruk_ (Korean _koji_), the contents of ethyl nonanoate, ethyl lactate, 2-phenyl ethanol, and acetaldehyde were higher by more than two times in _soju_ made under atmospheric distillation than in that made under vacuum distillation [7]. Distilled _soju_ uses various types of raw materials such as glutinous rice, barley, non-glutinous rice, wheat, sorghum, and corn. Various types of longer-chain esters such as ethyl decanoate, ethyl dodecanoate, ethyl tetradecanoate, ethyl hexadecanoate, ethyl 9-octadecenoate, and ethyl octadecanoate were major aroma ingredients in _soju_ manufactured using sweet potatoes [10]. Commercial Japanese _shochu_ did not show significant differences in aroma patterns according to the raw materials--rice and barley [11]. Ethyl decanoate, ethyl dodecanoate, ethyl octanoate, isoamyl alcohol, and isobutyl alcohol were major volatile components in _soju_ made with potato [12]. The volatile components of distilled _soju_ aging in different containers (oak, pottery, or stainless steel) over 18 months were investigated using HS-SPME [13]. _Soju_ aged in an oak barrel was higher in aldehydes, ketones, and miscellaneous compounds than that in other aging containers [13]. In addition, in the case of oak barrel aging, furfural and oakactone were detected, differently from the _soju_ aged in stainless steel, which are known to have burnt, oaky, woody, and coconut flavors [14]. These esters, alcohols, and other volatiles are widely found in distilled _soju_, beer, _shochu_, rice wine, etc., and have a great influence on the quality of liquors [15]. Depending on various manufacturing conditions, there is a big difference not only in aroma compounds but also in sensory characteristics [15]. In the study of seven commercial distilled _soju_, bitterness, bitter aftertaste, sake odor/flavor, corn silk odor, acetone odor, and soy sauce odor were determined as major sensory characteristics [16]. In addition, a sensory descriptive analysis of nine locally famous distilled _soju_ was performed with more detailed sensory characteristics such as eight aroma, four flavor/taste, and six mouthfeeel-related attributes [17].