## 1 Introduction  
As a unique beverage in China with a long history of more than 2000 years, baijiu has always been deeply loved by people [1]. Baijiu can be divided into 12 flavor types in line with dissimilar production processes [2]. Traditionally, baijiu is made from grains as raw materials. After a series of complicated processes, such as cooking, adding qu, saccharification, fermentation, and distillation, a traditional Chinese breved beverage with a complex aroma and rich flavor are ultimately obtained [3, 4]. Baijiu uses a starter called \"jiuqu\" for saccharification and fermentation in the brewing process. In general, jiuqu can be categorized as daqu, xiaoqu, and fuqu [5]. One of the most commonly employed is daqu. In accordance with the culture temperature, daqu can be divided into three categories: low-temperature type, medium-temperature type, and high-temperature type [6], which are adopted to produce diverse flavor types of baijiu. Light-flavor baijiu uses low-temperature daqu with the highest product temperature of 40-50 \({}^{\circ}\)C. Strong-flavor baijiu generally uses medium-temperature daqu with the highest product temperature of 50-60 \({}^{\circ}\)C, and sauce-flavor baijiu generally uses high-temperature daqu with the highest product temperature greater than 60 \({}^{\circ}\)C. Daqu is made of grain and is shaped like rectangular bricks. After artificial caiqu, these bricks are placed in a daqu house. At this time, bacteria, mold, yeast, and other microorganisms began to work. [7]. Within one month, the anqu duration will normally vary in line with the climate and temperature. When daqu is in the anqu process,rich flavors can be obtained through the fermentation and culture of microorganisms, making the flavor of baijiu diversified. Apart from being employed as a fermentation starter during the production processes of baijiu, daqu can also be employed as a \"flavoring agent\" to directly or indirectly provide baijiu with various flavors [8]. Dissimilar fermentation environments will give rise to the diversification of microorganisms when producing daqu, which will affect the flavor and fermentation metabolism of daqu [9]. Strong-flavor baijiu is one of the important styles of baijiu, and some of its flavor components are provided by daqu. Discussing the changes in volatile components in the production process of daqu can provide ideas and references for the study of flavor substances in strong-flavor baijiu. Notwithstanding the fundamental fact that scholars have conducted a myriad of studies on the microbial community in daqu, primarily exploring the correlation between microbes and flavor components [10; 11], but the variation in volatile compounds has been rarely discussed. In this study, the aim is to analyze the volatile compounds in the production process of daqu. There are many methods currently applied to the study of volatile compounds in the field of baijiu. The direct injection method (DI) involves the direct injection of the sample into a gas chromatograph (GC) for separation and detection without or with simple treatment. Liquid-liquid extraction (LLE) is the extraction and concentration of trace components in baijiu using organic solvents to enrich volatile compounds. HS-SPME is a solvent-free extraction method for the extraction of volatile compounds from baijiu by coating adsorption. Solvent-assisted flavor evaporation (SAFE) is a combination of a distillation unit and a high vacuum pump to achieve the separation and analysis of flavor substances. In addition to the above methods, other methods for the extraction of volatile compounds include liquid-liquid microextraction (LIME), dynamic headspace adsorption (DHS), etc. Based on the characteristics of the various methods and the aim of this study, which is to achieve rapid detection of flavor and rapid determination of maturity level in the production process of daqu, HS-SPME, which is simple to operate and less expensive, was finally selected for this study.