The world's alcoholic beverage culture reflects the geographical characteristics and customs of each country (Wilms et al., 2023). Obviously, distilled spirits are highly accepted and enjoyed by consumers from all over the world with their unique taste and long-lasting aroma (Shi et al., 2022). Chinese Baijiu, as one of the six major distilled spirits in the world, plays an important role in the development of the world's alcoholic beverage culture (Jin et al., 2017). Baijiu is generally fermented with various sorts of cereals, such as wheat, which is the spontaneous solid-state fermentation (SSF) process using natural fermentation starter Daqu (Li et al., 2023), which allows microorganisms to grow and multiply on solid grains. Based on the top culture temperature, the types of Daqu can be classified into low-temperature Daqu (40–50 °C), medium-temperature Daqu (50–60 °C), and high-temperature Daqu (60–65 °C) (Zheng et al., 2023). Among them, high-temperature Daqu is used as a solid starter for maotai-flavor liquor brewing with rich microbiota and higher enzyme activities, which the continuous gas stage and a small amount of liquid water exist in the spaces between the grains, making Daqu a microbial carrier and saccharification agent for Baijiu fermentation (Jia et al., 2020).

Most prominently, the HTD uses pure wheat as a substrate and the core microbial community metabolism produces protease, amylase, cellulase, lipase and pectinase etc., which in turn constitute a unique enzyme system that could break down large molecules in wheat into small molecules and promote subsequent fermentation and metabolism (Zeng et al., 2022). Previous studies have revealed that the use of wheat-derived microbiota is a key factor in the assembly of microbial communities during HTD incubation (Zhang et al., 2022). Furthermore, wheat germ is degraded into for protein hydrolysis, which can be used to prepare bioactive peptides and seeds through producing metabolic enzyme activity (Weng et al., 2021). For example, the effect of Lactobacillus rhamnosus on the physicochemical properties of fermented plant materials, it is clarified that different plants' own proteases and small molecule fractions synergize with microbial overall fermentation (Masiá et al., 2020). However, the effect of metabolic transport from the wheat have not been independently study in HTD fermentation. Thus, it is necessary to reveal the influence of wheat on the colonization and succession patterns of microbiota in fermentation substrates.

On the other hand, most of the SSF processes use aerobic microorganisms, RQ is an important indicator that reflect the respiratory and physiological metabolic state of microorganisms during fermentation (Banat et al., 2021), the flux distribution of intracellular metabolic pathways and measure the level of fermentation (Hardie et al., 2012). Among them, microorganisms ingest carbon sources through aerobic respiration to produce CO2, and the various carbon source substrate reduction degree tends to make the catabolic process of oxygen consumption varies. For example, the mass transfer effect of water assisted microorganisms (Lactococcus lactis in bacteria) with respiratory metabolism RQ indicators fluctuating around 1 (Gelpi et al., 2023). However, in addition to the movement of the bacterium, both the active transportation of the raw material metabolic substances and the turnover of macromolecules within the cell affect the RQ, which suggesting that respiration and fermentation are specifically adjusted to substrate energy consumption (Zivieri and Pacini, 2017). Accordingly, RQ is introduced to quantify the difference between wheat metabolism and overall metabolism and conversion rates in Daqu different fermentation stages.

In this study, the wheat for Daqu fermentation and microbial metabolism patterns were investigated. Substrate temperature variation was set in segments to explore the trend of substrate carbon source and nitrogen source refinement; elemental analysis and high-throughput sequencing technique were combined to characterize the relative abundance and type of microbial populations, the equilibrium amounts of O2 and CO2 were measured in the respiratory bottle for RQ calculations, and heatmap was used to characterize the correlation between fermentation index and microbial activity; finally redundancy analysis and construction of metabolic network model were used to predict the metabolic functions of wheat and microorganisms. This study provides a theoretical basis for the relationship between the contribution of wheat metabolism and fermentation quality in the Daqu production process.