Diversity and Succession of Acetic Acid-Related Fungi in Fermented Grains for Nongxiangxing Baijiu and Their Distribution in the Brewing Environment

In this study, high-throughput sequencing and headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) were applied to investigate the composition of acetic acid-related fungi in fermented grains for nongxiangxing baijiu in northern China and their distribution characteristics in the brewing environment during fermentation (from Days 7 to 67) in different seasons (spring and summer). The results indicated that the content of acetic acid in fermented grains increased with fermentation time and was significantly higher in summer than in spring (P < 0.05). Correlation analysis revealed that 122 genera of fungi were significantly correlated with acetic acid synthesis (P < 0.05). Among them, Thermomyces, Issatchenkia, Zygosaccharomyces, Kazachstania and Rhizopus were the dominant genera. Thermomyces and Zygosaccharomyces were positively correlated with acetic acid (P < 0.05), while Issatchenkia and Kazachstania showed significant negative correlations with acetic acid (P < 0.05). In addition, complex correlations existed between the acetic acid-related fungi and the dominant fungi. Source tracking results indicated that the fungi in the fermented grains originated from the ground (20.3%), tools (20.3%), pit mud (18.0%), Daqu^L (16.2%), raw materials (16.2%) and Daqu^F (9.1%). Moreover, six dominant genera of fungi significantly correlated with acetic acid were found in samples from both the ground and tools. In the fermented grains, Kazachstania, Thermomyces and Issatchenkia originated from pit mud, Issatchenkia from Daqu^L, Zygosaccharomyces from raw materials, and Rhizomucor and Rhizopus from Daqu^F. This study provides a theoretical basis for establishing strategies to regulate the content of acetic acid in fermented grains for Nongxiangxing Baijiu during the fermentation process.