2.2. Determination of physicochemical parameters and volatile flavor substances

The temperature of the fermented grains was measured by inserting a thermometer (RE‐W5007; People's Electric Group Co.) into the pit at a depth of 1 m. The pH of the fermented grains was determined using a pH meter (Model UB‐7; Denver Instrument Company), and the total acidity was determined using acid–base titration according to the national standard for food safety. Alcohol content was measured using the alcohol meter method (Alcolyzer Wine; Anton Paar Co.). The reducing sugar was determined via direct titration according to a previously published method (Kang et al., ²⁰²²). The moisture content was determined using the constant‐temperature drying method. The concentration of volatile flavor substances in fermented grains was measured via headspace solid‐phase microextraction coupled with gas chromatography–mass spectrometry (Clarus 600; PerkinElemer; Pang et al., ²⁰¹⁸).