Abstract: In this study, pH and the contents of four major organic acids, calcium and magnesium ions in stratified bottom-pit-mud (BPM) from young and old pits used for the production of Chinese strong-flavor Baijiu, as well as the turbidity of their centrifugal supernatant/filtrate were measured, and the correlations between these parameters were analyzed. The physicochemical properties of BPM from old pits showed no horizontal variations, whilst in the vertical direction, the increase in pit depth resulted in a higher pH (5.86–9.22), and lower contents of lactic acid (LA, 2 380.07–19.48 mg/kg), caproic acid (CA, 3 134.59–1 904.76 mg/kg), butyrate acid (BA, 559.10–103.95 mg/kg), acetic acid (AA, 2 737.79–536.48 mg/kg), Ca2+ (292.1–35.0 mg/kg), and Mg2+ (258.7–13.3 mg/kg). For BPM from young pits at the center and quarter positions, the increase in pit depth resulted in an increase in pH (4.24–6.42), and a decrease in LA (38 146.5–6 664.7 mg/kg) and Mg2+ (326.70–183.91 mg/kg), but no difference in Ca2+ (1 448.6 mg/kg); at the corner position, it caused an increase in pH (4.13–7.89), and a decrease in LA (14 461.0–81.1 mg/kg), Ca2+ (878.3–362.8 mg/kg) and Mg2+ (197.1–106.3 mg/kg); for each position, the contents of CA (6 663.69 mg/kg), BA (1 403.47 mg/kg) and AA (1 347.00 mg/kg) showed no significant difference when the pit depth increased. Correlation analysis showed that for both young and old BPM, pH was strongly correlated with LA (?0.686 2 vs ?0.789 3), indicating the importance of LA for the pH of BPM. LA content was strongly correlated with Ca2+ (0.418 6 vs 0.717 6) and Mg2+ (0.731 6 vs 0.738 5), indicating the importance of LA for Ca2+ and Mg2+. Compared with young BPM, stronger correlation of LA content with CA (?0.126 8 vs 0.466 0), BA (?0.092 5 vs 0.537 1) and AA (?0.078 0 vs 0.443 3) was observed in old BPM indicatingg stronger LA catabolism in old BPM. The difference in the turbidity of centrifugal supernatant (48.63 NTU vs 62.16 NTU) and filtrate (14.21 NTU vs 24.12 NTU) between young and old BPM indicated more micro-nano particles present in the old BPM. Therefore, this study illustrates that reducing LA content is beneficial to improve the quality of BPM and that the presence of more micro-nano particles in BPM is beneficial to the metabolism of LA-degrading bacteria.

Key words: Chinese strong-flavor Baijiu; stratified bottom-pit-mud; pH value; major organic acids; calcium ion; correlation; turbidity