Raw material–dependent changes in bacterial and compositional profiles are involved in insufficient pH decrease in natural lactic fermentation of Brassica rapa leaves.

• Sunki pickle fermentation was performed using turnip leaves from different origins. • A sample showed insufficient pH decrease despite high lactic acid bacterial counts. • High Limosilactobacillus / Lactobacillus ratio was related to ammonia accumulation. • In vitro study showed strong correlation between pH and ammonia changes (R = 0.892). • Ammonia–Gln-Glu correlations implied glutaminase activity suppressed the drop in pH. Sunki is an unsalted lactic fermented pickle made from red turnip leaves in the Kiso district, Japan. Accidental insufficient decrease in pH during sunki fermentation seriously reduces the product quality. To obtain insights into how the insufficient decrease occurs, we comprehensively analyzed differences in the microbiological and chemical properties of sunki made from three different turnip harvests and found a significant difference in their final pH. Microbiota and metabolome analyses revealed that the insufficient pH decrease showed strong relationships with the chemical composition (low lactic acid and high ammonia levels) and bacterial community structure (low Lactobacillus and high Limosilactobacillus). In vitro sunki fermentation experiments demonstrated that accumulated ammonia was associated with a decrease in glutamine and an increase in glutamic acid. Limosilactobacillus reuteri , a species of lactic acid bacteria possessing heterolactic metabolism, was suggested to be mainly responsible for insufficient decrease in pH related to accumulated ammonia during sunki fermentation. [ABSTRACT FROM AUTHOR]