Improving nattokinase activity and sensory properties of natto via co-fermentation with Bacillus subtilis natto and Lactobacilli

Natto is a nutrient-rich fermented soybean product with health-promoting functions due to its nattokinase content, which confers multiple benefits, including cardiovascular protection, antioxidant activity, and improved gastrointestinal digestion. In this study, Bacillus subtilis natto TS06 was co-fermented separately with Lactobacillus acidophilus 6005 and Lactobacillus helveticus TS6024 to improve natto quality. Optimal fermentation conditions using L. acidophilus 6005 were 5% glucose, 4% inoculum level, and incubation at 39 °C for 28 h, whereas those using L. helveticus TS6024 were 4% glucose, a 4% inoculum level, and incubation at 40 °C for 28 h. Under these conditions, nattokinase activities inoculated with only B. subtilis natto TS06 and those co-fermented with L. acidophilus 6005 and L. helveticus TS6024 were 1482.37, 2818.67, and 2880.84 IU/g, respectively, and total biogenic amines were 2208.25, 3450.48, and 1651.19 μg/kg. Addition of lactobacilli significantly enhanced nattokinase activity (p < 0.05) and sensory properties (p < 0.05) of natto. Biogenic amine levels significantly decreased after adding TS6024 (p < 0.05), whereas those using 6005 significantly increased (p < 0.05); however, all levels complied with food safety guidelines. Both lactobacilli can be used as auxiliary starter cultures to improve the functional properties and flavor profile of natto, thereby providing a foundation for the development of innovative natto products with high NK activity and improved sensory attributes.