Impact of Enzyme–Microbe Combined Fermentation on the Safety and Quality of Soy Paste Fermented with Grass Carp By-Products

Freshwater fish processing produces 30–70% nutrient-rich by-products, often discarded or undervalued. Grass carp by-products, rich in protein, offer potential as raw materials for fermented seasonings. This study explores the use of these by-products—specifically, minced fish and fish skin—in soybean fermentation to evaluate their effects on the quality of the resulting seasonings. Tetragenococcus halophilus was used as a starter culture alongside food-grade protease to assess their combined impact on the safety and flavor of soy fish paste and soy fish skin paste. The findings revealed that natural fermentation resulted in higher protein hydrolysis in soy fish skin paste compared to soy fish paste. Across all fermentation conditions, amino acid nitrogen levels increased, while total volatile basic nitrogen levels decreased in both pastes, indicating improved quality. Additionally, microbial fermentation significantly reduced biogenic amine content in soy fish paste, enhancing safety. Enzymatic fermentation further enriched the flavor of both pastes by boosting key compounds such as 2-methylbutanal and ethyl acetate. Notably, enzyme-microbe co-fermentation harnessed the strengths of both methods, achieving improved safety and enhanced flavor profiles while elevating overall product quality. These findings suggest a promising way to transform freshwater fish by-products into high-value condiments, advancing sustainable food processing.