Preventing physical destabilization and bioactive isoflavone loss in germinated soy milk using nisin

Germinated soy milk (GSM) is a promising functional beverage rich in bioavailable aglycone isoflavones. However, its nutrient-dense matrix renders it highly susceptible to microbial spoilage and rapid physical destabilization. This study evaluated the efficacy of nisin as a non-thermal biopreservation intervention to retard quality deterioration in GSM at ambient temperature (25 °C).

The untreated control spoiled rapidly within 3 days, characterized by a microbial surge (total colony count reaching 6.43 log CFU mL^-1 by day 7), a precipitous pH drop (from 6.48 to 5.33), macroscopic gelation, and severe particle enlargement (volume-weighted mean diameter D[4, 3] increased from ~1.0 μm to >64.0 μm). Conversely, nisin intervention (0.4 g kg^-1) effectively suppressed microbial growth, maintaining total colony count at a safe level of 2.67 log CFU mL^-1 on day 7. This microbial inhibition sustained the system pH at ~5.99, successfully preventing acid-induced protein aggregation and preserving emulsion homogeneity (D[4, 3] at 6.0 μm). Furthermore, nisin significantly mitigated the microbial-driven degradation of phytochemicals, reducing the loss rate of aglycone isoflavones from 56.63% (control) to 29.29% after 7 days.

By effectively blocking the 'microbial proliferation-acidification-aggregation' deterioration cascade, nisin extended the ambient shelf life of GSM to 5 days. This targeted biopreservation strategy provides a robust framework for developing physically stable, clean-label, and nutritionally enhanced plant-based functional beverages. © 2026 Society of Chemical Industry.