Development of fermented milk using food-grade recombinant Lactococcus lactis NZ3900

Fermented milk has been associated with the improvement of human health for decades. Food-grade Lactococcus lactis NZ3900 is a potential host for producing and delivering various biologics, including oral vaccines, hence it could enhance the functionality of fermented milk as a starter culture. A cultured fermented milk product was developed in this study using the food-grade recombinant L. lactis NZ3900 harboring plasmid pNZ8149. Response surface methodology was used to optimize the inoculum concentration (5–15% v/v), fermentation temperature (23–37 °C), and fermentation time (18–30 h) to enhance the viability, acidification activity, viscosity, and minimize syneresis of the L. lactis NZ3900-fermented milk using response surface methodology. The optimal inoculum concentration and fermentation conditions for L. lactis NZ3900-fermented milk are 12.5% (v/v) inoculum, 28 °C fermentation temperature, and 30 h of fermentation time, resulting in higher viability (9.64 log₁₀ CFU/mL), acidification activity (pH 4.42, 0.78% titratable acidity), viscosity (11,096.9 mPa·s), and comparable syneresis (63.62%) against commercial fermented milk products. This shows that food-grade L. lactis NZ3900 can be used as a starter culture, and NZ3900-fermented milk could be used as a functional drink for oral vaccine delivery.