Comprehensive optimization of composite cryoprotectant for Saccharomyces boulardii during freeze-drying and evaluation of its storage stability

Saccharomyces boulardii (S. boulardii) is widely adopted in the diarrhea treatment for humans or livestock, so guaranteeing the survival rate of S. boulardii is the critical issue during freeze-drying process. In this study, the survival rate of S. boulardii with composite cryoprotectants during freeze-drying procedure and the subsequent storage were investigated. With the aid of response surface method, the composite cryoprotectants were comprehensively optimized to be lactose of 21.24%, trehalose of 22.00%, and sodium glutamate of 4.00%, contributing to the supreme survival rate of S. boulardii of 64.22 ± 1.35% with the viable cell number of 9.5 ± 0.07 × 109 CFU/g, which was very close to the expected rate of 65.55% with a number of 9.6 × 109 CFU/g. The accelerated storage test demonstrated that the inactivation rate constant of the freeze-dried S. boulardii powder was k-18 = 8.04 × 10-6. In addition, the freeze-dried goat milk powder results exhibited that the inactivation rate constants were k4 = 4.48 × 10-4 and k25 = 9.72 × 10-3 under 4 and 25 °C, respectively. This work provides a composite cryoprotectant formulation that has a good protective effect for the probiotic S. boulardii during freeze-drying process, possessing the potential application prospect in food, medicine, and even feed industry.