Optimization of cryoprotectants for improving the freeze-dried survival rate of potential probiotic Lactococcus lactis ZFM559 and evaluation of its storage stability
Sitong Ge, Jiarun Han, Qiaoyu Sun, Zhongdu Ye, Qingqing Zhou, Ping Li, Qing Gu
Abstract
In this study, the cryoprotectant ratio of Lactococcus lactis ZFM559 (L. lactis ZFM559) was optimized by response surface methodology, and its protective mechanism was also explored. The results showed that the freeze-drying survival rate of L. lactis ZFM559 reached 81.02 ± 0.32% under the combination of 4.2% trehalose, 2.0% mannitol, 11.9% skim milk and 4.1% glutamic acid monosodium as cryoprotectant. The presence of cryoprotectant maintained the shape of L. lactis ZFM559, and provided an external protection. Moreover, the freeze-dried powder exhibited an increased glass transition temperature, effectively mitigating mechanical damage caused by ice crystals during the freeze-drying process. In addition, cryoprotectants could effectively maintain the integrity of bacterial cell membranes and keep Na+/K+- ATP protease activity on cell membranes. The viability of bacterial cell powder was also assessed after storage at -20˚C, 4˚C, and 25˚C for 30 days, with the freeze-dried bacterial powder exhibiting relatively higher storage stability at -20˚C. Our research established a cryoprotectant formulation and demonstrated the protective effect on L. lactis ZFM559 during the freeze-drying process, providing a theoretical foundation for industrial production and the development of functional foods of L. lactis ZFM559.