Effect of pretreatment, lyophilization parameters and different cryoprotectants on the efficiency of probiotic freeze-drying immobilization

Lyophilization is an excellent process to increase the shelf life of food products or preserve probiotics. Living cells prefer mild conditions, and any deviation (vacuum, high or low temperatures) leads to cell damage. This paper examined the influence of different freeze-drying process parameters on the survival of probiotic Lactobacillus plantarum immobilized on the activated charcoal pad. In specific, the process included several phases in which different pretreatments, freezing temperatures, cryoprotectants and phase durations were investigated. Activation of L. plantarum in De Man, Rogosa and Sharpe (MRS) broth before freezing, resulted in the increased initial number of living cells, but also positively affected the cell survival after the lyophilization process. Freezing the culture in liquid nitrogen did not significantly affect cell viability after lyophilization compared to deep freezing at -80?C, while incubation of the culture in a refrigerator for 2 h before lyophilization increased the probiotic viability. Also, the prolonged duration of lyophilization from 5 to 48 h had a slight impact on probiotic viability. The use of milk showed a significant increase in culture survival, while sucrose, maltose, and trehalose showed cryoprotectant ability, but significantly lower than that of milk. The best lyophilization protocol resulting in the highest viability of L. plantarum included the culture activation by MRS incubation, followed by either deep freezing in liquid nitrogen or by precooling for 2 h at 7?C and then deep freezing at -80?C in milk, and ending with lyophilization for 5 h.