Effect of protective agents on the viability of Lactococcus lactis subjected to freeze-thawing and freeze-drying

The effect of different protectants and the impact of the initial cell density on the viability of Lactococcus lactis Sr. 3.54 subjected to freeze-thawing and freezedrying was studied. Several additives were tested as protective agents against freezing and drying injuries. Maximum viability of the cells after freeze-thawing was obtained with sucrose and skim milk mixtures as protective agents (78% viability). Freeze-drying with protectants based on skim milk or MRS-broth were most effective (survival levels >60%). The effect of the initial cell load on the final recovery was dependent on the protectant. Every sample showed an increase in viability when a high initial cell concentration (10cfu ml) was used. The blank showed a 1500 fold increase, skim milk/sucrose based lyophilisates an 1,7 fold increase in viability when the initial cell load was changed from 10 to 10cfu ml. The use of 10cfu ml as initial cell concentration and sucrose/skim milk as protectant yielded a lyophilisate with 71% viability. Results suggest the possibility of producing freeze dried powders of Lactococcus lactis with high viability for the food industry.