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Abstract
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 (1010cfu ml-1) 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 1009 to 1010cfu ml-1. The use of 1010cfu ml-1 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.
Highlights
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
As expected lyophilisates obtained with skim milk and MRSbroth based protective agents yielded the best results
This study showed the impact of protective additives on the viability after freeze-thawing and freeze-drying
Summary
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. 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 1009 to 1010cfu ml-1. The use of 1010cfu ml-1 as initial cell concentration and sucrose/skim milk as protectant yielded a lyophilisate with 71% viability. As water is removed from the biomembrane, the headgroups of the lipids are brought closer together and this results in increased van der Waals interaction between the acyl chains This forces the biomembrane into the gel phase at room temperature. It has been suggested that sugars depress the phase transition in dry phospholipids by forming hydrogen bounds with the polar headgroups This is known as the water replacement hypothesis.
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