The biomass produced from each organism after fermentation
was collected and centrifuged at 10 000 rpm in a Beckman Model
J2 e 21 (Beckman Coulter, Halfway House, South Africa), Rotor JA 14
for 10 min. After centrifugation, the pellet was collected and
weighed. Glycerol (2%) and maltodextrin (5%) were added to each
pellet as cryoprotectants. The amount of glycerol and maltodextrin
added was calculated as a percentage of the mass of the cell
biomass (pellet) to which they were added. A sample was taken for
plate counts on MRS agar to evaluate cell viability. Cell viability of
the pellet was assessed by transferring 0.1 g of pellet to 0.9 g
(900 mL) of sterile water and diluting this sample in a ten-fold
dilution series, with subsequent spread plating on MRS agar and
incubating plates at 30 C for 48 h. The cell material with cryoprotectants
added was then freeze dried.
The freeze-drying process was carried out in a Genesis 25 L
freeze-dryer (VirTis, United Scientific, Gardiner, New York State,
USA) at a vacuum set point of 80 mT. The freeze-dried material
obtained varied from strain to strain, but generally ranged from ca.
1e5 g and was packaged into sterile aluminium bags and stored
at 20 C. The viability of the freeze-dried biomass was evaluated
as above. Plate counts were done in triplicate on MRS agar and
mean values were calculated.