Effect of different protectants on viability of thermotolerant acetic acid bacterium Acetobacter sengalenisis

Production of vinegar at high temperature (>37° C) needs special processes and equipments; one
of the key elements in the process, is the accessibility of active and stable starters. In this study
the influences of different cryo-protective agents on some steps (freezing, drying and storage) of
starter production were investigated. To achieve this goal, Acetobacter senegalensis, was used
as a thermotolerant acetic acid bacterium.
Glucose was used as carbon source in fermentor to produce biomass. Different cryo-protectants
(manitol (20%), glycerol (3%), sucrose (10%), trehalose (5%), glutamate (3%), maltodextrin
(10%), skimmed milk (10%) and spent growth medium) were added to washed and unwashed
biomass. The lyophilized cells (92-93% water content) were stored in darkness under different
temperatures (-20° C, +4° C and 35° C). The viability of cells after rehydration, activity of
glucose dehydrogenase, gluconate dehydrogenase and soluble protein contents were determined
up to 6 months.
According to the results, washing of cells by tap water has no effect on viability of cells during
freezing and more than 97% of cells are alive in all treatments. After lyophilization, unwashed
cells showed higher viability in all treatments in comparison to washed cells. On the basis of
residual viable cells, manitol, maltodextrin, and spent growth medium showed the highest
protective effects (92.3%, 88.2% and 82.1% survival, respectively) on cells during drying
process whereas glycerol had the lowest protective effect on viability (15.4% survival).
During storage of lyophilized cells at 35° C, 100% of cells are dead in all treatments after 15
days. Unwashed cells treated with manitol, maltodextrin and spent growth medium showed
79.2%, 68.3% and 62.7% viability, respectively after keeping at 4°C for 6 months.
There is direct relationship between the soluble protein contents of cells and storage temperature.
Cells stored at -20° C showed highest soluble protein contents after 6 months of storage while
the lowest amount of soluble protein contents was detected in cells stored at 35° C. On the other
hand, glucose dehydrogenase and gluconate dehydrogenase activities decreased during storage of
cells at 4°C, whereas more than 90% of the enzymes activity remained during storage of
different cells at -20° C, so it can be assumed that higher temperature can inactivate cell proteins.
In conclusion, lyophilization of Acetobacter senegalensis by the mentioned methods can provide
a promising and economic tool for production of stable and active vinegar starters.