Survival and preservation after freeze-drying process of thermoresistant acetic acid bacteria isolated from tropical products of Subsaharan Africa

Abstract

Two thermoresistant acetic acid bacteria (TAAB) were previously isolated and selected for a sustainable development of vinegar fermentation in Subsaharan Africa. Their use as a starter culture in vinegar manufactures in such regions could reduce considerably water cooling expenses. For optimising biomass preservation, the effect of 20% w/w mannitol as cryoprotectant on the cells viability after freeze-drying process and during storage was evaluated. Results showed that freeze-dried cells could be conserved at 4 °C for at least 6 months without loss of viability. The main reasons were that cryoprotectant tends to lower the water activity ( a w) and to maintain a temperature of product weaker than that of the glass transition temperature T g. Furthermore, the heat resistance of freeze-dried cells during storage was all the more increased that strains were cryoprotected. In addition, intrinsically, an increase of saturated fatty acids with the temperature is the essential modification in the lipidome level of membrane cells when the fermentation occured at a temperature of 30 °C. Tolerance to heat during storage was significantly enhanced under such mechanisms.