Critical water activity and amorphous state for optimal preservation of lyophilised lactic acid bacteria

► The relationships between water activity, glass transition temperature, and acidification activity of bacteria were investigated. ► The stability of freeze-dried lactic acid bacteria appears related to the monolayer water coverage estimated from the sorption sorption. ► The degradation of acidification activity of bacteria increases at very low moisture contents. ► A promising tool for optimising freeze-drying process and predicting bacteria stability was proposed.