State diagrams and sorption isotherms of bacterial suspensions and fermented medium

Abstract

Physical properties of lactic acid bacteria suspensions were determined by means of state diagrams and sorption isotherms. Differential scanning calorimetry (DSC) was used to establish the thermal transitions of concentrated “fresh” (before freeze-drying) and freeze-dried bacterial suspension equilibrated at different relative humidities. The bacterial cells and the re-suspending medium (fermented culture medium) were studied separately in order to determine each individual effect. No glass transition ( T g) was detected in thermal profiles of washed bacterial cells. Correspondingly, T g curves were similar for fermented medium and bacterial suspension. Simple glass transitions (one heat capacity step) were observed at T g in the case of low moisture samples. For samples containing more than 60.7% water, two steps remained in the glass transition region ( T g1′ and T g2′), even after annealing treatments. Water plasticising effect was well predicted by Gordon–Taylor equation. The invariant point ( C g′, T g′), characteristic of maximally freeze-concentrated samples, was estimated from the intersection of the T g curve and the melting curve T m. A linear relationship was proposed to rapidly predict the T g′ region of complex aqueous solutions, from T g′ values of single solute aqueous solutions. Water sorption properties of fermented medium and bacterial suspension were determined at 25°C and described by the Guggenheim–Anderson–de Boer equation.