State diagrams and sorption isotherms of bacterial suspensions and fermented medium

Physical properties of lactic acid bacteria suspensions were determined by means of state diagrams and sorption isotherms. Differential scanning calor imetry (DSC) was used to establish the thermal transitions of concentrated "fresh" (before freeze drying) and freeze-dried bacterial suspension equili brated at different relative humidities. The bacterial cells and the resusp ending medium (fermented culture medium) were studied separately in order t o 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 tr ansitions tone 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 r emained in the glass transition region (T-g1' and T-g2'), even after anneal ing treatments. Water, plasticising effect was well predicted by Gordon-Tay lor 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 ra pidly predict the T-g' region of complex aqueous solutions, from T-g' value s of single solute aqueous solutions. Water sorption properties of fermente d medium and bacterial suspension were determined at 25 C and described by the Guggenheim-Anderson-dr Beer equation. (C) 2001 Elsevier Science B.V. Al l rights reserved.