title: Impact of water activity, temperature, and physical state on the storage stability of Lactobacillus paracasei ssp. paracasei freeze-dried in a lactose matrix.
abstract: The aim of this study was to determine whether the combined effect of water activity and temperature on inactivation rates of freeze-dried microorganisms in a lactose matrix could be explained in terms of the glass transition theory. The stabilized glass transition temperature, Tg, of the freeze-dried products was determined by differential scanning calorimetry at two different temperatures, T (20 and 37 degrees C), and different water activities (0.07-0.48). This information served as a basis for defining conditions of T and water activity, which led to storage of the bacteria in the glassy (T < Tg) and nonglassy (T > Tg) states. The rates of inactivation of the dry microorganisms subjected to different storage conditions were determined by plate counts and could be described by first-order kinetics. Rates were analyzed as a function of water activity, storage temperature, and the difference between Tg and T. Inactivation below Tg was low; however, Tg could not be regarded as an absolute threshold of bacteria stability during storage. When the cells were stored in the nonglassy state (T > Tg), inactivation proceeded faster, however, not as rapid as suggested by the temperature dependence of the viscosity above the glass transition temperature. Furthermore, the first-order rate constant, k, was dependent on the storage temperature per se rather than on the temperature difference between the glass transition temperature and the storage temperature (T - Tg).
authors.affiliation: Food Process Engineering and Dairy Technology, Technische Universität München, Weihenstephaner Berg 1, D-85354 Freising, Germany.
authors.forename: Bettina
authors.initials: B
authors.lastname: Higl
authors.forename: Lone
authors.initials: L
authors.lastname: Kurtmann
authors.forename: Charlotte U
authors.initials: CU
authors.lastname: Carlsen
authors.forename: Jennifer
authors.initials: J
authors.lastname: Ratjen
authors.forename: Petra
authors.initials: P
authors.lastname: Först
authors.forename: Leif H
authors.initials: LH
authors.lastname: Skibsted
authors.forename: Ulrich
authors.initials: U
authors.lastname: Kulozik
authors.forename: Jens
authors.initials: J
authors.lastname: Risbo
mesh_terms: D001419:Bacteria / Q000378:metabolism; D001709:Biotechnology / Q000295:instrumentation* / Q000379:methods*; D003080:Cold Temperature; D005612:Freeze Drying; D005898:Glass; D007700:Kinetics; D007778:Lactobacillus / Q000378:metabolism*; D007785:Lactose / Q000737:chemistry*; D013048:Specimen Handling; D013696:Temperature; D014867:Water / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D014867:Water; D007785:Lactose