We hypothesized that there would be a difference among a_w conditions in glass transition temperature (T_g) for bacterial cells. Differential scanning calorimetry (DSC) is widely used as a method for measuring T_g [29,38,39]. However, it is difficult to measure T_g of a composite using DSC because the thermogram shows intricate thermal responses . Therefore, here, thermal rheological analysis (TRA) was used to measure T_g. TRA, which measures T_g by attaching a temperature control device to a rheometer, is based on the principle of thermal mechanical analysis [28–30]. Previous studies used by TRA investigated the effect of water content on the T_g of cookies [29, 40], hazelnuts , and deep-fried food . To conduct the measurements, a sample is compressed at a temperature below T_g, and heated above T_g with compression. Then, the T_g of the sample can be determined as a force drop induced by the glass transition. This is a useful method to apply to amorphous powders. By determining T_g values, we could confirm the glass transition of bacterial cells. In addition, we sought to elucidate the influence of a_w on bacterial survival and its relationship with T_g. Finally, we aimed to resolve the relationship between the state change of several Salmonella serotypes that is known to be present in low water activity foods due to glass transition and the changes in thermal resistance in a desiccation environment. The results obtained here will help to understand bacterial survival in a dry environment, which has not been clarified.

Bacterial stresses and you may culturing

Salmonella enterica Typhimurium (RMID 1985009 throughout the Lookup Institute having Microbial Diseases of Osaka University; remote out of people within the sporadic instance), S. enterica Chester, S. enterica Oranienburg (regarding Aomori Prefectural Search Lab out-of Societal Fitness; separated from dehydrated squid chips associated with an outbreak within the 1999), S. enterica Stanley (RIMD 1981001 throughout the Lookup Institute having Bacterial Ailment out of Osaka University; isolated away from patients inside the sporadic situation), and S. enterica Enteritidis (RIMD 1933001 regarding the Browse Institute getting Microbial Ailment off Osaka University; isolated out-of patients from inside the sporadic circumstances) were used in this study.

This type of serovars was in fact maintained on -80°C in the tryptic soya broth (TSB, Merck, Darmstadt, Germany) containing ten% glycerol. This new strains have been triggered once incubating during the 37°C to possess twenty-four h to the tryptic soy ager (TSA, Merck) dishes. A remote nest each and every bacteria was then gone to live in 5 mL away from TSB during the an excellent sterile centrifuge tube, incubated at the 37°C getting twenty four h, and a 100 ?L aliquot regarding cultured bacterium was added so you can 400 mL TSB and you will incubated on 37°C having forty eight h. The fresh new cultured structure was indeed accumulated by centrifugation (step 3,one hundred thousand ? logowanie bondagecom g, ten minute) and the pellets was indeed resuspended for the 5 mL out of clear water. Bacterial-mobile pellets were obtained by pipetting from the extreme water and you will accumulated toward a plastic dish. New plates had been frozen at the -80°C for twenty-four h in advance of drying to possess twenty four h having fun with a great freeze dryer (FDU-2200, EYELA, Tokyo, Japan). Dried microbial cells was in fact surface, placed in an environment-rigorous basket in the wanted cousin moisture (% RH), that was produced using soaked sodium aqueous possibilities (43% RH: potassium carbonate, 57% RH: salt bromide, 75% RH: sodium chloride, and you can 87% RH: potassium chloride), and you may kept at 4°C to have forty eight h. The water craft and you may temperatures floating around-strict basket was basically consistently featured playing with temperature recorder (TR-72wf, T and D, Nagano, Japan). And the liquid craft of bacteria try verified of the good liquids passion meter (Aqualab 4TE, Decagon Equipment, Arizona, USA).

Devotion out-of cup transition temperature (Tg)

Thermal rheological analysis (TRA) was used to measure T_g by attaching a temperature control device to a rheometer (EZ-SX, SHIMADZU, Kyoto, Japan) (illustrated in Fig 1); the analysis is based on the principle of thermal mechanical analysis [28–30]. A dried bacterial cell sample (ca. 100 mg) was placed in the forming die (? = 3 mm) and compacted with a rheometer at ca. 10 MPa. Subsequently, the sample was compressed at ca. 5 MPa ca. for 1 to 3 min and then heated at a rate of approximately 3°C/min until the temperature reached 120°C. Pressure-time data were collected with software attached to the rheometer. In parallel, a thermocouple was attached to the bottom of the forming die and time-temperature data were collected every second using a data logger. Since pressure reduction begins at the point at which the bottom temperature of the sample reaches the mechanical T_g, the onset temperature of pressure reduction could be regarded as the T_g of the sample .