We hypothesized that there would be a difference among aw conditions in glass transition temperature (Tg) for bacterial cells. Differential scanning calorimetry (DSC) is widely used as a method for measuring Tg [29,38,39]. However, it is difficult to measure Tg of a composite using DSC because the thermogram shows intricate thermal responses . Therefore, here, thermal rheological analysis (TRA) was used to measure Tg. TRA, which measures Tg 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 Tg of cookies [29, 40], hazelnuts , and deep-fried food . To conduct the measurements, a sample is compressed at a temperature below Tg, and heated above Tg with compression. Then, the Tg 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 Tg values, we could confirm the glass transition of bacterial cells. In addition, nazwa użytkownika amolatina we sought to elucidate the influence of aw on bacterial survival and its relationship with Tg. 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 challenges and you can culturing
Salmonella enterica Typhimurium (RMID 1985009 regarding Look Institute for Microbial Disease off Osaka University; isolated of patients inside the sporadic instance), S. enterica Chester, S. enterica Oranienburg (throughout the Aomori Prefectural Look Lab from Public Wellness; isolated out of dried squid chips on the an episode within the 1999), S. enterica Stanley (RIMD 1981001 in the Lookup Institute having Bacterial Diseases from Osaka University; remote away from customers when you look at the sporadic situation), and you may S. enterica Enteritidis (RIMD 1933001 on Search Institute to possess Bacterial Illness out-of Osaka University; separated out-of clients when you look at the sporadic circumstances) were used in this study.
Such serovars were managed on -80°C during the tryptic soya broth (TSB, Merck, Darmstadt, Germany) which has had 10% glycerol. The new challenges was indeed triggered immediately following incubating at 37°C getting 24 h for the tryptic soy ager (TSA, Merck) plates. A remote nest of every bacterium ended up being moved to 5 mL out-of TSB inside a sterile centrifuge pipe, incubated during the 37°C to have 24 h, and a hundred ?L aliquot off cultured bacteria are additional to eight hundred mL TSB and incubated within 37°C to own forty-eight h. The fresh new cultured tissue had been accumulated from the centrifugation (step 3,000 ? grams, ten minute) additionally the pellets had been resuspended into the 5 mL from uncontaminated water. Bacterial-cellphone pellets have been gotten because of the pipetting from the a lot of drinking water and you will collected to the a synthetic dish. This new dishes was frozen within -80°C to possess twenty-four h before drying out getting twenty four h using a freeze drier (FDU-2200, EYELA, Tokyo, Japan). Dehydrated microbial cells was basically crushed, placed in an atmosphere-tight container at wished cousin humidity (% RH), which had been lead using saturated sodium aqueous possibilities (43% RH: potassium carbonate, 57% RH: sodium bromide, 75% RH: salt chloride, and you can 87% RH: potassium chloride), and you will held within 4°C having 48 h. The water activity and you will heat in the air-strict container was basically consistently searched having fun with thermo recorder (TR-72wf, T and D, Nagano, Japan). Additionally the liquids passion of one’s bacteria are verified by the a beneficial liquids activity meter (Aqualab 4TE, Decagon Gadgets, Washington, USA).
Determination out-of cup transition heat (Tg)
Thermal rheological analysis (TRA) was used to measure Tg 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 Tg, the onset temperature of pressure reduction could be regarded as the Tg of the sample .