Correctly, the relationship ranging from stress and temperature research through the heating is determined

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 strains and culturing

Salmonella enterica Typhimurium (RMID 1985009 on the Search Institute to have Microbial Sickness out-of Osaka University; isolated out-of customers into the sporadic circumstances), S. enterica https://datingranking.net/pl/asiandate-recenzja/ Chester, S. enterica Oranienburg (from the Aomori Prefectural Look Laboratory away from Societal Wellness; separated out-of dried squid potato chips of this an outbreak into the 1999), S. enterica Stanley (RIMD 1981001 regarding Lookup Institute for Bacterial Disease away from Osaka University; remote of people from inside the sporadic circumstances), and you will S. enterica Enteritidis (RIMD 1933001 about Search Institute having Microbial Sickness regarding Osaka University; isolated out of clients for the sporadic circumstances) were chosen for this study.

These types of serovars was basically was able in the -80°C during the tryptic soy broth (TSB, Merck, Darmstadt, Germany) that contains 10% glycerol. The fresh new strains were triggered once incubating within 37°C to possess twenty-four h into the tryptic soya ager (TSA, Merck) dishes. An isolated nest of each bacterium ended up being gone to live in 5 mL regarding TSB in the a good sterile centrifuge tube, incubated in the 37°C getting 24 h, following a 100 ?L aliquot off cultured germs was additional so you’re able to 400 mL TSB and incubated at 37°C to own forty eight h. The fresh new cultured structure was indeed collected by centrifugation (step 3,one hundred thousand ? grams, ten minute) and also the pellets had been resuspended inside the 5 mL from uncontaminated water. Bacterial-mobile pellets was indeed received from the pipetting off the too much liquid and you can accumulated on the a synthetic dish. The new dishes was indeed suspended in the -80°C to have twenty four h before drying getting twenty four h playing with a good frost dryer (FDU-2200, EYELA, Tokyo, Japan). Dried microbial cells was basically soil, placed in an atmosphere-rigorous container at the wanted cousin humidity (% RH), that has been put using over loaded sodium aqueous possibilities (43% RH: potassium carbonate, 57% RH: salt bromide, 75% RH: sodium chloride, and you will 87% RH: potassium chloride), and you can held at the cuatro°C getting forty-eight h. Water craft and you may temperatures in the air-rigid container was consistently appeared playing with thermo recorder (TR-72wf, T and you will D, Nagano, Japan). Therefore the water passion of one’s micro-organisms was verified from the a good h2o activity meter (Aqualab 4TE, Decagon Products, Arizona, USA).

Devotion out-of glass change temperatures (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 .