Appropriately, the connection ranging from pressure and you may heat studies throughout the temperature was determined
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, 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.
Microbial strains and you may culturing
Salmonella enterica Typhimurium (RMID 1985009 about Lookup Institute for Microbial Ailment from Osaka University; separated off clients into the sporadic circumstances), S. enterica Chester, S. enterica Oranienburg (regarding Aomori Prefectural Look Lab out-of Societal Wellness; remote regarding dehydrated squid chips associated with an outbreak in 1999), S. enterica Stanley (RIMD 1981001 on the Research Institute for Microbial Infection out of Osaka University; separated of patients inside sporadic instance), and you can S. enterica Enteritidis (RIMD 1933001 about Browse Institute to possess Microbial Sickness out-of Osaka University; remote out-of customers in the sporadic instance) were chosen for this study.
Such serovars have been maintained on -80°C within the tryptic soy broth (TSB, Merck, Darmstadt, Germany) with ten% glycerol. This new strains was in fact triggered shortly after incubating from the 37°C to own 24 h https://datingranking.net/pl/blackpeoplemeet-recenzja/ to your tryptic soy ager (TSA, Merck) plates. An isolated nest of each and every bacterium was then moved to 5 mL from TSB during the an effective sterile centrifuge tube, incubated within 37°C to have 24 h, following a 100 ?L aliquot away from cultured germs was extra so you’re able to eight hundred mL TSB and incubated in the 37°C for forty eight h. New cultured tissues was basically compiled by the centrifugation (step three,000 ? grams, ten minute) therefore the pellets was in fact resuspended in the 5 mL out-of uncontaminated water. Bacterial-cell pellets have been acquired by pipetting off of the continuously h2o and you will gathered to the a plastic dish. This new dishes was in fact suspended during the -80°C to have twenty four h ahead of drying to own twenty-four h using an effective freeze dryer (FDU-2200, EYELA, Tokyo, Japan). Dried microbial tissues was basically soil, placed in an environment-tight container from the desired cousin moisture (% RH), that was delivered having fun with soaked sodium aqueous selection (43% RH: potassium carbonate, 57% RH: sodium bromide, 75% RH: salt chloride, and you will 87% RH: potassium chloride), and kept at 4°C for forty-eight h. The water passion and heat floating around-strict container was continuously searched having fun with temperature recorder (TR-72wf, T and D, Nagano, Japan). While the water activity of your micro-organisms was confirmed by the a great water passion meter (Aqualab 4TE, Decagon Products, Washington, USA).
Commitment out-of glass change 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 .