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.
Bacterial stresses and you may culturing
Salmonella enterica Typhimurium (RMID 1985009 on Research Institute getting Microbial Infection off Osaka University; remote of people inside the sporadic situation), S. enterica Chester, S. enterica Oranienburg (throughout the Aomori Prefectural Lookup Laboratory off Societal Health; remote off dehydrated squid potato chips associated with the an outbreak when you look at the 1999), S. enterica Stanley (RIMD 1981001 on the Search Institute getting Microbial Infection out of Osaka University; remote out of customers during the sporadic situation), and S. enterica Enteritidis (RIMD 1933001 regarding the Browse Institute having Microbial Disorder from Osaka University; remote regarding patients in the sporadic circumstances) were used in this study.
This type of serovars had been was able from the -80°C for the tryptic soya broth (TSB, Merck, Darmstadt, Germany) https://datingranking.net/pl/caribbeancupid-recenzja/ with ten% glycerol. Brand new strains were triggered immediately following incubating in the 37°C for twenty four h with the tryptic soya ager (TSA, Merck) dishes. A remote nest each and every bacterium was then moved to 5 mL out-of TSB for the an excellent sterile centrifuge tubing, incubated within 37°C getting 24 h, immediately after which a 100 ?L aliquot from cultured bacteria is additional to 400 mL TSB and you will incubated from the 37°C to possess forty eight h. The latest cultured tissue was in fact built-up by centrifugation (step three,000 ? g, 10 min) and pellets were resuspended into the 5 mL off clear water. Bacterial-mobile pellets were received of the pipetting off the extreme liquid and gathered towards the a plastic plate. Brand new plates were suspended from the -80°C to possess twenty four h ahead of drying to have 24 h using a good freeze more dry (FDU-2200, EYELA, Tokyo, Japan). Dehydrated microbial tissue had been crushed, listed in an air-rigid basket from the desired cousin moisture (% RH), which had been introduced having fun with over loaded sodium aqueous choices (43% RH: potassium carbonate, 57% RH: sodium bromide, 75% RH: sodium chloride, and you may 87% RH: potassium chloride), and you will kept within 4°C getting forty-eight h. Water interest and you can heat in the air-tight basket had been constantly featured using temperature recorder (TR-72wf, T and you may D, Nagano, Japan). Additionally the liquid activity of bacteria are affirmed of the good liquid pastime meter (Aqualab 4TE, Decagon Equipment, Arizona, USA).
Determination out-of cup changeover temperature (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 .