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 http://datingranking.net/pl/chatib-recenzja obtained here will help to understand bacterial survival in a dry environment, which has not been clarified.
Microbial challenges and you can culturing
Salmonella enterica Typhimurium (RMID 1985009 in the Browse Institute to own Microbial Disorder off Osaka University; separated regarding customers in the sporadic circumstances), S. enterica Chester, S. enterica Oranienburg (in the Aomori Prefectural Lookup Lab off Social Wellness; separated regarding dehydrated squid chips on the an outbreak for the 1999), S. enterica Stanley (RIMD 1981001 throughout the Look Institute to own Microbial Illness regarding Osaka University; separated off customers when you look at the sporadic instance), and S. enterica Enteritidis (RIMD 1933001 on the Research Institute to have Bacterial Diseases away from Osaka University; remote regarding patients from inside the sporadic case) were used in this study.
Such serovars had been maintained at the -80°C for the tryptic soya broth (TSB, Merck, Darmstadt, Germany) with 10% glycerol. The new stresses were triggered shortly after incubating at 37°C to have 24 h to your tryptic soy ager (TSA, Merck) plates. An isolated nest of every micro-organisms was then transferred to 5 mL out of TSB when you look at the a beneficial sterile centrifuge tubing, incubated from the 37°C to have twenty four h, then a 100 ?L aliquot out-of cultured germs was additional to eight hundred mL TSB and incubated from the 37°C getting 48 h. Brand new cultured cells had been amassed by centrifugation (step 3,100000 ? g, ten minute) therefore the pellets had been resuspended inside 5 mL out of clear water. Bacterial-cellphone pellets was basically acquired by the pipetting off of the too-much water and gathered into a plastic material dish. New dishes was indeed suspended from the -80°C to have twenty four h just before drying to have twenty four h playing with an excellent freeze dryer (FDU-2200, EYELA, Tokyo, Japan). Dried microbial tissues was indeed crushed, placed in an air-tight container from the wanted cousin dampness (% RH), which had been delivered using saturated salt aqueous alternatives (43% RH: potassium carbonate, 57% RH: salt bromide, 75% RH: sodium chloride, and you can 87% RH: potassium chloride), and stored on cuatro°C to possess forty eight h. The water activity and you will heat in the air-rigorous container were continuously checked playing with temperature recorder (TR-72wf, T and you may D, Nagano, Japan). Together with water activity of your micro-organisms is verified of the an excellent water passion meter (Aqualab 4TE, Decagon Products, Washington, USA).
Dedication out of mug 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 .