Accordingramsly, the relationship ranging from pressure and you will heat analysis during the heat try 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.
Bacterial challenges and culturing
Salmonella enterica Typhimurium (RMID 1985009 regarding the Search Institute getting Bacterial Diseases from Osaka University; isolated off clients for the sporadic circumstances), S. enterica Chester, S. enterica Oranienburg (in the Aomori Prefectural Lookup Lab out-of Societal Wellness; isolated out of dehydrated squid potato chips for the an episode within the 1999), S. enterica Stanley (RIMD 1981001 on Look Institute to own Microbial Problems out of Osaka University; isolated out of clients in the sporadic circumstances), and you can S. enterica Enteritidis (RIMD 1933001 about Look Institute for Bacterial Diseases regarding Osaka University; separated of customers inside sporadic instance) were used in this study.
This type of serovars were handled from the -80°C into the tryptic soy broth (TSB, Merck, Darmstadt, Germany) with 10% glycerol. New challenges were activated shortly after incubating within 37°C to own twenty four h toward tryptic soya ager (TSA, Merck) plates. A remote nest of each micro-organisms ended up being relocated to 5 mL out of TSB in the good sterile centrifuge tube, incubated in the 37°C to possess twenty four h, then a 100 ?L aliquot out of cultured micro-organisms are extra so you’re able to 400 mL TSB and you may incubated at 37°C for forty eight h. The brand new cultured tissues were collected of the centrifugation (step 3,one hundred thousand ? grams, ten min) and also the pellets have been resuspended inside 5 mL regarding clear water. Bacterial-mobile pellets had been acquired because of the pipetting from the too much water and you can collected into a vinyl plate. The new dishes was suspended in the -80°C for twenty-four h ahead of drying out getting twenty-four h using a good freeze dryer (FDU-2200, EYELA, Tokyo, Japan). Dried bacterial cells was basically soil, placed in an air-rigorous container on desired relative humidity (% RH), which was produced playing with saturated salt aqueous solutions (43% RH: potassium carbonate, 57% RH: salt bromide, 75% RH: salt chloride, and 87% RH: potassium chloride), and you may held in the cuatro°C for forty-eight h. Water hobby and temperature floating around-rigorous container were consistently looked playing with temperature recorder (TR-72wf, https://datingranking.net/pl/chatstep-recenzja T and you can D, Nagano, Japan). And drinking water interest of germs was verified from the an effective liquid passion meter (Aqualab 4TE, Decagon Devices, Arizona, USA).
Commitment away from mug change temperatures (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 .