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 you may culturing
Salmonella enterica Typhimurium (RMID 1985009 from the Browse Institute having Microbial Illness out-of Osaka University; separated out of people for the sporadic circumstances), S. enterica Chester, S. enterica Oranienburg (on Aomori Prefectural Lookup Research out of Social Fitness; remote regarding dried squid chips for the an outbreak when you look at the 1999), S. enterica Stanley (RIMD 1981001 in the Browse Institute to possess Microbial Ailment of Osaka University; remote regarding clients from inside the sporadic circumstances), and S. enterica Enteritidis (RIMD 1933001 throughout the Look Institute to have Microbial Sickness out of Osaka University; remote off customers during the sporadic instance) were used in this study.
These types of serovars was basically maintained within -80°C during the tryptic soy broth (TSB, Merck, Darmstadt, Germany) which includes 10% glycerol. New challenges had been triggered after incubating during the 37°C to have twenty-four h to the tryptic soy ager (TSA, Merck) plates. An isolated colony of each bacterium ended up being moved to 5 mL off TSB within the an excellent sterile centrifuge tubing, incubated at the 37°C having 24 h, right after which an one hundred ?L aliquot out-of cultured micro-organisms try added so you can 400 mL TSB and incubated on 37°C getting 48 h. Brand new cultured structure was obtained by centrifugation (step 3,100000 ? g, 10 minute) together with pellets had been resuspended within the 5 mL off clear water. Bacterial-mobile pellets were received from the pipetting off of the continuously drinking water and you may compiled with the a plastic plate. Brand new plates was indeed suspended in the -80°C to possess 24 h ahead of drying for 24 h having fun with an excellent freeze dryer (FDU-2200, EYELA, Tokyo, Japan). Dehydrated bacterial cells was indeed surface, placed in an atmosphere-rigorous basket on desired cousin humidity (% RH), that was introduced using over loaded sodium aqueous selection (43% RH: potassium carbonate, 57% RH: sodium bromide, 75% RH: salt chloride, and https://datingranking.net/cs/the-inner-circle-recenze/ you can 87% RH: potassium chloride), and you may held at 4°C having forty eight h. The water interest and you can temperature floating around-strict container was continuously appeared having fun with thermo recorder (TR-72wf, T and you will D, Nagano, Japan). Additionally the drinking water hobby of your own bacterium is actually confirmed by the good drinking water hobby meter (Aqualab 4TE, Decagon Gizmos, Washington, USA).
Commitment off mug change 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 .