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 culturing
Salmonella enterica Typhimurium (RMID 1985009 in the Search Institute having Microbial Diseases from Osaka University; separated from customers for the sporadic situation), S. enterica Chester, S. enterica Oranienburg (throughout the Aomori Prefectural Search Laboratory out-of Social Fitness; remote regarding dried squid potato chips of the an outbreak inside 1999), S. enterica Stanley (RIMD 1981001 from the Look Institute to have Bacterial Illness regarding Osaka University; remote regarding clients in the sporadic situation), and S. enterica Enteritidis (RIMD 1933001 regarding Lookup Institute to have Microbial Disease www.datingranking.net/pl/adventist-singles-recenzja/ out of Osaka University; separated of clients in the sporadic circumstances) were used in this research.
Such serovars was basically was able from the -80°C within the tryptic soy broth (TSB, Merck, Darmstadt, Germany) which has 10% glycerol. The fresh new stresses was activated after incubating in the 37°C to have 24 h on tryptic soy ager (TSA, Merck) plates. A remote colony of each germs ended up being moved to 5 mL out-of TSB into the a sterile centrifuge tubing, incubated at 37°C for 24 h, following a hundred ?L aliquot out-of cultured bacteria was added to help you 400 mL TSB and you will incubated on 37°C to have 48 h. This new cultured muscle had been amassed from the centrifugation (3,100000 ? grams, 10 min) while the pellets was in fact resuspended in the 5 mL from pure water. Bacterial-telephone pellets have been acquired because of the pipetting off of the excessive liquids and you may accumulated to your a synthetic dish. The latest dishes were suspended at the -80°C for 24 h prior to drying out having twenty-four h using good frost drier (FDU-2200, EYELA, Tokyo, Japan). Dehydrated bacterial cells had been ground, placed in an air-strict basket at wished cousin dampness (% RH), which had been put using soaked sodium aqueous choices (43% RH: potassium carbonate, 57% RH: salt bromide, 75% RH: salt chloride, and you may 87% RH: potassium chloride), and stored during the cuatro°C to have forty-eight h. Water interest and heat floating around-tight basket was consistently seemed using temperature recorder (TR-72wf, T and you will D, Nagano, Japan). Therefore the liquid craft of the micro-organisms was confirmed of the a beneficial drinking water activity meter (Aqualab 4TE, Decagon Devices, Arizona, USA).
Dedication from mug changeover 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 .