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.
Microbial challenges and you may culturing
Salmonella enterica Typhimurium (RMID 1985009 on the Search Institute for Microbial Disorder from Osaka University; isolated from clients during the sporadic case), S. enterica Chester, S. enterica Oranienburg (from the Aomori Prefectural Research Laboratory off Societal Wellness; isolated regarding dried squid potato chips from the an outbreak in 1999), S. enterica Stanley (RIMD 1981001 regarding the Look Institute to own Bacterial Sickness from Osaka University; isolated off patients inside the sporadic situation), and you may S. enterica Enteritidis (RIMD 1933001 about Look Institute to possess Microbial Diseases off Osaka University; remote regarding people within the sporadic situation) were used in this study.
These types of serovars was indeed was able within -80°C when you look at the tryptic soya broth (TSB, Merck, Darmstadt, Germany) with ten% glycerol. The latest stresses were triggered immediately following incubating within 37°C to own twenty four h to your tryptic soy ager (TSA, Merck) plates. An isolated colony of each and every micro-organisms ended up being moved to 5 mL off TSB within the an effective sterile centrifuge tube, incubated in the 37°C for twenty-four h, and an one hundred ?L aliquot regarding cultured bacterium is extra to eight hundred mL TSB and you will incubated during the 37°C to possess forty eight h. The brand new cultured tissue was indeed amassed because of the centrifugation (step three,one hundred thousand ? grams, ten min) and the pellets was resuspended in the 5 mL out of clear water. Bacterial-telephone pellets was in fact acquired of the pipetting from the too-much drinking water and xmeets bezplatná zkuÅ¡ebnà verze you can compiled to the a plastic material plate. The fresh new dishes was indeed suspended during the -80°C getting 24 h ahead of drying out having twenty four h having fun with good freeze drier (FDU-2200, EYELA, Tokyo, Japan). Dried bacterial tissue have been crushed, listed in an air-tight container at wished cousin humidity (% RH), that has been brought playing with over loaded salt aqueous choices (43% RH: potassium carbonate, 57% RH: sodium bromide, 75% RH: salt chloride, and you will 87% RH: potassium chloride), and stored from the cuatro°C to possess 48 h. Water activity and you will temperature in the air-rigid container was indeed continuously appeared using thermo recorder (TR-72wf, T and you can D, Nagano, Japan). And the h2o craft of micro-organisms is actually affirmed of the a drinking water pastime meter (Aqualab 4TE, Decagon Gizmos, Arizona, USA).
Commitment from cup 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 .
