Effect of Water Activities of Heating and Recovery Media on Apparent Heat Resistance of Bacillus cereus Spores

Abstract

Spores of Bacillus cereus were heated and recovered in order to investigate the effect of water activity of media on the estimated heat resistance (i.e., the D value) of spores. The water activity (ranging from 0.9 to 1) of the heating medium was first successively controlled with three solutes (glycerol, glucose, and sucrose), while the water activity of the recovery medium was kept near 1. Reciprocally, the water activity of the heating medium was then kept at 1, while the water activity of the recovery medium was controlled from 0.9 to 1 with the same depressors. Lastly, in a third set of experiments, the heating medium and the recovery medium were adjusted to the same activity. As expected, added depressors caused an increase of the heat resistance of spores with a greater efficiency of sucrose with respect to glycerol and glucose. In contrast, when solutes were added to the recovery medium, under an optimal water activity close to 0.98, a decrease of water activity caused a decrease in the estimated D values. This effect was more pronounced when sucrose was used as a depressor instead of glycerol or glucose. When the heating and the recovery media were adjusted to the same water activity, a balancing effect was observed between the protective influence of the solutes during heat treatment and their negative effect during the recovery of injured cells, so that the overall effect of water activity was reduced, with an optimal value near 0.96. The difference between the efficiency of depressors was also less pronounced. It may then be concluded that the overall protective effect of a decrease in water activity is generally overestimated.It has been recognized that the heat resistance of bacterial spores depends on the medium in which the spores are heated. The maximum thermostability of most microorganisms was found in the range of between 0.2 and 0.4 water activity (1,3,27,28,29). In typical ranges of water activities which are found in foodstuffs (a w Ͼ 0.8), the heat resistance of microorganisms generally increases at decreasing water activities. However, the apparent effect of the water activity of the medium on spores or vegetative cells is complicated by the specific effect of solutes which are used as depressors. It is generally agreed that the occurrence of such solutes in the medium reduces the heat resistance of microorganisms. This antagonism between the protective effect of an increase in water activity and the opposite specific effect of depressors can explain conflicting data from various authors.The influence of salt on the thermostability of microorganisms is disputed and depends on the heated type of microorganism. Some authors found no effect of the sodium chloride concentration on the heat resistance of bacteria (9,29,32,42). Others observed a reduced heat resistance of microorganisms at increasing salt concentrations (7,12,22,23). On the contrary, a protective effect of salt was found in several studies (6,14,26,35,38,39,40). Corry (14) deduced from his data that sodium chloride had a therm...

Inactivation of Salmonella enterica by UV-C Light Alone and in Combination with Mild Temperatures

“…It has been hypothesized that lowering the water activity of the treatment medium led to water loss within the cells, which increased DNA condensation and increased UV light resistance (41). The protective effect of sucrose to most inactivating technologies was demonstrated to be more pronounced than that of glycerol (7,31). To avoid misinterpretations, we studied the effect of water activity in the same media added with sucrose (a w values from 0.94 to Ͼ0.99) and reached the same conclusion as with glycerol (data not shown).…”

nfluences of physicochemical stresses on injury and inactivation behaviour of Listeria innocua

“…Several studies demonstrated that the water activity effect depends on the treatment temperature. Coroller et al (2001) and Chiewchan, Pakdee, and Devahastin (2007) concluded that a w effect was more pronounced at higher heating temperatures. Mattick et al (2001), when studying the heat tolerance of Salmonella spp., demonstrated that for temperatures equal or higher than 70°C the bacterial cells were more heat tolerant at low a w than those at higher a w .…”

“…Data available in literature revealed that when a w decreases, a marked increase in microbial thermal resistance is often observed. Results also suggested that the heat resistance of bacteria depends both on strain and type of solute chosen (Mazas, Martinez, Lopez, Alvarez, & Martin, 1999;Coroller, Leguerinel, & Mafart, 2001). The pH of the heating media is also an important factor that influences the thermal microbial inactivation.…”

Impact of environmental water activity on E. faecium thermoresistance and possibility of regeneration of heat-damaged cells in pasteurised canned meat

“…In addition, it also makes allowances for the regeneration of thermally damaged bacterial cells of such pH and water activity that occurred during heating because it corresponds to conditions which occurred in canned materials during their storage. Until recently, the probability of regeneration of microorganisms was checked on media with optimal parameters (pH and a w ), and therefore, the obtained D values were overestimated [10][11][12][13].…”

“…So far, such coefficients were determined for bacteria from Bacillus and Clostridium genus, that is, for sporulating bacteria which should be taken into consideration during the sterilisation process [8,9,11,13,14].…”