Temperature in agar plates and its influence on the results of quantitative microbiological food analyses

The numbers of colony forming units (cfu) of some strains of Enterobacteriaceae growing in Violet Red bile agar at 44 degrees C can vary considerably depending on incubation conditions and location of an individual agar plate in a stack. The reason is that the heating-up rates of the agar in plates incubated at elevated temperatures are slower if the incubators do not have an air circulator and/or if the plates are located in the centre of a stack. Variability in agar temperatures among different plates after 24 h of incubation at 44 degrees C was higher if plates were placed in an incubator with an air circulator than in an incubator without one. This effect could be avoided if the plates were incubated enclosed in a plastic bag.