The effects of combined preservative systems on bacterial spores and non-sporeformers
The effects of the combination of heat, chemicals and pH on Staphylococcus aureus (NCIB 6571), Escherichia coli (NCIB 8114) and particularly Bacillus cereus (NCIB 6349,7464) and B. stearothermophilus (NCIB 8919,8920) spores were studied in five test systems. The systems contained (a) potassium sorbate (0 - 1%) or (b) sodium chloride (0 - 10%) or (c) sodium benzoate (0 - 3%) or (d) combination of (a) and (b) or (e) combination of (a) and (c) at varying pH levels (4.2 - 7.1). The test systems were heated at varying temperatures ranging from 500 - 95°C at different times. In general inhibition of all tests strains by all systems increased with increasing levels of chemicals used and with decrease in pH. Low temperature heat treatment in the presence of sorbate at low pH caused almost total inhibition of S. aureus and E. coli but had marginal to moderate inhibition on bacterial spores. In the presence of either sodium chloride or benzoate at low pH temperatures of 85° - 95°C provided marginal to moderate inhibition of these spores. Total inhibition of the spores and synergistic inhibitory effects were obtained only with combination of sorbate and sodium chloride or combination of sorbate and benzoate at low pH with low heat treatment. The recovery of surviving spores was slightly inhibited by incubation temperatures of 37° and 60°C for B. cereus and B. stearothermophilus respectively wheras 44°C markedly inhibited both spores after exposure to selected combination treatments. The potential of the Direct Fluorescence Microscopy technique for the enumeration of spores after exposure to combination preservative systems was indicated. Evidence from Direct Fluorescence Microscopy, Scanning Electron Microscopy and spore extraction studies revealed changes in spore wall structure. Possible mechanisms explaining the observed inhibition of spores by the combination treatments are postulated. A computer program is described for the presentation of data generated from exposure of spores to combination treatments in the form of 3D-histograms.