Factors inhibiting the growth and toxin formation of clostridium botulinum types A and B in "Tsuyu" (Japanese noodle soup)

Abstract

The objective of this study was to evaluate the safety of five different "tsuyu" (Japanese noodle soups) and the effects of incubation temperature, salt concentration, and pH by pursuing the fate of Clostridium botulinum types A and B spores inoculated into them. C. botulinum did not produce toxin in three concentrated types (pH 5.0-5.3, NaCl 8-12%) of "tsuyu" within 3 months at 30°C and in two ready-to-use types (pH 4.83-4.92, NaCl 3.95-4.05) within 6 months at 20 and 30°C. In addition to the "tsuyu" products, positive controls of ready-to-use "tsuyu" with pH-salt combinations adjusted with a sterile NaOH solution to pH 7.0, 6.5, 6.0, and 5.5 and diluted with sterile distilled water to 3, 2, and 1% salt concentrations were included in this study. Growth and toxin production by C. botulinum in positive controls were dependent upon incubation temperature, pH, and salt concentration. That is, no botulinum toxin would occur in "tsuyu" with 4% salt at pH below 6.5 at 20°C; with 1% salt at pH below 5.0 at 30°C; 2 or 3% salt at pH below 5.5 at 30°C; 4% salt at pH below 6.0 incubated at 30°C, even if contaminated with the spores.