Thermal susceptibility of Streptococcus faecium strains isolated from frankfurters

Abstract

The heat resistance of nine strains of Streptococcus faecium isolated from frankfurters was determined at 63 and 68 degrees C in brain heart infusion broth. Exponential-phase cultures (approximately 10(7) colonies/mL) were used as inoculants. The heat resistance of S. faecium DP2181, a moderately resistant isolate, was further examined in broth (55, 63, and 68 degrees C) and frankfurter emulsion (63 and 68 degrees C). The decimal reduction times (D values) were determined by regression. In broth, both time-temperature combinations resulted in a 3-4 log decline in bacterial numbers for the nine S. faecium strains tested. For S. faecium DP2181, the survivor curves deviated from the logarithmic order of death at all three heating temperatures. An initial slow period of death was evident at 55 degrees C and a resistant tail of organisms was observed at 55, 63, and 68 degrees C. The D55D63, and, D68 values for the logarithmic portion of the corresponding survivor curves were 105.6, 9.36, and 3.34 min, respectively. The survival of DP2181 was enhanced by the frankfurter emulsion. The results indicate that populations of S. faecium existed that were very heat resistant and could survive normal frankfurter processing if initially present in high numbers.