Effect of Nitrite Concentration on Clostridium perfringens Growth during Extended Cooling of Cured Ham

Introduction: The Food Safety and Inspection Service (FSIS) provides Appendix B as a guideline to define cooling parameters needed to limit C. perfringens growth to < 1-log increase in cured (100 ppm NaNO₂) meat and poultry products.  Few published studies have compared the growth of C. perfringens in cured products formulated with nitrite concentrations greater than 100 ppm, along with extended cooling profiles which are relevant to products such as large diameter hams.

Purpose: To determine the inhibition of Clostridium perfringens growth in a 25-h biphasic cooling curve using various concentrations of nitrite.

Methods: Six treatments of ground ham were prepared with 0, 50, 75, 100, 150, or 200 ppm NaNO₂ and 547 ppm sodium erythorbate.  Formulations were inoculated with C. perfringens (3-strain mixture) to yield 3 log CFU/g.  Individual 50-g portions were vacuum-packed, cooked to 72°C, and cooled from 54.4°C to 26.7°C in 10 h, then from 26.7°C to 7.2°C in an additional 15 h.  Triplicate samples were assayed for C. perfringens growth at 0, 5, 7.5, 10, and 25 h by plating onto tryptose-sulfite-cycloserine agar.  The study was repeated three times.

Results: Populations of C. perfringens increased an average 6.4, 4.4, and 1.9 log at 7.5 h (~34°C) for the 0, 50, and 75 ppm nitrite treatments, respectively, but no additional growth was observed for the remaining 17.5 h cooling from 34 to 7.2°C.  In contrast, samples with 100, 150, and 200 ppm nitrite plus 547 ppm erythorbate prevented < 1.0-log increase for the duration of the 25-h experiment.

Significance: The study showed that combinations of > 100 ppm NaNO₂ plus 547 ppm erythorbate are effective in inhibiting C. perfringens in ham through a 25-h cooling period.  Therefore, it can be concluded cured meat and poultry products, such as ham, can be safely cooled following the longer cooling parameters identified in this study.