Purpose: Our objective was to evaluate E. faecium as a surrogate for Salmonella during peanut thermal processing.
Methods: Peanut kernels (unblanched, medium runner) were inoculated to ca. 7-8 log CFU/g, after drying to original % moisture and water activity (aw), with nalidixic acid resistant strains of S. Enteritidis phage type (SEPT) 30, S. Seftenberg, S. Tennessee or E. faecium (ATCC 8459). Kernels (50 g) were exposed to dry heat at 120 ± 1°C (20, 30, 40 min), 130 ± 1°C (10, 20, 30 min) or 140 ± 1°C (10, 20, 30 min) (n = 6). Populations were enumerated on selective and non-selective media supplemented with nalidixic acid (50 µg/ml). aw and % moisture were measured before inoculation, after inoculation, after drying and after each time-temperature combination.
Results: To be considered a good surrogate, log reductions of E. faecium must be significantly equal to or less than those of Salmonella, indicating it is more heat resistant. Under the conditions tested, log reductions of E. faecium were significantly higher (P < 0.05) than SEPT30 (120°C: 20 min; 130°C: 20, 30 min; 140°C: 10 min), S. Seftenberg (130°C: 20, 30 min; 140°C: 10 min) and S. Tennessee (130°C: 30 min). A log reduction exceeding 5 log CFU/g was only seen for S. Tennessee after 30 min at 140°C. Calculated D- and z-values were highest for SEPT30; D-values: 15.2 min at 120°C, 11.3 min at 130°C, and 7.9 min at 140°C and z-value 70.4°C. Decreases of up to 4.85 % moisture and 0.25 aw occurred following treatment.
Significance: Enterococcus faecium does not appear to be a valid surrogate for Salmonella during thermal processing of peanuts under stagnant dry air conditions; under forced dry air conditions, results may differ.