Comparison of different reducing agents for enhanced detection of heat-injured Listeria monocytogenes

The effect of different reducing agents (L-cysteine, Oxyrase, and Enterococcus faecium) and their combinations on the detection of heat-injured (62.8°C, 7.5 min or 10 min) Listeria monocytogenes was examined. The incorporation of L-cysteine (0.5 g/liter) yielded higher percentage detection than any of the other reducing agents (P < 0.05). The combination of Oxyrase (10 U/ml) and E. faecium (10³ CFU/ml) synergistically enhanced the detection of L. monocytogenes heat-injured for 10 min at 62.8°C (P < 0.05). Simultaneous addition of b-cysteine (0.5 g/liter) and Oxyrase (10 U/ml) significantly lowered the recovery of heat-injured L. monocytogenes (P < 0.05). Higher activities of Oxyrase (50 U/ml) inhibited the detection of heat-injured L. monocytogenes (P < 0.05). The rates of depletion of relative percentage O₂ were in the order: L-cysteine (0.5 g/liter; 6.63%/ min) > Oxyrase (10 U/ml; 5.00%/min) > E. faecium (10⁸ CFU/ml; 1.66%/min) > E. faecium (10³ CFU/ml; 0.20%/min). The final levels of redox potential (E(h)) achieved were -110.5 mV, -100 mV, -83.5 mV, and -25 mV for E. faecium (10⁸ CFU/ml), L-cysteine, Oxyrase, and E. faecium (10³ CFU/ml), respectively.