The recovery of heat-injured Listeria monocytogenes Scott A in Fraser broth (FB) supplemented with sodium thioglycolate, sodium pyruvate, L-(+)-cysteine hydrochloride, catalase or Oxyrase? was studied. After 3 h of incubation at 30°C, recovery was enhanced by all oxygen scavengers except sodium pyruvate. Oxyrase? (0.005 U ml-1) promoted the highest recovery (34%) compared to recovery in control broth (19%). All oxygen scavengers enhanced the recovery of injured L. monocytogenes in FB within 6 h of incubation. After 6 h at 30°C, 49 and 55% of injured cells underwent resuscitation in FB containing 2.5 mg of sodium pyruvate ml-1 and 400 μg of catalase ml-1, respectively, compared to 24% resuscitation in FB not supplemented with oxygen scavengers. The percentage recovery was increased as the incubation time was extended to 6 and 24 h. Nearly all injured cells were recovered within 24 h of incubation, regardless of supplementation of FB with oxygen scavengers. Fraser broth containing 2.5 mg of sodium pyruvate ml-1, 400 μg of catalase ml-1 or 0.01 U of Oxyrase? ml-1 were tested to determine the optimal incubation time and temperature for recovering heat-injured L. monocytogenes . Percentage recovery of injured cells increased with an increase in temperature from 25 to 30°C and from 30 to 35°C. The highest percentage of injured cells recovered was observed in FB containing 400 μg of catalase ml-1 (67%) and 0.01 U of Oxyrase? ml-1 (68%) within 6 h of incubation at 35°C. Catalase (400 μg ml-1) and Oxyrase? (0.01 U ml-1) in FB resulted in significantly higher recovery of injured cells from heated whole milk; however, recovery of injured cells from heated skim milk was not significantly higher. Enrichment in FB containing catalase or Oxyrase? has potential for recovering heat-injured L. monocytogenes cells within 6 h compared to 24 h required in conventional methods.
Keywords: heat injury; oxygen scavengers.