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.
Center for Food Safety and Quality Enhancement, Department of Food Science and Technology, University of Georgia, Griffin, Georgia 30223-1797
Publication date: March 1, 1995
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