Comparison of oxygen scavengers for their ability to enhance resuscitation of heat-injured Listeria monocytogenes

Abstract

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 ml1) 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 g 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 g 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 g of catalase ml-1 (67%) and 0.01 U of Oxyrase®ml-1 (68%) within 6 h of incubation at 35°C. Catalase (400 g 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. Copyright©, International Association of Milk, Food and Environmental Sanitarians.