Mild heat treatment of lettuce enhances growth of Listeria monocytogenes during subsequent storage at 5oC or 15oC

Abstract

Aims: The objective of this study was to determine the influence of mild heat treatment, storage temperature and storage time on the survival and growth of Listeria monocytogenes inoculated onto cut iceberg lettuce leaves. Methods and Results: Before or after inoculation with L. monocytogenes, cut iceberg lettuce leaves were dipped in water (20 or 50°C), containing or not 20 mg l -1 chlorine, for 90 s, then stored at 5°C for up to 18 days or 15°C for up to 7 days. The presence of 20 mg l -1 chlorine in the treatment water did not significantly (a ¼ 0AE05) affect populations of the pathogen, regardless of other test parameters. The population of L. monocytogenes on lettuce treated at 50°C steadily increased throughout storage at 5°C for up to 18 days. At day 10 and thereafter, populations were 1AE7-2AE3 log 10 cfu g -1 higher on lettuce treated at 50°C after inoculation compared with untreated lettuce or lettuce treated at 20°C, regardless of chlorine treatment. The population of L. monocytogenes increased rapidly on lettuce stored at 15°C. At 2 and 4 days, significantly higher populations were detected on lettuce that had been treated at 50°C, compared with respective samples that had been treated at 20°C, regardless of inoculation before or after treatment, or the presence of 20 mg l -1 chlorine in the treatment water. Conclusions:The results clearly demonstrated that mild heat treatment of cut lettuce leaves enhances the growth of L. monocytogenes during subsequent storage at 5 or 15°C. Significance and Impact of the Study: Mild heat treatment of cut lettuce may result in a prolonged shelf life as a result of delaying the development of brown discoloration. However, heat treatment also facilitates the growth of L. monocytogenes during storage at refrigeration temperature, thereby increasing the potential risk of causing listeriosis.

Effect of temperature and chlorination of pre-washing water on shelf-life and physiological properties of ready-to-use iceberg lettuce

“…Nevertheless, it should be noted that washing and sanitizing are unlikely to totally eliminate all pathogens of a contaminated produce . Several experiments had been conducted to investigate the survival and growth of L. monocytogenes and E. coli O157:H7 in fresh-cut iceberg lettuce during storage at different temperatures and under different atmospheres after decontamination with cold and warm, chlorinated water, respectively (Beuchat & Brackett, 1990;Delaquis, Stewart, Cazaux, & Toivonen, 2002;Francis & O'Beirne, 1997;Koseki et al, 2004;Li, Brackett, Chen, & Beuchat, 2001;Li, Brackett, Chen, & Beuchat, 2002). Enhanced growth under favourable conditions such as storage at an inadequate temperature was often ascribed to reductions or alterations in the competing indigenous microflora.…”

Reduction of Listeria monocytogenes contamination on produce – A quantitative analysis of common liquid fresh produce wash compounds

“…Some physical control measures including irradiation or pulsed UV light as well as certain biological means of control such as the use of bacteriophages or biocontrol agents have achieved reductions in experimental studies using L. monocytogenes (or other bacteria that may e6 log 10 cfu (Akbas & Olmez, 2007;Baert et al, 2009;Bari, Inatsu, Kawasaki, Nazuka, & Isshiki, 2002;Bari et al, 2003;Beuchat et al, 2004;Beuchat & Brackett, 1990;Beuchat et al, 1998;Burnett et al, 2004;Delaquis, Stewart, Cazaux, & Toivonen, 2002;Deza et al, 2003;Francis & O'Beirne, 2002;Hellstrom et al, 2006;Lang et al, 2004aLang et al, , 2004bLee, Yun, Fellman, & Kang, 2002;Li, Brackett, Chen, & Beuchat, 2002;Rodgers et al, 2004;Ruiz-Cruz et al, 2007;Stopforth et al, 2008;Su & D'Souza, 2012;Szabo, Simons, Coventry, & Cole, 2003;Udompijitkul et al, 2007;Walter et al, 2009;Wei et al, 2005;Zhang & Farber, 1996) Potassium hypochlorite is exempted from the requirement of a tolerance in or on all commodities (40 CFR 180.1300) Calcium hypochlorite is exempt from the requirement of a tolerance when used preharvest or postharvest in solution on all raw agricultural commodities and on grape when used as a fumigant postharvest by means of a chlorine generator pad (40 CFR 180.1054) Sodium hypochlorite is exempted from the requirement of a tolerance for residues and when used in accordance with good agricultural practices as a seed-soak treatment in the growing of the raw agricultural commodities vegetables, brassica, leafy , group 5 and radish, roots and radish, tops (40 CFR 180.1235;40 CFR 180.1070) Not all substances with chlorine are authorized as antimicrobials in the U.S. Calcium hypochlorite is authorized as food contact substance for use as an antimicrobial agent in water used to process fruits, vegetables, nuts, meat, and poultry and in pretreatment of water used in the manufacture of beverages...…”

Effects of gallotannin treatment on attachment, growth, and survival of Escherichia coli O157:H7 and Listeria monocytogenes on spinach and lettuce

“…The tannin treatment used in this study was superior to commercially employed treatments, including washing with chlorine solutions (200-250 mg/L) or modified atmosphere packaging (3 % O 2 , 97 % N 2 ) that did not influence growth of Listeria strains [27]. Mild heat treatment even enhanced growth of Listeria during subsequent storage at 5°C [28]. The use of anti-listerial tannins in the wash water may not eliminate bacterial cells that were internalized through the root system, through stomata, or from inoculated seeds, but will reduce cross-contamination during processing.…”