Evaluation of the impact of short-term temperature abuse on the microbiology and shelf life of a...

International Journal of Food Microbiology 59 (2000) 47–57www.elsevier.nl / locate / ijfoodmicro

Evaluation of the impact of short-term temperature abuse on themicrobiology and shelf life of a model ready-to-use vegetable

combination product

*Chris Thomas , David O’Beirne

Food Science Research Centre, Department of Life Sciences, University of Limerick, Limerick, Ireland

Received 13 March 2000; accepted 18 March 2000

Abstract

The growth dynamics of indigenous aerobic mesophilic populations (AMP), lactic acid bacteria (LAB) and inoculated(Listeria spp.) microbial populations on cooked and fresh vegetable products, packaged as separate entities and incombination, subjected to temperature fluctuation, were assessed. Microbial proliferation was temperature and productdependent, being most pronounced at 128C in all products with maximum growth rates of 0.140, 0.175 and 0.126 log10

CFU/g per h being identified for Listeria, aerobic mesophilic and LAB populations, respectively. Listeria spp. and AMPgenerally demonstrated higher rates of growth within products containing cooked vegetables. Prolonged storage at 38Cresulted in a reduced ability by AMP and Listeria spp. to proliferate upon exposure to growth temperatures; this was not thecase with LAB populations. Comparison of Listeria population estimates made using selective (Oxford) and non-selective(nutrient agar) identified reduced recovery on the former. The magnitude of the deviation increased with the duration ofexposure of Listeria populations to 38C with recoveries on selective systems being reduced by 6.3% immediately afterinoculation and 82.3% after 168 h at 38C, respectively. Growth of populations associated with exposure to abusetemperatures was not accompanied by significant changes in product colour (P , 0.05). 2000 Elsevier Science B.V. Allrights reserved.

Keywords: Listeria; Temperature abuse; Vegetables; Microbiology

1. Introduction preservation methods in favour of mild preservationtechniques to provide sufficient microbial control

Consumer demand for fresh, convenience foods (Schofield, 1992). The selective pressures imposedhas resulted in a relinquishment of traditional food upon microbial communities with the application of

such preservation techniques has inevitably resultedin an increase in the significance of micro-organisms

*Corresponding author. Food Safety and Medical Devicescapable of exploiting these ‘new’ environments. ForSection, National Standards Authority of Ireland, Glasnevin,example, increased reliance upon refrigeration andDublin 9, Ireland. Tel.: 1353-1-8073-848.

E-mail address: [email protected] (C. Thomas) reduced oxygen tension has presented new oppor-

0168-1605/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0168-1605( 00 )00290-7

48 C. Thomas, D. O’Beirne / International Journal of Food Microbiology 59 (2000) 47 –57

tunities to psychrotrophic facultatively anaerobic populations. For example, irrespective of the relativepopulations including pathogenic species of Listeria, independence of L. monocytogenes growth on veget-Clostridium and Bacillus. ables from storage atmosphere (3% O , 97% N )2 2

Minimally processed vegetable products (MPVs), (Berrang et al., 1989; Beuchat and Brackett, 1991),which traditionally include washed, processed fresh the increased shelf-life resulting from the reductionraw vegetables and may be combined with cooked of organoleptic deterioration increases the time avail-ingredients and dressings, demonstrate an ever-in- able for listerial growth (Brackett, 1992).creasing popularity (Reyes, 1996). The susceptibility Despite the crucial and integral role of temperatureof vegetable growing, harvesting, handling, process- in maintaining the microbial safety of foods, theing and storage procedures to microbial contamina- majority of studies have focused on the effects oftion and temperature abuse is well recognised long-term storage at fixed temperatures. Although(Beuchat and Ryu, 1997; Francis et al., 1999). These the findings from such studies have made significantfactors, combined with the relative inseverity of such contributions to our understanding of the microbialprocesses toward contaminating populations may risks associated with such temperature abuse profiles,facilitate the introduction and maintenance of micro- they bear little resemblance to the continual andorganisms including pathogens. short-term temperature variations evident throughout

Listeria monocytogenes is a foodborne pathogenic the chill-chain. A further weakness in some studiesbacterium which causes epidemic listeriosis, princi- has been the inoculation of pathogenic culturespally affecting the young, elderly and immunocom- which have been pre-incubated at temperatures,promised (Rocourt, 1994). The organism is ubiquit- optimal for their growth but grossly exceeding thoseous in soils and inevitably contaminates fresh of pre- and post-harvest environments.produce during both the pre- and post-harvest stages In this study, the impact of temperature abuseof production (Beuchat and Ryu, 1997; Thomas and profiles, typical of commercial practice, upon theO’Beirne, 1999). Reported contamination rates on survival and growth of artificially inoculated Listeria

´vegetables vary from 0 to 48% (Laine and Michard, populations, pre-incubated at environmental tempera-1988; Breer and Baumgartner, 1992; Gunasena et al., tures has been investigated within a model MPV1995). The potential for listerial survival and growth combination product. The dynamics of componentson MPV products has been demonstrated to vary of the indigenous microbial populations and thewith respect to the vegetable type, processing pro- shelf-life of the product were also assessed.cedures and storage temperature. However, as aresult of the scope and variability of these factors,inconsistencies still remain with respect to the mag-nitude of their individual significance toward listerial 2. Materials and methodspopulation dynamics.

Modified atmosphere packaging (MAP), is being 2.1. Preparation of vegetable productsincreasingly applied within preservation systems formany foods including MPVs. Passive MAP is regu- Sweetcorn (34 g) which had been cooked andlarly applied to MPVs sealed within bags and over- canned with no added preservatives was drained andwrapped with semi-permeable films, harnessing the weighed out into transparent plastic trays (180 3

natural respiratory activity of living vegetable tissue 130 3 25 mm). Raw beansprouts (Vigna radiata) (66to naturally modify atmospheric conditions. Under g) were then added to the canned sweetcorn and thesuch conditions, reductions in product respiration combination product was overwrapped using a lowand enzymatic browning rates are achieved, reducing migration catering cling film (Wrap Film Systems,organoleptic deterioration and extending the shelf- Telford, UK) and heat sealed. The film, with alife of the produce (O’Beirne, 1990). Despite being thickness of 8–10 mm, was identified to haveprimarily designed to prolong product shelf-life, permeabilities to oxygen and carbon dioxide of

3 22 21 21modified atmospheres potentially influence the 15 000 and 90 000 cm m day atm at 238C,growth and activity of contaminating microbial respectively. Products were also assessed when

C. Thomas, D. O’Beirne / International Journal of Food Microbiology 59 (2000) 47 –57 49

separately packaged using a standard product weight temperature abuse period after 38C for 24 h wasof 100 g. referred to as ‘Phase 1’, after 96 h ‘Phase 2’, and 168

h ‘Phase 3’. Temperature transition times approx-2.2. Strains and inocula preparation imating 48 and 54 min were identified for the upshift

(3–128C) and downshift (12–88C), respectively.Listeria monocytogenes strains ATCC 19114, Control tests were also performed, storing the

NCTC 11994 and two strains (ULFL1 and ULFL2; product at 38C for the complete 10-day duration ofUniversity of Limerick Food Laboratory Culture the experiment and removing samples for analysis atCollection) isolated from raw beansprouts and L. regular intervals.innocua (NCTC 11288) were maintained at 2 208Cin nutrient broth No. 2 (Oxoid; CM67, Unipath, 2.5. Microbiological analyses of Listeria, aerobicHampshire, UK) supplemented with glycerol at a mesophilic populations (AMP) and lactic acidfinal concentration of 15% (v/v). Resuscitation was bacteria (LAB)achieved by thawing the culture at room temperature(18–228C) and inoculation into nutrient broth No. 2 Microbiological analysis was performed upon theand onto plate count agar (Oxoid; CM325) followed individual components of both combination andby incubation at 128C for 96 h. Population density single product (100 g) packs. Samples (10 g) werewas assessed spectrophotometrically at 550 nm and aseptically removed, diluted (1 /10) with 90 ml of

7adjusted to approximately 10 CFU/ml prior to sterile peptone water (Oxoid; CM9) and homogen-centrifugation (4000 3 g), washing, and re-suspen- ised with a Colworth stomacher lab blender (Seward,sion in equal volumes of sterile distilled water. Model 400, London, UK) for 1 min. The homoge-

nates were decimally diluted in sterile peptone water2.3. Product inoculation and plated in duplicate on appropriate culture media.

All inoculated packs were assessed for ListeriaStudies were performed separately, in triplicate, spp. by surface plating (100 ml) in duplicate on

for each isolate. Prior to over-wrapping, either an Oxford selective agar ((Oxoid; CM856 and SR140);entire product or a specific component of the product 358C for 48 h). Uninoculated packs were assessed forwas inoculated with ten 10-ml aliquots of the pre- AMP and LAB by surface plating (100 ml) inpared suspension, which were distributed randomly duplicate on plate count agar ((Oxoid; CM325);over the product surface. In the case of the combina- 308C for 48 h) and de Man Rogosa Sharpe agartion product, both constituents were inoculated and (MRS (Oxoid; 359); 308C for 72 h in candleanalysed separately, these being designated as sweet- extinction jars (approximately 10% residual O )),2

corn (mix) for the sweetcorn component and respectively.beansprout (mix) for the beansprout component. To assess the occurrence of sub-lethal injury as aInoculated products were over-wrapped, gently result of chilled storage, the listerial populationshaken to assist inoculum distribution, and trans- density of sweetcorn immediately after inoculationferred to the assigned storage conditions. and following 168 h of storage at 38C was assessed

by surface plating (100 ml) in duplicate on Oxford2.4. Product storage regimes selective agar and nutrient agar. The number of

colonies on each medium type was calculated andThe impact of several temperature abuse regimes the difference between them taken as a measure of

using both individual and combined vegetable prod- the degree of sub-lethal injury (Busch and Donnelly,ucts were investigated. Products were stored at 38C 1992). The experiment was performed in duplicate.for 24, 96 or 168 h prior to transfer to 128C for 12 h In the case of Listeria spp., typical black coloniesand were then transferred to 88C for a further 48 h. were enumerated following the prescribed incubationFor the ease of identification whilst facilitating period, and recorded as CFU/g of product. Randomcomparisons the different points of temperature colonies were selected and subjected to biochemicalabuse were allocated different ‘phases’. That is the confirmation tests (Golden et al., 1988).


2.6. Headspace gas analysis periods are presented in Figs. 1 and 2. Compositegrowth rates for these populations during the phases

At each sampling point, the gaseous composition of temperature abuse are recorded in Table 1.(O and CO ) of the headspace atmosphere within Throughout experimentation, Listeria populations2 2

two identically prepared and stored packs was ana- were seen to exceed, or approximate those evidentlysed using a gas analyser (Systech Instruments; immediately following product inoculation. Indeed,Gaspace2, Oxfordshire, UK). all applied conditions resulted in the proliferation or

maintenance of Listeria spp. populations. Further-2.7. Colour analysis more, and in accord with the other microbial popula-

tions examined, temperature alteration resulted in aThe visual spoilage of the beansprout component detectable change in listerial growth rate, regardless

of the products was assessed by taking six measure- of storage history or gaseous concentrations.ments of L (lightness /darkness), a (greenness / red- The storage of all vegetable products at 38Cness), b (yellowness /blueness) using a Chromameter prevented the extensive proliferation of all Listeria(Minolta; Model 508i, Osaka, Japan) fitted with a strains. A slightly higher Listeria growth rate at this10-mm aperture plate. The chromameter was cali- temperature was observed in the beansprout com-brated using a white tile (L 5 98.88; a 5 2 0.18; ponent of the combination product although, with theb 5 2 0.51). These parameters were then used to exception of beansprouts, all products demonstrated

scalculate colour difference (DE) DE 5 statistically equivalent growth rates (P.0.05), and]]]]]2 2 2Œ dDL 1 Da 1 Db . no significant differences were detected between thepopulation density present upon the different prod-

2.8. Population density and growth rate analysis ucts at the completion of the trial.Listerial growth rates on the vegetable products

Data were analysed using analysis of variance and were seen to vary significantly (P,0.05) as a resultleast-significant difference testing on SPSS for Win- of temperature and the duration of storage at 38Cdows (SPSS; Release 7.0, Chicago, IL, USA). Mi- (Table 1). Growth rates in all product types werecrobial growth rates were determined by calculation highest at 128C and particularly on sweetcorn. Sig-of the slope of the best line fitted to the data. Where nificantly, the addition of sweetcorn to beansproutsgrowth was observed to be non-linear, growth rates prompted growth rates on the latter to be approxi-evident during the linear portion of such plots were mately twice those observed when the product wascalculated. packaged separately.

When comparing the impact of applying tempera-ture up-shift to 128C following varying exposure

3. Results times at 38C, the magnitude of growth rate increasewas generally observed to decline with increased

3.1. Microbiological studies storage at 38C, but this was observed to be producttype dependent. In the case of sweetcorn, exposure to

All strains of Listeria spp. demonstrated similar refrigeration temperatures (38C) for different periodssurvival and growth characteristics irrespective of of time had no significant effect (P.0.05) on theproduct composition or applied temperature abuse growth rates of Listeria observed following up-shiftregime. Indeed, the application of statistical analysis to 128C. However, with the other products, growthfailed to identify consistent significant differences rates were generally observed to decline significantly(P,0.05) between the growth and survival charac- (P,0.05) with increasing time at 38C. However, interistics (population density or growth rates) of any the cases of the sweetcorn combination componentof the strains. Composite populations, of the in- and beansprouts, no significant differences (P.0.30)dividual Listeria strains (mean values of the five were detected between growth rates evident withinstrains each performed in triplicate), AMP and LAB the 128C component of phases 2 and 3 of tempera-(means of duplicate experiments) within the array of ture abuse. Similarly, no significant differences (P.

vegetable combinations and temperature abuse 0.05) were detected between phases 1 and 2 in the


Fig. 1. Changes in population density of Listeria spp. in (a) sweetcorn packaged separately (j) or in combination with beansprouts (h)and, (b) beansprouts packaged separately (m) or in combination with sweetcorn (^) during storage under temperature abuse regimes at 38C(—) with upshift to 128C (- - -) and subsequent downshift to 88C (— - —).

case of the beansprout combination product com- individually packaged sweetcorn, rates within phasesponent. 1 and 3 were significantly different (P,0.01) whilst

At 88C, growth rates were, as expected, inter- for the beansprout combination product all phasesmediary between those observed at 3 and 128C. The were significantly different except 2 and 3 (P.0.05).different lengths of exposure to 38C were seen to Initial levels of AMP and LAB on beansprouts

6.5have no significant effect (P.0.05) on the sub- were seen to be variable and be in the region of 102.5sequent growth rates of Listeria following tempera- and 10 CFU/g, respectively (Fig. 2). Growth rates

ture downshift from 12 to 88C within the sweetcorn of AMP were, in common with Listeria spp., in-combination or the beansprout product. In the case of fluenced by temperature and product type. This was


Fig. 2. Changes in population density of (a) aerobic mesophilic populations and (b) lactic acid bacteria during storage in beansproutspackaged separately (m) and, in the sweetcorn (h) or beansprout (^) components of a combination product during storage undertemperature abuse regimes at 38C (—) with upshift to 128C (- - -) and subsequent downshift to 88C (— - —).

not the case with LAB populations where growth the phases in the other products AMP growth rates atrates on the different products at 8 or 128C were 128C were significantly reduced with increased stor-approximately equivalent. age time at 38C. Growth rates of LAB populations

Significant differences in AMP and LAB growth were also unaffected by duration at 38C for eachrates were detected at 128C between phase 3 and the vegetable type. However, AMP growth rates inother phases for the sweetcorn component of the sweetcorn combination product were significantlycombination product (Table 1). Although no signifi- reduced with prolonged storage at 38C, whilst onlycant differences were identified for the latter between the growth rate evident in phase 3 was significantly


Table 1Growth rates of microbial populations within different vegetable products during periods of storage at different temperatures

aTemperature Phase Product Population flux (log CFU/g per h)10

b b(8C) Listeria AMP LABc c3 – Sweetcorn 0.003 nd nd

Sweetcorn–combi 0.003* 0.031 0.027Beansprouts ,0.001 0.016 0.007Beansprouts–combi 0.004*,** 0.019 0.017

12 1 Sweetcorn 0.127Sweetcorn–combi 0.120* 0.175 0.107Beansprouts 0.063 0.086 0.095Beansprouts–combi 0.101*,**,*** 0.138 0.108

2 Sweetcorn 0.140Sweetcorn–combi 0.082 0.145 0.108Beansprouts 0.035** 0.033 0.100Beansprouts–combi 0.079 0.027 0.086

3 Sweetcorn 0.120Sweetcorn–combi 0.070 0.047 0.073Beansprouts 0.016 0.005 0.126Beansprouts–combi 0.031*** 0.007 0.095

8 1 Sweetcorn 0.035Sweetcorn–combi 0.012 0.049 0.028Beansprouts 0.003 0.018 0.012Beansprouts–combi 0.014** 0.023 0.018

2 Sweetcorn 0.027Sweetcorn–combi 0.008 0.021 0.015Beansprouts 0.004**,*** 0.011 0.019Beansprouts–combi 0.007**,*** 0.016 0.017

3 Sweetcorn 0.018Sweetcorn–combi 0.009 0.003 0.019Beansprouts 0.001 20.002 0.016Beansprouts–combi 0.005**,*** 0.001 0.020

a Storage at 38C for 24 h (1), 96 h (2) or 168 h (3) before temperature abuse.b AMP, aerobic mesophilic population; LAB, lactic acid bacteria. Combi, refers to individual components of the combination product.c Not determined.Differences between the listerial growth rates on the various products are indicated by ‘*, **, ***’. ‘*’ indicating no significant difference

(P.0.05) with the rate identified in Sweetcorn; ‘**’ with that in Sweetcorn-combi; ‘***’ with that in Beansprouts.

Table 2different from phases 1 and 2 beansprouts, whether Listeria populations on selective agar (Oxford formulation) as aseparate or in combination with sweetcorn (Table 1). percentage of that present upon non-selective agar (nutrient agar)

immediately after inoculation and following exposure to 38C for168 h3.2. Chill injuryIsolate Recovery

Selective culture systems were seen to reduce the Immediately after After 168 hnumbers of Listeria spp. recovered when compared inoculation at 38Cto simultaneously applied non-selective systems

L. monocytogenes ACTC19114 93.3 19.6(Table 2). Selective systems recovered more than L. monocytogenes NCTC11994 91.1 18.993% of the population yield from non-selective L. monocytogenes ULFL1 94.6 16.3

L. monocytogenes ULFL2 94.1 17.1systems when used to quantify freshly inoculatedL. innocua NCTC11288 95.1 16.5cultures. However, population recovery on selectiveMean6S.D. (n52) 93.661.6 17.761.5media was significantly reduced following storage at


38C for 168 h, recovering less than 18% of the period. Beansprouts packaged in combination withpopulation quantified using non-selective agar. sweetcorn were observed to demonstrate equivalent

rates of colour change as those packaged separately.3.3. Flux in headspace gas concentrations

Headspace gas concentration (O and CO ) modi- 4. Discussion2 2

fication was seen to vary with respect to product typeand storage temperature (result not shown). As Raw vegetables are inevitably contaminated withexpected, no significant modifications (P.0.05) to micro-organisms of spoilage and pathogenic poten-the gaseous concentrations within un-inoculated tial through agricultural cultivation and unhygienicsweetcorn packages were detected. This was not the handling, processing and storage practices. In thecase in products containing beansprouts. Both absence of additional preservation methods, thebeansprout containing products behaved similarly potential for contaminating microbial populations towith significant modifications (P,0.05) of gaseous proliferate and result in product quality and safetyconcentrations occurring within 24 h of packaging. deterioration is very much dependent upon strictFollowing 96 h of storage (38C), O levels had temperature control throughout the entire product-2

decreased to approximately 5% and CO increased to ion–supply chain. However, this is difficult to2

approximately 15%. These gas levels were main- achieve and products are regularly subjected to abusetained for the 10-day duration of storage at 38C. temperatures approximating 8–128C (Scandella et

Rates of gaseous exchange were fastest with al., 1990), together with temperature upshifts andexposure to 128C (results not shown). In the case of downshifts during processing, distribution and stor-CO , no significant differences were detected be- age. In an attempt to quantify the significance of2

tween the different phases of exposure, with an such temperature abuse profiles, the impact of vari-average increase of 6.760.62 percentage points ous time–temperature combinations on the micro-being recorded. Maximum CO levels, approximat- biology and sensory qualities of a minimally pro-2

ing 22–30%, were recorded at the completion of cessed vegetable product were examined.each phase of temperature abuse. Oxygen consump- The potential for L. monocytogenes to grow on alltion rates following transfer to 128C were seen to be vegetable products at 38C clearly indicates thelower in phase 3 than phase 2 and similarly between significance of the organism with respect to refriger-phase 2 and phase 1 with a maximum/minimum flux ated food products of extended shelf life. However,of 7.10 and 3.28 percentage points being identified in the low growth rates of Listeria populations onphases 1 and 3, respectively. In the case of phases 2 beansprouts at 38C in comparison to those of theand 3, temperature upshift to 128C was sufficient to assessed autochthonous populations may indicate,produce anoxic conditions within beansprout con- that the low nutrient levels and activity of thetaining packs. autochthonous vegetable microflora hinders the inte-

gration and establishment of contaminating micro-3.4. Colour changes organisms during post-harvest handling. Nonetheless,

the potential for listerial growth at refrigerationThe first and most obvious symptom of beansprout temperatures combined with the prolonged shelf life

deterioration was the development of a musty odour of MPV products at this temperature creates aand a visually detectable change in colour. Colour potential health risk (Dykes and Withers, 1999;change (DE), was observed to increase at a rate of Francis et al., 1999). However, it has been suggested0.05 units /h with storage at 38C. The rates of colour that pathogenic attenuation occurs with long-termchange during exposure to elevated temperatures chilled storage, which perhaps indicates that long-were not significantly different (P.0.05) from those term refrigerated storage does not necessarily de-at 38C during either phases 1 or 2. However, in the crease the safety of such products (Buncic et al.,case of phase 3, the rate of colour change during the 1996).entirety of phase 3, 0.08 units /h, significantly ex- Large populations of micro-organisms are fre-ceeded (P,0.05) that observed at 38C over the same quently present on beansprouts (Patterson and Wood-


burn, 1980; Splittstoesser et al., 1983); however, decreased nutrient availability, the accumulations oftheir significance from the microbial safety perspec- anti-microbial compound(s) and susceptibility to chilltive is limited (Piernas and Guirand, 1998). In injury. However, the fact that such trends were notcommon with Listeria, growth rates of indigenous observed in the lactic acid bacterial populations maypopulations increased with the addition of sweetcorn. indicate that this latter population is slightly moreConsequently it is clear that the inclusion of cooked resistant to the prevailing conditions than the otherproducts with raw vegetables increases the prolifer- populations assessed. Furthermore, the restriction ofation potential of both the spoilage and pathogenic LAB growth on raw vegetables (endive) through themicrobial populations of such products. absence of essential growth limiting nutrients pro-

Under refrigeration conditions, the growth rates of posed by Nguyen-the et al. (1996) was not evidentthe indigenous microflora clearly exceed those of with respect to beansprouts.Listeria, indicating that under well-controlled refrig- Lactic acid bacteria demonstrate a relatively higherated conditions the product is likely to spoil prior tolerance to the inhibitory activities of carbon diox-to the accumulation of a hazardous listerial popula- ide (Molin, 1983). Furthermore, their abilities totion. Although, it should be considered that rates of grow at low temperatures and production of anti-product deterioration may demonstrate independence microbial fermentation end products and bacteriocinsfrom storage temperature (Thomas et al., 1999), and are well documented (Klaenhammer, 1993; Bennikthat other factors, e.g., the gaseous transmission et al., 1997; Meghrous et al., 1999). These latterproperties of the applied packaging material, may properties may explain the decreased rates of growthalso be highly influential (Thomas, 1999). demonstrated by non-LAB populations. Indeed, sys-

Regardless of the duration of time spent at 38C, tems harnessing the anti-microbial activities of LABthe temperature upshift to 128C prompted increased are currently under development as a bio-preserva-rates of growth in all assessed populations. Rates of tion system for vegetables (Bennik et al., 1999).growth at this temperature were again maximised Other members of the autochthonous microflorawith the inclusion of sweetcorn. Listerial growth capable of exploiting environments of reduced oxy-rates in the same products held continuously at 128C gen tension, e.g., yeasts, are also likely to influenceover a 10-day period (Thomas et al., 1999) were the colonisation and growth of contaminating patho-significantly lower than those observed during the genic populations within food environments such aspresent study. With the exception of temperature, those examined here (Brackett, 1994).similar storage conditions were used in both studies. The increased magnitude of the difference be-Hence these observations may indicate that the tween population densities determined using selec-different pre-inoculation temperature history of the tive and non-selective systems with prolonged stor-cultures (temperature at which the inoculum was age at 38C illustrates the potential for sub-lethalcultivated; 308C in Thomas et al. (1999); 128C in the injury of Listeria spp. to occur on MPV productspresent study) has significant influence on the sub- during long-term chilled storage. These observationssequent survival characteristics of the organism. correspond with previously published studies usingSuch observations are in accord with those of alternative milieu and stress factors and indicate thatprevious studies investigating the impact of pre- selective media under-estimate the survival of Lis-inoculation temperature history on listerial survival teria in sub-optimal environments (Sheridan et al.,within broth culture (Membre et al., 1999) and 1994; Hoffmans et al., 1997; Dykes and Withers,modelled chicken, milk and cheese environments 1999).(Walker et al., 1990; Gay and Cerf, 1997). However, Nonetheless, the observation of listerial growth atsome conflicting evidence has been published (Buch- refrigeration conditions would seem to suggest that aanan and Klawitter, 1991). proportion of the inoculated Listeria population can

Although slight variations were observed on the adapt to such conditions whilst other componentsdifferent vegetables, the extent of proliferation by incapable of growth, undergo sub-lethal injury. Thelisterial and aerobic mesophilic populations declined adaptability of individual cells may be the result ofwith prolonged storage at 38C. This phenomenon is their age, history and physiological condition.perhaps the result of numerous factors including The potential impact of pre-exposure of con-


Buncic, S., Avery, S.M., Rogers, A.R., 1996. Listeriolysin Otaminating pathogens to low temperature conditionsproduction and pathogenicity of non-growing Listeria mono-and the consequences of their integration into thecytogenes stores at refrigeration temperature. Int. J. Food.

indigenous microflora of vegetables on their sub- Microbiol. 31, 133–147.sequent survival characteristics on MPV products Busch, S.V., Donnelly, C.W., 1992. Development of a repair-

enrichment broth for resuscitation of heat-injured Listeriarequires examination. When the added dimensions ofmonocytogenes and Listeria innocua. Appl. Environ. Mi-chill chain fragility, sub-lethal injury, growth undercrobiol. 58, 14–20.

refrigeration conditions and attenuated pathogenicity Dykes, G.A., Withers, K.M., 1999. Sub-lethal damage of Listeriaas a consequence of chilled storage are considered, it monocytogenes after long-term chilled storage at 48C. Lett.is clear that many questions remain to be resolved Appl. Microbiol. 28, 45–48.

Francis, G., Thomas, C., O’Beirne, D., 1999. The microbiologicalwith respect to the survival properties of pathogenssafety of minimally processed vegetables. Int. J. Food. Sci.and their significance within refrigerated food prod-Technol. 34, 1–22, Review paper.

ucts. Gay, M., Cerf, O., 1997. Significance of temperature and pre-incubation on survival of Listeria monocytogenes at pH 4.8.Lett. Appl. Microbiol. 25, 257–260.

Golden, D.A., Beuchat, L.R., Brackett, R.E., 1988. Evaluation ofAcknowledgementsselective direct plating media for their suitability to recoveruninjured, heat injured and freeze-injured Listeria monocyto-

This work was funded by the Irish Department of genes from foods. Appl. Environ. Microbiol. 54, 1451–1456.Agriculture, Food and Forestry as part of the Non- Gunasena, D.K., Kodikari, C.P., Ganepola, K., Widanapathirana,Commissioned Food Research Programme. S., 1995. Occurrence of Listeria monocytogenes in food in Sri

Lanka. J. Nat. Sci. Coun. Sri Lanka 23, 107–114.Hoffmans, C.M., Fung, D.Y.C., Kastner, C.L., 1997. Methods and

resuscitation environments for the recovery of heat-injuredReferences Listeria monocytogenes: a review. J. Rapid Methods Automa-

tion Microbiol. 5 (4), 249–268.Bennik, M.H.J., Smid, E.J., Gorris, L.G.M., 1997. Vegetable- Klaenhammer, T.R., 1993. Genetics of bacteriocins produced by

associated Pediococcus parvulus produces pediocin PA-1. lactic acid bacteria. FEMS Microbiol. Rev. 12, 39–86.´ ´Appl. Environ. Microbiol. 63, 2074–2076. Laine, K., Michard, J., 1988. Frequence et abondance des Listeria

´ ´ ´ ˆBennik, M.H.J., van Overbeek, W., Smid, E.J., Gorris, L.G.M., dans les legumes frais decoupes prets a l’emploi. Microbiol.1999. Bio-preservation in modified atmosphere stored mung- Aliments Nutr. 6, 329–335.bean sprouts: the use of vegetable-associated bacteriocinogenic Meghrous, J., Lacroix, C., Simard, R.E., 1999. The effects onlactic acid bacteria to control the growth of Listeria monocyto- vegetative cells and spores of three bacteriocins from lacticgenes. Lett. Appl. Microbiol. 28, 226–232. acid bacteria. Food Microbiol. 16, 105–114.

Berrang, M.E., Brackett, R.E., Beuchat, L.R., 1989. Growth of Membre, J.-M., Ross, T., McMeekin, T., 1999. Behaviour ofListeria monocytogenes on fresh vegetables stored under Listeria monocytogenes under combined chilling processes.controlled atmosphere. J. Food. Prot. 52, 702–705. Lett. Appl. Microbiol. 28, 216–220.

Beuchat, L.R., Brackett, R.E., 1991. Behaviour of Listeria mono- Molin, G., 1983. The resistance to carbon dioxide of some foodcytogenes inoculated onto raw tomatoes and processed tomato related bacteria. Eur. J. Appl. Microbiol. Biotech. 18, 214–217.products. Appl. Environ. Microbiol. 57, 1367–1371. Nguyen-the, C., Halna-du-Fretay, B., Abreu da Silva, A., 1996.

Beuchat, L.R., Ryu, J., 1997. Produce handling and processing The microbiology of mixed salad containing raw and cookedpractices. Emerg. Infect. Dis. 3 (4), 459–465. ingredients without dressing. Int. J. Food Sci. Technol. 31,

Brackett, R.E., 1992. Shelf stability and safety of fresh produce as 481–487.influenced by sanitation and disinfection. J. Food. Prot. 55, O’Beirne, D., 1990. Modified atmosphere packaging of fruits and808–814. vegetables. In: Gormley, T.R. (Ed.), Chilled Foods: The State

Brackett, R.E., 1994. Microbiological spoilage and pathogens in of the Art, Elsevier, Amsterdam, pp. 183–199.minimally processed refrigerated fruits and vegetables. In: Patterson, J.E., Woodburn, M.J., 1980. Klebsiella and otherWiley, R.C. (Ed.), Minimally Processed Refrigerated Fruit and bacteria on alfalfa and beansprouts at the retail level. J. Food.Vegetables, Chapman Hall, New York, pp. 269–312. Sci. 45, 492–495.

Breer, C., Baumgartner, A., 1992. Vorkommen und Verhalten von Piernas, V., Guirand, J.P., 1998. Control of microbial growth on¨L. monocytogenes auf Salaten and Gemusen sowie in rice sprouts. Int. J. Food Sci. Technol. 33, 297–305.

¨ ¨frischgeprebten Gemusesaften. Arch. Lebensmittelhyg. 43, Reyes, V.G., 1996. Improved preservation systems for minimally108–110. processed vegetables. Food Aust. 48 (2), 87–90.

Buchanan, R.L., Klawitter, L.A., 1991. Effect of temperature Rocourt, J., 1994. Listeria monocytogenes: the state of the science.history on the growth of Listeria monocytogenes Scott A at Dairy Food. Environ. Sanitation 14, 70–82.refrigeration temperatures. Int. J. Food. Microbiol. 12, 235– Scandella, D., Derens, E., Bennahmias, R., 1990. Ive gamme, al

ˆ246. chaine du froid. Infos-CTIFL 59, 17–22.


Schofield, G.M., 1992. Emerging food-borne pathogens and their Thomas, C., O’Beirne, D., 1999. Microbiological issues in mini-significance in chilled foods. J. Appl. Bacteriol. 72, 267–273. mally processed vegetables. Int. Food. Hyg. 10 (1), 17–23.

Sheridan, J.J., Duffy, G., Buchanan, R.I., Mc Dowell, D.A., Blair, Thomas, C., Prior, O., O’Beirne, D., 1999. Survival and growth ofI.S., 1994. The use of selective and non-selective enrichment Listeria species in a model ready-to-use vegetable productbroths for the isolation of Listeria monocytogenes species from containing raw and cooked ingredients as affected by storagemeat. Food Microbiol. 11, 439–446. temperature and acidification. Int. J. Food. Sci. Technol. 34,

Splittstoesser, D.F., Queale, D.T., Andaloro, B.W., 1983. The 317–324.microbiology of vegetables sprouts during commercial pro- Walker, S., Archer, P., Banks, J.G., 1990. Growth of Listeriaduction. J. Food. Safety 5, 79–86. monocytogenes at refrigeration temperatures. J. Appl. Bac-

Thomas, C., 1999. Microbiological safety of ready-to-use veget- teriol. 68, 157–162.ables. In: Proceedings: Fresh and Prepared Vegetables andFruits: Trends and Opportunities. University of Limerick,Ireland.

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