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FOOD-BORNE PATHOGENS
Effects of vegetable type, package atmosphere and storage
temperature on growth and survival of Escherichia coli O157:H7
and Listeria monocytogenes
GA Francis and D O’Beirne
Food Science Research Centre, Department of Life Sciences, University of Limerick, Limerick, Ireland
The survival and growth of Escherichia coli O157:H7 (ATCC 43888 and NCTC 12900 ) and Listeria monocytogenes
(ATCC 19114 and NCTC 11994) during storage ( 4 and 88C ) on ready - to -use ( RTU ) packaged vegetables (lettuce,
swedes ( rutabaga ) , dry coleslaw mix, soybean sprouts ) were studied. The vegetables were sealed within oriented
polypropylene packaging film, and modified atmospheres developed in packs during storage due to produce
respiration. Survival and growth patterns were dependent on vegetable type, package atmosphere, storage
temperature and bacterial strain. Populations of L. monocytogenes and E. coli O157:H7 increased (P<0.05, by
1.5 to 2.5 log cycles, depending on strain ) during a 12-day storage period on shredded lettuce (88C).
L. monocytogenes populations also increased ( by 1 log cycle) on packaged swedes, did not change
significantly ( P > 0.05) in packages of soybean sprouts and decreased by 1.5 log cycles ( P<0.05) on coleslaw
mix ( 88C). E. coli O157:H7 populations on packaged coleslaw and soybean sprouts increased ( by 1.5 to 2.5 log
cycles) up to day 5, but declined during subsequent storage ( 88C ) . On packaged swedes (88C), populations of
E. coli O157:H7 strain ATCC 43888 increased ( by 1 log cycle) during storage, whereas populations of strain
12900 increased between days 2 and 5, and declined during subsequent storage. Reducing the storage
temperature from 8 to 48C reduced the growth of L. monocytogenes and E. coli O157:H7 on packaged RTU
vegetables. However, viable populations remained at the end of the storage period at 48C. Journal of Industrial
Microbiology & Biotechnology ( 2001 ) 27, 111–116.
Keywords: foodborne pathogens; food safety; fresh-cut produce; modified atmosphere packaging; refrigeration
Introduction
Ready-to-use ( RTU) vegetables may be intact, washed vegeta-
bles ( e.g., soybean sprouts) or may consist of trimmed, peeled,
sliced and washed vegetables ( e.g., cut lettuce, dry coleslaw mix ).
These products are usually sealed within semipermeable packages
and stored at refrigeration temperatures [24] .
RTU vegetables harbor large and diverse populations of
microorganisms. Pathogens, such as Listeria monocytogenes
and Escherichia coli O157:H7 may also contaminate vegeta-
bles, and a number of outbreaks of foodborne disease have
been traced to vegetables [ 15,24,30 ] . For example, foodborne
outbreaks due to contamination with E. coli O157:H7 have
been associa- ted with elettuce, alfalfa sprouts and apple juice,
and an outbreak due to nterotoxigenic E. coli has been
linked to carrots [ 5 ] . L. monocytogenes and E. coli O157:H7
survive and / or grow on a range of RTU vegetables, such as
shredded lettuce [1,6,7,16,20,21,27,32 ] , minimally processed
endive [ 10,27 ], cut cabbage [ 9,28 ] and sliced cucumbers
[1].
Modified atmosphere packaging ( increased CO
2
, decreased
O
2
) is widely used, in combination with refrigeration, to
retard product respiration, delay physiological ageing and
thereby extend the shelf life of RTU vegetables. Vegetable
respiration alone will also decrease O
2
and increase CO
2
levels
inside the package, thereby passively modifying the in- pack
atmosphere [ 24 ] . Extending the shelf life increases the time
available for pathogens, if present, to grow and overextending
the shelf life may allow the development of significant
populations. A number of workers have shown that modified
atmosphere packaging can enhance survival and growth of
L. monocytogenes and E. coli O157:H7 on vegetables [1,11,
16,21,30] .
Storage temperature is probably the single most important
factor affecting the growth of microorganisms in RTU vegetables.
Although L. monocytogenes is capable of growth at low
temperatures, reducing the storage temperature will extend the
lag phase and reduce the rate of growth [ 7,10 ] . The mesophilic
status of E. coli O157:H7 makes growth in RTU vegetables
unlikely where temperature control is adequate ( 48C or less ).
However, temperature abuse during storage and distribution may
allow growth, and growth has been reported at 88C on minced
beef [34 ] .
In this work, the survival and growth of L. monocytogenes
and E. coli O157:H7 on packaged RTU vegetables were
studied. The effects of vegetable type ( lettuce, swedes, dry
coleslaw mix, soybean sprouts ) , passively modified atmo-
spheres, storage temperature (4 and 88C ) and bacterial strain
were investigated.
Correspondence: Dr GA Francis, Food Science Research Centre, Department of Life
Sciences, University of Limerick, Limerick, Ireland
Received 25 May 2000; accepted 21 September 2000
Journal of Industrial Microbiology & Biotechnology (2001) 27, 111–116
D 2001 Nature Publishing Group 1367-5435/01 $17.00
www.nature.com/jim
Materials and methods
Preparation of the model vegetable products
Lettuce:
Heads of Irish iceberg lettuce were purchased from a
local supplier. They were processed as follows: outer and damaged
leaves as well as the core of the lettuce heads were removed and
discarded. Inner leaves were sliced manually using a sharp knife to
approximately 10- mm strips.
Swedes: Irish swedes (rutabaga ) , from a local supplier, were
hand- peeled and diced into approximately 10- mm
2
-cube pieces
using a sharp knife.
Dry coleslaw and soybean sprouts: Dry coleslaw mix
( 80% shredded cabbage, 20% shredded carrot) and raw soybean
sprouts was obtained from a local supplier.
Intact ( in the case of soybean sprouts ) or shredded /diced
vegetable portions were washed in distilled water for 5 min with
agitation by hand. Washed vegetables were left to drain on
absorbent paper for 15 min.
Vegetable portions ( 25 g ) were transferred aseptically into bags
(10 cm10 cm) , composed of 35- m oriented polypropylene
packaging film (Cannings Packaging, Dublin, Ireland ) , which
were later heat - sealed. According to the manufacturer, this film had
a permeability to O
2
of 1200 ml /m
2
/day / atm and to CO
2
of 4000
ml/ m
2
/ day/ atm.
Strains and preparation of inocula
L. monocytogenes ( strains ATCC 19114 and NCTC 11994) and
two nontoxigenic E. coli O157:H7 strains ( ATCC 43888 and
NCTC 12900 ) were maintained at 208C in nutrient broth
(Oxoid CM 1 ) supplemented with 15% (v / v ) glycerol.
Resuscitation was achieved by thawing cultures at room tem-
perature (17 to 228C ) followed by a loop- transfer in tryptone
soya broth (10 ml TSB in screw - cap tubes, Oxoid CM 129 ) and
static incubation at 378C for 24 h. Cultures were centrifuged
(4000g, 15 min ) , the cells were washed twice with sterile
distilled water, resuspended and diluted in sterile distilled water to
desired concentrations (510
5
CFU ml
1
) to allow for conta-
mination of vegetables at initial levels of approximately 10
4
CFU
g
1
of vegetable.
Figure 1 Survival and growth of ( a ) L. monocytogenes ( strains
ATCC 19114 and NCTC 11994 ) and ( b ) E. coli O157:H7 ( strains
43888 and 12900 ) on diced packaged swedes ( rutabagas ) during
storage at 88C and 48C. Reported populations represent the means of
four values. Error bars show standard deviations.
Figure 2 Survival and growth of ( a ) L. monocytogenes ( strains
ATCC 19114 and NCTC 11994) and ( b ) E. coli O157:H7 ( strains
43888 and 12900 ) on shredded packaged lettuce during storage at 88C
and 48C. Reported populations represent the means of four values.
Error bars show standard deviations.
Survival of pathogens on packaged vegetables
GA Francis and D O’Beirne
112
Inoculation of vegetables and storage
After appropriate dilution, ten 10- l aliquots of the cell suspen-
sions were distributed over the vegetables contained within the
packages. After sealing, packs were gently shaken to assist
inoculum distribution. Immediately after inoculation and sealing,
samples were transferred to storage temperatures of 4 and 88C and
stored for a period of 12 days.
Microbiological analyses
Microbiological analyses were carried out on day 0 ( day of
inoculation ) and at regular intervals throughout the storage period
( days 2, 5, 7, 10 and 12 ) . At each sampling, duplicate packs from
the same experiment were analysed for L. monocytogenes or E. coli
O157:H7 populations. The 25 - g sample from each pack was
aseptically transferred into a stomacher bag. Samples were
homogenized for 2 min at high speed with 225 ml sterile
peptone– water ( Oxoid CM 9 ) using a Seward laboratory
stomacher (Model 400, AGB Scientific, Ireland ) . Serial dilutions
of each homogenized sample were made in peptone – water and
were surface spread (100 l/ plate ) in duplicate onto appropriate
media. Numbers of L. monocytogenes were determined on
Listeria- selective agar ( LSA, Oxoid CM 856) after incubation
for 48 h at 378C. Colony confirmation was performed on
characteristic black, aesculin- producing colonies by Gram stain,
catalase, motility and biochemical tests ( API - Listeria strips,
Biomerieux ). Populations of E. coli O157:H7 were determined
on sorbitol MacConkey agar ( Oxoid CM 813 ) , supplemented with
cefixime ( 50 g / l ) and potassium tellurite ( 2.5 mg/ l ) , after
incubation at 378C for 24 h [ 18 ] . Sorbitol nonfermenting
(colorless ) colonies were selected for confirmatory testing with a
rapid latex agglutination assay for E. coli O157 (Oxoid DR 620).
Analyses of the gaseous atmospheres inside the
packages
On each sampling date (days 0, 2, 5, 7, 10, 12 ) , gases within three
of each package type were analysed using an O
2
and CO
2
gas
analyser (PBI - Dansensor, PBI Development, Denmark, Model
TIA- III LV ) .
Statistical analyses
All experiments were carried out in duplicate and replicated twice.
At each analysis time material from duplicate packs was serially
Figure 3 Survival and growth of ( a ) L. monocytogenes ( strains
ATCC 19114 and NCTC 11994 ) and ( b ) E. coli O157:H7 ( strains
43888 and 12900 ) on packaged dry coleslaw mix during storage of 88C
and 48C. Reported populations represent the means of four values.
Error bars show standard deviations.
Figure 4 Survival and growth of ( a ) L. monocytogenes ( strains
ATCC 19114 and NCTC 11994 ) and ( b ) E. coli O157:H7 ( strains
43888 and 12900 ) on packaged soybean sprouts during storage at 88C
and 48C. Reported populations represent the means of four values.
Error bars show standard deviations.
Survival of pathoge ns on packaged vegetables
GA Francis and D O’Beirne
113
diluted and plated in the duplicate. Reported populations therefore
represent the means of four values. Means were separated using the
least significant difference test at the 5% level. Figures 1– 5 show
the means ± standard deviation. Statistical differences were deter-
mined using Student’s t - test after analysis of variance.
Results
Figure 1 shows the survival and growth of two strains of L.
monocytogenes and of E. coli O157:H7 on diced packaged swedes
during storage at 4 and 88C. L. monocytogenes populations
gradually increased ( P < 0.05 ) during storage in packs of diced
swedes stored at 88C. Populations had increased by approximately
1 log cycle by day 12 of storage. The two strains of E. coli O157:H7
behaved differently on swedes, with strain ATCC 43888 generally
surviving and growing better at 88C than strain NCTC 12900.
Numbers of E. coli O157:H7 strain ATCC 43888 increased ( by
approximately 1 log cycle) during storage on packaged swedes at
88C, whereas populations of strain NCTC 12900 slightly increased
between days 2 and 5, but decreased thereafter. L. monocytogenes
populations did not change significantly ( P > 0.05) on swedes held
at 48C between days 0 and 10, but slightly increased between days
10 and 12, whereas E. coli O157:H7 populations declined ( by 0.5
log cycles) during storage at 48C.
Survival and growth of ( a ) L. monocytogenes and (b ) E. coli
O157:H7 on shredded packaged lettuce during storage at 4 and
88C are shown in Figure 2. Populations of L. monocytogenes and
E. coli O157:H7 significantly increased ( P < 0.05 ) during storage
on lettuce held at 88C. L. monocytogenes populations had
increased by approximately 1.5 log cycles during the 12 - day
storage period, whereas final population densities of E. coli
O157:H7 were 6.5– 7.0 log
10
CFU/ g. Populations of L. mono-
cytogenes and E. coli O157:H7 did not change significantly
(P > 0.05 ) on lettuce stored at 48C.
Survival of L. monocytogenes and E. coli O157:H7 on
packaged dry coleslaw mix during storage is shown in Figure 3.
L. monocytogenes numbers in packs of coleslaw mix decreased by
1.5 log cycles during storage for 12 days at 88C, and by
approximately 2.0 log cycles when samples were held at 48C.
Populations of E. coli O157:H7 on coleslaw mix had increased by
approximately 1.5 log cycles by day 5, but populations declined
by 1 to 2 log cycles, depending on strain, with further storage at
88C. At 48C, populations of E. coli O157:H7 decreased (by 1 to
1.5 log cycles ) , but viable cells were still detected at the end of the
storage period.
Survival and growth of L. monocytogenes and E. coli O157:H7
on packaged soybean sprouts during storage at 4 and 88Care
shown in Figure 4. L. monocytogenes numbers on soybean
sprouts did not change significantly during storage at 88C, but
decreased by 1.5 log cycles ( P < 0.05 ) when samples were stored
at 48C. Populations of E. coli O157 strain 43888 on soybean
sprouts ( 88C ) increased ( by approximately 2.5 log cycles)
between days 0 and 2, did not change significantly ( P >0.05)
between days 2 and 5, and declined during subsequent storage at
88C. Numbers of strain 12900 also increased ( by approximately
2.5 log cycles ) between days 0 and 2, but did not change
significantly ( P > 0.05 ) for the remainder of the storage period.
When soybean sprouts were stored at 48C, numbers of strain
43888 did not change significantly (P > 0.05 ) , whereas popula-
tions of strain 12900 gradually decreased ( by approximately 0.5
log cycles) during storage.
The RTU vegetables were sealed in packages initially
enclosing air. During storage, the gas atmospheres within
packages were modified, mainly as a result of the respiration
of the packaged vegetables. The concentrations of O
2
and CO
2
Figure 5 Changes in gas ( O
2
and CO
2
) levels inside packages of ( a ) shredded lettuce, ( b ) diced swedes, ( c ) soybean sprouts and ( d ) dry
coleslaw mix during storage at 88C. Values are the means of six determinations. Error bars show standard deviations.
Survival of pathogens on packaged vegetables
GA Francis and D O’Beirne
114
achieved within the packs varied with the packaged product
( Figure 5 ) . With lettuce, levels of CO
2
increased to 10 –12% and
O
2
levels fell to 3 – 4%. During the same period, O
2
levels fell to
2% and CO
2
levels had risen to 8 – 10% in packs of diced
swedes. Dry coleslaw mix has a relatively high respiration rate,
and higher levels of CO
2
were reached in packages of coleslaw
than those attained in packs of lettuce or swedes. Inside packs of
coleslaw, CO
2
levels rose to 25– 27% and O
2
levels fell to 0 –1%
during the 12 - day storage period. During the same time, packs
containing soybean sprouts achieved similar gas levels to packs
containing coleslaw.
Discussion
During these investigations, the two strains of L. monocytogenes
generally behaved similarly on vegetable products and at both
temperatures. E. coli O157:H7 survived and grew better than L.
monocytogenes on most packaged vegetables, and strain ATCC
43888 generally survived better than strain NCTC 12900.
Survival and growth patterns for L. monocytogenes and E. coli
O157:H7 were dependent on product type, package atmosphere
and storage temperature.
The ability of L. monocytogenes to survive and grow on RTU
vegetables has previously been demonstrated [7,9,10,21,28,32 ] .
The rate of growth was significantly reduced when the storage
temperature was decreased [7,10 ] . The effect of storage tempera-
ture (5, 12, 218C ) on survival and growth of E. coli O157:H7
inoculated on shredded iceberg lettuce was examined [1] .
Populations of viable E. coli O157:H7 significantly decreased on
lettuce stored at 58C and significantly increased during storage at 12
or 218C.
Gas atmospheres within packages were modified, mainly as a
result of the respiration of the packaged vegetables. Gas atmo-
spheres within packs of lettuce and swedes (CO
2
levels of 9% to
12% and O
2
levels of 2% to 4% ) were not inhibitory to growth of L.
monocytogenes or E. coli O157:H7. Previous work showed that
CO
2
concentrations of 5% to 10% did not inhibit growth of L.
monocytogenes, and increasing the CO
2
concentration to 20%
reduced the growth rate, but not the maximum population densities
reached [4,22 ] . Hao and Brackett [ 25 ] concluded that growth of
E. coli O157:H7 was not inhibited by gas mixtures containing up to
10% CO
2
at 5 or 108C. Other workers reported that CO
2
concentrations of 30% had no inhibitory effect on growth of E.
coli O157:H7 on shredded lettuce stored at 13 or 228C[16].
Coleslaw was largely unsuitable for growth of Listeria
presumably due to a combination of factors, such as the
antibacterial effects of carrot on Listeria species [ 8,31] , competi-
tion from the extensive indigenous microbial population ( 10
7
to
10
8
CFU g
1
on day 0, data not shown ) of the cabbage/ carrot
mixture, and inhibitory effects from the relatively high CO
2
levels
( 25 to 30%) within the packs. The inhibitory effects of CO
2
are
related to reduction of the intracellular and extracellular pH, and
interference with cellular metabolism [ 14,17,19,26 ]. The quantity
and quality of the background microflora may also affect growth of
L. monocytogenes. Strains of lactic acid bacteria, enterobacteria and
pseudomonads inhibit the growth of L. monocytogenes and other
pathogens on minimally processed vegetables [ 3,12,22,23,33 ] .
The inclusion of shredded carrots in the dry coleslaw mix may
have also affected survival and growth of E. coli O157:H7. Abdul -
Raouf et al [ 1] reported that populations of E. coli O157:H7
numbers on shredded carrots (128C ) increased during initial days
of storage, and subsequently declined on extended storage. A
known carrot phytoalexin, 6 - methoxymellein, inhibits the growth
of several fungi and bacteria, and may also be inhibitory to E. coli
O157:H7 [1,29 ] .
Soybean sprouts did not support growth of L. monocytogenes,
due presumably to competition from the high populations of
background microflora ( 10
8
CFU g
1
on day 0, data not shown ) ,
inhibition from the relatively high levels of CO
2
present within
packs (25 – 30% ) , and the more limited nutrient availability of
intact vegetables. E. coli O157 grew better on soybean sprouts than
L. monocytogenes. Packaging vegetables under an atmosphere
containing 3% O
2
and 97% N
2
had no apparent effect on growth of
populations of E. coli O157:H7, in comparison to vegetables that
were sealed in packs initially enclosing air [1 ] . In addition, CO
2
had no inhibitory effect on growth of E. coli O157:H7 on shredded
lettuce stored at 13 or 228C, and growth increased in an atmosphere
of O
2
/CO
2
/N
2
: 5/ 30/ 65, compared to growth in air [16 ] . E. coli
O157:H7 was more competitive against spoilage microorganisms
than Salmonella [ 21] , and due to its high acid tolerance [ 2,13] , is
hypothetically more resistant to the fermentation end- products of
lactic acid populations.
Conclusions
Survival and growth patterns for L. monocytogenes and E. coli
O157:H7 were dependent on product type, package atmosphere,
storage temperature and bacterial strain. E. coli O157:H7
generally survived and grew better than L. monocytogenes on
packaged vegetables. Storage at 48C enabled survival of both
pathogens on all products throughout the storage period. The
work underlines the importance of strict temperature control from
processing to consumption. Refrigerated temperatures must be
maintained during transportation, distribution, storage or handling
in supermarkets and by consumers. It is essential that
contamination of produce be minimized through the use of good
agricultural and strict hygiene practices, and that HACCP
programs specific for the pathogen of concern be applied at all
stages of production.
Acknowledgements
The authors acknowledge the financial assistance of the Irish
Department of Agriculture, Food and Forestry.
References
1 Abdul - Raouf UM, LR Beuchat and MS Ammar. 1993. Survival and
growth of E. coli O157:H7 on salad vegetables. Appl Environ Microbiol
59: 1999– 2006.
2 Benjamin MM and AR Datta. 1995. Acid tolerance of enterohemor-
rhagic Escherichia coli. Appl Environ Microbiol 61: 1669 – 1672.
3 Bennik MHJ, HW Peppelenbos, C Nguyen - the, F Carlin, EJ Smid and
LGM Gorris. 1996. Microbiology of minimally processed, modified
atmosphere packaged chicory endive. Postharvest Biol Technol 9:
209 – 221.
4 Bennik MHJ, EJ Smid, FM Rombouts and LGM Gorris. 1995. Growth
of psychrotrophic foodborne pathogens in a solid surface model system
under the influence of carbon dioxide and oxygen. Food Microbiol 12:
509 – 519.
5 Beuchat LR. 1996. Pathogenic microorganisms associated with fresh
produce. J Food Prot 59: 204 – 216.
Survival of pathoge ns on packaged vegetables
GA Francis and D O’Beirne
115
6 Beuchat LR. 1999. Survival of enterohemorrhagic Escherichia coli
O157:H7 in bovine feces applied to lettuce and the effectiveness of
chlorinated water as a disinfectant. J Food Prot 62: 845 – 849.
7 Beuchat LR and RE Brackett. 1990. Survival and growth of Listeria
monocytogenes on lettuce as influenced by shredding, c hlorine
treatment, modified atmosphere packaging and temperature. J Food
Sci 55: 755 – 758, 870.
8 Beuchat LR and RE Brackett. 1990. Inhibitory effect of raw carrots on
Listeria monocytogenes. Appl Environ Microbiol 56: 1734 – 1742.
9 Beuchat LR, RE Brackett, DY- Y Hao and DE Conner. 1986. Growth
and thermal inactivation of Listeria monocytogenes in cabbage and
cabbage juice. Can J Microbiol 32: 791– 795.
10 Carlin F, C Nguyen - the and A Abreu da Silva. 1995. Factors affecting
the growth of Listeria monocytogenes on minimally processed fresh
endive. J Appl Bacteriol 78: 636– 646.
11 Carlin F, C Nguyen - the, A Abreu da Silva and C Cochet. 1996. Effects
of carbon dioxide on the fate of Listeria monocytogenes, of aerobic
bacteria and on the development of spoilage in minimally processed
fresh endive. Int J Food Microbiol 32: 159 – 172.
12 Carlin F, C Nguyen- the and CE Morris. 1996. The influence of the
background microflora on the fate of Listeria monocytogenes on
minimally processed fresh broad leaved endive ( Cichorium endivia
var. latifolia ). J Food Prot 59: 698 – 703.
13 Conner DE and JS Kotrola. 1995. Growth and survival of E. coli
O157:H7 under acidic conditions. Appl Environ Microbiol 61:
382 – 385.
14 Daniels JA, R Krishnamurthi and SSH Rizvi. 1985. A review of the
effect of carbon dioxide on microbial growth and food quality. J Food
Prot 48: 532– 537.
15 De Roever C. 1998. Review: Microbiological safety evaluations and
recommendations on fresh produce. Food Control 9: 321 – 347.
16 Diaz C and JH Hotchkiss. 1996. Comparative growth of E. coli
O157:H7, spoilage organisms and shelf life of shredded iceberg lettuce
stored under modified atmospheres. J Sci Food Agric 70: 433– 438.
17 Dixon NM and DB Kell. 1989. The inhibition by CO
2
of the growth
and metabolism of micro- organisms: a review. J Appl Bacteriol 67:
109 – 136.
18 Duffy G, P Garvey and JJ Sheridan. 2000. E. coli O157:H7; and
European perspective. Ir J Agric Food Res 39: 173 – 182.
19 Farber JM. 1991. Microbiological aspects of modified atmosphere
packaging technology: a review. J Food Prot 5: 58 – 70.
20 Farber JM, SL Wang, Y Cai and S Zhang. 1998. Changes in
populations of Listeria monocytogenes inoculated on packaged fresh -
cut vegetables. J Food Prot 61: 192– 195.
21 Francis GA and D O’Beirne. 1997. Effects of gas atmosphere,
antimicrobial dip and temperature on the fate of Listeria innocua and
Listeria monocytogenes on minimally processed lettuce. Int J Food Sci
Technol 32: 141 – 151.
22 Francis GA and D O’Beirne. 1998. Effects of storage atmosphere on
Listeria monocytogenes and competing microflora using a surface
model system. Int J Food Sci Technol 33: 465 – 476.
23 Francis GA and D O’Beirne. 1998. Effects of the indigenous microflora
of minimally processed lettuce on the survival and growth of L.
monocytogenes. Int J Food Sci Technol 33: 477– 488.
24 Francis GA, C Thomas and D O’Beirne. 1999. Review paper: the
microbiological safety of minimally processed vegetables. Int J Food
Sci Technol 34: 1 – 22.
25 Hao YY and RE Brackett. 1993. Influence of modified atmosphere
on growth of vegetable spoilage bacteria in media. J Food Prot 56:
223 – 228.
26 Hudson JA, SJ Mott and N Penney. 1994. Growth of Listeria
monocytogenes, Aeromonas hydrophila and Yersinia enterocolitica on
vacuum and saturated carbon dioxide controlled atmosphere - packaged
sliced roast beef. J Food Prot 57: 204 – 208.
27 Jacxsens L, F Devlieghere, P Falcato and J Debevere. 1999. Behavior
of Listeria monocytogenes and Aeromonas spp. on fresh - cut produce
packaged under equilibrium- modified atmosphere. J Food Prot 62:
1128 – 1135.
28 Kallender KD, AD Hitchins, GA Lancette, JA Schmieg, GR Garcia,
HM Solomon and JN Sofos. 1991. Fate of Listeria monocytogenes in
shredded cabbage stored at 5 and 258C under a modified atmosphere.
J Food Prot 54: 301 – 304.
29 Kurosaki F and A Nishi. 1983. Isolution and antimicrobial activity of
the phytoalexin 6 - methoxymellein from cultured carrot cells. Phyto-
chemicals 22: 669 – 673.
30 Nguyen - the C and F Carlin. 1994. The microbiology of minimally
processed fresh fruits and vegetables. Crit Rev Food Sci Nutr 34:
371 – 401.
31 Nguyen - the C and BM Lund. 1991. The lethal effect of carrot on
Listeria species. J Appl Bacteriol 70: 479 – 488.
32 Steinbruegge EG, RB Maxcy and MB Liewen. 1988. Fate of L.
monocytogenes on ready to serve lettuce. J Food Prot 51: 596 – 599.
33 Vescovo M, S Torriani, C Orsi, F Macchiarolo and G Scolari. 1996.
Application of antimicrobial - producing lactic acid bacteria to control
pathogens in ready - to - use vegetables. J Appl Bacteriol 81: 113– 119.
34 Weeratna RD and MP Doyle. 1991. Detection and production of
verotoxin 1 of E. coli O157:H7 in food. Appl Environ Microbiol 57:
2951 – 2955.
Survival of pathogens on packaged vegetables
GA Francis and D O’Beirne
116
