Microbiological quality of bagged cut spinach and lettuce mixes.
ABSTRACT Analysis of 100 bagged lettuce and spinach samples showed mean total bacterial counts of 7.0 log(10) CFU/g and a broad range of < 4 to 8.3 log10 CFU/g. Most probable numbers (MPN) of > or = 11,000/g coliforms were found in 55 samples, and generic Escherichia coli bacteria were detected in 16 samples, but no E. coli count exceeded 10 MPN/g.
- SourceAvailable from: Peter C H Feng[Show abstract] [Hide abstract]
ABSTRACT: Analysis of fresh produce showed that enterotoxigenic Escherichia coli (ETEC) strains are most often found in cilantro and parsley, with prevalence rates of approximately 0.3%. Some ETEC strains also carried Shiga toxigenic E. coli (STEC) genes but had no STEC adherence factors, which are essential to cause severe human illness. Most ETEC strains in produce carried stable toxin and/or labile toxin genes but belonged to unremarkable serotypes that have not been reported to have caused human illnesses.Journal of food protection 05/2014; 77(5):820-823. · 1.83 Impact Factor
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ABSTRACT: The aim of this study was to evaluate the bacterial and fungal quality of minimally-processed vegetables (MPV) and sprouts. A total of 116 samples of fresh-cut vegetables, ready-to-eat salads, and mung bean and wheat sprouts were randomly collected and analyzed. The load of aerobic mesophilic bacteria was minimum and maximum in the fresh-cut vegetables and fresh mung bean sprouts respectively, corresponding to populations of 5.3 and 8.5 log CFU/g. E. coli O157:H7 was found to be absent in all samples; however, other E. coli strains were detected in 21 samples (18.1%), and Salmonella spp. were found in one mung bean (3.1%) and one ready-to-eat salad sample (5%). Yeasts were the predominant organisms and were found in 100% of the samples. Geotrichum, Fusarium, and Penicillium spp. were the most prevalent molds in mung sprouts while Cladosporium and Penicillium spp. were most frequently found in ready-to-eat salad samples. According to results from the present study, effective control measures should be implemented to minimize the microbiological contamination of fresh produce sold in Tehran, Iran.Journal of Health Population and Nutrition 09/2014; 32(3):391-9. · 1.12 Impact Factor
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ABSTRACT: This study was conducted to investigate spoilage bacteria on organic and conventional fresh produce in Korea. Three samples (perilla leaf, cabbage, and romaine lettuce) of organic and conventional fresh produce were stored at for 14 days and examined for spoilage bacteria on TSA. Isolated bacteria from organic and conventional fresh produces were identified using 16S rRNA sequencing method. Population of total aerobic bacteria on conventional perilla leaf, cabbage, and romaine lettuce were 7.59, 7.01, and , and populations of total aerobic bacteria were 6.72, 6.15, and , for organic perilla leaf, cabbage, and romaine lettuce, respectively. Major spoilage bacteria of organic and conventional fresh produces were similar however their levels were little different. For example, a major spoilage bacterium resulting the highest level on conventional perilla leaf was Stenotrophomonas maltophilia whereas that was Microbacterium sp. for organic produce. From these results, microflora or spoilage microorganism could be different depending on their cultivation types as conventional or organic produces and this information might be used for developing effective preservation method for different types of fresh produce.Journal of Food Hygiene and Safety. 01/2013; 28(4).
APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Feb. 2008, p. 1240–1242
Vol. 74, No. 4
Microbiological Quality of Bagged Cut Spinach and Lettuce Mixes?
Iris Valentin-Bon, Andrew Jacobson, Steven R. Monday, and Peter C. H. Feng*
Division of Microbiology, United States Food and Drug Administration, College Park, Maryland 20740
Received 4 October 2007/Accepted 13 December 2007
Analysis of 100 bagged lettuce and spinach samples showed mean total bacterial counts of 7.0 log10CFU/g
and a broad range of <4 to 8.3 log10CFU/g. Most probable numbers (MPN) of >11,000 /g coliforms were found
in 55 samples, and generic Escherichia coli bacteria were detected in 16 samples, but no E. coli count exceeded
Increases in the worldwide consumption of ready-to-eat
(RTE) produce have resulted in increases in food-borne illness
associated with these products (11). In the United States in
2006, a multistate outbreak of Escherichia coli O157:H7 was
traced to bagged spinach (1) and, a few months later, another
outbreak implicated bagged lettuce in fast-food restaurants,
thus raising concerns about the microbiological quality of RTE
The quality of RTE leafy greens has recently been surveyed
in the United Kingdom (8, 10), Spain (12), and Brazil (3), but
there are no recent data on the microbial quality of bagged
leafy greens in the United States. In this study, we tested 100
bags of RTE leafy greens purchased in 2007 from stores in the
Washington, DC, metropolitan area for total, coliform, and
generic E. coli bacterial counts to assess their microbiological
quality. Although recent outbreaks had implicated specific
brands and products, we opted to do a broad survey rather
than to focus on specific produce types or brands. Hence, the
100 samples, many labeled “triple-washed” or “ready to eat,”
consisted of 45 spinach and 55 lettuce mixes (12 different
varieties), including organic products. The samples included
five brands, of which 20% were local store brands that may
only have regional distribution, but the rest were national
brands that are widely available. The samples were tested using
the U.S. FDA Bacteriological Analytical Manual (2; http://www
.cfsan.fda.gov/?ebam/bam-4.html) methods. Briefly, 50 g of
the product was blended with 450 ml of buffered peptone
water, from which 1:10 serial dilutions were made. Total bac-
terial counts were done by standard plate count, where 0.1 ml
of each dilution was plated in duplicate on Trypticase soy agar
(BD Diagnostics, Sparks, MD). The coliform and E. coli bac-
terial counts were done with the most probable number (MPN)
method with ColiComplete discs (Biocontrol, Bellevue, WA)
(AOAC official method 992.30). The discs contain X-Gal (5-
bromo-4-chloro-3-indolyl-?-D-galactopyranoside), which in co-
liforms are cleaved by ?-galactosidase to yield a blue product.
The discs also contain 4-methylumbelliferone-?-D-glucu-
ronide, which in E. coli bacteria is cleaved by ?-glucuronidase
to yield blue fluorescence (365-nm UV). After 48 h at 37°C, a
combination of coliform-positive (blue) and E. coli-positive
(fluorescence) tubes was used to estimate the levels of each
indicator from the MPN table.
The mean total count of the samples examined was around
7.0 log10CFU/g, with a broad range of ?4 to 8.3 log10CFU/g
(Table 1). Our data are consistent with the results of a 1998
U.S. study of 52 bagged salads which found a mean total
bacterial count of 7.0 log10CFU/g (4). Similarly, a study from
Spain showed that the total bacterial counts for 140 RTE
lettuce samples at 16 university restaurants ranged from 3.01 to
7.81 log10CFU/g (12), while an analysis of 133 RTE leafy
salads in Brazil found that 51% had counts that were ?6.0
log10CFU/g (3). Interestingly, we saw a large variation in
counts not only among the samples, but also within same-brand
products that had identical “use by” dates and were tested on
the same day. For example, five such seemingly “identical”
romaine and spinach samples had bacterial counts that ranged
from 5.3 to 7.0 log10CFU/g and ?4.0 to 7.4 log10CFU/g,
respectively. Also, except for 23 samples that came in plastic
tubs, condensation moisture was observed at the bottom of
many bags. Since water is essential for microbial growth, con-
densation moisture in the bags would be expected to promote
microbial growth; hence, 80% of the bags were opened and
sampled from the bottom. The top-sampled bags had a mean
total count of 6.96 log10CFU/g, with a range of 4.8 to 7.86 log10
CFU/g. In contrast, the bottom-sampled bags had a mean total
count of 7.65 log10CFU/g and a range of ?4 to 8.3 log10
CFU/g. Although bottom sampling yielded higher counts,
these findings are preliminary, and additional studies, in which
the same bags are sampled from both the top and bottom, are
needed to fully establish the effects of condensation moisture
on bacterial counts.
The coliform counts of the samples we tested ranged from
* Corresponding author. Mailing address: HFS-711, FDA, 5100
Paint Branch Parkway, College Park, MD 20740. Phone: (301) 436-
1650. Fax: (301) 436-2644. E-mail: email@example.com.
?Published ahead of print on 21 December 2007.
TABLE 1. Total microbiological counts of 100 bagged spinach and
Product and type
?0.47 to ?4.0 log10MPN/g (Table 2), similar to the range of
?0.47 to 3.38 log10MPN/g reported for RTE lettuce in Spain
(12). We were unable to obtain a mean coliform count, as
55/100 samples exceeded our counting limit of ?11,000
MPN/g. Generic E. coli bacteria were detected in 10 lettuce
and 6 spinach samples (Table 2). The highest E. coli level
found was 9.2 MPN/g, but 12/16 samples had counts of 3.6
MPN/g or less. Most of the E. coli-positive samples (11/16) had
total counts of ?6.0 log10CFU/g, and 14/16 samples had co-
liform counts of ?11,000 MPN/g. There are no E. coli limits for
bagged produce in the United States, but guidelines and limits
exist in other countries. The Brazilian standard for salads that
are minimally processed before consumption has a fecal coli-
form limit of 100 CFU/g, and analysis of 133 salad samples
showed that 73% exceed this limit (3). The guidelines of the
United Kingdom Public Health Laboratory Service for RTE
foods, including bagged produce, have set E. coli count limits
of ?20 CFU/g as satisfactory, 20 to ?100 CFU/g as acceptable,
and ?100 CFU/g as unsatisfactory (9). In two large surveys in
the United Kingdom, 3,200 organic and 3,852 conventional
RTE salads were tested and it was found that 0.5% exceeded
the 100-CFU/g E. coli limit and were unsatisfactory (8, 9).
Although we found 16% of our samples to have E. coli bacte-
ria, none exceeded 10 MPN/g, but whether low E. coli counts
are prevalent in other bagged leafy greens in the United States
remains to be determined.
These surveys show that the microbial flora and content of
RTE produce are highly variable and complex. One study (6)
looked at the microbiological quality of fresh, uncut produce
from production through packing and showed that indicator
levels in mustard greens and spinach remained fairly constant
but that for cilantro, parsley, and, especially, cantaloupe, the
indicator levels actually increased during packing, hence show-
ing that the microbial load can vary by processing but also
depends on the produce type. The processing for whole pro-
duce is probably distinct from that for cut, bagged produce, so
it is uncertain if the high counts or the variations in counts we
observed in the bagged leafy greens are due to produce type
variations or to microbial growth, if any, during processing.
Our finding that there are wide ranges, as well as large varia-
tions, in counts among samples and even among seemingly
“identical” samples, coupled with the rare observation of a
spinach leaf with visible filth (Fig. 1), suggests that differences
or inconsistencies in processing parameters may also have an
effect on microbial load. Lastly, the finding that bagged leafy
greens (3, 4; this study) can have ranges of total (4 to 7 log10
CFU/g) and coliform (1 to 4 log10MPN/g) bacterial counts
similar to those for produce sampled in the field (5, 7) also
suggests that, microbiologically, these products may be very
difficult to clean and process.
We thank Deanne Deer and John Callahan for their assistance in
procuring the produce samples used in this study.
1. Centers for Disease Control and Prevention. 2006. Ongoing multistate out-
break of Escherichia coli serotype O157:H7 infections associated with con-
sumption of fresh spinach—United States, September 2006. Morb. Mortal.
Wkly. Rep. 55:1045–1046.
2. Feng, P., S. D. Weagant, and M. A. Grant. September 2002. Enumeration of
Escherichia coli and the coliform bacteria. In R. I. Merker (ed.), Bacterio-
logical analytical manual, 8th ed., revision A. U.S. Food and Drug Admin-
istration, College Park, MD. http://www.cfsan.fda.gov/?ebam/bam-4.html.
3. Fro ¨der, H., C. G. Martins, K. L. De Souza, M. Landgraf, B. D. Franco, and
M. T. Destro. 2007. Minimally processed vegetable salads: microbial quality
evaluation. J. Food Prot. 70:1277–1280.
4. Hagenmeaier, R. D., and R. A. Baker. 1998. A survey of the microbial
population and ethanol content of bagged salad. J. Food Prot. 61:357–359.
5. Johnston, L. M., L. A. Jaykus, D. Moll, J. Anciso, B. Mora, and C. L. Moe.
2006. A field study of the microbiological quality of fresh produce of domes-
tic and Mexican origin. Int. J. Food Microbiol. 112:83–95.
6. Johnston, L. M., L. A. Jaykus, D. Moll, M. C. Martinez, J. Anciso, B. Mora,
and C. L. Moe. 2005. A field study of the microbiological quality of fresh
produce. J. Food Prot. 68:1840–1847.
7. Mukherjee, A., D. Speh, E. Dyck, and F. Diez-Gonzalez. 2004. Preharvest
evaluation of coliforms, Escherichia coli, Salmonella, and Escherichia coli
O157:H7 in organic and conventional produce grown by Minnesota farmers.
J. Food Prot. 67:894–900.
8. Sagoo, S. K., C. L. Little, and R. T. Mitchell. 2001. The microbiological
examination of ready-to-eat organic vegetables from retail establishments in
the United Kingdom. Lett. Appl. Microbiol. 33:434–439.
9. Sagoo, S. K., C. L. Little, and R. T. Mitchell. 2003. Microbiological quality
FIG. 1. Photograph of a spinach leaf obtained from a bagged
TABLE 2. Coliform and E. coli bacterial contents of 100 bagged spinach and lettuce mixes
Product and type
(no. of samples)
Coliform bacteria (MPN/g)E. coli bacteria
No. of positive samplesRange (MPN/g)
VOL. 74, 2008 MICROBIOLOGICAL QUALITY OF BAGGED LEAFY GREENS1241
of open ready-to-eat salad vegetables: effectiveness of food hygiene training
of management. J. Food Prot. 66:1581–1586.
10. Sagoo, S. K., C. L. Little, L. Ward, I. A. Gillespie, and R. T. Mitchell. 2003.
Microbiological study of ready-to-eat salad vegetables from retail establish-
ments uncovers a national outbreak of salmonellosis. J. Food Prot. 66:403–
11. Sivapalasingam, S., C. R. Friedman, L. Cohen, and R. V. Tauxe. 2004. Fresh
produce: a growing cause of outbreaks of foodborne illness in the United
States, 1973 through 1997. J. Food Prot. 67:2342–2353.
12. Soriano, J. M., H. Rico, J. C. Molto ´, and J. Man ˜es. 2006. Assessment of the
microbiological quality and wash treatments of lettuce served in university
restaurants. Int. J. Food Microbiol. 58:123–128.
1242 VALENTIN-BON ET AL.APPL. ENVIRON. MICROBIOL.