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 American Journal of  
Food Science and Technology (AJFST)

The Safety of  Leafy Vegetables in Oman
Mohamed Al-Farsi1*, Haroon Muhammad Ali1, Mohammed Al-Omairi1

Volume 3 Issue 1, Year 2024
ISSN: 2834-0086 (Online)

DOI: https://doi.org/10.54536/ajfst.v3i1.2674
https://journals.e-palli.com/home/index.php/ajfst

Article Information ABSTRACT

Received: May 04, 2024

Accepted: June 10, 2024

Published: June 14, 2024

Leafy vegetables are potential carriers of  foodborne diseases that threaten the community’s 
well-being. Therefore, monitoring leafy vegetable’s microbial and heavy metal contamination 
is crucial. This study evaluates the safety of  leafy vegetables consumed in Oman by 
examining pathogenic bacteria such as Escherichia coli, Staphylococci, Salmonella, and 
Listeria, as well as the heavy metals concentration. Results indicate high levels of  microbial 
contamination in all samples, exceeding the Gulf  Standard Organization permissible levels 
for E. coli and Staphylococci. The highest levels of  E. coli were 5.96, 6.08, 6.09, 6.25, and 
6.01 log CFU/g in Arugula, Radish, Lettuce, Cabbage, and Spring onion, respectively. While 
for Staphylococcus the highest values were 5.59, 5.71, 4.33, 5.07, and 4.46 log CFU/g in 
Arugula, Radish, Lettuce, Cabbage, and Spring onion, respectively. No Salmonella or Listeria 
colonies were found in any samples even after several days of  incubations. The Arsenic 
and Lead concentrations in all samples were significantly higher than the permissible levels 
set by FAO/WHO, while Chromium was high in some samples. The concentrations of  
Copper and Iron in most samples fell below their respective permissible levels, while Zinc 
was undetectable in all samples. These findings underscore inadequate hygiene, harvesting, 
storage, and agricultural practices, raising concerns about the health implications of  
consuming contaminated leafy vegetables.

Keywords

Microbiological, Heavy 
Metal, Contamination, Leafy 
Vegetables

1 Natural & Medical Sciences Research Center, University of  Nizwa, Oman
* Corresponding author’s e-mail: malfarsi@unizwa.edu.om

INTRODUCTION
Vegetables are the vital constituents of  the human 
eating regimen providing essential dietary components 
preserving prime health. Among diverse varieties, leafy 
vegetables stand out at a distinct position due to their 
nutrient balance and culinary diversity. The consumption 
of  leafy vegetables continuously increases in Oman due to 
their promising nutrient profile. Leafy vegetables such as 
Arugula, Cabbage, Lettuce, Spring onions, and Radish are 
sources of  certain vital minerals (iron, calcium, potassium, 
and magnesium), vitamins (K, C, E, and many types of  B 
vitamins), and phytonutrients (beta-carotene, lutein, and 
zeaxanthin) (Gupta et al., 2022) . Settaluri et al. (Settaluri et 
al., 2015) reported the nutritional constituents of  Omani 
leafy vegetables such as Lettuce, Cabbage, and Arugula, 
they found that all the leafy vegetables possessed high 
contents of  starch, sucrose, and other biomolecules vital 
for biological functions. Saiwal et al. (Saiwal et al., 2019) 
also found that leafy vegetables are excellent sources of  
antioxidants, omega-3 fats, selenium, and other healthy 
minerals. Ismail & Cheah (Ismail & Sook, 2003) proved 
in their study that Cabbage contains a high content of  
beta-carotene, riboflavin, vitamin C, and vitamin K 
per serving. However, irrespective of  their exceptional 
dietary advantages, leafy vegetables are potential carriers 
of  foodborne diseases that threaten dietary safety and 
community well-being. 
Consuming contaminated foods can result in foodborne 
infections posing challenges to public health and 
the global economy. Contaminated foods have been 
associated with outbreaks of  foodborne diseases, due 

to which contaminated leafy vegetables are feared to be 
the potential cause of  foodborne outbreaks. According 
to the World Health Organization (WHO), as many as 
600 million people, or 1 in every 10 people suffer from 
foodborne diseases every year and almost 420,000 people 
die (Li et al., 2020). Another study in 2016 discovered 
that most of  the leafy vegetables sold in Finland 
contained E.coli, Listeria, and Salmonella, highlighting 
the poor microbial safety of  leafy vegetables in Finland 
(Nousiainen et al., 2016). Among various pathogenic 
bacterial strains, Escherichia coli, staphylococcus, 
salmonella, and listeria are the agents responsible for the 
contamination of  leafy vegetables and possible suspects 
of  launching foodborne outbreaks (Marshall et al., 2020). 
Similarly, Yafetto et al. (Yafetto et al., 2019) also reported 
8.29 and 8.09 log CFU/g bacterial counts on nutrient agar 
for cabbage and lettuce, respectively. The resilience and 
aggressiveness of  these pathogenic culprits highlight the 
cruciality of  microbiological analysis of  leafy vegetables 
guaranteeing their safety and preventing the consumers 
from serious health consequences. A threshold of  2 and 
3 Log CFU/g for E. coli and Staphylococcus respectively 
in leafy vegetables has been established by the Gulf  
Standard Organization (GSO 1016), which is a regulatory 
body within the Gulf  Cooperation Council (GSO, 2019). 
This threshold is 0 for Salmonella and Listeria, which 
shows the zero-tolerance policy of  GSO for Salmonella 
and Listeria in leafy vegetables.
Apart from the microbial hazards, leafy vegetables also 
host a variety of  heavy metals sourced from soil, water, and 
agricultural products. Plants absorb heavy metals through 



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different trails such as contaminated water, irrigation, 
agrochemicals, and atmospheric depositions (Liu et al., 
2005, Hardaway et al., 2016). Additionally, sewage sludge 
and the application of  fertilizers, pesticides, and manure 
may also influence plants’ intake of  trace elements. It was 
found that the uptake of  metals by crops is directly linked 
to the heavy metal concentration in the soil (McBride, 
2003). It is vital to ensure that the concentrations of  
heavy metals in vegetables are less than the permissible 
limits set by FAO/WHO (WHO, 1989). The permissible 
concentrations for Arsenic (As), copper (Cu), zinc (Zn), 
iron (Fe), lead (Pb), and chromium (Cr) are 0.2 mg/kg, 
73 mg/kg, 99 mg/kg, 425 mg/kg, 0.3 mg/kg, and 2.3 
mg/kg, respectively. Several studies have reported higher 
concentrations of  heavy metals in vegetables than those 
allowed by FAO/WHO. For instance, Mensah et al. 
(Mensah et al., 2009) reported in their study that all heavy 
metals except nickel have surpassed the FAO/WHO 
standardized heavy metals concentrations. Similarly, much 
higher concentrations of  heavy metals were reported in 
Palestine vegetables by Bawwab et al. (Bawwab et al., 2022) 
compared to the FAO/WHO limits. Excessive intake of  
certain heavy metals including cadmium, lead, copper, 
and chromium can cause serious adverse effects including 
bone, neurological and cardiovascular disorders, renal 
impairment, and gastrointestinal disturbances. Depending 
on the exposure time, the toxicity due to heavy metal 
exposure may be acute or chronic (Jean-Lou Dorne et al., 
2010).
To ensure food safety, it is extremely important to monitor 
the microbial and heavy metal contamination in leafy 
vegetables along with their associated health risks for the 
consumers. Therefore, this study aimed to evaluate the 
safety of  leafy vegetables consumed in Oman. For this 
purpose, we determined the microbiological quality of  
leafy vegetables by determining the number of  colonies 
of  pathogenic bacterial strains that are Escherichia coli, 
Staphylococci, Salmonella, and Listeria in leafy vegetables. 
Additionally, we have also determined the heavy metals 
concentration in leafy vegetables.

MATERIALS AND METHODS
Samples Collection
Twenty-five samples (approximately 500 g each) of  
edible portions of  five leafy vegetables including Lettuce 
(Lactuca sativa), Cabbage (Brassica oleracea), Arugula, 
(Eruca sativa), Radish (Raphanus sativus), and Spring 
onion (Allium fistulosum) were collected in sterile sample 
bags from different Hypermarkets in Nizwa, Oman. 
The samples were stored at 5 ◦C until analysis. The 
microbiological analysis was conducted within 6 hours 
of  the collection. To keep the sources of  the samples 
anonymous, they were represented by numbers i.e.; 1, 2, 
3, 4, and 5 along with the first letter of  the vegetable’s 
common name. 

Microbiological Analysis
The samples were cut down into small pieces using a 

sterile knife. A sample of  10g was then added to 90 ml of  
sterile Buffered Peptone Water (BPW), which was made 
from10 g of  peptone powder, 5g of  sodium chloride, 3.5 
g of  disodium phosphate, and 1.5 g of  mono-potassium 
phosphate were added to 1000 ml of  water and mixed 
well using hot-plate stirrer for 5 minutes. For Salmonella 
and Listeria, the sample was incubated in BPW overnight. 
However, no overnight incubation was given for E.coli 
and Staphylococcus as it can affect the threshold colony 
count of  these strains. Different selective mediums were 
used for all four types of  microorganisms used in this 
study.

Escherichia Coli Count
MacConkey agar (15.45 g in 300 mL H2O) was used for 
the isolation of  E. coli. The media was heated using a 
hot plate stirrer until boiling followed by sterilization at 
121 oC for 15 minutes using an autoclave. The media was 
poured into sterile petri plates and let solidify at room 
temperature. Then, three different volumes (0.1 mL, 0.2 
mL, and 0.3 mL) of  the 1:10 diluted sample were spread 
on the agar plates using a sterile spreader. After culturing 
all the samples, the Petri plates were covered with parafilm 
to prevent cross-contamination during incubation. All the 
plates were then incubated overnight at 44 ◦C. After 24 
hours of  incubation, the number of  colonies on all the 
plates was counted (Degaga et al., 2022). 

Staphylococci Count
To isolate Staphylococci, Mannitol salt agar was prepared 
(33.3 g in 300mL H2O). Similar to E. coli isolation, the 
media was heated and autoclaved. The same volumes and 
dilutions were added to the plates and incubated at 37 ◦C for 
24 hours before counting the colonies (Degaga et al., 2022).

Salmonella Count
Xylose-lysine-deoxycholate (XLD) was prepared (17.004 
g of  XLD was added to 300 mL dH2O) for the selective 
isolation of  Salmonella. The medium was heated at a very 
low temperature and then transferred to a water bath for 
a few minutes at 45-50 oC before use. The same volumes 
and dilutions were added to the plates and incubated at 
37 ◦C for 24 hours before counting the colonies (Degaga 
et al., 2022, Sant’Ana et al., 2011).

Listeria Count
For Listeria isolation, 20.66 g of  PALCAM agar 
was dissolved in H2O. The medium was heated till 
boiling, then sterilized at 121oC for 15 minutes. After 
sterilization, 0.6 mL of  selective supplement (Polymyxin 
B, acriflavine, and ceftazidime combined) was also added 
to PALCAM agar for the selective isolation of  Listeria. 
The same volumes and dilutions were added to the plates 
and incubated at 37 ◦C for 24 hours before counting the 
colonies (Degaga et al., 2022, Abatcha et al., 2020).

Heavy Metal Analysis
The collected leafy vegetables were chopped into small 



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pieces, dried in an oven at 60 ◦C until constant weight, 
and crushed to powder form using a regular blender. 
For the accurate analysis of  heavy metals, complete 
digestion is essential. For this purpose, 0.5 g of  sample 
was weighed and 5mL of  nitric acid (HNO3) was added. 
Then the sample was introduced to a microwave digestion 
apparatus (model: ULTRAWAVE, SN:16040687, 
Milestone srl, Sorisole, Italy). After digestion in the 
microwave combustion system, the solution was filtered 
using Whatman filter paper. Then the filtrate was analyzed 
with Inductively Coupled Plasma-Optical Emission 
Spectroscopy (ICP-OES, OPTIMA 8000, PerkinElmer, 
Massachusetts, America). Five calibration standards were 
taken for each sample. The calibration concentration 
ranged from 0.07-3 for As, 1-20 for Cu, 1-100 for Zn, 
1-150 for Fe, 1-20 for Pb, and 1-20 mg/L for Cr.  The 
amounts of  As, Cu, Zn, Fe, Pb, and Cr, were determined 
in mg/kg. All the samples were experimented on twice 
and the resulting values are the average of  the duplicates 
(Bingöl et al., 2010).

Statistical Analysis
The data were evaluated statistically using Microsoft 
Excel for the mean value and standard deviation (SD). 
Analysis of  variance (ANOVA) was used to determine 
the level of  significance (P < 0.05). 

RESULTS AND DISCUSSION
Microbiological Analysis
In this study, five different leafy vegetables i.e., Arugula, 
cabbage, onion, lettuce, and radish from five different 
hypermarkets, were evaluated for their microbiological 
qualities. Table 1 represents the log values ± standard 
deviation of  the calculated colonies per gram compared 
to the standard values outlined by GSO 1016 for each 
bacterium (GSO, 2019). The results showed that all the 
samples had high microbial contamination levels, and their 
log CFU/g values were much higher than the standard 
values for E. coli and Staphylococci. This highlights 
the poor hygiene, harvesting, storage, and agricultural 
practices. However, the log CFU/g values varied among 
the varieties of  leafy vegetables and the hypermarkets 
from where the samples were collected. 
The E. coli counts in all samples were significantly higher 
than the permissible limits set by GSO 1016 (2.0 Log 
CFU/g) (GSO, 2019). The E. coli count ranged between 
5.21-5.96 in Arugula, 5.46-6.08 in Radish, 5.32-6.25 in 
Lettuce, 3.77-6.09 in Cabbage and 5.30-6.01 in Spring 
onion. Even the lowest Log CFU/g values in this study 
are much higher than those previously reported by Khalil 
(Khalil, 2016) for arugula which was 4.51 for E. coli. 
Our results are also in line with those of  Hashemi et al. 
(Hashemi et al., 2019) who reported 4.2 Log CFU/g for 
E. coli in radish. These values are higher than the average 
Log CFU/g values of  3.34 for E. coli in lettuce reported 
by Choi et al. (Choi et al., 2016). Among the Spring onions 
of  all the brands, O2 and O5 were the most contaminated 
with 6.01 Log CFU/g each for E.coli. O1 exhibited the 

next highest Log CFU/g for E.coli, leaving O3 to be the 
least contaminated among all the samples for E. coli. In a 
previous study reported by Xu et al. (Xu et al., 2013), the 
initial E.coli contamination in onion leaves was 5.2 log 
CFU/g, which is comparatively less than the one found 
in our study.
Also, the Staphylococci counts in all samples were 
significantly higher than the permissible limits set by GSO 
1016 (3.0 Log CFU/g) (GSO, 2019). The Staphylococci 
counts ranged between 4.59-5.59 in Arugula, 4.62-5.71 in 
Radish, 3.10-5.07 in Lettuce, 3.27-4.33 in Cabbage and 
3.48-4.46 in Spring onion. Our results showed higher log 
CFU/g values than those of  Hashemi et al. (Hashemi et 
al., 2019) who reported 3.3 Log CFU/g for Staphylococci 
in radish. Even though the values for Staphylococci in 
the cabbage sample are surprisingly much less than the 
rest of  the sample, they are almost similar to the log 
CFU/g values for both E.coli and Staphylococci (4.44) 
reported by Kothe et al. (Kothe et al., 2019) when stored 
for 5 days at 30oC. They also reported Log CFU/g values 
for E. coli much less than the values that resulted in 
our study. However, their values increased significantly 
when incubated for a whole week. This indicates that 
improper storage of  vegetables for extended periods 
provides microorganisms optimal conditions for growth 
resulting in the microbial contamination and spoilage 
of  vegetables. These values are higher than the average 
Log CFU/g values of  4.62 for Staphylococci in lettuce 
reported by Choi et al. (Choi et al., 2016). Sample O5 
resulted in being the most contaminated with 4.46 Log 
CFU/g for Staphylococci among the onion samples. The 
sample O2 exhibited the next highest Log CFU/g in 
Staphylococci, leaving O4 to be the least contaminated 
among all the samples for Staphylococci.
The overall results showed that leafy vegetables from 
hypermarkets 2 and 5 were the most contaminated 
with E. coli and Staphylococci when compared to the 
samples from the rest of  the Omani hypermarkets. 
However, all the samples showed significantly higher 
microbial contamination than the GSO 1016 standards 
(GSO, 2019). This raises a critical concern regarding 
the microbial quality of  leafy vegetables from all the 
hypermarkets. Our study highlights the questionable 
transportation, distribution, storage, and handling of  leafy 
vegetables in hypermarkets. Encouragingly, not a single 
colony of  Salmonella and Listeria was observed in any of  
the samples from all the hypermarkets even after several 
days of  incubation. Both strains are very pathogenic and 
have the potential to cause severe health complications 
in consumers. Unlike the previous studies of  Aytaç et al. 
(Aytac et al., 2010), Abadias et al. (Abadias et al., 2008), and 
Fröder et al. (Fröder et al., 2007), who reported the high 
prevalence of  Salmonella and Listeria in leafy vegetables, 
our results suggest that Omani leafy vegetables are free 
of  Salmonella and Listeria contamination. The reason 
behind this might be due to competition between 
Salmonella and Listeria, and the background bacteria 
which in this case are E. coli and Staphylococcus. This 



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hypothesis was also stated by Abadias et al. (Abadias et al., 
2012), who suggested that different bacterial species on 
a single food sample can compete for physical space as 
well as nutrient availability. Similarly, Babic et al. (Babic et 
al., 1997) also reported in their study that the background 
microbiota such as E.coli inhibited the growth of  Listeria 
in spinach.
This indicates that leafy vegetables with elevated 
levels of  one bacterium can cause growth inhibition 
of  other pathogenic bacteria. The absence of  these 
strains highlights the appreciable food safety of  Omani 
hypermarkets. However, the presence of  E. coli and 

staphylococcus at such a significantly high number 
cannot be neglected. The high prevalence of  E.coli 
and Staphylococcus in leafy vegetables raises concerns 
regarding safety and public health. Both these bacteria are 
pathogenic, making these leafy vegetables the potential 
source of  food-borne outbreaks. These leafy vegetables 
contaminated with these bacterial strains can cause 
severe food-borne communicable illnesses, especially 
the local consumers unaware of  the consequences of  
contaminated food. Protective measures should be 
taken by the food safety authorities to address the raised 
concerns and ensure public health.

Table 1: The average microbial count of  selected leafy vegetables collected from markets in Nizwa, Oman
Samples E. coli Log CFU/g Staphylococci Log 

CFU/g
Salmonella Log 
CFU/g

Listeria Log 
CFU/g

GSO 1016 2.0a 3.0a 0 0
Arugula:
A1 5.73±0.11b 5.33±0.24b ND ND
A2 5.21±0.25c 5.59±0.15b ND ND
A3 5.75±0.23b 4.90±0.13c ND ND
A4 5.27±0.04c 4.59±0.12d ND ND
A5 5.96±0.14b 5.43±0.16b ND ND
Radish:
R1 5.46±0.21b 5.65±0.16b ND ND
R2 5.63±0.11b 5.08±0.16c ND ND
R3 5.62±0.03b 4.62±0.08d ND ND
R4 5.98±0.19b 5.30±0.11b ND ND
R5 6.08±0.13b 5.71±0.14b ND ND
Lettuce:
L1 5.71±0.15b 3.72±0.16e ND ND
L2 6.25±0.20b 4.51±0.12d ND ND
L3 5.32±0.13c 4.84±0.06c ND ND
L4 5.80±0.09b 3.10±0.17a ND ND
L5 5.74±0.21b 5.07±0.08c ND ND
Cabbage:
C1 3.77±0.15d 3.43±0.13f ND ND
C2 5.85±0.15b 4.33±0.11d ND ND
C3 5.65±0.11b 3.27±0.14a ND ND
C4 6.09±0.20b 4.07±0.10j ND ND
C5 5.63±0.17b 3.69±0.13e ND ND
Spring Onion:
O1 5.84±0.14b 4.23±0.12d ND ND
O2 6.01±0.14b 4.11±0.12j ND ND
O3 5.49±0.11b 3.48±0.16e ND ND
O4 5.30±0.03c 3.91±0.06j ND ND
O5 6.01±0.19b 4.46±0.09d ND ND

Means ± SD followed by the same letter, within a column, are not significantly different (P > 0.05). Samples 1-5: samples from 5 
distinct locations, ND: Not detected.



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Heavy Metal Analysis
Table 2 represents the concentrations of  As, Cu, Zn, Fe, 
Pb, and Cr per Kg of  leafy vegetables from five different 
hypermarkets in Oman. The concentrations of  As, and Pb, 
in all the leafy vegetable samples from all the hypermarkets 
were significantly higher than the permissible limits set 
by FAO/WHO (WHO, 1989) while some samples had 
higher concentrations of  Cr. The concentrations of  Cu 
and most of  the Fe samples fell below their respective 
permissible levels, while Zn was non-detective in all the 
samples. However, the concentrations of  heavy metals 
varied between the samples. 
The results showed that the concentrations of  As range 
from 1.67 to 2.14 mg/kg in arugula, 1.89 to 2.19 mg/kg 
in radish, 1.98 to 2.19 mg/kg in lettuce, 1.95 to 2.26 mg/
kg in cabbage, and 1.65 to 1.92 mg/kg in spring onion. All 
these values are far higher than the 0.2 mg/kg permissible 
concentration set by FAO/WHO (WHO, 1989). Our 
study completely contradicts previous studies reporting 
As concentrations in leafy vegetables. For instance, Ma 
et al. (Ma et al., 2007) reported 0.09 and 0.14 mg/kg of  
As in lettuce and spring onion. However, Gezahegn et al. 
(Gezahegn et al., 2017) also previously reported 1.4 mg/
kg of  As in cabbage. such elevated concentrations of  As 
are associated with cardiovascular problems in humans 
such as hypertension, atherosclerosis, and peripheral 
cardiovascular diseases (Balarastaghi et al., 2022). The 
consumption of  these vegetables contaminated with As 
can lead to serious health issues. 
Cu concentrations in all the samples are significantly 
lower than the permissible limit of  73 mg/kg for leafy 
vegetables. The Cu concentration ranged from 13.8 to 
19.4 mg/kg in arugula, 13.9 to 23.7 mg/kg in radish, 
12.7 to 18.4 in lettuce, 9.1 to 13.2 mg/kg in cabbage, 
and 12.6 to 18.7 mg/kg in spring onion. Our results 
align with those of  Kananke et al (Kananke et al., 2014), 
who reported that the concentrations of  Cu fell below 
the permissible limits. Even though Cu is important for 
several body functions such as central nervous system 
maintenance and anemia prevention, its elevated levels 
can be toxic to the human body such as liver damage, 
gastrointestinal symptoms, and kidney damage (Kananke 
et al., 2014). Fortunately, our study reassures the Cu levels 
below the threshold associated with toxicity. 
Even though Fe has a very high threshold concentration 
of  425 mg/kg in leafy vegetables, most of  the arugula 
samples still crossed that limit. The samples in which the 
Fe concentrations were higher than the FAO/WHO limit 
are A1 (665 mg/kg), A2 (699 mg/kg), A4 (744 mg/kg), 
and A5 (633 mg/kg) in arugula, R1 (479 mg/kg) in radish, 
and O1 (524 mg/kg) in spring onion. Fe concentrations in 
cabbage were non-detectable. All the remaining samples 
have permissible levels of  Fe. Interestingly, lettuce 
samples from all the hypermarkets have Fe concentrations 
below the threshold levels. These results agree with the 
previous studies (Uriu-Adams & Keen, 2005, Hanif  et 
al., 2006, Zwolak et al., 2019), which reported that Fe 

levels in lettuce fall within the permissible limits. Fe is 
important for many biological functions in the body such 
as oxygen transport, energy production, and immune 
functions (Demi-Rezen & Aksoy, 2006). However, some 
of  the tested samples, especially arugula possess elevated 
levels of  Fe that can potentially cause negative effects 
such as increased oxidative stress, pancreas damage, and 
cardiomyopathy (Lieu et al., 2001, Sampaio et al., 2014)
Pb concentrations ranged from 2.86 to 15.89 mg/kg in 
arugula, 2.8 to 6.01 mg/kg in radish, 3.55 to 4.85 mg/
kg in lettuce, 4.07 to 5.46 mg/kg in cabbage, and 2.97 
to 3.32 mg/kg in spring onion. These concentrations 
are significantly higher than the maximum permitted 
by FAO/WHO for leafy vegetables. However, these 
concentrations are far lower than the 12.3 mg/kg of  
Pb in cabbage reported by Gezahegn et al. (Gezahegn et 
al., 2017). These elevated levels of  Pb in vegetables can 
result either from the ink-contaminated water absorbed 
by the soil or from the water used for watering the plants 
directly, especially the water leaked from industries as 
Gezahegn et al. (Gezahegn et al., 2017) suggested that the 
high levels of  Pb in cabbages can be attributed to the ink 
leakage from industries to the water. Such elevated levels 
of  Pb can have negative impacts on the human body such 
as kidney dysfunction, and brain damage as it is toxic to 
the central and peripheral nervous system (Murphy & 
Oudit, 2010).  
Most of  the tested samples possess elevated Cr 
concentrations exceeding the permissible limit of  2.3 mg/
kg for vegetables. A2 exhibited the highest concentration 
of  11.65 mg/kg, while the lowest Cr concentration of  
1.13 mg/kg was exhibited by C5. Arugula, was found to 
be the most contaminated with Cr with an average Cr 
concentration of  6.7 mg/kg, followed by radish, lettuce, 
and Spring Onion with average concentrations of  3.73 
mg/kg, 3.51 mg/kg and 3.27 mg/kg respectively. Kananke 
et al. (Kananke et al., 2014) reported a much lower average 
Cr concentration of  1.79 mg/kg in leafy vegetables that 
falls within the permissible level. Such high levels of  Cr 
can be attributed to the using waste for the irrigation 
of  leafy vegetables (Nava-Ruíz & Méndez-Armenta, 
2013). Due to the persistence and high toxicity of  Cr, 
it is considered to be very toxic for human biological 
processes (Soumi et al., 2016). 
Some of  the heavy metals such as Cu, Zn, and Fe are vital 
for the biological functions of  the human body. However, 
the elevated levels of  these metals can exhibit adverse 
effects and can be toxic to the human body. Therefore, 
it is crucial to ensure the concentrations of  heavy metals 
are not above the FAO/WHO recommended levels in 
foods (leafy vegetables in this study). There could be 
multiple reasons for the elevated contents of  such toxic 
heavy metals in vegetables but fertilizers and irrigation 
water are the most convincing reasons. Bawwab et 
al. (Bawwab et al., 2022) suggested that sometimes 
compost contaminated with industrial waste is utilized 
as a fertilizer containing high contents of  heavy metals. 



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Mensah et al. (Mensah et al., 2009) argued that apart 
from fertilizers, the use of  contaminated water can also 
increase the heavy metals absorption in plants. When 
roots absorb Pb, it cannot go through the root’s endo-
derms. However, the results from the study of  Domergue 
& Vedy (Domergue & Védy, 1992) showed that leafy 
vegetables not only accumulate Pb from the soil but 
also use leaves to accumulate Pb from the atmosphere. 
Other factors such as the plant species, the type of  soil, 
growth phase, and the surrounding environment greatly 
affect the accumulation of  heavy metals in plants. Even 

the atmospheric deposition and marketing can be the 
potential reasons for the elevated heavy metals contents. 
The elevated samples are directly associated with the 
health consequences of  consumers. As we discussed 
above, intake of  vegetables with such prominent levels 
of  heavy metals can lead to many serious health issues 
such as hypertension and cardiovascular diseases due 
to As, gastrointestinal, kidney, and liver damage due to 
copper, cardiomyopathy, and pancreas damage due to Fe, 
compromised central and peripheral nervous system due 
to Pb, and kidney dysfunction due to Cr. 

Table 2: The heavy metal concentration of  selected leafy vegetables collected from markets in Nizwa, Oman
Samples Arsenic 

mg/kg
Copper mg/
kg

Zinc mg/
kg

Iron mg/kg Lead mg/kg Chromium 
mg/kg

WHO/FAO limits 0.2a 73a 99 425a 0.3a 2.3a

Arugula
A1 ND 19.4±0.4b ND 665±67b 4.36±0.39b 10.40±0.89b

A2 1.84±0.19b 18.3±0.7c ND 699±95b 15.89±1.62c 11.65±1.48c

A3 1.90±0.34b 16.8±1.5d ND 149±11c 4.16±0.51b 2.17±0.11d

A4 2.14±0.15c 14.8±0.7e ND 744±17d 2.86±0.12d 4.86±0.11e

A5 1.67±0.01d 13.8±0.1f ND 633±38b 3.43±0.0.39e 4.43±0.04f

Radish
R1 1.89±0.29b 15.2±0.8g ND 479±62e 2.8±0.28d 6.8±1.1g

R2 1.92±0.10b 14.3±0.3e ND 263±23f 3.34±0.22e 3.54±0.18h

R3 2.19±0.10c 23.7±1.8h ND 271±10f 6.01±0.91f 3.12±0.42i

R4 2.06±0.03c 13.9±0.3f ND 343±25g 3.26±0.79e 2.13±0.54d

R5 1.92±0.02b 15.3±0.6g ND 106±8h 4.34±0.69b 3.07±0.17i

Lettuce
L1 2.04±0.14c 12.7±0.1i ND 207±13i 4.49±0.32b 2.90±0.38i

L2 2.05±0.01c 18.4±0.1c ND 16± j 4.85±0.78g 1.49±0.08 j

L3 2.11±0.15c 13.5±1.1f ND 52±5k 3.55±0.15e 4.29±0.21f

L4 1.98±0.22b 12.9±0.2i ND 106±15h ND 6.55±0.01g

L5 2.19±0.20c 13.5±1.1f ND 40±8l 4.44±0.72b 2.34±0.40k

Cabbage
C1 1.95±0.24b 13.2±0.5f ND ND 4.44±0.41b 3.42±0.77h

C2 2.19±0.21c 12.7±2.3i ND ND 4.07±0.68h 1.72±0.15l

C3 1.96±0.11b 9.1±1.5j ND ND 4.77±0.73g 1.21±0.05m

C4 2.26±0.09c 11.8±1.2k ND ND 5.46±0.21i 1.59±0.02j

C5 2.05±0.07c 11.1±1.4k ND ND 4.73±0.60g 1.13±0.10m

Spring Onion
O1 1.91±0.27b 14.2±2.9e ND 524±4k 3.28±0.68e 7.71±0.12n

O2 1.65±0.01d 12.6±0.2i ND ND 3.07±0.08e 1.44±0.07j

O3 1.92±0.02b 18.7±0.6b ND 62±2m 3.32±0.49e 2.47±0.40k

O4 1.91±0.04b 15.6±0.7g ND 233±12n 2.97±0.14d 2.76±0.03o

O5 1.9±0.03b 18.5±0.3b ND 121±5o 2.97±0.04d 1.99±0.32q

Means ± SD followed by the same letter, within a column, are not significantly different (P > 0.05). Samples 1-5: samples from 5 
distinct locations, ND: Not detected.



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Am. J. Food. Sci. Technol. 3(1) 42-50, 2024

CONCLUSION
It can be concluded from the present study that leafy 
vegetables have high microbiological and heavy metals 
contamination. Most of  the samples used in this study 
were contaminated with E. coli and Staphylococci as well 
as some toxic heavy metals such as As, Pb, and Cr. This 
study highlights the questionable production, irrigation, 
fertilization, harvesting, packaging, transportation, 
handling, and storage of  leafy vegetables in Oman. The 
consumption of  vegetables with such microbiological 
and heavy metals content can lead to concerning health 
issues among the consumers. Further work needs to be 
conducted to ensure microbiological safety, assessing 
more food samples for even more pathogenic bacterial 
strains. Additionally, the heavy metals accumulation by 
other vegetables and fruits also needs to be explored. 

Acknowledgments
The authors acknowledge the role of  the Food Science 
and Technology Lab and Natural and Medical Sciences 
Research Centre (NMSRC) administration in providing 
a conducive environment for research activities. The 
authors also acknowledge the role of  the Microbiology 
lab and DARIS in providing support and facilities 
necessary for conducting experiments and data analysis.

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