Nitrates are present naturally in soils, water, plants (particularly in vegetables) as a consequence of nitrogen fixation. The wide use of nitrogen based fertilizers in agriculture contributes to the total nitrate present in the environment as well. The significance of nitrate to human health derives from the fact that nitrate can be converted in vivo to nitrite producing toxic effects.
In addition, nitrite ion can react with secondary or tertiary amines to form
N-nitroso compounds, some of them being implicated in the etiology of cancers
(Sebecic and Vedrina-Dragojevic, 1999). Nitrate contami-nation
in vegetables occurs when crops absorb more than they require for their sustainable
growth. Spinach, lettuce, broccoli, cabbage, celery, radish, beetroot, etc.,
possess the tendency to accumulate nitrates. On the other hand, vegetables such
as carrots, cauliflower, French beans, peas and potatoes seldom accumulate nitrates.
Nitrate content of vegetables may range from 1-10,000 mg kg-1 (MAFF,
1998; Ximenes et al., 2000). Concentrations
of nitrate in vegetables depend on agricultural practicfes, storage conditions,
the temperature and light in which they are grown and the concentrations of
nitrate in the soil, fertilizers and water used to grow the vegetables (National
Research Council, 1981; Duncan and John, 2006).
The main concern for the public health is the link between nitrates and stomach
cancer. Nitrite is a precursor in the formation of nitrosamines (Tannenbaum
and Correa, 1985). Another important concern is that vegetables are an important
part of most babies diets (Huarte-Mendicoa et al.,
1997). Young babies with low stomach acidity may suffer from infantile methemoglobinemia
due to excessive nitrates in their diet where nitrite is substituted for oxygen
in hemoglobin and death may occur (Ezeagu, 1996; Gundimeda
et al., 1993). Even after such a high risk on public health there
is no data available on South Province of Turkeys commonly consumed vegetables.
This forms the basis for the regular monitoring of nitrate and nitrite levels
in commonly consumed South Province of Turkeys fresh vegetables.
MATERIALS AND METHODS
Preparation of samples: Samples were washed with tap water. The edible
part of the samples was used for analyses. For each vegetable, a composite sample
of many individuals (e.g., spinach, cabbage, leek, lettuce, parsley and radish)
was used. After washing the vegetables were chopped into small sections and
homogenized in a blender mill.
Extraction and analysis: Sen and Donaldson (1978)
method was used for the extraction of nitrate-N and nitrite-N in the present
study. Homogenized sample (10 g) was accurately weighed and blended for 5 min
with 70 mL of water. Then 12 mL of 2% NaOH was added while pH ca. About 8 was
adjusted with 2% NaOH (avoiding excess NaOH). The slurry was transferred to
a 200 mL volumetric flask and heated on water bath (50-60°C) with occasional
swirling until the temperature of the suspension reached about 50 C. ZnSO4
(10 mL) was added and temperature of the suspension maintained at about 50°C
for further 10 min. If a white precipitate of Zn (OH)2 did not appear,
2-5 mL of 2% NaOH was added (avoiding excess NaOH). Contents were cooled to
room temperature by immersing flask in cool water bath. The solution was diluted
to a fixed volume with water and mixed thoroughly. Then the solution was filtered
through a 0.45 mL membrane filter. The first zone of filtrate was discarded
in order to overcome possible nitrate contamination from the filter-papers.
The limit of detection was 5 mg kg-1 for nitrate and 0.3 mg kg-1
Nitrate analysis: An aliquot of 10 mL filtrate was mixed with 5.0 mL
NH4Cl buffer and passed through the cadmium column. This solution was reacted
with sulphanilamide and N-(1-naphthyl) ethylene-diamine and the absorbance of
the violet azo compound was measured at 540 nm (Sen and
Nitrite analysis: The nitrite was determined calorimetrically by diazotization of sulfanilamide and subsequent coupling with N-(lnapthyl)-ethylenediamine to form a pink azodye whose absorbance was measured at 540 nm against aqueous prepared standards.
Quality assurance: The method was shown to provide accurate results
by participation in the UK Food Analysis Performance Scheme (FAPAS) exercises.
The results for the nitrate contents of 1052 mg kg-1 spinach puree
were Z:-0.7 and robust mean was 1097.3 mg kg-1 (FAPAS
RESULTS AND DISCUSSION
Nitrate contents of vegetables: The nitrate levels of all vegetables
measured in this study are shown in Table 1. Generally, nitrate
contents of all samples were found very high compared with nitrite values. It
was shown that radish; spinach, lettuce and parsley contained a higher level
of nitrates whilst cabbage and leek contained a lower level of nitrates. The
mean contents of nitrates are summarized as follows (mg kg-1): Cabbage,
510; leek, 91; lettuce, 1439; parsley, 1070; spinach, 1132; radish, 3428. The
content of nitrates can vary from 1-10000 mg kg-1, depending not
only on genetic factors such as kinds or strains of the vegetables but also
on environmental factors including the places or conditions of cultivation and
storage (European Commission, 1995; World
Health Organization, 1995).
Generally, the levels of accumulated nitrates needed for subsequent survival
and growth fluctuate in different vegetables and in the different parts of the
vegetables. A higher nitrate level was observed in greenish-yellow leaf vegetables
(European Commission, 1995; World
Health Organization, 1995). Generally, a lower level of nitrate was observed
in the vegetable groups of cucumbers and tomatoes (<500 mg kg-1)
while higher levels were observed in the groups of lettuce, spinach, radish
and Chinese cabbage (Scharpf, 1991). The highest levels
of nitrate were in radish followed by lettuce, spinach, parsley, cabbage and
Nitrite contents of vegetables: As shown in Table 1, the mean nitrite contents in most vegetables were <15 mg kg-1. Nitrite contents were the highest in lettuce and pollution levels decreased in the order of spinach, radish, cabbage, parsley and leek. All samples were detected for nitrite. The highest nitrite concentrations were mainly detected in lettuce from 2.92-8.80 mg kg-1 (mean value 11 mg kg-1). It was also detected in some only samples of spinach, radish and cabbage (from 0.06-0.23 for cabbage, from 0.15-0.32 for spinach and from 0.15-0.41 mg kg-1 for radish). In the samples of leek and parsley the least nitrite concentrations were detected.
Comparison with monitoring results of foreign countries: The levels
of the nitrate and nitrite in vegetables in some other parts of the worlds are
shown in Table 2. The results in this table are on the basis
of their fresh weight and NO3, NO2. Comparison with levels
in vegetables in the present study revealed that the levels are generally comparable.
The results obtained were compared with those of other countries (Table
2). Leafy vegetables (lettuce, spinach, parsley and radish) appear to contain
high levels of nitrate and nitrite. This trend is similar to those reported
in Korea (Chung et al., 2003) and in other countries;
Germany (Scharpf, 1991), England (MAFF,
1996), Italy (Santamaria et al., 1999). The
average nitrate levels of each vegetable varied depending on the country (European
Commission, 1995), the average levels of nitrate of lettuce and spinach
varied widely ranging from 907-4674 ppm and from 390-3383 ppm, respectively.
|| Nitrate and nitrite concentrations in vegetables (fresh weight,
mg kg-1, NO3¯, NO2¯)
|aSD: Standard Deviation
The minimum and maximum levels of nitrate and nitrite in lettuce were lower
than the amounts reported European Commission (1995).
However, the minimum nitrate levels in spinach were higher than those found
in EU while the maximum nitrate values were below the amount found European
Commission. The nitrite contents in all vegetables were higher than those detected
in Korea (Chung et al., 2003) and England (MAFF,
1996). The spinach grown in Danish contain higher levels of nitrite compared
with the values (while these spinach values were below the amount found in Danish)
while the values for nitrite levels in spinach were similar to that recorded
in Japan (Sumiko and Masako, 1993).
However, cabbage, leek and lettuce grown in Japan and Danish contain lower
levels of nitrite compared with the values. On the other hand, the nitrate and
nitrite concentrations in radish were higher than those detected in Korea (Chung
et al., 2003), Japan (Sumiko and Masako, 1993)
and England (MAFF, 1996). The values of nitrate in some
vegetables were lower than the amounts reported in Korea (Chung
et al., 2003), Germany (Scharpf, 1991), Japan
(Sumiko and Masako, 1993), Danish (Petersen
and Stoltze, 1999). However, the mean levels of nitrate in cabbage, lettuce
and radish were higher than those found Santamaria et
al. (1999) in similar vegetables. But the nitrate levels in other vegetables
(parsley and spinach) were lower compared with these obtained in Italy (Santamaria
et al., 1999).
It was apparent that nitrate levels of cabbage and spinach cultivated in England
(MAFF, 1996) and Japan (Sumiko and
Masako, 1993) were higher than those in present study however, the levels
in radish grown in these countries were lower compared with these obtained in
the study. Also, nitrate contents in cabbage, lettuce and spinach produced in
Korea (Chung et al., 2003) and Germany (Scharpf,
1991) were shown to be higher than those in the present papers similar
vegetable samples. But also the levels of nitrate in radish grown these counties
were lower than the values obtained in the study. Monitoring results from the
Danish (Petersen and Stoltze, 1999) showed that nitrate
contents were higher in cabbage, leek, lettuce and spinach compared with the
With the exception of radish, the levels of nitrates in vegetables grown in the present study were lower than the values obtained in other countries. However, the nitrite levels were higher or even similar than those detected in other countries. Moreover, it is believed that antioxidant agents ubiquitous in vegetables play a beneficial role as a inhibitor of the nitrosamine formation from nitrites. Therefore, nitrates consumed from vegetables are concluded to be harmless to human health. In conclusion, overall evaluation of all the studies performed abroad or domestically leads to the belief that it is necessary to establish limits of nitrate contents of vegetables cultivated in Turkey. This review of nitrate and nitrite contamination in vegetables in Turkey shows that this is a serious problem that will continue to grow as increasing amounts of nitrogenous fertilizers are applied.