Food Additives and Contaminants: Part B Vol. 3, No. 1, March 2010, 19–29

View Dataset Nitrate and nitrite content in organically cultivated vegetables M.C. Matallana Gonza´leza, M.J. Martı´ nez-Tome´b* and M.E. Torija Isasaa a

Universidad Complutense de Madrid, Dpto. de Nutricio´n y Bromatologı´a II: Bromatologı´a, Madrid, Spain; Universidad Miguel Herna´ndez, Instituto de Biologı´a Molecular y Celular, Avda. de la Universidad s/n, Elche 03202, Spain

b

(Received 15 August 2009; final version received 28 December 2009) The nitrate and nitrite content of leaf vegetables (Swiss chard, sea beet, spinach and cabbage), ‘‘inflorescence’’ vegetables (cauliflower) and fruit vegetables (eggplant and vegetable marrow) grown with organic fertilizers have been determined by a modified cadmium–Griess method. Samples were purchased from organic food stores as well as collected directly from an organic farm in Madrid (Spain). Nitrate levels were much higher in the leaf vegetables (especially Swiss chard species; average over the different samples and species of 2778.6  1474.7 mg kg1) than in inflorescence or fruit products (mean values between 50.2  52.6 and 183.9  233.6 mg kg1). Following Swiss chard species, spinach (1349.8  1045.5 mg kg1) showed the highest nitrate content, and nitrite was found above the limit of detection in some samples only (spinach, 4.6  1.0 mg kg1; sea beet, 4.2  0.7 mg kg1 and Swiss chard, 1.2  0.4 mg kg1). Some vegetables (spinach, cabbage and eggplant) had lower nitrate content in the samples harvested in summer, showing the influence of climatic conditions on the nitrate levels in a plant. The samples taken directly from the organic farm, with the exception of eggplant, had higher or slightly higher average nitrate values than samples purchased in the organic food stores, ranging from 117 to 1077%. Keywords: AAS; nitrate; nitrite; vegetables

Introduction In recent years, growing consumer interest in obtaining high quality, nutritive and more ‘‘natural’’ chemical-free foods has encouraged the development of organic agriculture and increased the demand of their products. Organic agriculture is an alternative to conventional (intensive) agricultural systems; it essentially rejects the use of synthetic chemical substances as fertilizers and in pest and disease control, and it looks for soil dynamics equilibrium (structure, microbial life, nutrient dynamics) with environmental preservation (Lampkin 1990; Schnug et al. 2006). Crop plants absorb nitrogen mainly in the form of nitrate ions. The nitrate and nitrite content in food and water is a concern due to the relationship of nitrite with the formation of carcinogenic nitrosamines and with infant methemoglobinemia (see Santamaria 2006 and references therein). Mineral nitrogen fertilizers (used in conventional agriculture) directly provide the nitrate, while many organic fertilizers gradually release their nitrogen content. Easily decomposable organic fertilizers, such as dried blood and sludge, make nitrogen readily available in ways similar to mineral nitrogen fertilizers. The amount of nitrate absorbed by plants depends on the nitrate dissolved in the soil solution and, therefore, the type of fertilizer is not the only *Corresponding author. Email: [email protected] ISSN 1939–3210 print/ISSN 1939–3229 online ß 2010 Taylor & Francis DOI: 10.1080/19440040903586299 http://www.informaworld.com

cause of nitrate accumulation in the plant. Incorrect practices, such as overfertilization with nitrogen, also favour this accumulation (Barker 1975; Termine et al. 1987; Muramoto 1999; Raigo´n et al. 2002). Many studies have demonstrated that organically grown crops have lower nitrate content than conventionally grown crops (Schuphan 1974; Barker 1975; Temperli et al. 1982; Lairon et al. 1984; Vogtmann et al. 1984; Muramoto 1999; Pussemier et al. 2006), although this conclusion is not uniformly supported (De Martin and Restani 2003; Guadagnin et al. 2005). Woese et al. (1997) reviewed 41 comparative studies of nitrate content in conventional and organically grown vegetables and concluded that, in general, higher nitrate levels are found in leaf, root and tuber vegetables with mineral fertilization. In contrast, differences in nitrate content of fruit, seed and bulb vegetables are much smaller. Worthington (2001) summarized the results of 18 studies comparing nitrate levels of organic and conventional fruit, vegetables and grains, and found that in 72% of the cases nitrate levels were higher in the conventional products, while in 24% of the cases nitrate levels were higher in the organic products. Heaton (2001) also found 14 studies reporting lower nitrate content in organically grown crops, with two studies showing no significant differences.

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M.C. Matallana Gonza´lez et al.

Table 1. Studied vegetables and nomenclature of the samples. Vegetable Leaves: Swiss chard Sea beet Spinach Cabbage ‘‘Inflorescences’’: Cauliflower Fruits: Eggplant Vegetable marrow Sample source Organic food stores Organic farm

Species

Variety

Spanish name

Nomenclature

Beta vulgaris L. subsp. cicla (L.) W.D.J.Koch Beta vulgaris L. subsp. maritima (L.) Arcang. Spinacia oleracea L. Brassica oleracea L. var. Capitata L.

Bressame Verde de cortar Viking Roxy

Acelga Acelga silvestre Espinaca Repollo

A As E R

Brassica oleracea L. var. botrytis L.

Stella

Coliflor

Co

Solanum melongena L. Cucurbita pepo L.

Black beauty Diamante

Berenjena Calabacı´ n

B Ca

Similar results were obtained by Bourn and Prescott (2002), who reviewed studies comparing the nutritional value of organically and conventionally grown food as purchased from retailers, as well as studies comparing the use of different types of fertilizers and of organic and conventional farms. Recently, a number of review articles concerning organically produced foods (Siderer et al. 2005; Magkos et al. 2006; Winter and Davis 2006; Rembialkowska 2007) have summarized these results. There are other factors that influence the nitrate content of a vegetable besides fertilization type. These include the edible plant portion, such as leaf, root, inflorescence and fruit, listed by decreasing nitrate content (Maynard et al. 1976; Santamaria et al. 1999), light intensity, soil type, temperature, harvest time and storage (Termine et al. 1987; Raigo´n et al. 2002). Therefore, nitrate and nitrite levels in vegetables available to the average consumer could be highly variable, so that differences between organic and conventional might be smoothed out. Further studies on the influence of organic agriculture practices on the accumulation of nitrate and nitrite in vegetables are needed. Our objective was to determine the nitrate and nitrite content in some of the most commonly consumed vegetables in Spain that are grown with organic fertilizers. The available literature on nitrate and nitrite content in some of the products studied in this work, such as eggplant and vegetable marrow, is scarce. We have studied samples purchased from two organic food stores as well as samples taken directly from an organic farm, and assessed the possible differences between them. We compared our results with those from relevant literature concerning organic and conventional vegetables. Materials and methods Materials Different vegetable species were selected for the analyses: (a) leaf vegetables, including Swiss chard

from C1 to C4 F5, F6

(Beta vulgaris L. subsp. cicla L.), sea beet (Beta vulgaris L. subsp. maritima L.), spinach (Spinacia oleracea L.) and cabbage (Brassica oleracea L. var. capitata); (b) inflorescences, i.e. cauliflower (Brassica oleracea L.); (c) fruit vegetables, i.e. eggplant (Solanum melongena L.) and vegetable marrow (Cucurbita pepo L.). For each species, homogenized laboratory samples numbered from 1 to 4 were obtained from vegetables purchased in two organic food stores in Madrid, while homogenized laboratory samples 5 and 6 were prepared from vegetables directly taken from an organic farm located in the region of Madrid. Sea beet was only available in the organic food stores. The nomenclature of the vegetables is shown in Table 1 and is used throughout this paper. RC1 refers to the first sample of cabbage (R1) that was purchased in an organic food store (C). EF5 indicates the first sample of spinach (E5) that was directly taken from an organic farm (F). The edible parts of the vegetables were first washed with running tap water, then with distilled water and dried with filter paper. Vegetable samples were sliced and homogenized in a blender to obtain a laboratory sample weighting 250 g. Results for each sample are reported as averages of analyses carried out in triplicate on the same homogenate.

Methods Nitrate and nitrite analysis The method [(Norme AFNOR 04.409, 04.410 (1974) modified by Bosch Bosch (1985) and Garcı´ a Mata (1985)] is based on the Griess reaction. It consists of the quantitative determination of nitrite by absorption spectrometry at a wavelength of 538 nm of the azo derivative obtained in the reaction of the nitrite with a primary amino (sulfanilamide) in an acid medium and then with an aromatic compound [N-(1-naphthyl)ethylendiamine hydrochloride]. The nitrate is reduced to nitrite passing through a porous metallic cadmium

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1  Table 2. Nitrate and nitrite content in organic Swiss chard, sea beet, spinach and cabbage (NO FM). 3 and NO2 mg kg

Food stores Sample AC1 AC2 AC3 AC4 AC (mean) AsC1 AsC2 AsC3 AsC4 AsC (mean) EC1 EC2 EC3 EC4 EC (mean) RC1 RC2 RC3 RC4 RC (mean)

Farm

Nitrate (X  SDn–1)

Nitrite (X  SDn–1)

Sample

1704.7  49.0 568.5  48.2 4763.4  197.6 3485.1  228.3 2630.4  1860.9 3676.1  192.3 1033.3  33.2 2202.3  134.8 3468.8  110.6 2595.1  1228.2 437.9  7.5 286.0  69.8 1675.2  128.4 2280.9  90.8 1170.0  967.3 357.1  16.2 LOD 22.6  0.4 27.3  1.4 101.8  170.7

0.9  1.6 5LOD 5LOD 5LOD 0.3  0.5 4.2  0.7 5LOD 5LOD LOD

AF5 AF6

3407.4  33.7 2742.3  0.1

LOD 1.5  0.7

AF (mean)

3074.9  470.4

0.7  1.1

EF5 EF6

2774.9  338.6 643.8  54.1

LOD LOD

EF (mean) RF5 RF6

1709.4  1506.9 329.8  66.8 223.1  21.3

RF (mean)

276.5  75.5

4.6  1.0 5LOD 5LOD LOD 1.1  2.3 5LOD 5LOD LOD 5LOD

Nitrate (X  SDn–1)

Nitrite (X  SDn–1)

LOD 5LOD

Notes: X ¼ mean triplicate. SDn–1 ¼ standard deviation. LOD, limit of detection. 5LOD and LOD are considered as zero to calculate the organic food stores and organic farm mean values.

column and then the total nitrite content is assayed. The cadmium column was re-activated before each use. The nitrate was calculated by subtracting the initial nitrite content (assayed directly without going through the cadmium reduction step) from the total nitrite content. Calibration curves for sodium nitrite and potassium nitrate were obtained over the concentration range 0.1–1.0 mg ml1. The linear correlation coefficients were 0.9982 and 0.9998, respectively. All nitrate and nitrite values are expressed in NO 3 1 and NO of fresh matter (FM). 2 mg kg Analytical quality assurance Blank samples were determined to evaluate the standard deviation of an empty sample. The limit of detection (LOD) was calculated as three times the standard deviation of the lowest standard solution according to the IUPAC (Thomsen et al. 2003), with concentration values obtained using the slope of the calibration curve. The respective LOD for nitrite and nitrate was found to be 0.5 and 7.6 mg kg1.

Statistical methods The experimental results were submitted to an ANOVA analysis of variance and to a Duncan test to check for significant differences among the analyzed vegetables with the nitrate content as the studied factor.

Furthermore, a comparison between the averages of the samples taken from the organic farm and from the organic food stores was performed using the Student’s ttest. The statistical analysis was performed in the Data Processing Center of the Complutense University of Madrid using the SAS v.8 computer package (SAS/ STAT 1999). Results and discussion Leaf vegetables The nitrate and nitrite content in Swiss chard and sea beet are given in Table 2. Nitrate concentrations were similar in the Swiss chard and sea beet samples coming from the organic food stores (2630.4 and 2595.1 mg kg1 FM on average, respectively). Values for the sea beet were more homogeneous, while we found a maximum level of 4763.4 mg kg1 FM in the Swiss chard. The organic farm samples of Swiss chard showed values near or higher than the ones from the organic food stores. Our results are similar to those for conventional Swiss chard as reported by Bosch Bosch (1985) 2754.0 mg kg1 FM, but lower than Elmadfa (2001) 4870 mg kg1 FM. However, other authors obtained values lower than ours, below 2417 mg kg1 FM (Table 3). On the other hand, Bosch Bosch et al. (1991) and Raigo´n et al. (2002) reported nitrate content for organically cultivated Swiss chard of 1190

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M.C. Matallana Gonza´lez et al.

Table 3. Nitrate and nitrite content in conventional Swiss 1  chard (NO FM). 3 and NO2 mg kg Data source Bosch Bosch (1985) Aubert (1990) Fritsch and Saint Blanquat (1990) Bosch Bosch et al. (1991) (1) Domı´ nguez (1994) Santamaria et al. (1999) Souci et al. (1999) Elmadfa (2001) Raigo´n et al. (2002) (2) Herencia et al. (2007) (3) Parks et al. (2008)

Nitrate

Nitrite

2754.0 (357.0–4997.8)* 2028

2.3 (0.5–8.4) 1.7

2077 (760 – 3923) 2417 (556 – 3384) 2363 (1299 – 4220) 1000.0 – 2200.0 4870 1076 870.4 – 2113.5 3002.6 (53.1 – 4774)

Notes: *Mean (range) (1) Nitrate value of organically grown Swiss chard: 1190 (177 – 2382) mg kg1 FM. (2) Nitrate value of organically grown Swiss chard: 376 mg kg1 FM. (3) Range of mean nitrate values of organically grown Swiss chard: 546.4 – 1274.3 mg kg1 FM.

and 376 mg kg1 FM, respectively. Herencia et al. (2007) reported mean values, depending on the crop rotational system used, in the range 780.4–2113.5 mg kg1 FM for mineral fertilization and 546.4–1274.3 mg kg1 FM for organic chard. With regard to nitrites, most of our samples showed either trace levels or they were not detected. However, we found values of 0.9 (AC1), 1.5 (AF6) and 4.2 (AsC1) mg kg1 FM (Table 2). The available literature concerning the nitrite content of this vegetable is scarce. For conventional Swiss chard, Bosch Bosch (1985) obtained a nitrite value of 2.3 mg kg1 FM and Fritsch and Saint Blanquat (1990) reported a level of 1.7 mg kg1 FM. Spinach, like Swiss chard, has been frequently characterised by its ability to accumulate high levels of nitrate (Silguy 1999). Our spinach samples had a high variability in nitrate levels: from 286.0 to 2280.9 mg kg1 FM in the samples from the organic food stores, and 643.8–2774.9 mg kg1 FM for those from the organic farm (Table 2). The average nitrate level was higher (1709.4 mg kg1 FM) in this latter case than in the organic food stores samples (1170.0 mg kg1 FM). Studies on conventionally fertilized spinach show a wide range of nitrate content (Table 4). In particular, Shokrzadeh et al. (2007) and Leclerc (1986) obtained results below 420 mg kg1 FM, close to the minimum value we found. On the other hand, our values are similar to those reported by most of the authors cited

in Table 4 with nitrate levels between 1055 mg kg1 (Escoı´ n et al. 1998) and 2820 (O¨ztekin et al. 2002) mg kg1 FM. The wide range of values found by Bosch-Bosch (1985), Domı´ nguez (1994), Chung et al. (1999), Souci et al. (1999), De Martin and Restani (2003) and Merino et al. (2006) should be noted. European Regulation (EC) 466/2001, modified by Regulation (EC) 563/2002, establishes the maximum content for certain contaminants (expressed in NO 3 mg kg1) to be found in a number of edible products. Depending upon the season, climatic conditions affect the nitrate content of certain vegetables and this is taken into account. The maximum nitrate level for the spinach harvested in the period between 1 November 1 while for and 31 March is set to 3000 NO 3 mg kg spinach harvested in the period from 1 April to 31 1 October the level is 2500 mg NO 3 kg . In no case did our results surpass the allowed maximum levels of mg kg1 the mean content for the 1902.6 NO 3 1 for winter-grown spinach and 804.9 NO 3 mg kg the summer-grown species. Under mineral fertilization Watanabe et al. (1994) obtained similar values for the summer and the autumn varieties of spinach (around 2400 mg kg1 FM, Table 4). However, Santamaria et al. (1999) reported higher levels in autumn–winter than in spring (2580 compared with 1622 mg kg1 FM; Table 4), with a nitrate mean value for the conventional spinach of 1845 mg kg1 FM. In our samples, we have also found this difference depending upon the season: batches EC1 and EC2, harvested in summer, present lower values than the others harvested in winter (EC3 and EC4) (Table 2). The concentration dependence with the season of the crop, therefore, shows the importance of different factors involved in nitrate accumulation. Conversely, Muramoto (1999) reported higher levels for conventional spinach in summer than in winter and lower concentrations for the organic vegetable with similar values in winter and summer time. The average levels for both the organic food store and the organic farm spinach are within the range of the mean values cited in the literature for this organic product, 395 (Raigo´n et al. 2002) and 1810 (Muramoto 1999) mg kg1 FM, respectively. The organic farm mean value was similar to the maximum reported by Muramoto (1999) and Pussemier et al. (2006) (Table 4). Other authors have compared conventional and organic spinach samples (Table 4). Some of them report a higher nitrate content in organically grown spinach, such as Rauter and Wolkerstorfer (1982) and Elmadfa (2001), while authors like Ahrens et al. (1983), Bakr and Gawish (1997), Muramoto (1999), Raigo´n et al. (2002), Malmauret et al. (2002) and Pussemier et al. (2006) obtained higher levels of nitrate in conventionally grown samples.

Food Additives and Contaminants: Part B

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1  Table 4. Nitrate and nitrite content in organic and conventional spinach (NO FM). 3 and NO2 mg kg

Nitrate Data source Siciliano et al. (1975) Rauter and Wolkerstorfer (1982) Causeret (1984) Bosch Bosch (1985) Leclerc (1986) Schuster and Lee (1987) Aubert (1990) Fritsch and Saint Blanquat (1990) Domı´ nguez (1994) Watanabe et al. (1994) Lenzzi et al. (1996) Rutkowska (1997) Escoı´ n et al. (1998) UK MAFF (1998) Chung et al. (1999) Muramoto (1999) Santamaria et al. (1999) Souci et al. (1999) Amr and Hadidi (2001) Elmadfa (2001) Yordanov et al. (2001) O¨ztekin et al. (2002) Raigo´n et al. (2002) Chung et al. (2003) Malmauret et al. (2002) (1) De Martin and Restani (2003) Merino et al. (2006) Pussemier et al. (2006) Tamme et al. (2006) Iqbal et al. (2007) Shokrzadeh et al. (2007)

Organic 970

Nitrite Conventional

Conventional

2220 840 442.2 (36.2 – 1148.2)* 1829.7 (120.8 – 4253.1) 411 2378.7 442

0.7 3.2 (0.3 – 17.3) (Traces – 682.7) Traces/non-detected 3.2

1906 (376 – 3418) 2500 (summer) 2400 (autumn) 1574 1456.8 1055 (max 2863) 1631 2788 (311 – 5522) 1810 (600 – 3000) 2540 (1500 – 3400) 1800 (890 – 2600) (winter) 2230 (1500 – 2900) (winter) 1820 (600 – 3000) (summer) 2850 (2000 – 3400) (summer) 1845 (547 – 3350) 2580 (autumm-winter) 1622 (spring) 1660.0 (20.0 – 6700.0) 25.0 (2.5 – 55.0) 1.0 (non-detected – 2.5) 970 840 860.1 7.44 2820 395 548 4259 1135 (median) (max: 3923) 1591 (median) (max 1901) 7.5 (max 10.1) 1757 (non-detected – 3720) 1747 (median) (47 – 5975) 1703 2637 2508 Non-detected 1140.0 – 1460.0 300

Notes: *Mean (range). (1) Nitrite value of organically grown spinach: 0.2 (max. 0.2) mg kg1 FM.

Schudel et al. (1979) studied the Nores and Nobeloriginal varieties of spinach under different fertilization practices. Figure 1 shows the nitrate levels for these two varieties along with our results obtained from the food stores and the farm samples, all expressed as dry matter. The Nores variety fertilized with composted manure showed lower nitrate content than did the mineral fertilized one. The Nobel-original variety, fertilized both with composted manure and vegetable compost, showed a much lower concentration than the samples grown with mineral fertilizer. Nitrite content in the analyzed spinach samples was either detected only at trace levels or was not detected, except for one of the samples (EC1), with 4.6 mg kg1 FM (Table 2). This fact is explained because vegetables only use nitrate for protein synthesis; however, conversion of nitrate into nitrite can occur in some cases, such as incorrect temperature and moisture condition

storage that are favourable to certain activity-reducing bacteria (Lin and Yen 1980; Hill 1991). Schuster and Lee (1987) and Tamme et al. (2006) also report trace or non-detectable levels for conventional spinach, while some authors have found nitrite levels from 0.7 mg kg1 FM (Siciliano et al. 1975) or 1.0 mg kg1 FM (Amr and Hadidi 2001) to 7.44 mg kg1 FM (Yordanov et al. 2001), with two works (Causeret 1984; Fritsch and Saint Blanquat 1990) obtaining 3.2 mg kg1 FM (Table 4). One study (Malmauret et al. 2002) has compared the nitrite content in organic and conventional spinach, recording 0.2 and 7.5 mg kg1 FM, respectively. The nitrate contents varied from traces to 357.1 mg kg1 FM considering all the cabbage samples studied. The average for the organic food stores samples was 101.8 mg kg1 FM, while for the organic farm ones it was 276.5 mg kg1 FM (Table 2).

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M.C. Matallana Gonza´lez et al.

1  Table 5. Nitrate and nitrite content in organic and conventional cabbage (NO FM). 3 and NO2 mg kg

Nitrate Data source

Organic

Siciliano et al. (1975) Nilsson (1979) Causeret (1984) Garcı´ a-Roche´ and Ilnitsky (1986) Ahonen et al. (1987) Schuster and Lee (1987) Lakhidov et al. (1994) Wieczorek and Traczyk (1995) Rutkowska (1997) Santamaria et al. (1999) Souci et al. (1999) Amr and Hadidi (2001) Elmadfa (2001) Yordanov et al. (2001) Ximenes et al. (2000) Zhong et al. (2002) Tamme et al. (2006) Susin et al. (2006)

171 393.8

740

Nitrite Conventional

Conventional

784 84 159.5 (2.5 – 791.3)* 407.9 (143.5 – 932.9) 422.0 671.6 666.7 – 918.74 177.2 728.4 400 (8 – 929) 1070.0 (10.0 – 3230.0) 34.0 (1.3 – 53.7) 1070 196.9 1829.0 1530 437 (74 – 1138) 881 (112 – 1864)

0.5 1.1 (0.1 – 10.1) 0.6 (0.0 – 2.4) Non-detected Non-detected

1.0 (Non-detected – 2.6) 1.02 Non-detected 0.2 (Non-detected – 0.4)

Note: *Mean (range).

1  Table 6. Nitrate and nitrite content in organic cauliflower (NO FM). 3 and NO2 mg kg

Food stores Sample CoC1 CoC2 CoC3 CoC4 CoC (mean)

Farm

Nitrate (X  SDn–1)

Nitrite (X  SDn–1)

Sample

Nitrate (X  SDn–1)

Nitrite (X  SDn–1)

8.6  4.0 22.6  0.9 89.5  2.5 31.4  3.2 38.1  35.6

LOD 5LOD 5LOD 5LOD

CoF5 CoF6

138.7  2.1 10.1  1.6

LOD 5LOD

CoF (mean)

74.4  91.0

Notes: X ¼ mean triplicate. SDn–1 ¼ standard deviation.

The lowest values were found in the cabbages harvested in summer, RC3 and RC4. Some authors reported mean values between 34.0 (Amr and Hadidi, 2001) and 1829.0 (Ximenes et al. 2000) mg kg1 FM for conventionally grown cabbage (Table 5). For organic cabbage, on the other hand, other researchers present mean levels from 171 mg kg1 FM (Nilsson 1979), similar to our mean value, to 740 mg kg1 FM (Elmadfa 2001), which is much higher than our results (Table 5). Most of the studies reviewed report significantly higher nitrate levels in conventionally grown cabbages than in organically grown species, for example, Elmadfa (2001) (Table 5) or Termine et al. (1984), Meier-Ploeger et al. (1989), Vogtmann et al. (1993) and Rutkowska (1999). Conversely, Nilsson (1979) found higher nitrate levels in organic cabbages than in those fertilized with soluble NPK, whereas Ahonen et al. (1987) and Stopes et al. (1988) did not obtain

significant differences between conventional and organic cabbages. The nitrite content in the cabbage samples was either not detected or only traces were obtained (Table 2). Our results support those obtained by Schuster and Lee (1987), Wieczorek and Traczyk (1995) and Tamme et al. (2006) for conventional cabbage and white cabbage. However, other authors obtained nitrite mean values in the range 0.2 (Susin et al. 2006) to 1.1 mg kg1 FM (Causeret 1984) for conventional cabbage (Table 5) as well.

‘‘Inflorescence’’ vegetables The average nitrate values for the cauliflower samples from the organic food stores was 38.1 mg kg1 FM with 74.4 mg kg1 FM for those taken from the organic farm (Table 6). Our total average value (50.2 mg kg1 FM) is similar to that indicated by Schuster and

Food Additives and Contaminants: Part B Table 7. Nitrate and nitrite content in conventional cauli1  flower (NO FM). 3 and NO2 mg kg Data source Leclerc (1986) Schuster and Lee (1987) Aubert (1990) Pommerening et al. (1992) Rutkowska (1997) Santamaria et al. (1999) Souci et al. (1999) Amr and Hadidi (2001) Elmadfa (2001) (1) Tamme et al. (2006)

Nitrate

Nitrite

383 45.4 107 ca. 200 364.2 202 (143 – 354)* 420.0 (40.0 – 1030.0) 1.3

Non-detected

420 287 (104 – 404)

Non-detected

Notes: *Mean (range). (1) Nitrate value of 350 mg kg1 FM.

organically

grown

cauliflower:

Lee (1987), and lower than the reported by most of the authors compiled in Table 7 for conventional cauliflower. Only Amr and Hadidi (2001) have reported levels smaller than ours, and Aubert (1990) obtained an average value within the range found by us. The nitrate content in organically cultivated cauliflower is compared with Elmadfa (2001) that reports a value of 350 mg kg1 FM (Table 7), much higher than ours. Kaniszewski and Rumpel (1998) studied nitrogen-fertilized cauliflowers and found a linear increase in the nitrate content with an increase of the amount fertilizer used. They deduced that nitrate content is influenced by the type of soil and the type of fertilizer (nitrogen) supplied. Nitrite was only found at trace levels in some of the studied cauliflower samples (Table 6). Amr and Hadidi (2001) and Tamme et al. (2006) did not detect nitrite in conventional cauliflower or they were below detection limits.

Fruit vegetables The nitrate content in the eggplant samples varies between 23.8 and 135.3 mg kg1 FM with a mean value of 89.6 mg kg1 FM for the organic food stores samples and 29.6 mg kg1 FM for the organic farm ones (Table 8). The lowest values were obtained in the batch harvested in summer (BC3) and in the batches taken from the organic farm. There is a lack of data in the literature about the nitrate content in eggplant but we can mention the results of some studies for conventional eggplant (Table 9), all with levels higher than ours. We have not found in the literature any study on the nitrate content in organic eggplant.

25

With regard to the nitrite content in eggplant (Table 8), we obtained trace levels in all the analyzed batches. Likewise, Siciliano et al. (1975) have given a value of 0.5 mg kg1 FM for conventional samples (Table 9). Nitrate concentration in the vegetable marrow samples showed high variability, from 13.2 to 594.4 mg kg1 FM (Table 8). Batches taken directly from the organic farm had higher nitrate values (465.3 mg kg1 FM) than the organic food stores samples (43.2 mg kg1 FM). Yordanov et al. (2001) reported an average value of 26.8 mg kg1 FM for conventional samples (Table 9). Nitrate content in organic vegetable marrow has been recently studied by Herencia et al. (2007) giving mean values in the range 249.1–405.4 and 221.9–884.6 mg kg1 FM for the conventional crop, depending on the utilized rotational system. Nitrite was either not detected or was found at trace levels in the vegetable marrow samples (Table 8). Yordanov et al. (2001) have obtained a mean value of 0.14 mg kg1 FM (Table 9).

Statistical analysis The ANOVA statistical analysis shows that the studied vegetables can be separated into three groups with statistically significant differences (95% confidence level) according to their nitrate content: firstly, Swiss chard (maximum of 4763.4 mg kg1 FM) and sea beet (maximum of 3676.1 mg kg1 FM) with the highest values; secondly, spinach (with a maximum of 2774.9 mg kg1 FM); to the third group belong the other studied vegetables, having the lowest nitrate content (Figure 2). This is in agreement with previous works (Siciliano et al. 1975; Woese et al. 1997; Silguy 1999) concluding that leaf vegetables have a higher capacity of nitrate accumulation, even more than root or tuber vegetables. The average nitrate values of the samples taken directly from the organic farm were higher or slightly higher than those samples purchased in the organic food stores (with the exception of eggplant), although the Student’s t-test showed no significant differences between the two groups.

Conclusions Nitrate levels were much higher in the leaf vegetables (especially Swiss chard species) than in fruits or inflorescence products. Nitrate levels in spinach were in the range found by other authors; the European limit value for the nitrate content in this vegetable was not exceeded in any of the samples. In general, the nitrate content of cabbage, cauliflower, eggplant and vegetable marrow (organic food stores) was lower than

26

M.C. Matallana Gonza´lez et al.

1  Table 8. Nitrate and nitrite content in organic eggplant and vegetable marrow (NO FM). 3 and NO2 mg kg

Food stores

Farm

Nitrate (X  SDn–1)

Sample BC1 BC2 BC3 BC4 BC (mean) CaC1 CaC2 CaC3 CaC4 CaC (mean)

Nitrite (X  SDn–1)

89.9  4.2 135.3  4.8 23.8  1.9 109.5  3.6 89.6  47.7 60.1  2.5 30.5  2.0 69.1  7.8 13.2  1.0 43.2  25.9

LOD LOD LOD LOD 5LOD LOD 5LOD LOD

Sample

Nitrate (X  SDn–1)

Nitrite (X  SDn–1)

BF5 BF6

34.4  1.7 24.8  1.4

LOD LOD

BF (mean) Ca. F5 Ca. F6

29.6  6.8 594.4  23.5 336.2  7.4

LOD LOD

CaF (mean)

465.3  182.6

Notes: X ¼ mean triplicate. SDn–1 ¼ standard deviation.

Table 9. Nitrate and nitrite content in conventional egg1  plant and vegetable marrow (NO FM). 3 and NO2 mg kg

5000

Data source

4000

Eggplant Siciliano et al. (1975) Souci et al. (1994) Elmadfa (2001) Zhong et al. (2002) Vegetable marrow Yordanov et al. (2001) Herencia et al. (2007) (1)

Nitrate

302 199.7 200 479

0.5

26.8 221.9 – 884.6

0.14

35000 NO3– (mg kg–1 DM)

30000

mean min.

3000 2000 1000

Note: (1) Range of mean nitrate values of organically grown vegetable marrow: 249.1–405.4 mg kg1 FM.

40000

max. NO3– (mg kg–1 FM)

Nitrate

0 Swiss chard

Sea beat

Spinach

Cabbage Cauliflower Eggplant Vegetable marrow

Figure 2. Average and range (vertical bars) of nitrate content  in the studied organic vegetables expressed in NO 3 and NO2 mg kg1 FM.

Nores variety Nobel - Original variety Organic food stores Organic farm

25000 20000 15000 10000 5000 0 Control C 100

C 300

Cv 100

Cv 300

NPK NPK 100 300

Figure 1. Average nitrate content (expressed dry matter) in organic spinach samples. Data from Schudel et al. (1979) and present work. C 100 ¼ composted manure 100 kg N ha1; C 300 ¼ composted manure 300 kg N ha1. Cv 100 ¼ vegetable compost 100 kg N ha1; Cv 300 ¼ vegetable compost 300 kg N ha1.

that reported for similar organic and/or conventionally grown vegetables in the literature. Conversely, Swiss chard showed higher levels than those reported in other studies. We are not aware of any other studies for

organic eggplant and there is one reference on vegetable marrow to compare with our results. Spinach, cabbage and eggplant had lower nitrate content in the samples harvested in summer, showing that climatic conditions also affect nitrate levels in a plant. The nitrite content was only quantified in Swiss chard and sea beet, as well as in spinach; the remaining vegetables had only trace levels of nitrite in the samples coming from the organic farm while it was not detected in the shop samples. The high variability found in nitrate levels within each vegetable type suggests that products available to the consumer from organic retailers do not necessarily have lower nitrate concentration than available products produced by conventional fertilization techniques. Comparison among studies with non-controlled or non-comparable cultivation conditions makes differences between organic and conventionally grown vegetables less apparent.

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