Analysis of 20 trace and minor elements in soy and dairy yogurts by ICP-MS

Analysis of 20 trace and minor elements in soy and dairy yogurts by ICP-MS

Microchemical Journal 102 (2012) 23–27 Contents lists available at SciVerse ScienceDirect Microchemical Journal journal homepage:

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Microchemical Journal 102 (2012) 23–27

Contents lists available at SciVerse ScienceDirect

Microchemical Journal journal homepage:

Analysis of 20 trace and minor elements in soy and dairy yogurts by ICP-MS E.J. Llorent-Martínez, M.L. Fernández de Córdova, A. Ruiz-Medina, P. Ortega-Barrales ⁎ Department of Physical and Analytical Chemistry, Faculty of Experimental Sciences, University of Jaén, Campus Las Lagunillas, E-23071 Jaén, Spain

a r t i c l e

i n f o

Article history: Received 5 October 2011 Received in revised form 9 November 2011 Accepted 9 November 2011 Available online 22 November 2011 Keywords: Yogurt Soy Trace elements ICP-MS Microwave digestion Nutritional value

a b s t r a c t The content of 20 trace and minor elements in soy and dairy yogurts consumed in Spain has been determined using inductively coupled plasma-mass spectrometry (ICP-MS) after microwave digestion. This work presents two goals: a) to determine the nutritional value in terms of minor elements content, using recommended daily allowance data; b) to determine the levels of trace toxic elements and compare them with the acceptable daily intake values. The developed analytical method was validated by using both milk certified reference materials and recovery experiments over different yogurt samples, obtaining satisfactory results in all cases. In addition, a comparison between the levels of minor elements found in soy and dairy yogurts was performed. Much higher concentrations of Cu and Mn were found in soybean products (up to 30-fold higher) and minor differences were observed in the content of Fe and Zn. Other minor element found at higher concentrations in soy yogurts was Ni (effect usually observed in plant-origin foods), which was determined at levels up to 450 ng g − 1 in some samples. Finally, the levels of toxic elements were very low, even below the detection limit in some cases. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Milk and dairy products are a good source of many valuable nutrients and minerals in human diet. Among these products, yogurt is gaining popularity due to its acceptability for consumers as well as its nutritional properties and potentially beneficial effects on human health. One of the reasons for its rising popularity is the increasing number of flavours and yogurts enriched with fruit pieces, cereals or other ingredients. Although dairy yogurts still represent the highest percentage of consumed yogurts, soy-based ones are gaining ground in the market. Soy yogurts are produced using soy milk (produced by soaking dry soybeans and grinding them with water), yogurt bacteria and sometimes additional sweeteners, in a similar way than dairy yogurts (we will refer only to yogurts obtained from cow's milk). In addition to nutritional minerals, undesirable potentially toxic elements can be present in these foods too. In fact, apart from those communities exposed to high levels of pollution by industrial effluents or emissions rich in heavy metals, for most of individuals, food is the most important source of potentially toxic elements, such as Cd, Hg, Ni or Pb. The exposure of the organism to high levels of these metals can cause different adverse effects, from contact dermatitis (Ni) to neurological or carcinogenic diseases (Cd) [1]. As a result, routine control of their levels is required to ensure the safe consumption of these products. There are numerous bibliographic references on the mineral content and levels of trace elements in bovine milk [2-9]. However, ⁎ Corresponding author. Tel.: + 34 953 212757; fax: + 34 953 212940. E-mail address: [email protected] (P. Ortega-Barrales). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.11.004

fewer studies have been made on other dairy products, such as yogurt [3,10,11] or other different types of milks [12,13]. Although the concentration of selected major and trace elements has been studied in some soy-based formulations [14-16], to the best of our knowledge, this study has not been performed in soy yogurt and milk. As a result, this work presents two different goals for comparing soy and dairy yogurts: a) to determine the levels of minor nutritional elements in both types of yogurts and compare their nutritional value using the recommended daily allowance; b) to determine the levels of potentially toxic trace elements and compare them with the acceptable ADI data. In addition, samples of cow's milk and soymilk have also been analysed and the obtained values compared, observing similar patterns to those ones found in yogurts. In this work, we made use of ICP-MS due to its well-known advantages of sensitivity, selectivity and multi-element analysis. ICP-MS has been widely used for the analysis of trace elements in a wide variety of food samples [17-20] with satisfactory results. However, to date the analysis of yogurts has been carried out only with atomic absorption spectrometry (AAS) or ICP with atomic emission spectrometry (ICP-AES). Here, the preparation of samples was carried out by microwave digestion, which has proven to be excellent in the analysis of trace metals in different food [21-23]. With this technique, the samples are enclosed in the vessels, so cross contamination and loss of volatiles, critical aspects in trace and ultratrace analysis, are eliminated. The proposed method was applied to the quantitative analysis of 20 elements (Ag, Al, As, Ba, Be, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Sb, Sn, Tl, V, Zn) in yogurt and milk samples after microwave digestion with nitric acid. The validation of the method was performed


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by determining these elements in two milk certified reference materials and carrying out additional recovery studies over different yogurts. The method was applied to the analysis of bovine and soybean-based milk and yogurt samples collected from Spanish supermarkets. Acceptable daily intake (ADI) and recommended daily allowance (RDA) data were used to make a critical comparison between the food from each origin.

2. Experimental 2.1. Instrumentation The quadrupole inductively coupled plasma mass spectrometer used in this work was an Agilent 7500a (Agilent Technologies, CA, USA) equipped with a Babington nebuliser, a Peltier-cooled quartz spray chamber and a standard torch (2.5 mm i.d.). Before each experiment, the instrument was tuned using an aqueous multi-element standard solution (Agilent, Madrid, Spain) of 10 ng mL − 1 each of Li, Y, Co, Ce and Tl for consistent sensitivity ( 7Li, 89Y and 205Tl) and minimum doubly charged and oxide species levels ( 140Ce). Operating conditions are shown in Table 1. The samples were digested using the following system for the microwave digestion: Milestone Ethos MicroSYNTH oven with programmable power control (10 W increments, maximum power 1000 W) with segmented rotor MPR-600 (operating pressure up to 35 bar maximum; operating temperature 260 °C maximum) with 10 reaction vessels.

2.2. Reagents All calibration standard solutions were prepared from 100 μg mL− 1 multi-element standard solution (SCP Science, Paris, France) by dilution with the suitable percentage of HNO3 in ultrapure water. In solution (1000 μg mL− 1) was obtained from SCP Science. Analytical reagent grade HNO3 65% (Sigma-Aldrich, Madrid, Spain) was additionally cleaned by sub-boiling distillation. The sub-boiling still was built up with components obtained from Savillex ( The certified reference materials, skim milk powder BCR-151 and skim milk powder BCR-063R were obtained from Sigma-Aldrich (Madrid, Spain). Ultrapure deionised water (18.2 MΩ cm) was obtained from a Milli-Q system (Millipore, Bedford, MA, USA). All plastic containers were soaked in 10% v/v sub-boiling HNO3 for at least 24 h, and then rinsed extensively with Milli-Q water prior to use. All kinds of glassware were avoided to prevent metal releases. All plastic containers, polyethylene flasks, pipette tips, PFA Teflon digestion vessels and reagents that came into contact with samples or standards were checked for contamination.

Table 1 Operating conditions for ICP mass spectrometer. Plasma conditions RF power Plasma Ar flow rate Auxiliary Ar flow rate Carrier Ar flow rate Torch horizontal alignment Torch vertical alignment Sampling depth Instrument Sampler cone Skimmer cone

1.2 kW 15 L min− 1 0.89 L min− 1 0.95–1.0 L min− 1 (0.5–1.0) mm 0.2–0.5 mm 6.0–8.0 mm

Nickel, 1.0 mm orifice diameter Nickel, 0.4 mm orifice diameter

2.3. Sample preparation and digestion The yogurt and milk samples used for the experiments were obtained from Spanish local supermarkets and consisted of 30 dairy yogurts (14 regular, 8 skim, 8 with fruits and cereals), 18 soy yogurts (10 regular, 8 with fruits and cereals), 10 cow's milks (7 whole, 3 skim) and 8 soy milks. The digestion of the certified reference materials was carried out by weighing 1 g of sample directly in the digestion vessel and adding 3 mL ultrapure water and 6 mL sub-boiled HNO3. For yogurt and milk samples, 4 g were weighed in the corresponding vessel and 6 mL subboiled HNO3 were used for the digestion. Then, they were left open for 15 min for reacting in order to reduce the built up of gases which could increase the pressure inside the vessels. The power of the microwave for each digestion step was optimised, from 500 up to 1000 W. In all cases, the lowest power that provided the required temperature was selected. The operating program for the microwave digestion system for the analytical batch is shown in Table 2. The digestion was complete in all cases, as checked by the analysis of reference materials and recovery studies. As a result, the addition of hydrogen peroxide was not required. After cooling at room temperature, all the digestion liquors were quantitatively transferred into plastic containers and diluted to 60 mL with ultrapure water and In was added as internal standard to yield a concentration of 2 μg L − 1. Although Ga and Y were also tested as internal standards, the best results were obtained using In and hence it was the chosen one. After each analytical batch, the vessels were cleaned using the same microwave operating program used for samples, adding 10 mL HNO3 to each digestion vessel. After cooling at room temperature, all vessels were thoroughly rinsed with Milli-Q water. 2.4. Calibration procedure For the quantitative analysis of the samples, calibration curves were built on six different concentrations. Standard solutions were prepared in 10% (v/v) sub-boiling HNO3 (the same percentage of acid present in the samples) by diluting a multi-element standard solution containing all the elements. The calibration curves were built from the quantification limit (QL) of the corresponding element up to 200 ng mL − 1 in all cases except Hg, which was prepared up to 5 ng mL − 1 in order to avoid memory effects. The absence of polyatomic interferences was checked by measuring several isotopes of the elements and checking the isotopic ratio in the digested solution of the samples. 3. Results and discussion The development of the method required the selection of the isotope masses for each element. When a given analyte had two or more isotopes, at least two of its isotopes were monitored to check the absence of interferences. The isotopes finally selected were the most abundant of each sought element showing less interference from the digested matrix: 107, 109Ag, 27Al, 75As, 137Ba, 9Be, 111, 114Cd, 59Co, 53 Cr, 63, 65Cu, 57Fe, 199,202Hg, 55Mn, 95,98Mo, 60,62Ni, 206,208Pb, 121,123Sb, 116,118,120 Sn, 203,205Tl, 51V, 66,68Zn. Table 2 Operating program for the microwave digestion. Microwave digestion program Step

Initial T (°C)

Final T (°C)

Time (min)

Power (W)

1 2 3 4

25 90 90 180

90 90 180 180

5 3 10 8

750 600 600 600

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3.1. Validation of the method

Table 4 Analysis of the skim milk certified reference material BCR-063R.

In order to check the applicability of the proposed method to the analysis of yogurt and milk samples, two certified reference materials (BCR-151 and BCR-063R) available for skim milk (in powder form) were analysed. In addition, taking into account that no certified material is available for yogurts, in this case recovery experiments were performed. On the one hand, the certified reference materials were analysed by triplicate using the microwave digestion procedure previously described. The experimental results obtained were compared with those ones provided by the manufacturer and they are depicted in Tables 3 and 4. The Student's t statistical test (P = 0.005) was performed in order to check if there were any significant differences between the certified and obtained values. For all the certified elements, no significant difference was observed. In addition, the indicative values were close to the ones obtained by the proposed method, although the values for Co and Zn were slightly different. On the other hand, recovery experiments were performed for the 20 selected elements (Ag, Al, As, Ba, Be, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Sb, Sn, Tl, V, Zn) in soy and dairy yogurts. For this study, two concentration levels were selected, 100 and 2000 ng g− 1. All elements were determined at the lowest level, except Al and Fe, due to their high QLs. In the case of Al, it is due to the high blank signal obtained and, in the second case, the isotope 56Fe could not be measured using a simple quadrupole and 57Fe had to be selected. In addition, the elements Cu, Mn and Zn were also studied at the 2000 ng g− 1 level, as they were expected to be found in higher concentrations than the other trace elements. The recoveries, depicted in Table 5, were in the range 85–110% with relative standard deviations (n = 3) lower than 7% in all cases. The obtained recoveries confirmed that no significant metal losses occurred during the digestion procedure. 3.2. Analytical parameters All the measurements were carried out using the full quantitative mode analysis. The correlation coefficients for all the calibration curves were at least 0.998, showing good linear relationships throughout the ranges of concentrations studied. The detection limits (DLs) for all the selected elements, calculated as the concentration yielding three times the blank signal, are shown in Table 6. As can be seen, the DLs allowed the determination of both trace and minor elements at the required levels. Inter-day (n = 5) and intra-day repeatability data were obtained for all the analysed elements. The inter-day data were generated on different days using new instrument tuning and new calibrations curves. The relative standard deviations were lower than 105 and 7% for all the elements for inter- and intra-day data, respectively. Table 3 Analysis of the skim milk certified reference material BCR-151. Element

Certified value (ng g− 1)

Observed valuea (ng g− 1)

Cd Cu Fe Hg Pb

101 ± 8 5230 ± 80 50100 ± 1300 101 ± 10 2002 ± 26

98 ± 5 5080 ± 100 53500 ± 3000 111 ± 3 1940 ± 50

Indicative value (ng g− 1)

Observed valuea (ng g− 1)

6 223 56 0.8 50000

18 ± 1 244 ± 20 54 ± 5 n.d. 45500 ± 200

Co Mn Ni Tl Zn n.d. = not detected. a n = 3.



Certified value (ng g− 1)

Observed valuea (ng g− 1)

Cu Fe Pb Zn

602 ± 19 2320 ± 230 18.5 ± 2.7 49000 ± 600

585 ± 30 2400 ± 300 20 ± 3 48000 ± 3000


n = 3.

3.3. Results The main goal of this study was to compare the levels of trace and minor elements (including toxic heavy metals) in soy and dairy yogurts. In addition, some milk samples (soy and cow) were analysed to try corroborating the differences found between soy and cow yogurts. Therefore, a higher number of yogurts were analysed in comparison to milk samples. The number of soy products analysed was lower than cow products due to their minor availability in the Spanish market. A total of 66 samples were analysed, being the exact number of each type indicated in Table 7. Each sample was independently digested and analysed by triplicate. The range of concentrations found for each type of sample is shown in Table 7. The levels of some trace elements (Ag, Be, Sn) were below DL in all the analysed samples. In other cases (Ba, Co, Cr, Mo, V) the observed levels were similar in dairy or soybean-based products. The most significant results will be discussed in more detail in following sub-sections. 3.3.1. Toxic trace elements The levels of potentially toxic trace elements (Al, As, Cd, Hg, Pb, Sb, Sn, Tl) were very low or even below DL in all yogurt samples, except Al, which was measured from 100 ng g − 1 up to 1.2 μg g − 1. However, it has been previously described that dairy and cereal products account for about 60% of the total dietary intake of Al and these levels present no risk for healthy people [1]. In the case of Pb, the only regulated metal in milk and related products [24], the found levels were below the maximum tolerated level, 20 ng g − 1, in all cases. 3.3.2. Minor elements As can be seen in Table 7, the main differences between dairy and soy products were observed in the levels of Ni and minor nutritional Table 5 Recoveries (%) for spiked yogurts. Element

Spike recovery (%) ± RSDa (%) Cow's yogurt

Ag Al As Ba Be Cd Co Cr Cu Fe Hg Mn Mo Ni Pb Sb Sn Tl V Zn a

n = 3.

Soy yogurt

100 ng g− 1

2000 ng g− 1

100 ng g− 1

2000 ng g− 1

88 ± 5 – 101 ± 4 107 ± 3 99 ± 4 91 ± 5 99 ± 2 106 ± 5 93 ± 6 – 108 ± 6 101 ± 4 90 ± 5 99 ± 3 101 ± 3 92 ± 4 98 ± 5 90 ± 6 105 ± 5 103 ± 4

– 109 ± 7 – – – – – – 106 ± 5 105 ± 7 – 96 ± 3 – – – – – – – 104 ± 5

92 ± 4 – 99 ± 3 98 ± 4 106 ± 5 89 ± 4 101 ± 3 103 ± 4 98 ± 7 – 110 ± 5 100 ± 3 93 ± 5 105 ± 4 100 ± 3 94 ± 5 105 ± 3 95 ± 6 106 ± 4 106 ± 4

– 106 ± 6 – – – – – – 109 ± 6 104 ± 6 – 102 ± 2 – – – – – – – 99 ± 5


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Table 6 Detection limits for the analysed elements. Element

DL (ng g− 1)


DL (ng g− 1)

Ag Al As Ba Be Cd Co Cr Cu Fe

0.7 80 3.0 1.0 1.5 1.5 1 5.0 1.5 35

Hg Mn Mo Ni Pb Sb Sn Tl V Zn

12 1.2 1.0 5.0 0.7 1.2 1.5 1.0 0.5 20

elements (Cu, Fe, Mn and Zn). The highest differences in the levels of nutritional elements were observed in Cu and Mn, which appeared at much higher levels in soybean yogurts. The representation of the average levels of these elements is shown in Fig. 1. Total dietary nickel intakes of humans vary with the amounts and proportion of food of animal (low-nickel) or plant (high-nickel) origin consumed, and with the amount of refined or processed foods in the diet. Some reports indicate intakes around 150 μg daily [25-28]. Taking into account the recommendations from the World Health Organization (WHO), the threshold level for toxicity can be set at less than 600 μg/day [1]. The average nickel levels found in dairy and soy yogurts were 14 and 195 ng g -1, respectively. Similar results were also found in milk samples. These results are consistent with the higher amount of Ni found in plant origin foods and other authors have reported approximately the same levels in Spanish dairy yogurts [29]. Considering the weight of one yogurt (approximately 125 g), the intake of Ni would be 1.75 μg and 24.3 μg when consuming one dairy or soy yogurt, respectively, which would not be considered any problem for the health. However, Ni levels up to 450 ng g − 1 were found in some soy yogurt (500 ng g − 1 in milk samples), and that would mean an intake of approximately 60 μg Ni, which is 10% of the toxicological threshold level. As a result, although no risk for health arises from these data, it is clear that routine analyses of trace elements are advisable for soy-related foods. Table 8 shows the results obtained for minor nutritional elements (Cu, Fe, Mn, Zn). In this table, the mean concentrations (μg g− 1 in fresh weight) are indicated and the percentage of contribution to the RDA

Table 7 Results obtained in the analysis of yogurt and milk samples available in Spanish supermarkets, expressed in ng g− 1. Element

Cow yogurt (n = 30)

Soy yogurt (n = 18)

Cow milk (n = 10)

Soybean milk (n = 8)

Ag Al As Ba Be Cd Co Cr Cu Fe Hg Mn Mo Ni Pb Sb Sn Tl V Zn

b DL 100–800 b DL-12 40–140 b DL b DL b DL-15 6–60 35–180 1500–3600 b DL 20–220 35–75 6–30 b DL-7 b DL b DL b DL - 2 0.5–8 2600–4500

b DL 100–1200 b DL-10 70–260 b DL b DL-6 1–32 4–75 300–1350 3000–7100 b DL 190–6000 50–150 14–450 b DL-6 b DL b DL b DL 0.5–11 1500–3500

b DL 60–550 2–4 80–160 b DL b DL 1–24 7–50 40–150 2400–3800 b DL-7 20–130 40–60 4–25 1–10 b DL b DL b DL 1–10 2900–3600

b DL 900–3000 2–4 150–500 b DL 1.5–5 5–40 10–55 700–1300 4000–9400 b DL-12 1300–2400 250–440 100–500 2–8 b DL-1.5 b DL b DL 6–40 2100–3200

Fig. 1. Average concentration of Cu, Ni, Fe, Mn and Zn in the different yogurts analysed.

is calculated for each element, considering the intake of one yogurt. The employed RDA data, which are the levels of intake of essential nutrients considered to be adequate to meet the needs of practically all healthy people, are those ones provided by the Commission of the European Communities in 2008 [30]. The levels found in dairy yogurt were similar to the ones reported in other works in which different types of yogurts were analysed [10,31]. The only difference was observed in the higher levels of Fe here found, which could be attributed not only to the addition of fruit pieces or cereals, which causes an increase in the concentration of the mineral [10], but also to the existence of Fe-enriched yogurts. In addition, the levels of Fe can vary depending on different factors and are not always constant [11]. The comparison between the mean levels found in yogurts of cow or soybean origin showed that soy-based ones presented higher levels of Cu, Mn and Fe, providing increased percentages of contribution to the RDA values, especially in the cases of Cu and Mn, where the increases were 8-fold and 30-fold, respectively. These results Table 8 Mean daily intake and percentage of contribution to RDA for minor nutritional elements in yogurts. Cu

-1 a

Level (μg g ) Mean daily intake (μg)b RDA (mg) % RDAb a b












0.1 12.5 1 1.25

0.8 100

2.9 362 14 2.6

4.6 575

0.06 6.25 2 0.3

1.8 150

3.5 437.5 10 4.4

2.5 312.5




Average level of the determined values (Table 7). Calculated on the basis of the intake of one yogurt (≈ 125 g).


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made sense taking into account that the levels of these elements are higher in vegetables than in bovine milk and support the introduction of soybean products in the diet. Finally, the differences observed between dairy and soy yogurts were similar to those ones found between cow's milk and soy milk (Table 7), supporting the previously discussed results. These differences in different kinds of milk samples had already been reported [32] and are analogous to the ones here mentioned. 4. Conclusions In this paper, the determination of trace elements and minor nutrients in yogurt of bovine and soybean origin was carried out using microwave digestion with nitric acid followed by ICP-MS analysis. Forty-eight yogurt samples were analysed and the results obtained were used to make a critical comparison between both types of yogurt in terms of RDA. To the best of our knowledge, this is the first time that this comparison has been reported in scientific literature. In minor nutritional elements analysed, much higher concentrations of Cu and Mn were found in soybean products and lower differences were observed in the content of Fe and Zn. The RDA data support the possibility of replacing dairy products with soybean-based ones for individuals with intolerance to lactose. However, the content of Ni was also higher in soybean yogurts and, therefore, they are not recommended for patients with severe allergies to Ni. Finally, it is worth mentioning that the levels of potentially toxic elements were always below the toxicity threshold. Acknowledgements The authors are grateful to Centro de Instrumentacion CientíficoTécnica (CICT) from the University of Jaén for the availability of the ICP-MS instrument. References [1] World Health Organization, Trace elements in human nutrition and health, WHO, Geneva, 1996. [2] A. Ataro, R.I. McCrindle, B.M. Botha, C.M.E. McCrindle, P.P. Ndibewu, Quantification of trace elements in raw cow's milk by inductively coupled plasma mass spectrometry (ICP-MS), Food Chem. 111 (2008) 243–248. [3] A. Ayar, D. Sert, N. Akin, The trace metal levels in milk and dairy products consumed in middle Anatolia–Turkey, Environ. Monit. Assess. 152 (2009) 1–12. [4] E. Coni, S. Caroli, D. Ianni, A. Bocca, A methodological approach to the assessment of trace elements in milk and dairy products, Food Chem. 50 (1994) 203–210. [5] I.R. do Nascimento, R.M. de Jesus, W.N.L. dos Santos, A.S. Souza, W.D. Fragoso, P.S. dos Reis, Determination of the mineral composition of fresh bovine milk from the milk-producing areas located in the State of Sergipe in Brazil and evaluation employing exploratory analysis, Microchem. J. 96 (2010) 37–41. [6] N. Herwig, K. Stephan, U. Panne, W. Pritzkow, J. Vogl, Multi-element screening in milk and feed by SF-ICP-MS, Food Chem. 124 (2011) 1223–1230. [7] F.A.R. Martino, M.L.F. Sánchez, A. Sanz-Medel, The potential of double focusingICP-MS for studying elemental distribution patterns in whole milk, skimmed milk and milk whey of different milks, Anal. Chim. Acta 442 (2001) 191–200. [8] C. Sola-Larrañaga, I. Navarro-Blasco, Chemometric analysis of minerals and trace elements in raw cow milk from the community of Navarra, Spain, Food Chem. 112 (2009) 189–196.


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