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Mineral Content in Baltic Herring and Baltic Herring Products
(2000) 13, 893}903 doi:10.006/jfca.2000.0933 Available online at http://www.idealibrary.com on
JOURNAL OF FOOD COMPOSITION AND ANALYSIS
ORIGINAL ARTICLE Mineral Content in Baltic Herring and Baltic Herring Products Raija Tahvonen, Tarja Aro, Johanna Nurmi, and Heikki Kallio Department of Biochemistry and Food Chemistry, University of Turku, FIN-20014 Turku, Finland Received September 27, 1999, and in revised form March 23, 2000
The contents of Fe, Cu, Zn, Mn, K, Mg, Ca, Na, As and Cd in gutted Baltic herring (Clupea harengus membras), in Baltic herring "llets and in two commercial ready-to-eat products made of these were investigated. The "sh was caught in 1996}1998 in the northern Baltic. The commercial products were fried "llets and "sh burgers. The e!ects of industrial processing on the mineral composition were studied by collecting samples at several consecutive stages. In the gutted "sh the levels of Mn, Ca and Cd were considerably higher than in the "llet mainly due to the bones. Signi"cant di!erences were also observed in the contents of Mg, Na, Cu and Zn. In the burger process the bones and skin were removed which caused a decrease in the Zn and Ca levels. Ingredients such as salt, spices and #our as part of the formula increased Na and Mn contents in the ready-to-eat products. In the "nal products the Na content was about 6 times higher than that of the fresh "sh. 2000 Academic Press Key =ords: Baltic herring; "sh products; Ca; Mg; K; Na; Fe; Mn; Cu; Zn; Cd; As; processing.
INTRODUCTION Baltic herring (Clupea harengus membras), a sub-species of Atlantic herring (Clupea harengus harengus) is the most important wild "sh in Finland, the catch having been about 90 000 t/year at the end of the 1990s. In 1997, almost half of the total catch occurred during May and June, with nearly 70% being caught in the Gulf of Bothnia. The major part of the Baltic herring was caught using trawls (Anon, 1998). The age of the "sh in trawl catches is usually over 3 years and in trap nets over 4 years. Due to size sorting, older "sh are mainly used in industrial processing (Parmanne, 1994). There has been a #uctuation in the Baltic herring population; the amount of "sh has increased but the size decreased. The small size is a problem especially in the "sh industry (Parmanne, 1994). Over the last few years, the growth has retarded remarkably in the Archipelago Sea and the Gulf of Finland, but in the Gulf of Bothnia this trend has not been so evident (Parmanne, 1992). Baltic herring from the south and south-west coast of Finland migrate after spawning to the southern parts of the Baltic. Thus, the population that remains in the Archipelago Sea and the Gulf of Finland in the autumn consists mainly of young, small "sh. The "sh caught after spawning in the Gulf of Bothnia are bigger because no special migration occurs from this area (Parmanne, 1990). To whom correspondence and reprint requests should be addressed. Tel: #358-2-333 6876. Fax: #358-2-333 6860. E-mail: tarja.aro@utu." 0889}1575/00/060893#11 $35.00/0
2000 Academic Press
894
TAHVONEN E¹ A¸.
In Finland only a small portion of the annual catch is used for food. The average consumption of Baltic herring in 1996 was 3.1 kg per capita equalling about 16 000 t of "sh (Anon, 1997). The Baltic herring could be better exploited, especially in the food industry by o!ering to the consumer new choices in "sh products. The present study was aimed at evaluating the mineral element composition and nutritional value of Baltic herring and the e!ects of industrial processing on the "sh products. In order to exclude the annual variation in the composition, the sample material was collected over 2.5 years from two "sh processing plants and one food factory. MATERIALS AND METHODS Sample Collection and Preparation The Baltic herrings were caught between May 1996 and September 1998 in the northern area of the Baltic Sea (The Archipelago Sea and the Gulf of Bothnia). Fishing took place during all seasons but the main part was caught in May or June. The "sh was sorted, mechanically headed and gutted in a processing plant (size class 0: 18}24 "sh/kg). In the case of "sh being prepared as "llets they were also deboned (size class 00: 12}17 "sh/kg). Two commercial Baltic herring products were processed in a local food factory (LaK nnen Tehtaat Oyj, Turku). One product was a fried double "llet with rapeseed oil, salt, rye breading and spices (e.g., dill). The other product was a "sh burger made of minced "sh, wheat breading, breadcrumbs, salt and spices. The "llets were dipped in rapeseed oil before frying whereas burgers were deep-fried. The stages of both processes are described in Figure 1. Altogether 33 batches of fried "llets and 11 of burger were collected for analysis. Each batch included "llets or gutted "sh from the processing plant and the corresponding samples from the food factory, samples from the processing line and ready-to-eat products. Iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), potassium (K), sodium (Na), calcium (Ca) and magnesium (Mg) levels were measured in all sub-samples during the "rst sampling year (10 batches of fried "llets and four of burgers). The e!ect of processing on mineral contents was investigated on these batches. Later, only ready-to-eat products and random samples of gutted and "lleted "sh from the food factory were analysed. Cadmium (Cd) contents were measured only in gutted or "lleted "sh and the "nal products. The number of samples analysed is shown in Tables 2 and 3. Ten lots of Baltic herring caught from the Archipelago Sea between May 1996 and December 1997 and 10 batches of burgers produced from the same "sh lots were collected for arsenic (As) measurements. The "sh in these samples were small (12}24 "sh/kg). All samples (about 2 kg) were packed into LD-polyethylene bags and transported to the laboratory in insulated containers together with solid carbon dioxide. The samples were stored in !703C until analysed. Before analysis, thawed samples were homogenized using a Robot Coupe R8 (Robot Coupe S.A., Bourgogne, France) homogenizer. Mineral Element Analysis The samples (about 2 g) were digested in a mixture of 65% nitric acid (5 mL pa#, Riedel-de-HaeK n, Seelze, Germany) and 30% hydrogen peroxide (2 mL, Mallinckrodt
MINERAL CONTENT IN BALTIC HERRING
895
FIGURE 1. Stages in the processing of fried Baltic herring "llets and Baltic herring burger.
Baker B.V., Deventer, Holland) in closed tetra#uormethaxil vessels of a Milestone MLS-1200 microwave (Milestone s.r.l., Sonisole (BG), Italy) digestion system. The digested samples were diluted to 25 mL in polypropylene volumetric bottles and stored in polyethylene vessels before analysis. For Ca and Mg determinations the samples were diluted with a LaCl solution to 0.1% w/v La and for Na and K determination with a CsCl solution to 0.5% w/v Cs. Concentrations were determined by a Varian SpectrAA 300 (Varian Techtron Pty. Limited, Mulgrave, Victoria, Australia) #ame system equipped with an autosampler (Ca, Mg, K, Na, Zn, Cu, Fe, Mn) or GTA-96-graphite furnace system equipped with an autosampler (As, Cd). Flame methods were the &&cook-book'' methods of the instrument. D -background correction was used in As, Cd, Fe and Zn analysis. Ni(NO ) (0.05%) was used as a matrix modi"er in As-determinations. All samples were analysed as duplicates. Several blanks were digested during each analysis period. Two commercial reference materials (GSV-3 poplar leaves, Institute of Geophysical and Geochemical Exploration, Langfang, P.R. China and DORM-1 dog"sh muscle, National Research Council of Canada, Ottawa) were used as quality control samples. All glassware and other vessels were washed with a detergent (Deconex, Borer Chemie Ag, Zuchwill, Switzerland), rinsed with reverse osmose water, soaked in 10% nitric acid overnight, rinsed with reverse osmose water and "nally rinsed with ultra-pure water (MilliQ). The microwave vessels were rinsed with ultra-pure water only, washed with 5 mL 30% nitric acid (pa) in the microwave digestion system using the digestion}heating programme and rinsed again with ultrapure water.
896
TAHVONEN E¹ A¸.
Statistical Analysis Statistical analysis was performed using SPSS software, version 7.5. The di!erences in raw materials ("sh "llets, gutted "sh) and in the "nal products (fried "llets, burger) were subjected to an ANOVA-test. The comparisons between the processing stages were tested using paired, two-tailed t-tests.
RESULTS AND DISCUSSION Analytical Quality Control The results for the analysis of reference materials and certi"ed values are shown in Table 1. The accuracy of the analytical procedures was satisfactory for most metal elements when compared to the certi"ed values for a dog"sh muscle (DORM-1). For Zn only 93% of the certi"ed value of DORM could be detected whereas in poplar leaves (GSV-3) the recovery was over 100%. Arsenic recovery in the reference material (DORM-1) was 76.3%, meaning that the As contents may be slightly underestimated. When compared to certi"ed values of poplar leaves, the analytical values of Mn and Zn were slightly out of the upper limit while the values of Fe and Na were lower than the certi"ed values. The matrix was rather di!erent compared to the "sh samples and, thus, a dog"sh muscle was a better reference material for this study. The detection limits for Cd, Mn, Cu, Zn, Fe, Mg, Ca, K and Na were 0.0012, 0.26, 0.44, 0.84, 1.5, 5.7, 124, 63 and 137 mg/kg, respectively. The detection limits (3 times the standard deviation of blanks) were calculated using an average sample weight. Mineral Contents in Gutted Baltic Herring and Baltic Herring Fillets The results for the mineral contents in gutted Baltic herring and "llets are shown in Table 2. The contents were studied using "llets and gutted "sh from the food factory. The ranges of the mineral levels reported for herring caught in di!erent areas of the Baltic Sea are also presented. Ca and K levels in the "sh "llets were slightly higher than those reported by Nuurtamo et al. (1980) and Torelm and Danielsson (1998). Nuurtamo et al. (1980) stated that incomplete deboning could lead to a wide variation in Ca contents. Torelm TABLE 1 Results of analysis for reference materials Analytical results DORM-1 Arsenic Cadmium Copper Iron Magnesium Manganese Potassium Sodium Zinc Note: Mean and
13.5$0.5 (5) 0.094$0.014 (12) 5.23$0.29 (18) 67.9$5.5 (18) 1234$51 (17) 1.42$0.25 (18) 15874$1019 (18) 7743$203 (17) 19.8$0.32 (18) S.D.
Certi"ed values DORM-1 17.7$2.1 0.086$0.012 5.22$0.33 63.6$5.3 1210$130 1.32$0.26 15900$1000 8000$600 21.3$1.0
in mg/kg. No. of analyses in parentheses.
Analytical results GSW-3
Certi"ed values GSW-3
9.5$0.0 (2) 226$6 (3)
9.3$0.5 274$10
48$1 (3) 13877$832 (3) 175$8 (7) 39$2 (2)
45$2 13800$400 200$10 37$1
897
MINERAL CONTENT IN BALTIC HERRING TABLE 2 Content (mean and range) of mineral elements in Baltic herring Fish "llets
Gutted "sh
Other studies
Ca
874? (21) 544}1158
3272 (10) 2310}5972
335}770
7, 11
K
3904 (21) 2993}4742
3789 (10) 3217}4492
2170}3720
7, 11
282 (21) 251}336
317 (10) 288}342
170}290
4, 7
Na
619 (21) 454}802
715 (10) 624}870
236}903
11
Cu
0.86 (21) 0.75}1.06
0.77 (10) 0.68}0.91
0.2}1.4
1, 2, 4, 7}9
Fe
8.1 (21) 6.5}9.1
8.5 (10) 6.9}11.3
5.2}17.2
4, 7, 9, 11
0.47 (21) 0.30}0.63
1.30 (10) 1.01}1.66
0.10}0.70
1, 4, 7, 9
Zn
24.0 (21) 18.7}30.5
21.0 (10) 17.3}24.9
3}23
1, 2, 4, 7}9
Cd
3.2 (10) 2.4}4.1
13.8 (7) 6.2}22.4
1}86
1}4, 7}10
0.26 (10) 0.06}0.65
0.3}1.4
1, 5, 6, 7
Mg
Mn
As
Ref.
Note: Contents are given in mg/kg fresh weight, except Cd, which is given in lg/kg. No. of samples analysed in parentheses. Di!erent superscripts in the same row are signi"cantly di!erent (P(0.05). References: (1) Engman and Jorhem (1998), (2) Haahti (1991), (3) Kiesvaara et al. (1992), (4) KrelowskaKulas (1995), (5) Liukkonen-Lilja (1993), (6) Liukkonen-Lilja (1996), (7) Nuurtamo et al. (1980), (8) PerttilaK et al. (1982), (9) Szefer and Falandysz (1985), (10) Tahvonen and Kumpulainen (1996), (11) Torelm and Danielsson (1998).
and Danielsson (1998) could not "nd any big seasonal or regional variations in Na, P, Fe, Ca and K levels in herring during 1 year in Sweden. The Fe and Mn agreed with those reported for herring from the Baltic Sea. Cu and Zn contents were higher than those found in some previous Finnish studies (Haahti, 1991; Nuurtamo et al., 1980; PerttilaK et al., 1982) but at the same level as has been reported for herring caught in the other areas of the Baltic (Engman and Jorhem, 1998). Haahti (1991) reminded that the "sh he studied were only 2-year-old, which could have been the reason for the low concentrations. PerttilaK et al. (1982) also studied 2-year-old "sh. The ages of "sh were not measured in the present study but, due to size sorting, the average age was probably more than 2 years. The Cd level in the "llets was low, 2.4}4.1 lg/kg only. Low Cd levels for Baltic herring, 1}10 lg/kg, have also been reported in previous Finnish studies (Haahti, 1991; PerttilaK et al., 1982; Nuurtamo et al., 1980; Tahvonen and Kumpulainen, 1996). However, Kiesvaara et al. (1992) found a Cd content of 55 lg/kg, in "sh caught from the eastern Gulf of Finland. Also Krelowska-Kulas (1995) determined high Cd contents, 40}86 lg/kg, in herring caught in the Baltic Sea. This was the largest level found in any salt- or fresh-water "sh in her study. In Atlantic herring the
898
TAHVONEN E¹ A¸.
concentration was 18}48 lg/kg. Higher levels of Cd in Baltic herring compared to Atlantic herring were also reported by Engman and Jorhem (1998). The As content in 10 lots of gutted "sh varied from 0.06 to 0.65 mg/kg. In earlier studies, the As level found in Finnish Baltic herring samples ranged from 0.3 to 1.4 mg/kg (Liukkonen-Lilja, 1993) and was later found to be 0.62 mg/kg in one pooled sample (Liukkonen-Lilja, 1996). Engman and Jorhem (1998) reported an As content of 0.69}1.0 mg/kg in Baltic herring. They studied several "sh species from fresh, brackish and marine waters and found the highest levels in the marine species. The chemical form of the As was not determined in the present study but usually the major part of the As in "sh exists as non- or low-toxic organic compounds (Branch et al., 1994; Lawrence et al., 1986). The concentrations of Cu and Zn were lower while the concentrations of Ca, Mg, Mn, Na and Cd were signi"cantly higher in the gutted "sh than in "llets. The same di!erences between Baltic herring "llets and Baltic herring with bones with respect to Ca, Mn and Cd levels have also been reported by Nuurtamo et al. (1980). From the gutted "sh only the head and internal organs were removed while from the "llets the major parts of the bones were also taken out. Deboning decreases the Ca content signi"cantly due to its mainly being located in the bones (Pigott and Tucker, 1990). Also the levels of Cd and Mn are quite high in the bones although the highest concentrations have been found in the brain and internal organs. Only a small portion of the total Cd and Mn is found in "sh muscle (Allen, 1995; Gomaa et al., 1995; Pentreath, 1976). Processing Mineral contents in the consecutive stages of processes are shown in Figures 2 and 3. As expected, there were no signi"cant di!erences in the mineral levels between the processing stages one and two, as between these stages the "sh was only transported from the processing plant to the food factory. In processing the burger, the "rst alteration in mineral concentrations occurred at stage three, where the skin and bones were removed (Fig. 2). As expected, the deboning caused a signi"cant decrease not only in the Ca level but also in the levels of Fe, Mg, Zn, Na and Mn. This seems to indicate that the skin and bones of Baltic herring contain a remarkable amount of these elements. Pentreath (1973b) found that a large part of Zn, Fe and Mn in ray (Raja clavata L.) was located in the skin. Also Singh et al. (1991) found that Fe, Zn and Cu levels were higher in "sh skin than in muscle tissue. In plaice (Pleuronectes platessa L.), the level of Fe was much higher in the bones and muscle than in the skin (Pentreath, 1973a). On the other hand, Martinez et al. (1998) reported that the addition of "sh bones did not increase the contents of Fe and Zn in "sh-based weaning foods while the content of Ca increased signi"cantly. The weaning foods were made of sole (Solea vulgaris vulgaris) and hake (Merluccius merlucius). The variation between studies may suggest that the distribution of minerals in muscle, bones and skin is dependent on "sh species. In both processes, the most important stages concerning mineral levels were those where other ingredients were added. Salt, spice and cereal products used in the formulas increased signi"cantly the contents of Fe, Cu, Mn and Na. This alteration was most notable in Mn and Na. The mean Mn concentration in fried "llets and burger was about 4 times that of raw "llets. The Mn level in cereals, especially in wheat #our, is high when compared to Baltic herring (KansanelaK kelaitos, 1993). As expected, the Na level increased remarkably due to the salt added during processing. The level in the "nal products was about 6 times higher than in the raw materials. This "nding is in agreement with the study of Julsham and Lied (1986).
MINERAL CONTENT IN BALTIC HERRING
899
FIGURE 2. Contents of selected minerals (mg/kg) in the processing of Baltic herring burger. Stages in the process: (1) gutted "sh in "sh processing plant, (2) gutted "sh transported to food factory, (3) minced "sh, (4) minced "sh with breadcrumbs, salt and spice, (5) formed, coated product, (6) Baltic herring burger. * Signi"cant di!erence between stages (P(0.05).
The rapeseed oil used in the processes had only a minimal e!ect on mineral levels due to oils being very poor sources of minerals. Atta et al. (1997) reported that baking decreased Cd, Cu, Pb and Zn contents in "sh. This was not observed in the present study, which could be due to other ingredients being added to the products. On the other hand, the levels of Mg, K and Cu increased due to frying during the "llet process. The levels of other minerals remained quite stable. Baltic Herring Products The mineral element composition of ready-to-eat products is shown in Table 3. When comparing fried "llets and burger, signi"cant di!erences were found in the contents of all minerals except those of Cu and Na. The main di!erences can be seen in the Zn, Ca and Cd levels. The Zn level in fried "llets was considerably higher as the fried "llets
900
TAHVONEN E¹ A¸.
FIGURE 3. Contents of selected minerals (mg/kg) in the processing of fried Baltic herring "llets. Stages in the process: (1) "llets in "sh processing plant, (2) "llets transported to food factory, (3) "llets with rye breading, salt, spice and oil, (4) fried "llets. * Signi"cant di!erence between stages (P(0.05).
contained skin while it was removed from the burger. The "nal Ca level was lower in burger than in fried "llets. This would seem to indicate that the "sh bones were separated very e!ectively during the burger process. On the other hand, fried "llets contained dill, the Ca content of which is rather high, 2600 mg/kg (KansanelaK kelaitos, 1993). The Cd level in burger was signi"cantly higher than in the fried "llets. A part of the Cd is located in the "sh bones and these were removed during the burger processing. In "sh muscle, the Cd level is negligible (Allen, 1995). Thus, the main part of Cd in both products probably came from the spice and #our added during processing. Wheat #our (included in wheat breading and breadcrumbs) used in the burger process contains more Cd than the rye #our used in the processing of fried "llets (KansanelaK kelaitos, 1993). According to Julsham and Lied (1986) processing increases the Cd level in "sh.
901
MINERAL CONTENT IN BALTIC HERRING TABLE 3 Content (mean and range) of mineral elements in Baltic herring products Fried "llets
Burger
Ca
811? (33) 559}1055
532 (11) 341}774
K
3768 (32) 3343}4611
3285 (11) 2972}3762
Mg
306 (33) 274}365
289 (11) 263}325
Na
3824 (33) 2557}4570
3938 (11) 3571}4460
Cu
1.00 (33) 0.78}1.30
0.95 (11) 0.79}1.22
Fe
8.8 (33) 7.3}10.6
9.8 (11) 8.6}11.1
Mn
2.34 (32) 1.79}3.00
2.57 (11) 2.06}2.92
Zn
22.2 (33) 17.9}26.3
12.0 (11) 9.1}16.3
Cd
4.5 (23) 3.1}8.0
13.9 (7) 9.2}17.4
As
0.16 (10) 0.07}0.26
Note: Contents are given in mg/kg fresh weight, except Cd, which is given in lg/kg. No. of samples analysed in parentheses. Di!erent superscripts in the same row are signi"cantly di!erent (P(0.05).
The mean As content in burgers was 0.16 mg/kg which was slightly less than in gutted "sh. Julshamn and Lied (1986) analysed the As levels in many frozen "sh products and found higher As levels, ranging from 0.63 to 8.7 mg/kg. CONCLUSIONS Baltic herring "llets are a good source of many major and essential elements. Fe, Zn, Mn, Ca and K levels are very high when compared to the recommended daily allowances in the Nordic countries (Nordiska Ministerras det, 1996). In addition, the levels of non-essential elements, such as Cd and As, are typically low. If the skin and bones of Baltic herring are removed, the levels of Ca and Zn decrease signi"cantly. However, the levels of these elements still remain relatively high. During industrial processing the levels of Mn and Na may increase due to salt and other ingredients being added to the products. ACKNOWLEDGEMENTS The authors wish to thank LaK nnen Tehtaat Oyj (Turku) for the supply of the products, Ms Tarja Lammila, Mr Tomi Mattila, Ms Taru Hallenberg, Ms Anne Kinanen and Ms Tiina-Kaisa Ritvanen for their technical assistance and Ms Hanna Kivini for statistical analysis. This study was funded by the Ministry of Agriculture and Forestry.
902
TAHVONEN E¹ A¸.
REFERENCES Allen, P. (1995). Accumulation pro"les of lead and cadmium in the edible tissues of Oreochromis aureus during acute exposure. J. Fish Biol. 47, 559}568. Anon. (1997). Kalavirrat. ¹ietoa kalan tarjonnasta ja kaK ytoK staK . Finnish Game and Fisheries Research Institute. SVT YmpaK ristoK Vol. 13 (in Finnish). Anon. (1998). Ammattikalastus merialueella vuonna 1997. Finnish Game and Fisheries Research Institute. SVT YmpaK ristoK , Vol. 12 (in Finnish). Atta, M. B., El-Sebaie, L. A., Noaman, M. A., and Kassab, H. E. (1997). The e!ect of cooking on the content of heavy metals in "sh (¹ilapia nilotica). Food Chem. 58, 1}4. Branch, S., Ebdon, L., and O'Neill, P. (1994). Determination of arsenic species in "sh by directly coupled high-performance liquid chromatography-inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 9, 33}37. Engman, J. and Jorhem, L. (1998). Toxic and essential elements in "sh from Nordic waters with the results seen from the perspective of analytical quality assurance. Food Addit. Contam. 15, 884}892. Gomaa, M. N. E., Abou-Arab, A. A. K., Badawy, A., and Khayria, N. (1995). Distribution pattern of some heavy metals in Egyptian "sh organs. Food Chem. 53, 385}389. Haahti, H. (1991). Concentrations of harmful substances in "sh in the northern Baltic. Memoranda Soc. Fauna Flora Fennica 67, 15}20. Julsham, K. and Lied, E. (1986). Nutrient contents in Norwegian frozen "sh products major and minor elements. Fiskeridirektoraetes Skrifter, Serie Ern!ring 11, 167}178. KansanelaK kelaitos (1993). In Ruoka-aineiden ravintosisaK ltoK (Nutrient Composition of Foods) (M. Rastas, R. SeppaK nen, L.-R. Knuts, R.-L. Karvetti, and P. Varo, Eds.). Vammalan Kirjapaino Oy, Vammala, Finland. Kiesvaara, M., HeinioK , R.-L., Mustaranta, A., Hattula, T., Vartiainen, T., and Hallikainen, A. (1992). Silakka ravintona (Baltic Herring as Food). Research Notes 11/1992, National Food Administration, Helsinki, Finland (in Finnish, summary in English). Krelowska-Kulas, M. (1995). Content of some metals in mean tissue of salt-water and fresh-water "sh and their products. Die Nahrung 39, 166}172. Lawrence, J. F., Michalik, P., Tam, G., and Conacher, H. B. S. (1986) Identi"cation of arsenobetaine and arsenocholine in Canadian "sh and shell"sh by high-performance liquid chromatography with atomic absorption detection and con"rmation by fast atom bombardment mass spectrometry. J. Agric. Food Chem. 34, 315}319. Liukkonen-Lilja, H. (1993). Arseeni elintarvikkeissa (Arsenic in Food). Research Notes 12/1993, National Food Administration, Helsinki, Finland (in Finnish, summary in English). Liukkonen-Lilja, H. (1996). Arseeni elintarvikkeissa. Jatkotutkimus (Arsenic in foods. Continued study). Research Notes 2/1996, National Food Administration, Helsinki, Finland (in Finnish, summary in English). Martinez, I., Santaella, M., Ros, G., and Periago, M.J. (1998). Content and in vitro availability of Fe, Zn, Mg, Ca and P in homogenized "sh-based weaning foods after bone addition. Food Chem. 63, 299}305. Nordiska Ministerras det (1996). Nordiska naK ringsrekommendationer 1996. Nord Vol. 28. Nordiska ministerras det, KoK penhamn (in Swedish). Nuurtamo, M., Varo, P., Saari, E., and Koivistoinen, P. (1980). Mineral element composition of Finnish foods. VI Fish and Fish Products. Acta. Agr. Scand., Suppl. 22, 77}87. Parmanne, R. (1990). Growth, morphological variation and migrations of herring (Clupea harengus L.) in the northern Baltic Sea. Finnish Fish. Res. 10, 1}48. Parmanne, R. (1992). Changes in the growth of herring in the northern Baltic Sea in 1970}1991. International Council for the Exploration of the Sea (ICES) C.M. 1992/J, 42. Parmanne, R. (1994). Silakka muuttuvassa ympaK ristoK ssaK . >mpaK ristoK ja ¹erveys 25 (7}8), 9}14 (in Finnish). Pentreath, R. J. (1973a). The accumulation and retention of Fe and Co by the plaice, Pleuronected platessa L. J. Exp. Mar. Biol. Ecol. 12, 315}326. Pentreath, R. J. (1973b). The accumulation from sea water of Zn, Mn, Co and Fe by the thronback ray, Raja clavata L. J. Exp. Mar. Biol. Ecol. 12, 327}334. Pentreath, R. J. (1976). Some further studies on the accumulation and retention of Zn and Mn by the plaice, Pleuronected platessa L. J. Exp. Mar. Biol. Ecol. 21, 179}189. PerttilaK , M., Tervo, V., and Parmanne, R. (1982). Heavy metals in Baltic herring and cod. Mar. Pollut. Bull. 1, 391}393.
MINERAL CONTENT IN BALTIC HERRING
903
Pigott, G. M. and Tucker, B. W. (1990) Components of seafood. In Seafood: E+ects of ¹echnology on Nutrition (G. M. Pigott and B. W. Tucker, Eds.), pp. 32}65. Marcel Dekker, Inc., New York. Singh, J. G., Chang-Yen, I., Stoute, V. A., and Chatergoon, L. (1991). Distribution of selected heavy metals in skin and muscle of "ve tropical marine "shes. Environ. Pollut. 69, 203}215. Szefer, P. and Falandysz, J. (1985). Trace metals in muscle tissue of "sh taken from the southern Baltic. Z. ¸ebensm ;nters. Forsch. 181, 217}220. Tahvonen, R. and Kumpulainen, J. (1996). Contents of lead and cadmium in selected "sh species consumed in Finland in 1993}1994. Food Addit. Contam. 13, 647}654. Torelm, I. and Danielsson, R. (1998). Variations in major nutrients and minerals in Swedish foods: a multivariate, multifactorial approach to the e!ects of season, region, and chain. J. Food Comp. Anal. 11, 11}31.
JOURNAL OF FOOD COMPOSITION AND ANALYSIS
ORIGINAL ARTICLE Mineral Content in Baltic Herring and Baltic Herring Products Raija Tahvonen, Tarja Aro, Johanna Nurmi, and Heikki Kallio Department of Biochemistry and Food Chemistry, University of Turku, FIN-20014 Turku, Finland Received September 27, 1999, and in revised form March 23, 2000
The contents of Fe, Cu, Zn, Mn, K, Mg, Ca, Na, As and Cd in gutted Baltic herring (Clupea harengus membras), in Baltic herring "llets and in two commercial ready-to-eat products made of these were investigated. The "sh was caught in 1996}1998 in the northern Baltic. The commercial products were fried "llets and "sh burgers. The e!ects of industrial processing on the mineral composition were studied by collecting samples at several consecutive stages. In the gutted "sh the levels of Mn, Ca and Cd were considerably higher than in the "llet mainly due to the bones. Signi"cant di!erences were also observed in the contents of Mg, Na, Cu and Zn. In the burger process the bones and skin were removed which caused a decrease in the Zn and Ca levels. Ingredients such as salt, spices and #our as part of the formula increased Na and Mn contents in the ready-to-eat products. In the "nal products the Na content was about 6 times higher than that of the fresh "sh. 2000 Academic Press Key =ords: Baltic herring; "sh products; Ca; Mg; K; Na; Fe; Mn; Cu; Zn; Cd; As; processing.
INTRODUCTION Baltic herring (Clupea harengus membras), a sub-species of Atlantic herring (Clupea harengus harengus) is the most important wild "sh in Finland, the catch having been about 90 000 t/year at the end of the 1990s. In 1997, almost half of the total catch occurred during May and June, with nearly 70% being caught in the Gulf of Bothnia. The major part of the Baltic herring was caught using trawls (Anon, 1998). The age of the "sh in trawl catches is usually over 3 years and in trap nets over 4 years. Due to size sorting, older "sh are mainly used in industrial processing (Parmanne, 1994). There has been a #uctuation in the Baltic herring population; the amount of "sh has increased but the size decreased. The small size is a problem especially in the "sh industry (Parmanne, 1994). Over the last few years, the growth has retarded remarkably in the Archipelago Sea and the Gulf of Finland, but in the Gulf of Bothnia this trend has not been so evident (Parmanne, 1992). Baltic herring from the south and south-west coast of Finland migrate after spawning to the southern parts of the Baltic. Thus, the population that remains in the Archipelago Sea and the Gulf of Finland in the autumn consists mainly of young, small "sh. The "sh caught after spawning in the Gulf of Bothnia are bigger because no special migration occurs from this area (Parmanne, 1990). To whom correspondence and reprint requests should be addressed. Tel: #358-2-333 6876. Fax: #358-2-333 6860. E-mail: tarja.aro@utu." 0889}1575/00/060893#11 $35.00/0
2000 Academic Press
894
TAHVONEN E¹ A¸.
In Finland only a small portion of the annual catch is used for food. The average consumption of Baltic herring in 1996 was 3.1 kg per capita equalling about 16 000 t of "sh (Anon, 1997). The Baltic herring could be better exploited, especially in the food industry by o!ering to the consumer new choices in "sh products. The present study was aimed at evaluating the mineral element composition and nutritional value of Baltic herring and the e!ects of industrial processing on the "sh products. In order to exclude the annual variation in the composition, the sample material was collected over 2.5 years from two "sh processing plants and one food factory. MATERIALS AND METHODS Sample Collection and Preparation The Baltic herrings were caught between May 1996 and September 1998 in the northern area of the Baltic Sea (The Archipelago Sea and the Gulf of Bothnia). Fishing took place during all seasons but the main part was caught in May or June. The "sh was sorted, mechanically headed and gutted in a processing plant (size class 0: 18}24 "sh/kg). In the case of "sh being prepared as "llets they were also deboned (size class 00: 12}17 "sh/kg). Two commercial Baltic herring products were processed in a local food factory (LaK nnen Tehtaat Oyj, Turku). One product was a fried double "llet with rapeseed oil, salt, rye breading and spices (e.g., dill). The other product was a "sh burger made of minced "sh, wheat breading, breadcrumbs, salt and spices. The "llets were dipped in rapeseed oil before frying whereas burgers were deep-fried. The stages of both processes are described in Figure 1. Altogether 33 batches of fried "llets and 11 of burger were collected for analysis. Each batch included "llets or gutted "sh from the processing plant and the corresponding samples from the food factory, samples from the processing line and ready-to-eat products. Iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), potassium (K), sodium (Na), calcium (Ca) and magnesium (Mg) levels were measured in all sub-samples during the "rst sampling year (10 batches of fried "llets and four of burgers). The e!ect of processing on mineral contents was investigated on these batches. Later, only ready-to-eat products and random samples of gutted and "lleted "sh from the food factory were analysed. Cadmium (Cd) contents were measured only in gutted or "lleted "sh and the "nal products. The number of samples analysed is shown in Tables 2 and 3. Ten lots of Baltic herring caught from the Archipelago Sea between May 1996 and December 1997 and 10 batches of burgers produced from the same "sh lots were collected for arsenic (As) measurements. The "sh in these samples were small (12}24 "sh/kg). All samples (about 2 kg) were packed into LD-polyethylene bags and transported to the laboratory in insulated containers together with solid carbon dioxide. The samples were stored in !703C until analysed. Before analysis, thawed samples were homogenized using a Robot Coupe R8 (Robot Coupe S.A., Bourgogne, France) homogenizer. Mineral Element Analysis The samples (about 2 g) were digested in a mixture of 65% nitric acid (5 mL pa#, Riedel-de-HaeK n, Seelze, Germany) and 30% hydrogen peroxide (2 mL, Mallinckrodt
MINERAL CONTENT IN BALTIC HERRING
895
FIGURE 1. Stages in the processing of fried Baltic herring "llets and Baltic herring burger.
Baker B.V., Deventer, Holland) in closed tetra#uormethaxil vessels of a Milestone MLS-1200 microwave (Milestone s.r.l., Sonisole (BG), Italy) digestion system. The digested samples were diluted to 25 mL in polypropylene volumetric bottles and stored in polyethylene vessels before analysis. For Ca and Mg determinations the samples were diluted with a LaCl solution to 0.1% w/v La and for Na and K determination with a CsCl solution to 0.5% w/v Cs. Concentrations were determined by a Varian SpectrAA 300 (Varian Techtron Pty. Limited, Mulgrave, Victoria, Australia) #ame system equipped with an autosampler (Ca, Mg, K, Na, Zn, Cu, Fe, Mn) or GTA-96-graphite furnace system equipped with an autosampler (As, Cd). Flame methods were the &&cook-book'' methods of the instrument. D -background correction was used in As, Cd, Fe and Zn analysis. Ni(NO ) (0.05%) was used as a matrix modi"er in As-determinations. All samples were analysed as duplicates. Several blanks were digested during each analysis period. Two commercial reference materials (GSV-3 poplar leaves, Institute of Geophysical and Geochemical Exploration, Langfang, P.R. China and DORM-1 dog"sh muscle, National Research Council of Canada, Ottawa) were used as quality control samples. All glassware and other vessels were washed with a detergent (Deconex, Borer Chemie Ag, Zuchwill, Switzerland), rinsed with reverse osmose water, soaked in 10% nitric acid overnight, rinsed with reverse osmose water and "nally rinsed with ultra-pure water (MilliQ). The microwave vessels were rinsed with ultra-pure water only, washed with 5 mL 30% nitric acid (pa) in the microwave digestion system using the digestion}heating programme and rinsed again with ultrapure water.
896
TAHVONEN E¹ A¸.
Statistical Analysis Statistical analysis was performed using SPSS software, version 7.5. The di!erences in raw materials ("sh "llets, gutted "sh) and in the "nal products (fried "llets, burger) were subjected to an ANOVA-test. The comparisons between the processing stages were tested using paired, two-tailed t-tests.
RESULTS AND DISCUSSION Analytical Quality Control The results for the analysis of reference materials and certi"ed values are shown in Table 1. The accuracy of the analytical procedures was satisfactory for most metal elements when compared to the certi"ed values for a dog"sh muscle (DORM-1). For Zn only 93% of the certi"ed value of DORM could be detected whereas in poplar leaves (GSV-3) the recovery was over 100%. Arsenic recovery in the reference material (DORM-1) was 76.3%, meaning that the As contents may be slightly underestimated. When compared to certi"ed values of poplar leaves, the analytical values of Mn and Zn were slightly out of the upper limit while the values of Fe and Na were lower than the certi"ed values. The matrix was rather di!erent compared to the "sh samples and, thus, a dog"sh muscle was a better reference material for this study. The detection limits for Cd, Mn, Cu, Zn, Fe, Mg, Ca, K and Na were 0.0012, 0.26, 0.44, 0.84, 1.5, 5.7, 124, 63 and 137 mg/kg, respectively. The detection limits (3 times the standard deviation of blanks) were calculated using an average sample weight. Mineral Contents in Gutted Baltic Herring and Baltic Herring Fillets The results for the mineral contents in gutted Baltic herring and "llets are shown in Table 2. The contents were studied using "llets and gutted "sh from the food factory. The ranges of the mineral levels reported for herring caught in di!erent areas of the Baltic Sea are also presented. Ca and K levels in the "sh "llets were slightly higher than those reported by Nuurtamo et al. (1980) and Torelm and Danielsson (1998). Nuurtamo et al. (1980) stated that incomplete deboning could lead to a wide variation in Ca contents. Torelm TABLE 1 Results of analysis for reference materials Analytical results DORM-1 Arsenic Cadmium Copper Iron Magnesium Manganese Potassium Sodium Zinc Note: Mean and
13.5$0.5 (5) 0.094$0.014 (12) 5.23$0.29 (18) 67.9$5.5 (18) 1234$51 (17) 1.42$0.25 (18) 15874$1019 (18) 7743$203 (17) 19.8$0.32 (18) S.D.
Certi"ed values DORM-1 17.7$2.1 0.086$0.012 5.22$0.33 63.6$5.3 1210$130 1.32$0.26 15900$1000 8000$600 21.3$1.0
in mg/kg. No. of analyses in parentheses.
Analytical results GSW-3
Certi"ed values GSW-3
9.5$0.0 (2) 226$6 (3)
9.3$0.5 274$10
48$1 (3) 13877$832 (3) 175$8 (7) 39$2 (2)
45$2 13800$400 200$10 37$1
897
MINERAL CONTENT IN BALTIC HERRING TABLE 2 Content (mean and range) of mineral elements in Baltic herring Fish "llets
Gutted "sh
Other studies
Ca
874? (21) 544}1158
3272 (10) 2310}5972
335}770
7, 11
K
3904 (21) 2993}4742
3789 (10) 3217}4492
2170}3720
7, 11
282 (21) 251}336
317 (10) 288}342
170}290
4, 7
Na
619 (21) 454}802
715 (10) 624}870
236}903
11
Cu
0.86 (21) 0.75}1.06
0.77 (10) 0.68}0.91
0.2}1.4
1, 2, 4, 7}9
Fe
8.1 (21) 6.5}9.1
8.5 (10) 6.9}11.3
5.2}17.2
4, 7, 9, 11
0.47 (21) 0.30}0.63
1.30 (10) 1.01}1.66
0.10}0.70
1, 4, 7, 9
Zn
24.0 (21) 18.7}30.5
21.0 (10) 17.3}24.9
3}23
1, 2, 4, 7}9
Cd
3.2 (10) 2.4}4.1
13.8 (7) 6.2}22.4
1}86
1}4, 7}10
0.26 (10) 0.06}0.65
0.3}1.4
1, 5, 6, 7
Mg
Mn
As
Ref.
Note: Contents are given in mg/kg fresh weight, except Cd, which is given in lg/kg. No. of samples analysed in parentheses. Di!erent superscripts in the same row are signi"cantly di!erent (P(0.05). References: (1) Engman and Jorhem (1998), (2) Haahti (1991), (3) Kiesvaara et al. (1992), (4) KrelowskaKulas (1995), (5) Liukkonen-Lilja (1993), (6) Liukkonen-Lilja (1996), (7) Nuurtamo et al. (1980), (8) PerttilaK et al. (1982), (9) Szefer and Falandysz (1985), (10) Tahvonen and Kumpulainen (1996), (11) Torelm and Danielsson (1998).
and Danielsson (1998) could not "nd any big seasonal or regional variations in Na, P, Fe, Ca and K levels in herring during 1 year in Sweden. The Fe and Mn agreed with those reported for herring from the Baltic Sea. Cu and Zn contents were higher than those found in some previous Finnish studies (Haahti, 1991; Nuurtamo et al., 1980; PerttilaK et al., 1982) but at the same level as has been reported for herring caught in the other areas of the Baltic (Engman and Jorhem, 1998). Haahti (1991) reminded that the "sh he studied were only 2-year-old, which could have been the reason for the low concentrations. PerttilaK et al. (1982) also studied 2-year-old "sh. The ages of "sh were not measured in the present study but, due to size sorting, the average age was probably more than 2 years. The Cd level in the "llets was low, 2.4}4.1 lg/kg only. Low Cd levels for Baltic herring, 1}10 lg/kg, have also been reported in previous Finnish studies (Haahti, 1991; PerttilaK et al., 1982; Nuurtamo et al., 1980; Tahvonen and Kumpulainen, 1996). However, Kiesvaara et al. (1992) found a Cd content of 55 lg/kg, in "sh caught from the eastern Gulf of Finland. Also Krelowska-Kulas (1995) determined high Cd contents, 40}86 lg/kg, in herring caught in the Baltic Sea. This was the largest level found in any salt- or fresh-water "sh in her study. In Atlantic herring the
898
TAHVONEN E¹ A¸.
concentration was 18}48 lg/kg. Higher levels of Cd in Baltic herring compared to Atlantic herring were also reported by Engman and Jorhem (1998). The As content in 10 lots of gutted "sh varied from 0.06 to 0.65 mg/kg. In earlier studies, the As level found in Finnish Baltic herring samples ranged from 0.3 to 1.4 mg/kg (Liukkonen-Lilja, 1993) and was later found to be 0.62 mg/kg in one pooled sample (Liukkonen-Lilja, 1996). Engman and Jorhem (1998) reported an As content of 0.69}1.0 mg/kg in Baltic herring. They studied several "sh species from fresh, brackish and marine waters and found the highest levels in the marine species. The chemical form of the As was not determined in the present study but usually the major part of the As in "sh exists as non- or low-toxic organic compounds (Branch et al., 1994; Lawrence et al., 1986). The concentrations of Cu and Zn were lower while the concentrations of Ca, Mg, Mn, Na and Cd were signi"cantly higher in the gutted "sh than in "llets. The same di!erences between Baltic herring "llets and Baltic herring with bones with respect to Ca, Mn and Cd levels have also been reported by Nuurtamo et al. (1980). From the gutted "sh only the head and internal organs were removed while from the "llets the major parts of the bones were also taken out. Deboning decreases the Ca content signi"cantly due to its mainly being located in the bones (Pigott and Tucker, 1990). Also the levels of Cd and Mn are quite high in the bones although the highest concentrations have been found in the brain and internal organs. Only a small portion of the total Cd and Mn is found in "sh muscle (Allen, 1995; Gomaa et al., 1995; Pentreath, 1976). Processing Mineral contents in the consecutive stages of processes are shown in Figures 2 and 3. As expected, there were no signi"cant di!erences in the mineral levels between the processing stages one and two, as between these stages the "sh was only transported from the processing plant to the food factory. In processing the burger, the "rst alteration in mineral concentrations occurred at stage three, where the skin and bones were removed (Fig. 2). As expected, the deboning caused a signi"cant decrease not only in the Ca level but also in the levels of Fe, Mg, Zn, Na and Mn. This seems to indicate that the skin and bones of Baltic herring contain a remarkable amount of these elements. Pentreath (1973b) found that a large part of Zn, Fe and Mn in ray (Raja clavata L.) was located in the skin. Also Singh et al. (1991) found that Fe, Zn and Cu levels were higher in "sh skin than in muscle tissue. In plaice (Pleuronectes platessa L.), the level of Fe was much higher in the bones and muscle than in the skin (Pentreath, 1973a). On the other hand, Martinez et al. (1998) reported that the addition of "sh bones did not increase the contents of Fe and Zn in "sh-based weaning foods while the content of Ca increased signi"cantly. The weaning foods were made of sole (Solea vulgaris vulgaris) and hake (Merluccius merlucius). The variation between studies may suggest that the distribution of minerals in muscle, bones and skin is dependent on "sh species. In both processes, the most important stages concerning mineral levels were those where other ingredients were added. Salt, spice and cereal products used in the formulas increased signi"cantly the contents of Fe, Cu, Mn and Na. This alteration was most notable in Mn and Na. The mean Mn concentration in fried "llets and burger was about 4 times that of raw "llets. The Mn level in cereals, especially in wheat #our, is high when compared to Baltic herring (KansanelaK kelaitos, 1993). As expected, the Na level increased remarkably due to the salt added during processing. The level in the "nal products was about 6 times higher than in the raw materials. This "nding is in agreement with the study of Julsham and Lied (1986).
MINERAL CONTENT IN BALTIC HERRING
899
FIGURE 2. Contents of selected minerals (mg/kg) in the processing of Baltic herring burger. Stages in the process: (1) gutted "sh in "sh processing plant, (2) gutted "sh transported to food factory, (3) minced "sh, (4) minced "sh with breadcrumbs, salt and spice, (5) formed, coated product, (6) Baltic herring burger. * Signi"cant di!erence between stages (P(0.05).
The rapeseed oil used in the processes had only a minimal e!ect on mineral levels due to oils being very poor sources of minerals. Atta et al. (1997) reported that baking decreased Cd, Cu, Pb and Zn contents in "sh. This was not observed in the present study, which could be due to other ingredients being added to the products. On the other hand, the levels of Mg, K and Cu increased due to frying during the "llet process. The levels of other minerals remained quite stable. Baltic Herring Products The mineral element composition of ready-to-eat products is shown in Table 3. When comparing fried "llets and burger, signi"cant di!erences were found in the contents of all minerals except those of Cu and Na. The main di!erences can be seen in the Zn, Ca and Cd levels. The Zn level in fried "llets was considerably higher as the fried "llets
900
TAHVONEN E¹ A¸.
FIGURE 3. Contents of selected minerals (mg/kg) in the processing of fried Baltic herring "llets. Stages in the process: (1) "llets in "sh processing plant, (2) "llets transported to food factory, (3) "llets with rye breading, salt, spice and oil, (4) fried "llets. * Signi"cant di!erence between stages (P(0.05).
contained skin while it was removed from the burger. The "nal Ca level was lower in burger than in fried "llets. This would seem to indicate that the "sh bones were separated very e!ectively during the burger process. On the other hand, fried "llets contained dill, the Ca content of which is rather high, 2600 mg/kg (KansanelaK kelaitos, 1993). The Cd level in burger was signi"cantly higher than in the fried "llets. A part of the Cd is located in the "sh bones and these were removed during the burger processing. In "sh muscle, the Cd level is negligible (Allen, 1995). Thus, the main part of Cd in both products probably came from the spice and #our added during processing. Wheat #our (included in wheat breading and breadcrumbs) used in the burger process contains more Cd than the rye #our used in the processing of fried "llets (KansanelaK kelaitos, 1993). According to Julsham and Lied (1986) processing increases the Cd level in "sh.
901
MINERAL CONTENT IN BALTIC HERRING TABLE 3 Content (mean and range) of mineral elements in Baltic herring products Fried "llets
Burger
Ca
811? (33) 559}1055
532 (11) 341}774
K
3768 (32) 3343}4611
3285 (11) 2972}3762
Mg
306 (33) 274}365
289 (11) 263}325
Na
3824 (33) 2557}4570
3938 (11) 3571}4460
Cu
1.00 (33) 0.78}1.30
0.95 (11) 0.79}1.22
Fe
8.8 (33) 7.3}10.6
9.8 (11) 8.6}11.1
Mn
2.34 (32) 1.79}3.00
2.57 (11) 2.06}2.92
Zn
22.2 (33) 17.9}26.3
12.0 (11) 9.1}16.3
Cd
4.5 (23) 3.1}8.0
13.9 (7) 9.2}17.4
As
0.16 (10) 0.07}0.26
Note: Contents are given in mg/kg fresh weight, except Cd, which is given in lg/kg. No. of samples analysed in parentheses. Di!erent superscripts in the same row are signi"cantly di!erent (P(0.05).
The mean As content in burgers was 0.16 mg/kg which was slightly less than in gutted "sh. Julshamn and Lied (1986) analysed the As levels in many frozen "sh products and found higher As levels, ranging from 0.63 to 8.7 mg/kg. CONCLUSIONS Baltic herring "llets are a good source of many major and essential elements. Fe, Zn, Mn, Ca and K levels are very high when compared to the recommended daily allowances in the Nordic countries (Nordiska Ministerras det, 1996). In addition, the levels of non-essential elements, such as Cd and As, are typically low. If the skin and bones of Baltic herring are removed, the levels of Ca and Zn decrease signi"cantly. However, the levels of these elements still remain relatively high. During industrial processing the levels of Mn and Na may increase due to salt and other ingredients being added to the products. ACKNOWLEDGEMENTS The authors wish to thank LaK nnen Tehtaat Oyj (Turku) for the supply of the products, Ms Tarja Lammila, Mr Tomi Mattila, Ms Taru Hallenberg, Ms Anne Kinanen and Ms Tiina-Kaisa Ritvanen for their technical assistance and Ms Hanna Kivini for statistical analysis. This study was funded by the Ministry of Agriculture and Forestry.
902
TAHVONEN E¹ A¸.
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MINERAL CONTENT IN BALTIC HERRING
903
Pigott, G. M. and Tucker, B. W. (1990) Components of seafood. In Seafood: E+ects of ¹echnology on Nutrition (G. M. Pigott and B. W. Tucker, Eds.), pp. 32}65. Marcel Dekker, Inc., New York. Singh, J. G., Chang-Yen, I., Stoute, V. A., and Chatergoon, L. (1991). Distribution of selected heavy metals in skin and muscle of "ve tropical marine "shes. Environ. Pollut. 69, 203}215. Szefer, P. and Falandysz, J. (1985). Trace metals in muscle tissue of "sh taken from the southern Baltic. Z. ¸ebensm ;nters. Forsch. 181, 217}220. Tahvonen, R. and Kumpulainen, J. (1996). Contents of lead and cadmium in selected "sh species consumed in Finland in 1993}1994. Food Addit. Contam. 13, 647}654. Torelm, I. and Danielsson, R. (1998). Variations in major nutrients and minerals in Swedish foods: a multivariate, multifactorial approach to the e!ects of season, region, and chain. J. Food Comp. Anal. 11, 11}31.