Mineral and trace element composition of the roe and muscle tissue of farmed rainbow trout (Oncorhynchus mykiss) with respect to nutrient requirements

Abstract

Background

Due to excellent nutritional properties of fish and fish products, dietitians recommend their larger share in the diet, for both the prevention and treatment of diet-related diseases. In this context particular attention should be given to fish eggs (roe). The aim of the study was to compare the content of selected minerals and trace elements in the roe and muscle tissue of aquaculture-reared rainbow trout. Moreover, the extent to which an edible portion of 100 g of fish product (roe or fillet) satisfies the requirements for these elements in adult consumers (women and men) and children was determined.

Methods

The research material was represented by 20 farmed rainbow trout females (Oncorhynchus mykiss). In muscle tissue and roe the proximate composition, energy value, and nutritional quality index were examined. Concentrations of K, Na, Ca, Mg, Zn, Fe, Mn and Cu were determined by flame atomic absorption spectrometry analysis. The levels of minerals and trace elements in fish products were used to calculate the percentages of the recommended dietary reference values for adults and children in portion of 100 g.

Results

The roe had higher content of 7 elements including, Zn (29.40 ± 1.38, 27.74–30.91 mg/kg), Mn (1.66 ± 0.08, 1.55–1.73 mg/kg), Fe (11.17 ± 0.35, 10.75–11.60 mg/kg), Cu (1.24 ± 0.04, 1.21–1.30 mg/kg), Ca (546.90 ± 23.68, 522.02–578.96 mg/kg), Mg (546.97 ± 23.68, 522.02–578.96 mg/kg), Na (403.61 ± 5.53, 397.29–407.57 mg/kg) compared to muscle tissue, respectively, Zn (2.73 ± 0.97, 0.94–3.77 mg/kg, Mn (0.22 ± 0.10, 0.14–0.46 mg/kg), Fe (2.19 ± 1.44, 1.02–5.50 mg/kg), Cu (0.24 ± 0.06, 0.17–0.36 mg/kg), Ca (164.07 ± 71.72, 93.24–306.32 mg/kg), Mg (240.00 ± 73.37, 127.92–316.41 mg/kg), Na (273.37 ± 99.65, 183.93–524.41 mg/kg). In turn, the muscle tissue contained 3 times as much K (5159.87 ± 974.70, 4004.63–6343.91 mg/kg) as the roe (1713.65 ± 58.68, 1643.97–1764.14 mg/kg). The roe covered the recommended nutrition standards (population reference intake, PRI and adequate intake, AI) to a greater extent than the fillet for all analysed trace elements and minerals except K. For children, the contribution of roe to the reference intake of such elements as Mg, Zn, Fe, Mn and Cu was particularly high. Therefore the roe should be considered as a source of Mg for all consumer groups, and as high in Zn for women and children. The suggested portion of fillet and roe provided only 2–3 % of the daily requirement for Na.

Conclusions

Despite the differences found, both roe and fillet from rainbow trout are a good source of analysed elements. The results indicate the need to continue research in order to determine the influence of other factors on the content of minerals and trace elements in roe and the possibility of using gonads to enrich other food products with elements.

Introduction

Consumption of fish, due to their excellent nutritional properties and sensory characteristics, is showing an upward trend. Dietitians recommend a larger share of fish in the diet, for both the prevention and treatment of diet-related diseases, including cancer [1,2]. Consumers generally prefer the muscle tissue of fish [3], which is a source of biologically active compounds, including long-chain polyunsaturated omega-3 fatty acids (n-3 LC-PUFA), amino acids, vitamins and essential elements [4]. Deficiencies of these compounds in the diet and associated health problems continue to occur all over the world, even in developed countries [5]. Many scientific studies confirm the high nutritional value of edible by-products of fish, including the liver, roe, skin and gills [[6], [7], [8]]. These raw materials (together with the fish bone) are also a source of amino acids, omega-3 and omega-6 fatty acids, Ca, P, Mg and Fe [3,8], as well as such bioactive compounds as peptides and hydroxyapatite [9]. Thus, inclusion of fish by-products as part of a balanced diet may increase the nutritional potential of consumed foods.

Among fish products, particular attention should be paid to fish eggs (roe), which are generally processed into three products: whole ovaries, individual eggs (caviar), and pâté, pastes and other products [10]. The most popular form of fish eggs is caviar, which can be described as fish eggs that have been separated from the connective tissue of the ovaries and then salted and cured [11,12]. Roe is most commonly obtained from sturgeon (over 20 species), wild and farmed salmonids, and other species (mullet, whitefish, cod, herring and others) [10]. Due to increased production of rainbow trout and the upward trend in commercial fish weight, as well as aquaculture rearing of single-sex fish stock, farms specializing in fish fattening usually treat rainbow trout roe as a by-product or even a waste product [13].

Assessment of the elemental composition of food is important due to the impact of minerals and trace elements on human health, in terms of both nutritional value and toxicological hazards [8]. Optimal content of such trace elements as Mn, Zn, Fe, Cu, Co, I, Mo and Se is indispensable to human health [14]. On the other hand, the excessive concentration of elements such as Hg, Cd, Pb and As, as well as biogenic elements (as, for example, Cr, Ni, Al), are toxic and can cause serious damage to the human body [15]. It is worth noting that farmed rainbow trout available in Poland is safe for consumers with regard to toxic metals [16] and other contaminants [17]. Moreover, even if fish products were to be consumed at recommended levels, which exceed current consumption by more than 50 %, no hazard would exist [18].

The aim of the study was to compare the content of selected elements in the roe and muscle tissue of aquaculture-reared rainbow trout. The results were used to determine the extent to which an edible portion of 100 g of fish product (roe or fillet) satisfies the requirements for these elements in adult consumers (women and men) and children.