Effect of Different Cooking Methods on Minerals, Vitamins, and Nutritional Quality Indices of Rainbow Trout (Oncorhynchus mykiss)

Abstract

In this study, the influence of four cooking methods (baking, boiling, microwaving, and frying) on the nutritional value of rainbow trout was determined. Proximate, fatty acid composition, vitamin, mineral contents, and nutritional quality indices of rainbow trout were investigated (i.e., before and after cooking treatments). In all samples, there was reduction of total n-3 than raw fish fillets. Considering overall nutritional quality indices, vitamin, and mineral contents; baking and boiling were the best cooking methods among other methods used in this study.

Keywords:

• Cooking method
• Rainbow trout
• Fatty acids
• Nutritional value
• Vitamins
• Minerals

Introduction

Cooking of fish leads to enhanced flavor and taste and inactivation of pathogenic microorganisms. Furthermore, it causes macro and micro nutrients distortion because of their sensitivity to heat, light, oxygen, pH, or a combination of these.^[1] Major changes occur in proximate, vitamins, minerals contents, and fatty acid composition of fish during cooking.^[2] One of the most important healthy aspects of fish consumption is due to complex fatty acid profile, the major groups of fatty acids based on the number of double bonds include, saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), and polyunsaturated fatty acids (PUFA). The expert consultation recognized that individual fatty acids within each broad classification of fatty acids may have unique biological properties and health effects.^[3] It is evident that the long-chain poly unsaturated fatty acids (LC-PUFA) such as arachidonic acid (ARA), eicosapentaenoic acid (EPA), and docosahexaenoic fatty acid (DHA, 22:6n-3) have markedly different physiological properties and biological functions compared to the shorter chain PUFAs linoleic acid (18:2n-6) and α-linolenic acid (ALA,18:3n-3).^[4] Indeed, the polyunsaturated acids are more susceptible to oxidation during heating than their saturated analogues; however, several studies mentioned that EPA and DHA contents remained stable in some fish species in certain types of cooking.

Due to different effects of fatty acids on health, it is necessary to define nutritional quality index (NQI) with regard to fatty acids profile and biological functions of them. NQI was estimated by several indices of fatty acid composition: the indices of atherogenicity (AI) and thrombogenicity (TI), according to Turan et al.^[5] and the hypocholesterolemic/hypercholesterolemic fatty acid ratio (HH) according to Testi et al.,^[6] EPA + DHA, PUFA/SFA-stearic acid according to Unusan,^[7] PUFA/SFA ratio according to Kalogeropoulos et al.,^[8] and Marques et al.,^[9] ratios n-3/n-6 PUFA according to Marques et al.,^[9] and ultimately ARA/EPA and UFA/SFA ratio according to Larsen et al.^[4]

Fish is a potential source of vitamins and minerals and regardless of distortion effects of cooking on nutrients, the way of cooking may be an important factor for final content of the nutrients in the fish.^[10] Vitamin A (Retinol) and vitamin D (Calciferol) are from fat soluble vitamins. Vitamin A is necessary for visual system in humans and vitamin D plays an important role at the maintenance of normal blood levels of calcium and phosphate, bone formation, and remodeling in higher vertebrates.^[11] Fat-soluble vitamins are less heat-labile than the water-soluble vitamins such as vitamin B groups but in the presence of oxygen they can be degraded at high temperatures. Vitamin B1 (Thiamin) is the most susceptible to thermal degradation of the B vitamins. Fish contains less vitamin B1 than beef but it is a good resource for vitamin B3 (Niacin). Vitamin B1 (Thiamin) deficiency results in Beriberi, polyneuritis, and Wernicke-Korsakoff syndrome. In contrast to thiamin, vitamin B3 is a heat stable vitamin. Pellagra is a disease caused by dietary lack of vitamin B3.^[12,13] Five macro (sodium: Na; potassium: K; magnesium: Mg; calcium: Ca; phosphorus: P) and four trace elements (iron: Fe, manganese: Mn, copper: Cu; zinc: Zn) are necessary for regulation of healthy functions in human body. The major minerals, Ca, P, and Mg, are involved in bone health. Fe is the most abundance trace element in the human body that its insufficient daily intake results in anemia. Mg, Mn, and Zn are responsible for activity regulation of several enzymes.^[14] Thus, the aim of this research was to investigate proximate, fatty acid composition, NQI, vitamins, and minerals of raw and cooked rainbow trout with different methods.

Materials and methods

Sample Preparation and Cooking

Samples of rainbow trout (n = 50, 1–1.2 kg weight) were randomly selected from a local market (Noor, Iran). They were kept on ice and immediately transported to the laboratory within 1 h. Heads, scales, and the viscera were removed, and a total of 50 fish fillets were divided into five groups of ten fish. The first group was a control and was not cooked, while the four others were employed for each of the cooking methods. The fish fillets were cooked in a microwave oven (2450 MHz, 13 min), boiled in water for 5 min, baked in an oven (180°C, 30 min), or fried in sunflower seed oil in a frying pan (150°C, 15 min). After cooking, the skin and backbone of the samples were removed. Samples of each group were homogenized in a food blender and then distributed into three batches (n = 3) to achieve the statistical analysis.

Proximate Composition Analysis

Moisture, protein, fat, and ash contents of cooked and uncooked fish samples were measured in triplicate according to American Officials of Analysis (AOAC) methods. Moisture was measured by oven drying at 105°C to constant weight.^[15] The crude protein content was measured by the Kjeldahl procedure. In this procedure, percent of nitrogen (N) was determined and converted to protein using the factor 6.25. Total lipid was extracted by the AOAC method using the Soxhlet system.^[15] Ash was measured gravimetrically in a furnace by heating at 550°C to constant weight.^[15]

Analysis of Fatty Acids

Fatty acids were extracted from fish samples according to the method of Zuraini et al.^[16] Fatty acids of the extracts were then converted to fatty acid methyl esters (FAME) by the AOAC (2005) method.^[15] FAME were then analyzed using gas chromatography (Hewlett Packard, BPX70, capillary column of 60 m × 0.25 mm, i.e., 0.32 μm film thickness) with a flame ionization detector. The injection and detector temperature were set at 270 and 280°C, respectively, and the carrier gas was N2 at 2 mL/min. After injection (1 μL, split ratio 30:1) the column temperature was held at 190°C for 35 min, then increased to 240°C at 50°C/min and held at this temperature for 5 min. The fatty acids were reported as g/100 g of dry weight.

Analysis of Vitamins

B₁ and B₃ vitamins

Water soluble vitamins (B₁ and B₃) were determined according to Ersoy and Özeren, and high-performance liquid chromatography (HPLC; Agilent model 1200) methods.^[13] HPLC conditions are expressed as follows: Wavelength: 254 nm, Flowing rate: 1.0 mL/min, Injection volume: 20 μL, Mobile phase: 1000 mL phosphate solvent, 360 mL methanol, Pressure: 150–160 bar, running time: 22 min.

A and D vitamins

Lipid soluble vitamins (A and D) were determined by HPLC (Agilent model 1200) according to the Ersoy and Özeren (2009) and Lu et al. (2007), respectively.^[13,17] The vitamin D fraction chromatographed on a straight phase HPLC. Collected fractions were chromatographed on a reverse-phase HPLC to quantify the vitamin D2 and vitamin D3 content based on the UV absorption according to Lu et al (2007).^[17]

Analysis of minerals

The contents of Na, K, Ca, Mg, Fe, Mn, Cu, and Zn were measured after digestion in HNO3/HClO by atomic absorption spectrophotometry (Shimadzu model AA-7000) according to Gokoglu et al.(2004)^[18] P content was measured by a spectrophotometer after color development of the samples in Barton solution.^[18]

Statistical Analyses

The data were analyzed by one-way analysis of variance ANOVA using the SPSS Statistic Program Version 19.0 (SPSS Inc., Chicago, IL, USA). The Tukey’s test was used to compare means when a significant variation was highlighted by the one-way ANOVA.

Results and Discussion

Proximate Composition

The changes in moisture, protein, fat, and ash contents of fish fillets after cooking process are shown in Table 1. The moisture content of the fish fillets varied from 63.24 to 72.33 g/100 g sample, decreasing after cooking. Raw fillets had the lowest protein content (18.59 g/100 g), while fried fillets had the highest (29.30 g/100g). Raw fish fillets contained a higher level of moisture compared to cooked samples and fried fillets had the lowest rate of moisture between cooked ones. The moisture content of fish sample found to be inversely related to the total lipid content. This can be attributed to the oil penetration on the food after water is evaporated during cooking.^[19] Generally, the protein and ash contents increased after cooking in all evaluated methods (p < 0.05). According to Ersoy and Özeren, increases in protein, fat, and ash contents could be explained by the reduction in moisture.^[13] These results are similar to findings of other researchers (Larsen et al., 2010, Gokoglu et al., 2004).^[4,18]

TABLE 1 Proximate composition (g/100 g) in rainbow trout after different ways of cooking: mean values ± SE (standard error; n = 3)

Cooking method	Moisture	Protein	Fat	Ash
Raw (control)	72.33 ± 1.911a	18.59 ± 0.771d	3.58 ± 0.087c	1.07 ± 0.04c
Baked	65.35 ± 1.435bc	26.62 ± 0.694b	4.62 ± 0.47bc	1.14 ± 0.036bc
Boiled	68.36 ± 1.275b	24.82 ± 0.12c	3.94 ± 0.686c	1.22 ± 0.283b
Microwaved	66.54 ± 0.672b	27.72 ± 0.73b	5.25 ± 0.464b	1.68 ± 0.11a
Fried	63.24 ± 0.972c	29.30 ± 0.802a	7.92 ± 1.087a	1.72 ± 0.087a

Means within the same column having different superscripts are significantly different (p < 0.05).

Fatty Acid Composition

Table 2 shows the most important fatty acids profile in rainbow trout. The most abundant fatty acid found in raw samples were palmitic acid (C16:0), stearic acid (C18:0), oleic acid (18:1n-9), DHA (C22:6), and descending order were: C18:1 > C16 > C22:6 n-3 > C18:0. Fatty acid contents present in raw fish (with 3.58% total fat) followed a relative pattern with MUFA > SFA > PUFA. This pattern was quite different from the fatty acid signatures obtained for fish in the fatty category (e.g., <4%), such as walleye pollock, smelt, canary rockfish, and pink salmon, followed a relative pattern with PUFA > MUFA > SFA.

TABLE 2 Prominent fatty acid content (g/100 g of dry weight) in rainbow trout after different ways of cooking:mean values ± SE (standard error; n = 3)

Fatty acid (%)	Raw (control)	Baked	Boiled	Microwaved	Fried
C14:0	0.21 ± 0.11a	0.2 ± 0.02a	0.15 ± 0.013b	0.16 ± 0.008b	0.21 ± 0.007a
C16:0	2.52 ± 0.174a	2.27 ± 0.126b	1.96 ± 0.025b	2.02 ± 0.209b	2.3 ± 0.533ab
C18:0	1.1 ± 0.088a	0.89 ± 0.094b	0.51 ± 0.172c	0.72 ± 0.089b	1.07 ± 0.236ab
C24:0	0.13 ± 0.004a	0.119 ± 0.1a	0.07 ± 0.06b	0.09 ± 0.011b	0.044 ± 0.03b
ΣSFA	3.96 ± 0.26a	3.46 ± 0.284ab	2.69 ± 0.262b	2.99 ± 0.276ab	3.6 ± 0.808a
C16:1	0.53 ± 0.028a	0.47 ± 0.122ab	0.33 ± 0.049b	0.32 ± 0.027b	0.43 ± 0.081ab
C18:1	4.69 ± 0.255a	4.19 ± 0.205b	3.32 ± 0.152c	3.44 ± 0.394c	4.29 ± 0.199b
ΣMUFA	5.21 ± 0.276a	4.66 ± 0.315b	3.66 ± 0.169c	3.76 ± 0.418c	4.71 ± 0.275b
C18:2n6	0.67 ± 0.027b	0.81 ± 0.16b	0.61 ± 0.105b	0.61 ± 0.049b	4.94 ± 0.196a
C20:4n6	0.18 ± 0.014b	0.21 ± 0.102ab	0.14 ± 0.017c	0.29 ± 0.028a	0.21 ± 0.082ab
Σ n6	0.85 ± 0.027b	1.02 ± 0.26b	0.76 ± 0.121b	0.9 ± 0.073b	5.14 ± 0.268a
C18:3 n3	0.63 ± 0.064a	0.56 ± 0.034ab	0.32 ± 0.094b	0.44 ± 0.176ab	0.38 ± 0.083b
C20:5 n3	0.18 ± 0.054a	0.13 ± 0.056ab	0.08 ± 0.004b	0.14 ± 0.002ab	0.15 ± 0.027ab
C22:6 n3	1.7 ± 0.107a	1.25 ± 0.19b	1.15 ± 0.024b	1.15 ± 0.101b	1.06 ± 0.112b
Σn3	2.52 ± 0.14a	1.94 ± 0.084b	1.55 ± 0.105b	1.73 ± 0.21b	1.58 ± 0.222b
ΣPUFA	3.36 ± 0.266b	2.96 ± 0.542b	2.3 ± 0.244b	2.63 ± 0.356b	6.74 ± 0.5a

Means within the same column having different superscripts are significantly different (p < 0.05).

Fried fishes had high level of linoleic acid content and other cooking methods had no significant effect on linoleic acid. Sunflower oil is the most popular oil for frying in Iran and is a good source of linoleic acid. Due to oil absorption of fish during frying, high level of linoleic acid in fried fish was attributed to fatty acid composition of sunflower oil. Boiling caused significantly reduction of the ARA content but microwave cooking lead to an increase while baking and frying had no significant effect. None of cooking methods had a significant effect on n-6, except frying which increased it. The results are in accordance with the reports of other researchers.^[7,20] Weber et al.^[20] showed that frying in soy oil caused a higher increasing of n-6 content in comparison with hydrogenated vegetable oil and canola oil due to the higher content of sum of n-6 in soy oil. α-linoleic acid (ALA; 18:3n-3), EPA: C20:5n-3, DHA: C 22:6 n-3 were the most important n-3 fatty acids that analyzed in fish. EPA and DHA found in marine sources such as fish, mussel, oyster, shrimp. (ALA, 18:3n-3) found in plant and can be converted to the EPA and DHA within human body.^[21] The conversion of ALA to EPA and DHA in human is very low, Wall et al. (2010) reported it is about 5%, but the importance of these acids is great.^[22] It was observed that DHA is the most abundant n-3 fatty acid in fish. DHA/EPA ratio in raw fish was 9.93. All cooking methods significantly reduced content of total n-3. Conversely, Unusan^[7] showed that baking and microwave cooking had no significant effect on the n-3 and n-6 fatty acids content and Weber et al.^[20] reported that frying in hydrogenated vegetable oil caused a reduction in n-3 content. Domiszewski et al.^[22] have reported that boiling (without salt) had no effect on total n-3 content and other cooking methods (boiling with salt, microwave cooking, and frying) caused significant reduction. Reduction of n-3 content during the cooking method was due to high amounts of unsaturated fatty acids and low amounts of natural antioxidants, auto-oxidizes at a much more rapid rate than other kinds of lipids.^[22] It seems that the most important factor which reduces total n-3 content during frying is oil absorption by fish, in contrast to other cooking methods (boiling, baking, and microwave cooking) on reduction of n-3 content depends on fatty acids composition and sensitivity of them to oxidation. Testi et al.^[6] defined peroxidis ability index (PI) for evaluating the relationship of fatty acid composition and its sensitivity to oxidation as follows:

A high level of PI shows a higher sensitivity of fatty acids to oxidation. The PI index value of 158.73 was obtained in this study. PI values for Weber et al.^[20] and Unusan^[7] studies were 74.55 and 133.94, respectively. In this study, the PI value was 158.73. Thus, our fish samples were more sensitive to oxidation during cooking, comparing with previous studies. Consequently, suggested reason for the mild effect of cooking on total n-3 content reported in previous studies can be related to lower amount of PI index of samples.

NQI

PUFA/SFA, PUFA/(SFA-stearic acid), and UFA/SFA

The greatest increase in PUFA /SFA, PUFA/ (SFA-stearic acid) and UFA/SF ratios was observed in fried samples, due to absorption of oil by the fish samples (Table 3). The evidences indicate that effect of SFA on raising low-density lipoprotein cholesterol (LDLC) depends on chain length, thus C12:0, C14:0, and C16:0 are most active while longer lengths have little or no effect on serum cholesterol levels. Of the SFA, only those with a chain length of 12, 14, or 16 C atoms have a cholesterol-raising effect and are thus atherogenic. SFA with a chain length of 14, 16, or 18 C atoms have been suggested to be thrombogenic.^[23] On the other hand, MUFA has benefit effect on health by raising high-density lipoprotein cholesterol (HDLC).^[8]

TABLE 3 Nutritional quality indices (NQI) in rainbow trout after different ways of cooking: mean values ± SE (standard error; n = 3)^[28,30]

Nutritional index	Raw (control)	Baked	Boiled	Microwaved	Fried
PUFA/SFA	0.84 ± 0.008b	0.84 ± 0.073b	0.85 ± 0.005b	0.87 ± 0.026b	1.9 ± 0.292a
PUFA/(SFA-stearicacid)	1.17 ± 0.014b	1.14 ± 0.048b	1.05 ± 0.015b	1.15 ± 0.01b	2.72 ± 0.435a
UFA/SFA	2.16 ± 0.032b	2.2 ± 0.009b	2.22 ± 0.106b	2.13 ± 0.023b	3.25 ± 0.533a
n-3/n-6	2.95 ± 0.122a	1.94 ± 0.231b	2.08 ± 0.18b	1.91 ± 0.146b	0.30 ± 0.026c
EPA+DHA(g/100 g of dry weight)	1.89 ± 0.088a	1.38 ± 0.075b	1.23 ± 0.024b	1.29 ± 0.111b	1.21 ± 0.14b
HH	3.14 ± 0.057b	3.08 ± 0.084b	2.82 ± 0.089b	2.93 ± 0.01b	4.65 ± 0.716a
AI	0.82 ± 0.013a	0.83 ± 0.011a	0.91 ± 0.037a	0.88 ± 0.01a	0.63 ± 0.098b
TI	0.28 ± 0.005a	0.3 ± 0.001a	0.3 ± 0.004a	0.29 ± 0.0006a	0.23 ± 0.04b
ARA/EPA	1.04 ± 0.247ac	1.58 ± 0.112abc	1.74 ± 0.125ab	2.069 ± 0.17b	1.36 ± 0.306ac
DHA/EPA	9.93 ± 2.496ab	10.56 ± 3.391ab	14.38 ± 0.419a	8.20 ± 0.604b	7.13 ± 0.545b

Means within the same column having different superscripts are significantly different (p < 0.05).

n-3/n-6

n-3/n-6 in raw rainbow trout, was 2.95 (Table 3). This ratio in different fish species was mostly found to range from 0.24 to 4.1. But in some fishes, such as cod, this ratio is up to 43 and in herring is about 21.^[24] It was suggested that a ratio of 1:1–1:5 would be healthy for human nutrition.^[25] In this study, all cooking methods significantly decreased n-3/n-6 ratio and frying had the highest reduction among other methods. Despite of decreasing this ratio during different cooking procedures, all fish samples had n-3/n-6 ratio within the World Health Organization (WHO) recommended range.^[26] Some researchers suggested that, ARA/EPA ratio is better NQI than n-3/n-6 ratio. Increasing of ARA/EPA ratio reduces nutritional value of fish oil.^[4] In this study, microwave cooking increased significantly (p < 0.05) ARA/EPA ratio and other cooking methods had no significant effect on this index.

EPA + DHA

EPA and DHA are long chain n-3 fatty acids that are precursors of hormones known as eicosanoids which play important roles in biological processes within the body.^[27] According to American Heart Association, about 1.0 g/day of EPA+DHA, or two servings of fatty fish per week reduce the risk of death from coronary heart disease.^[28] Consequently, “EPA + DHA” is one of the most important NQI. In our study, all of the cooking methods significantly (p < 0.05) reduced the “EPA+DHA” content of cooked fish. In another study, Hosseini et al.^[29] have shown in cooked kutum roach (Rutilus frisii kutum) the “EPA+DHA” content reduced during cooking procedure which supports our findings about reduction of “EPA+DHA” content.

HH

In this study, the HH acid ratio was 3.14 in raw fish. In other studies, the ratio of HH was found to range from 0.25 to 3.23 for some other species.^[6,29] Higher amounts of HH ratio are desirable, considering the specific effects of fatty acids on cholesterol metabolism, indicates that rainbow trout has high nutritional value. All of the cooking methods significantly reduced HH ratio, except frying which increased it.

AI and TI

Ulbricht and Southgate have been proposed two indices including AI and TI which might better characterize the atherogenic and thrombogenic potential of the fatty acids than other indices.^[30] AI and TI, in different sea foods, ranged from 0.33 to 2.37 and from 0.01 to 1.18, respectively.^{[5,8,29,31,32]} The lower values of both indices show the better nutritional quality of fatty acids, thus diets with low AI and TI values could reduce the potential risk of coronary heart disease (CHD). In this study, AI and TI, indices of raw fish, were 0.82 and 0.28, respectively. All of the cooking methods significantly increased AI and TI, except frying which decreased them. These results about AI and TI were similar to findings of Rosa et al.^[31] in African catfish. They found lowest AI values in fried samples and TI values of all samples were similar to the raw sample.^[31]

Vitamin Contents

Table 4 shows Vitamin composition of samples. Frying and microwave cooking had no significant effect on content of vitamin B1, while baking and boiling reduced it significantly. It seems that during boiling, leaching of vitamin B1 (thiamin) into the water caused significant reduction. Reduction of thiamin during baking was attributed to thermal breakdown of the vitamin.^[33] Lynch and Young reported that thiamin is mostly preserved after frying and microwaving due to the short transit times.^[33] Except during boiling, the niacin content was constant during different cooking methods. Vitamin A (Retinol) content of baked and boiled fish showed no significant difference to raw fish (p < 0.05), so that frying and microwave reduced them significantly. Vitamin D content of baked and microwaved fish showed no significant difference to raw fish (p < 0.05), so that frying and boiling reduced them significantly. The study of Kumar and Aalbersberg^[34] on different foods has shown that higher retention of this vitamin was observed in microwave cooked samples.

TABLE 4 Vitamin content in rainbow trout after different ways of cooking: mean values ± SE (standard error; n = 3)

Cooking method	B1 (Thiamin) mg/100 g	B3 (Niacin) mg/100 g	A (Retinol) μg/100 g	D (Calciferol) μg/100 g
Raw (control)	0.12 ± 0.02a	6.21 ± 0.48a	43.09 ± 3.78a	17.66 ± 3.56a
Baked	0.04 ± 0.01b	5.36 ± 0.48ab	37.88 ± 3.26ab	13.38 ± 0.69ab
Boiled	0.05 ± 0.04bc	4.48 ± 0.87b	37.91 ± 2.92ab	10.52 ± 0.73bc
Microwaved	0.08 ± 0.01abc	5.37 ± 0.45ab	34.42 ± 2.57b	14.19 ± 0.47ab
Fried	0.1 ± 0.01ac	5.41 ± 0.23ab	24.48 ± 3.05c	7.67 ± 1.55c

Means within the same column having different superscripts are significantly different (p < 0.05).

Mineral Contents

In Table 5, the mineral contents of raw and cooked fish are presented. According to this table, the Na content of raw fish was 501.5 mg/kg. Microwave and frying had no significant effect on Na content, while boiling decreased it and baking increased the content of Na. The results of Gokoglu et al.^[18] showed that Na content of microwave cooking rainbow trout increased, while it is decreased in boiled fish significantly. Ersoy and Özeren^[13] reported that Na content of baked, microwave cooked, and fried African cat fish significantly increased, but in grilled samples decreased.

TABLE 5 Mineral content (mg/kg) in rainbow trout after different ways of cooking: mean values ± SE (standard error; n = 3)

Mineral	Raw (control)	Baked	Boiled	Microwaved	Fried
Na	501.5 ± 61.24b	662.68 ± 46.24a	383.67 ± 77.04c	546.89 ± 39.36b	530.91 ± 70.96b
K	3754 ± 183c	4457 ± 41.7b	3719 ± 195c	4975 ± 329ab	5018 ± 40.4a
Ca	188.29 ± 25.16b	492.64 ± 22.63a	483.65 ± 69.74a	202.20 ± 36.24b	479.30 ± 48.70a
Mg	195.38 ± 7.05c	235.38 ± 13.85ab	226.15 ± 12.21b	255.38 ± 7.05a	205.33 ± 8.73c
P	2608.5 ± 87.47b	2671.69 ± 125.98b	2329.39 ± 40.04c	2405.66 ± 83.73c	3148.65 ± 56.46a
Fe	6.18 ± 0.99a	5.11 ± 0.26a	6.29 ± 1.11a	5.30 ± 1.22a	7.34 ± 1.01a
Mn	0.19 ± 0.08b	0.25 ± 0.01b	0.16 ± 0.04b	0.31 ± 0.07ab	0.42 ± 0.06a
Cu	0.35 ± 0.09a	0.17 ± 0.04b	0.30 ± 0.05ab	0.33 ± 0.03ab	0.25 ± 0.05ab
Zn	2.96 ± 0.31b	3.29 ± 0.42b	2.69 ± 0.33b	3.73 ± 0.34ab	4.79 ± 0.75a

Means within the same column having different superscripts are significantly different (p < 0.05).

According to Table 5, the K content of raw fish was found to be 3754 mg/kg. Boiling had no significant effect on K content while other cooking methods increased. Our results for microwaved fishes are similar to that of Gokoglu et al.^[18] reported in rainbow trout. According to Table 5, the Ca and Mg content of raw fish were 188.29 and 195.38 mg/kg, respectively. Except for the microwave that had no significant effect on Ca content, other cooking methods increased significantly (p > 0.05). Badiani et al.^[10] reported that the Ca content of European sea bass did not alter upon cooking. In our study, except for frying which had no significant effect on Mg content, other cooking methods increased significantly (p > 0.05). Gokoglu et al.^[18] reported that the Mg content of rainbow trout significantly decreased after frying, grilling, baking, and boiling but not for the microwave cooked samples.^[18] According to Ersoy and Özeren^[13] and Rosa et al.^[31] the Mg content of cooked African catfish increased significantly during cooking. Our findings were similar to the Ersoy and Özeren^[13] and Rosa et al.^[31]

According to Table 5, the P content of raw fish was found to be 2608.5 mg/kg. Boiling and microwave significantly decreased and frying increased the content of P, but effect of baking was insignificant. Base on Table 5 data, the Fe content of rainbow trout was 6.18 mg/kg. All of the cooking methods had no significant effect on the content of Fe (p < 0.05). Our results were similar to others^[13,18,31] demonstrated results in Table 5 shown Mn content of raw fish was found to be 0.19 mg/kg. Frying significantly increased content of Mn and other cooking methods had no significant effect. Gokoglu et al.^[13] reported that Mn content of rainbow trout significantly decreased after cooking by all cooking methods.^[18] Our findings were opposite to these results.

According to Table 5, the Cu content of raw rainbow trout was found to be 0.35 mg/kg. Baking significantly decreased content of Cu and other cooking methods had no significant effect. Ersoy and Özeren^[13] reported that none of the cooking methods had significant effect on Cu content of African catfish and according of Gokoglu et al.^[18] the Cu content of fried rainbow trout increased significantly but effect of other cooking methods was insignificant.

Zn content of raw rainbow trout was found to be 2.96 mg/kg (Table 5). Frying increased the content of Zn significantly, meanwhile, other cooking methods had no significant effect (p > 0.05). Gokoglu et al.^[18] reported that the decrease in Zn content of rainbow trout was significant for all cooking methods. Ersoy and Özeren^[13] reported that except for grilling, none of the cooking methods had significant effect on Zn content of African catfish.

Conclusions

The n-3/n-6 ratio, EPA + DHA, HH, AI, and TI are the best nutritional quality indices in fish. Boiling, frying, and microwave cooking methods changed the fatty acids composition and NQIs of rainbow trout samples, meanwhile baking had no effect on NQIs, there was an effect only on n-3/n-6 and EPA+DHA content, so was the best cooking method for rainbow trout. All of cooking methods evaluated in the present study changed the fatty acids composition and NQIs of rainbow trout samples. Frying has the greatest effect on fish composition due to oil absorption. From the point of human health, frying is not usually recommended over the other three investigated cooking methods, and in particular baking. Although frying with sunflower oil caused an increase in the HH index and a reduction in the AI and TI indices of rainbow trout; however, it may also lead to reductions in the n-3/n-6 ratio. Frying and microwave cooking had no significant effect on thiamin and niacin and frying had the greatest effect on reduction of vitamin A and D content. All of the cooking methods had no significant effect on content of Fe. Baking significantly increased content of Na, K, Ca, and Mg, decreased content of Cu, and had no significant effect on Zn and P. Boiling significantly increased content of Ca and Mg, decreased content of Na and P, and had no significant effect on content of Zn, K, Mn, and Cu. Microwave cooking significantly increased the content of K and Mg, had no effect on the content of Na, Ca, Mn, and Zn, and decreased the content of P. Frying significantly increased the content of K, Ca, Mn, Zn, P and had no effect on content of Na, Mg, and Cu. Considering overall nutritional quality indices, the vitamin and mineral content of cooked samples demonstrated baking and boiling are the best cooking method for rainbow trout, among the different cooking methods.