Lipid Quality of Anchovy (Engraulis Encrasicholus) Fillets Affected by Different Cooking Methods

Abstract

The influence of cooking methods (baking, grilling, or frying) on lipid oxidation and fatty acid composition of anchovy (Engraulis encrasicholus) fillets was evaluated. All of the treatments reduced the free fatty acid content of the fillets (P < 0.05) and minimal values were determined in grilled (3.30 g/100 g lipid) and fried (2.84 g/100 g lipid) fillets. The peroxide value and thiobarbituric acid value of the fillets were also increased by the cooking process (P < 0.05). While the maximum peroxide value content of anchovy fillets was determined in grilled fillets as 16.23 meq O₂/kg lipid (P < 0.05), the highest thiobarbituric acid values were determined as 3.63 mg MDA/kg fillet and 2.96 mg MDA/kg fillet for grilled and baked fillets, respectively. In the grilled fillets, the increase of thiobarbituric acid value exceeded the permitted limiting value. All of the cooking methods used caused modifications in the fatty acid compositions and only frying caused modification in the n-3/n-6 ratio of the fillets. The modifications in the fatty acid compositions were more prominent in the fried fillets. These results indicated that grilling may be hazardous because of the formation of lipid oxidation products as a result of thermal degradation of lipids during cooking.

Keywords:

• Cooking methods
• Lipid quality
• Lipid oxidation
• Fatty acids
• Anchovy

INTRODUCTION

Anchovy (Engraulis encrasicholus) is the most frequently consumed fish in Turkey. It comprises around 50% of the total fishery products in Turkey. Total production was reported as 385,000 tons in 2007.[1] An important portion of this production is utilized for human consumption and the remainder is used for producing fish meal and fish oil. It is usually cooked in traditional methods, such as baking, grilling, and frying, before consumption. The main purpose of heating food is to improve its digestibility and enhance its sensorial properties, such as flavor and taste. Moreover, heating is applied to inactivate enzymes and pathogenic microorganisms, reduce water activity, and increase shelf-life.[2,3]

The fish lipids containing high amounts of polyunsaturated fatty acids are known to be highly susceptible to oxidative breakdown and heat strongly catalysts the initiation of lipid peroxidation and formation of oxidation products.[3–5] The oxidation process reduces the nutritional value of lipids greatly and leads to the development of undesirable rancidity and potentially toxic reaction products.[6] Furthermore, the oxidation of fish lipids decreases the amount of polyunsaturated fatty acids having important functions on health.[7]

Many studies have shown that cooking causes lipid oxidation and affects fatty acid compositions. Al-Saghir et al.[4] showed that cooking procedures (steaming, pan-frying without oil, with olive oil, with corn oil, or with partially hydrogenated plant oil) have little impact on the lipid oxidation and fatty acids content of the salmon fillets. Tokur[5] reported that heating (frying, oven-baking, barbecuing, smoking) accelerates lipid oxidation in rainbow trout. Bakar et al.[8] indicated that microwave oven, grilling, steaming, and shallow fat-frying processes increased free fatty acids, peroxide value (POV), and thiobarbituric acid (TBA) value of Indo-Pacific king mackerel fish. In addition, saturated fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids contents were stable. Turkkan et al.[9] reported that cooking had a considerable effect on the fatty acid composition, and baking and microwave cooking were found to be the best cooking methods for healthy eating. Weber et al.[3] indicated that the cooking process increased the lipid oxidation of silver catfish but this increase did not reach the limiting value for human consumption; and grilling and canola oil frying were found to be the most compatible processes concerning oxidative stability and fatty acid profile. Reports on the impact of cooking methods, such as baking, grilling, and frying, on lipid oxidation and fatty acids content of anchovy have not been published yet. This study was undertaken to evaluate the influence of cooking methods (baking, grilling, and frying) on lipid oxidation (free fatty acid, peroxide value, and thiobarbituric acid value) and fatty acid composition in anchovy.

MATERIALS AND METHODS

Sample Preparation and Cooking

Anchovies (Engraulis encrasicholus) (mean weight 8.62 ± 0.92 g, mean length 11.03 ± 0.47 cm, dry matter 31.70 ± 0.98 g/100 g, protein 18.61 ± 0.65 g/100 g, lipid 10.87 ± 0.43 g/100 g, and ash 1.23 ± 0.02 g/100 g) caught from the Black Sea, Turkey in December 2007 were used in this research. Fishes were purchased from Samsun Fish Bazaar on the day of catch, boxed in ice, and quickly transported to the laboratory. They were beheaded, eviscerated, washed with tap water, and then divided into four portions (300 g each). One portion was analyzed as a raw sample, the second portion was baked in a 170°C electrical oven (Mini Oven MF 6, 1200 W, 220 V, 50 Hz, Arcelik A.S., Istanbul, Turkey) for 30 min, the third portion was grilled by using an electrical grill (Mini Oven MF 6, 2400 W, 220 V, 50 Hz, Arcelik A.S., Istanbul, Turkey) (the distance between heat source and the samples was 5 cm) for a total of 10 (6 min on one side and 4 min on the other side) and the fourth portion was fried in a Teflon pan containing 20 mL of corn oil at 150°C for a total of 10 min (6 min on one side and 4 min on the other side). Corn oil was used in frying because it is one of the commonly used oils in Turkey. Three replicates were made for each cooking treatment.

Analysis

Prior to analysis, the raw and cooked samples were hand-deboned, ground in a glass mortar to ensure its homogeneity, and then put into glass jars. The homogenized samples were analyzed in triplicate and the result of each replication was given as the mean value of two parallels. All reagents were of analytical grade, and deionized water was used throughout.

Dry matter, protein (N × 6.25) and ash contents of raw anchovies were determined according to AOAC[10] procedure. Total lipid was extracted by the method of Bligh and Dyer.[11] Free fatty acids (FFA) and POV were determined according to AOAC[10] procedure in the Bligh and Dyer extracts. TBA value was determined according to the water vapor distillation method of Tarladgis et al.[12] Fatty acid composition in the Bligh and Dyer extracts was determined after methylation[13] using a Shimadzu (Model GC-2010; Shimadzu Corporation, Kyoto, Japan) gas chromatograph with an Optima-FFAP column (60 m × 0.25 mm I.D., 0.25 μm) (Macherey-Nagel, Düren, Germany). The temperature of the injector port and detector was held at 270 and 280°C, respectively. The injected volume was 1.0 μL. The carrier gas was helium at a pressure of 150 kPa. The split used was 100:1. The temperature of the column was held at 140°C for 2 min, raised to 170°C at 10°C/min and 215°C at 10°C/min, held at 215°C for 5 min, raised again to 230°C at 5°C/min, held at 230°C for 5 min, and finally raised to 240°C at 10°C/min, held at 240°C for 35 min. Fatty acids were identified by comparison of their retention times with those of authentic standards (Supelco 37 Components FAME Mixture, Cat. No. 18919-1AMP; Supelco, Bellefonte, PA, USA) and reported as the percentage of total fatty acids determined. The data obtained from three replications were analyzed by ANOVA using the SPSS statistical package program, and differences among the means were compared using the Duncan's Multiple Range test.[14] A significance level of 0.05 was chosen.

RESULTS AND DISCUSSION

In the experiment, FFA, POV, and TBA values were selected in order to characterize lipid oxidation in cooking anchovy fillets. By using a cooking process, a decrease was determined in FFA content of anchovy fillets (P < 0.05) and minimal values were determined in grilled (3.30 g/100 g lipid) and fried (2.84 g/100 g lipid) fillets (Fig. 1). The decline in FFA contents of cooked fish fillets is the result of thermal effect, because the enzymes, such as lipases and phospholipases leading to FFA products, are denatured during cooking.[3,5] In addition, this decline can be an outcome of the loss of volatile FFA probably occurred during heating.[3] Similar results were determined by different researchers.[3 ^–5] The determination of the lowest FFA content in fried anchovy fillets can be explained by dilution in the bath oil or FFA volatilization. Weber et al.[3] reported that the free fatty acid content in silver catfish fillets decreased with frying, by dilution in soybean oil, canola oil, or hydrogenated vegetable oil.

Figure 1 FFA contents of raw, baked, grilled, and fried anchovy fillets. Results are mean ± SD of three replicates. Bars that have no common letters are significantly different (P < 0.05).

The POV is a useful method to determine the primary lipid oxidation products. According to Fig. 2, POV contents of anchovy fillets were increased by the cooking process and maximum value was determined in grilled fillets as 16.23 meq O₂/kg lipid (P < 0.05). Baked and fried fillets had no significant differences in POV contents (P > 0.05). The increase in POV contents of cooked samples proved that lipid oxidation was a result of heat. Accordingly, fish lipids contain a great deal of polyunsaturated fatty acids, which are sensitive to oxidation and heat catalyzes the lipid oxidation strongly.[3,5] Different POV levels related to cooking methods might depend on cooking temperature and time. Moreover, the unstability of peroxides, which means that they exist only transiently, will be rapidly decomposed and can be a cause of this difference. Similar to this study results, Tokur[5] and Bakar et al.[8] indicated that the cooking process increased POV content of fish. The maximum acceptability limit of POV in fatty foods was reported as 25 meq O₂/kg lipid by Evranuz.[15] Hence, all samples evaluated were suitable for consumption.

Figure 2 POV of raw, baked, grilled, and fried anchovy fillets. Results are mean ± SD of three replicates. Bars that have no common letters are significantly different (P < 0.05).

TBA values represent the content of secondary lipid oxidation products, mainly aldehydes, which contribute to off-flavors in oxidized meat and fish. Cooking process had an important effect on the TBA value of anchovy fillets and TBA values were increased by cooking process (P < 0.05) (Fig. 3). While the highest TBA value was determined to be 3.63 mg MDA/kg sample and 2.96 mg MDA/kg sample for grilled and baked fillets, respectively, the lowest value was determined as 2.46 mg MDA/kg sample for fried fillets. However, differences in TBA values between fried and baked fillets were not significant (P > 0.05). These results suggested that cooking contributed to increase the levels of secondary lipid oxidation products. In agreement with the results of this study, Tokur[5] reported that TBA values of grilled and baked rainbow trout fish were higher than the fried ones. Except for grilling, similar results were determined by Weber et al.[3] According to Al-Kahtani et al.,[16] meat products can be considered in a good conservation state, concerning oxidative changes, when they have less than 3 mg MDA/kg sample. As the result, TBA values of grilled anchovy fillets were reported to be higher than that value. Such a situation indicates that the grilling process can be risky for the lipid oxidation.

Figure 3 TBA values of raw, baked, grilled, and fried anchovy fillets. Results are mean ± SD of three replicates. Bars that have no common letters are significantly different (P < 0.05).

Because of the high content of n-3 polyunsaturated fatty acids (PUFAs), fish lipids reduce the amount of cholesterol and triglycerides and the level of low density lipoprotein cholesterol in the blood serum and reduce the blood pressure and the tendency of the blood to form clots. As the result of these beneficial effects, fish lipids reduce the risk of atherosclerosis, cardiovascular disease, and arterial hypertension. Unfortunately, fish fats are the most sensitive of the nutritional fats to oxidation. Therefore, cooking and thermal processing carry with them a risk that the products of the oxidation of such beneficial fatty acids will also be consumed.[9] Lipid contents of raw and cooked anchovy fillets are given in Table 1. As can be seen from the table, the total lipid content, which was 10.87 g/100 g fillet in a raw sample, increased to 16.34 g/100 g fillet in baked samples and 14.99 g/100 g fillet in grilled samples. The increases in lipid contents of baked and grilled samples were most probably due to the loss of water during cooking. Lipid content of fried fillets (24.93 g/100 g fillet) increased significantly (P < 0.05) because of oil absorption from the cooking medium.

Table 1 Fatty acid composition of corn oil and raw, baked, grilled, and fried anchovy fillets (g/100 g total fatty acids)*

		Cooking methods
Fatty acids	Corn oil	Raw	Baked	Grilled	Fried
C12:0	0.04	0.08±0.01 b	0.09±0.01 a	0.10±0.01 a	0.06±0.01 c
C13:0	0.04	0.07±0.01	0.06±0.01	0.08±0.02	0.05±0.01
C14:0	0.17	6.92±0.08 a	7.06±0.27 a	7.11±0.39 a	5.09±0.19 b
C15:0	—	0.89±0.02 a	0.82±0.03 b	0.88±0.03 a	0.64±0.01 c
C16:0	10.84	21.48±0.18 b	21.85±0.74 ab	22.72±0.51 a	19.25±0.19 c
C17:0	0.08	1.01±0.01 b	1.11±0.06 a	1.05±0.01 ab	0.76±0.06 c
C18:0	1.81	4.31±0.01 a	4.46±0.14 a	4.48±0.08 a	3.77±0.04 b
C20:0	0.36	1.27±0.01 b	1.46±0.06 a	1.21±0.02 b	1.06±0.06 c
C21:0	—	0.07±0.01	0.04±0.02	0.06±0.01	0.05±0.01
C22:0	—	0.25±0.01 a	0.16±0.02 b	0.27±0.04 a	0.17±0.03 b
C24:0	—	0.43±0.02 bc	0.47±0.08 ab	0.56±0.04 a	0.34±0.02 c
ΣSFA	13.34	36.78±0.25 b	37.58±1.22 ab	38.52±0.87 a	31.24±0.49 c
C14:1	—	0.28±0.01 ab	0.27±0.01 b	0.29±0.01 a	0.19±0.01 c
C15:1	—	0.15±0.01 a	0.13±0.01 a	0.14±0.01 a	0.10±0.01 b
C16:1	0.11	6.44±0.10 b	6.95±0.20 a	6.74±0.21 ab	5.00±0.33 c
C17:1	—	0.30±0.01 b	0.36±0.02 a	0.30±0.02 b	0.24±0.02 c
C18:1n-9	27.93	19.88±0.04 b	19.44±0.53 b	19.70±0.14 b	21.32±0.04 a
C20:1n-9	0.22	0.48±0.01 ab	0.42±0.04 bc	0.51±0.03 a	0.40±0.03 c
C22:1n-9	—	0.08±0.01 ab	0.05±0.02 b	0.10±0.01 a	0.06±0.01 b
ΣMUFA	28.26	27.61±0.14	27.62±0.62	27.78±0.35	27.31±0.27
C18:2n-6	56.97	1.92±0.02 b	2.06±0.04 b	1.94±0.01 b	14.50±0.69 a
C18:3n-6	0.79	0.13±0.01 b	0.15±0.01 a	0.13±0.01 b	0.10±0.01 c
C18:3n-3	—	0.88±0.01 a	0.69±0.04 bc	0.67±0.02 c	0.73±0.01 b
C20:2	—	0.17±0.01 a	0.16±0.01 ab	0.18±0.01 a	0.14±0.01 b
C20:4n-6	—	0.18±0.01 a	0.18±0.02 a	0.16±0.01 ab	0.14±0.01 b
C20:3n-3	—	0.68±0.01 a	0.68±0.01 a	0.65±0.01 b	0.53±0.01 c
C20:5n-3	0.12	8.56±0.12 ab	8.98±0.35 a	8.20±0.19 b	6.69±0.24 c
C22:6n-3	—	12.03±0.23 a	12.49±0.56 a	11.37±0.11 b	9.98±0.02 c
ΣPUFA	57.88	24.55±0.20 b	25.39±0.86 b	23.30±0.34 c	32.81±0.44 a
Others	0.52	11.06±0.07	9.41±1.86	10.40±1.57	8.64±0.32
n-3/n-6	0.01	9.93±0.29 a	9.57±0.51 a	9.36±0.05 a	1.21±0.08 b
Total lipid (%)	—	10.87±0.43 d	16.34±0.79 b	14.99±0.68 c	24.93±0.81 a
*Values are mean ± SD of three replicates.
Means in a line with different letters are significantly different (P < 0.05).
SFA: saturated fatty acids; MUFA: monounsaturated fatty acids; PUFA: polyunsaturated fatty acids.

The fatty acid composition of raw and cooked anchovy fillets are given in Table 1. In raw anchovies, the predominant saturated fatty acids (SFAs) were myristic (14:0), palmitic (16:0), and stearic acid (18:0), the most abundant monounsaturated fatty acids (MUFAs) were 16:1 and 18:1n-9, and the predominant polyunsaturated fatty acids (PUFAs) were 20:5n-3 and 22:6n-3. Similar results were obtained by Guner et al.[17] in raw anchovies.

As can be seen in Table 1, cooking process had a significant effect on fatty acid content except for C13:0, C21:0, and ΣMUFA (P < 0.05). Baking had a minimal effect on fatty acid contents of anchovy fillets and this effect caused increases in some samples (C12:0, C17:0, C20:0, C16:1, C17:1, C18:3n-6), whereas led others to decrease (C15:0, C22:0, C18:3n-3). The minimal changes observed must be primarily a consequence of the low fat loss produced by this process. An oily fish like anchovy accumulates fat mainly as triglycerides in red and white muscle; thus, fatty acid changes have to be a consequence of the action of heat, mainly, on these muscle triglycerides, favoring the loss of the more accessible fatty acids.[2] These changes were similar to those found by Garcia-Arias et al.[2] in oven baked sardine and by Weber et al.[3] in oven-baked silver catfish fillets. Also, there have been changes in fatty acid composition of grilled anchovy fillets (Table 1) and generally while SFAs content increased, PUFAs content decreased. The increase in SFAs amount and the decrease in PUFAs amounts could be attributable to the oxidation of PUFAs during the cooking process.[18] These results show parallelism with POV and TBA values. Similar changes were identified by Bakar et al.[8] for grilled Indo-Pacific king mackerel fish. The most important changes during cooking were seen in fatty acid composition of fried anchovy fillets and 20 of 26 total examined fatty acids decreased in quantity whereas 2 of them (C18:1n-9, C18:2n-6) increased in quantity. This increase was especially clear in C18:2n-6 and raw samples' value of 1.92% climbed up to 14.50% by frying. This situation was probably caused by the fatty acid composition of frying oil (corn oil), since C18:2n-6 content of corn oil was found to be 56.97% (Table 1). Similar results were determined by Al-Saghir et al.[4] for farmed salmon fish cooked with corn oil. Moreover, lipid oxidation could also be a cause of changes in fatty acids during cooking. An n-3/n-6 ratio is an important nutritional index of fatty acid alteration during cooking.[3,4] There were no significant changes in this ratio for baked and grilled fillets as compared to control samples (P > 0.05), while decreased to 1.21 in fried fillets (P < 0.05). This was expected, due to the absorption of the n-6 fatty acids from the corn oil and is in agreement with the lower n-3/n-6 ratio of this oil.

CONCLUSION

All of the traditional cooking methods evaluated in this study changed FFA, POV, and TBA values, and fatty acid profile of the anchovy fillets. Changes in FFA were more prominent in grilled and fried fillets. Cooking increased POV and TBA values of anchovy fillets and this increase did not pose any risk with respect to POV, while it was above the limit value with respect to TBA value in grilled fillets. Fatty acid composition anchovy fillets were affected by cooking methods and this effect was especially clear in fried fillets due to the absorption of frying oil. These results show that grilling of anchovy fillets might be risky with respect to lipid oxidation.