Comparative study on the effect of different cooking methods on physicochemical properties and color characteristics of golden grey mullet (Liza aurata) fillets

M. Ghelichpour, B. Shabanpour and S. M. Hoseini

Faculty of Fisheries and Environment, Department of Fisheries, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran. E-mail:ml.ghelichpour@gmail.com

Abstract — The effect of different cooking methods (grilling, frying, steaming and microwave cooking) was investigated on proximate composition, cooking loss (CL), color and protein solubility (PS) of golden grey mullet (Liza aurata) fillet. Fillet's protein and ash contents significantly increased after cooking with all methods, compared to raw fillet. Higher protein content was related to microwave method, followed by fried, grilled and steamed, respectively. All treatments showed increase in protein and ash contents. Decrease in moisture and increase in fat contents was the most prominent changes in proximate composition. After cooking, PS of fillets decreased with decreasing pH with the minimum solubility was observed at a pH range 5-6 indicating the isoelectric point (PI) of fillets. Microwaved sample showed the lowest solubility compared to other cooking methods. All cooking processes reduced L* value and increased b* value. Fried and microwaved samples had significant differences in a* value compared to other treatments.

Keywords — Golden Grey Mullet; Cooking Methods; Proximate Composition; Protein Solubility; Cooking Loss; Color Determination.

I. INTRODUCTION

About three millions juveniles of Black sea mullets (Mugil cephalus, L. aurata and Liza saliens) were successfully introduced to Caspian Sea over 1930 to 1934; however, only the two last species have successfully acclimated, adapted and propagated in the Caspian Sea (Oren, 1981; Kosarev and Yablonskata, 1994). Nowadays, they provided one of the important fishing resources, especially in the southern Caspian Sea. Among the large groups of fish species which have been consumed in Iran, golden grey mullet is one of the most important fish in southern Caspian Sea which have extensively been consuming with occupant of Gilan and Mazandaran regions (Abdoli and Naderi, 2009).

Fish is a major source of animal protein that has been consumed by humans (Hall, 1992). The high protein levels, with good digestibility and also low fat content are advantages of seafood (Pigott and Tucker, 1990). Interest in the health benefits of seafood such as existence of omega-3 fatty acids, decreased risk of prostate cancer (Terry et al., 2001) and Alzheimer diseases (Huang et al., 2005) is rising in the recent decades. Fish is usually treated by various processes before consumption. Heating (grilling, frying, steaming and microwave cooking) is applied to food to enhance its flavor and taste, inactivated pathogenic microorganisms and increase shelf life (Bognar, 1998). On the other hand, the use of the microwave oven for cooking has increased greatly during recent decades (Arias et al., 2003). The nutritive values of fish might be affected by processing or cooking methods. The effects of different cooking methods on nutritive values such as proximate composition of different fish have been previously studied (Weber et al., 2008; Ersoy and Ozeren, 2009). Fish are exposed to different conditions during cooking process which may, in turn, result in changes in their carotenoid content and may lead to color modifications (Bhattacharya et al., 1994). Solubility of protein is a paramount functional property of dehydrated products. The method of processing affects the solubility of protein especially if they are exposed to heat (Kilara and Harwalkar, 1996). Although the pH-solubility relationship for proteins has been studied in mugilidae (Mohan et al., 2007) and some reports are available on the composition of the fish, there is lack of information on the chemical composition, fillet color and nutritional profile of cooked L. aurata in southern Caspian Sea.

This study was, therefore, conducted to determine the influence of four cooking methods (grilling, frying, steaming and microwave cooking) on physicochemical properties of L. aurata fillets.

II. MATERIALS AND METHODS

A. Sample preparation

Three golden grey mullets (L. aurata) were purchased from Bandar-Torkman fish market, Golestan, Iran, during the autumn 2009. They were about 500g in weight. The fish were transported to the laboratory for sample preparation and analysis. On arrival at the laboratory the fish were washed with tap water several times to remove adhering blood and slime, they were then eviscerated, beheaded and their backbone, skin, tail and fins were removed yielding two fillets. Finally, fillet of each fish were divided in to 5 pieces to obtain total of 15 pieces. These 15 pieces were divided to 5 group (each contain 3 pieces) and each group was considered as one of the control, grilling, frying, steaming and microwave treatments. Each of the 3 pieces of the groups was analyzed separately as one replication of the group.

B. Cooking methods

Common ways of cooking were used. The samples were cooked by frying, grilling, steaming and microwave cooking. The fish fillets were fried in frying vegetable oil (Bahar frying oil, Iran). The temperature of oil during the frying process was 150°C over a 10 min period in an automatic fryer (ADR2, Moulinex, Portugal). The grilling process was carried out with an electrically operated stainless steel grill (Bq100, Delongi, Germany) at 50-60 Hz frequency and maximum temperature level. The fillets were grilled for 20 min (10 min on each side), after attaining the desired temperature. Steaming was performed in a domestic steamer at approximately 98C for 20 min (Tefal Steam Cuisine, Berkshire, UK). Microwave cooking was carried out in the microwave oven (Panasonic, NN-CD987W) at 2450 MH for 13 min.

Samples of raw or cooked fish fillets were immediately homogenized using a kitchen blender and analyzed to determine proximate composition, CL, PS and color measurement.

C. Analyses

Proximate composition

Proximate composition analyses of cooked and uncooked fish fillets were performed in triplicate for moisture, protein, lipid and ash contents. The moisture was determined by oven-drying at 100-105°C until a constant weight was obtained (AOAC, 1993). The crude protein content was calculated by the Kjeldahl procedure using conversion factor of 6.25 (AOAC, 1993). Fat content was determined by the method described by AOAC (1990) using the Soxhlet System (416 SE, Gerhardt, Germany) with petroleum ether as the solvent. Ash was gravimetrically determined using a muffle furnace by heating at 500°C to constant weight (AOAC, 1993).

Cooking loss

CL was measured according to the method of Niamnuy et al. (2008) and was calculated from the differences in the mass of golden grey mullet fillets before and after each cooking methods (frying, grilling, steaming and microwave cooking). Calculation of CL is below:

(1)

Protein solubility

PS was determined according to the method of Lee et al. (1992), with some modification. 2g homogenized fillet sample was added to 40 ml of distilled water and the mixture was stirred using a magnetic stirrer at speed 2 at room temperature (RHB2, IKA, Germany). The pH of slurry was adjusted to desired pH (1-12) by the addition of 1 or 0.1 N HCl and 1 or 0.1 N NaOH to desired acidic and alkaline pH values, respectively. The volume was adjusted to 50 ml with distilled water. It was shaken for 1h at room temperature (about 27°C), centrifuged at 5000 rpm for 20 min at 4°C and the pH of the supernatant noted. Protein content of supernatants was determined using the Kjeldahl method. Percentages of soluble protein in the supernatant compared to the total protein were calculated at each pH value as follows:

(2)

The PI was estimated as the pH value corresponding to the minimum solubility percentage. All treatments were conducted on triplicate.

Color determination

The color of fillets was measured using a colorimeter (CR200, Minolta Camera Ltd, Osaka, Japan). The reading was performed at the surface of the samples, for L* value (lightness), a* value (redness), and b* value (yellowness). The equipment was standardized with white colored calibration tile.

Statistical analysis

The effects of cooking and pH and their interactions were evaluated using two way ANOVA followed by LSMeans test. The effect cooking proximate composition and color of the fillet was analyzed by one-way ANOVA. Analyses were performed using SAS learning edition 2.0 software. Differences were considered to be significant when P < 0.05. Data were presented as mean±STD.

III. RESULTS

A. Proximate composition

The changes in moisture, protein, fat and ash content as a result of cooking process has been shown in Table 1.

Table 1. Proximate composition of raw and cooked golden gray mullet fillet

Values are shown as mean ± standard deviation.
Within the column values with different letters are significantly different (P<0.05).

The proximate composition was significantly affected by all the cooking methods (P < 0.05). The moisture content of the fish fillets significantly decreased after cooking, where, the lowest value was related to fried, followed by microwaved, grilled and steamed samples, respectively (Table 1). Protein content increased after cooking in all methods; and higher value was related to microwaved, fried, grilled and steamed samples (Table 1). Ash levels increased, also, after cooking, where higher values were related to fried, grilled followed by steamed and microwaved, however, values of the two last group were not significantly differ (Table 1). While there was no significant difference between fat content in raw and steamed group, grilled and fried groups have higher fat content (Table 1).

B. Cooking loss

CL in golden grey mullet muscle was measured after each cooking treatment. The CL was different depending on the cooking process. Microwaved sample and steamed fillet showed significantly the highest (66.55%) and lowest (32.61%) CL, respectively. Grilled and fried fillets had whatever similar CL, 43.07% and 47.74%, respectively.

C. Protein solubility

PS was significantly affected by cooking methods, pH as well as their interaction (P < 0.0001) (Fig. 1). PS of all samples versus pH showed a specific behavior as the most solubility observed at acidic and alkaline and the least at PI. Heat processing affected the solubility of proteins. In general, raw samples possess higher PS than that of cooked ones. As it is seen in figure 1, the lowest PS exhibited in microwaved fillet, while the steamed samples showed maximum level of PS between the treatments.

Fig. 1: Changes in protein solubility in golden grey mullet to different cooking methods. P_(pH) <0.0001; P_(cooking) <0.0001; P_(pH*cooking) <0.0001.

Minimum PS (PI), in raw and cooked samples was occurred at pH 5-6. The PS of raw and cooked fillets showed decreasing pattern with increasing pH to PI then increased again to higher pH (Fig. 1). The protein were least soluble in raw, grilled, fried, steamed and microwaved fillets (16.5%, 9.72%, 10.09%, 11.09% and 4.06%, respectively) at pH 6, 5, 6, 5 and 5, respectively (Fig. 1). Effect of five cooking methods and 12 pH treatments as well as their interaction has been shown in Figure 1 (P < 0.0001).

D. Color determination

The influences of different cooking methods on the values of lightness (L*), redness (a*) and yellowness (b*) values are shown in Table 2. There was significant changes in L*, a* and b* values after cooking treatments compared to control (P < 0.05). The b* values increased in all cooked samples, while L* values decreased. Steamed and fried fillets had the minimum and maximum a* values respectively. Steam cooking method had higher L* and lower a* and b* values compared to the other cooking methods.

Table 2. Effect of different cooking methods on color properties of Grey golden mullet fillet.
Values are shown as mean standard deviation.
Within the column values with different letters are significantly different (P<0.05).

IV. DISCUSSIONS

A. Proximate composition

The decrease in the moisture content has been described as the most prominent change that makes the protein, fat and ash contents increase significantly in cooked fish fillets (García-Arias et al., 2003). Accordingly, the increase in ash, protein and fat content found in cooked golden grey mullet fillets is explained by the reduction in moisture. The fat content of fried golden mullet respect to other cooking methods was significantly higher than that of the raw fillets suggesting that the increase in fat content of the fried fish fillets is also related to oil absorption during the cooking process. Fat increase can be due to the oil penetration in to the food after water is partially lost by evaporation (Saguy and Dana, 2003). Similar results have been reported for African catfish fried in sunflower oil (Rosa et al., 2007) and fried rainbow trout in sunflower oil (Gokoglu et al., 2004).

B. Cooking loss

Aggregation and denaturation of protein in golden grey mullet muscle that is induced by heating leads to the loss in water holding capacity of proteins. As a result, drastic CL was observed. Similarly, Niamnuy et al. (2008) reported occurrence of drip loss in shrimp muscle throughout the boiling in salt solution.

C. Protein solubility

Signs of denaturation of protein are reflected in changes in solubility levels. Method of processing affects the solubility of protein especially if they are exposed to heat (Kilara and Harwalkar, 1996). As it has been shown in Figure 1, steamed samples which have received minimum heat during cooking, showed higher solubility compare to three other processing methods. It is concluded that this method has less negative effect on the nutritional parameters. Solubility of grilled and fried samples was whatever near to each other. Fried samples had higher solubility at alkalic values. Reduction in PS due to heat processing has been reported (Romero et al., 2009; Bourtoom et al., 2009). In PI and in neutral condition, electrostatic absorbance takes place between two neighbor ions which lead to orientation and also aggregation of molecules. On the other hand, hydrogen-bonding results in more adjacency of molecules to each other. Nevertheless, because of numerous hydrogen-bonding and electrostatic linkages at PI, there could be the most stability and minimum solubility. Also, Damodaran (1997) states that the minimum solubility occurs at about the PI of protein, and that the majority of food proteins are acidic proteins. Data of present study on PI were in agreement with Fatemi (2000). At pH above or below isoelectric pH, the proteins become negatively or positively charged depending on the pH, resulting in electrostatic repulsions between molecules and hydration charged residues, contributing to the solubility of proteins. Strong electrostatic repulsions among proteins molecules and the increased protein-solvent interaction at this pH could have contributed to this increased solubility (Mohan et al., 2007).

D. Color determination

Color is an important indicator of food quality. The consumer associates food color with good processing and safety. Although seafood color is a parameter normally not used by many consumers in their buying decision, it is very important when seafood is consumed. The results of changing after cooking treatment are in agreement with Moradi et al. (2009).

The mechanisms of those changes are not entirely clear. However, Sikorski et al. (1994) stated that denaturation and oxidation of proteins, as well as the formation of colored compounds with involvement of H2S released from amino acids and in Maillardtype reactions could be the reasons of color changes in cooked samples. Also in another study Bligh et al. (1989) reported that carbonyl compounds from lipid oxidation in the presence of heat can react with the e-amino group of lysine to complete the Millard reaction, in combination with oxypolymerisation of unsaturated fish oils was responsible for darkening of dehydrated fish.

V. CONCLUSIONS

Cooking of golden grey mullet fillets caused significant changes in proximate composition, PS, color and CL. Steaming of fillets had minimum effect on PS of samples and because in food systems, existence of protein in soluble form is necessary, thus indicates on a positive effect of using this method. In general it can be said that although steamed samples didn't have a good taste acceptability compared to other methods, they showed the least undesirable effects of heating such as protein denaturation and resulted in maintaining nutritional value of fish, can be selected as the best.

ACKNOWLEDGEMENTS
The authors wish to thank staff of chemistry laboratory, Gorgan University of Agricultural Sciences and Natural Resources for their valuable help.

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Received: January 21, 2011.
Accepted: July 7, 2011.
Recommended by Subject Editor Ricardo Gómez.