Effects of scale and skin on chemical and sensory quality of marinated sea bass filets ( Dicentrarchus labrax , L. 1758) in sunflower oil during storage at 4ºC

In this study, the effects of descaling and skin removal on chemical changes and sensory attributes of marinated sea bass filets ( Dicentrarchus labrax ) in sunflower oil stored at 4ºC were investigated. In terms of overall acceptability, although skinless sea bass fillets reached consumable limit values at the 70th day, scaly and descaled samples did not reach limits even at the end of the 90-day storage. During 90-day storage, (Total Volatile Basic Nitrogen) TVB-N and (Trimethylamine–nitrogen) TMA-N values of sea bass fillets were found to be below consumable limits. Thiobarbituric acid (TBA) value reached the consumable limit of 8 mg malonaldehyde/kg at the 56th day for skinned fillets and at the 70th day for descaled fillets. Scaly fillets did not reach the TBA limit during 90-day storage time. According to obtained results, scaly and descaling sea bass fillets were found to be more appropriate for marination in terms of brightness, juiciness, tenderness, acid and salt transition and hygiene. It was detected that descaling has some disadvantages on texture (acid and salt transition) and hygiene of fish. Comparing the scaly and descaled fillets, it was found that scales effect the brightness of brine solution and also cause undesirable colour changes as the scales stick to the fish skin.


Introduction
Marinating process is one of the oldest methods of conservation of fish and it is popular in Europe. Fatty fish like sardines, mackerel, herring, and anchovies, as well as several kinds of crustaceans and bivalves are usually used. The term ''marinades'' or ''marinated fish'' is used to define fish products which consists of fresh, frozen or salted fish or portions of fish processed by treatment with an edible organic acid, usually acetic acid, and salt and put into brines, sauces or oil (Duyar and Eke, 2008). Due to the increasing consumer demand for fresh refrigerated foods with prolonged shelf-life, many researches have focused on preservation techniques to control bacterial growth for safety purposes or for extending the shelf-life of the food (Sallam, 2007). Marination, a food-preservation technique, is based on treatment of muscle with solutions containing salt, spices, lemon juice, etc. and provides high sensory acceptability to a variety of meat products (Ozogul et al., 2009).
Marination is also used to tenderise or to change taste, textural and structural properties of raw material. Marinades stored at cooler temperatures (4-6°C) keep a long time such as four months. Marinades are semi-preserves; the preserving principal is the combination of acetic acid and salt. (Gokoglu et al., 2004;Sallam et al., 2007;Duyar and Eke, 2009). Salt and acetic acid uptake depends on many factors including species, muscle type, fish size, fillet thickness, weight, composition (lipid content and distribution), physiological state, salting method, brine concentration, duration of salting step, and fish-tosalt ratio, ambient temperature, freezing and thawing (Gallart-Jornet et al., 2007). The marinating process has many positive effects on the palatability and shelf-life of meat and seafood products. The main aims of marinating are to tenderize and enhance the flavour, safety and shelflife of meat products owing to inhibition of microbial growth (Bjö rkroth, 2005;Ozogul et al., 2008).
Anchovies are generally used for marination fish production; however, use of some reared fish, like gilthead seabream (Sparus aurata) and sea bass (Dicentrarchus labrax) has been recently proposed. Recently, in Italy, marination technology is also spreading to some reared fish, like gilthead seabream and sea bass (Giuffrida et al., 2007). Aquaculture of sea bass has major economic importance in Turkey and many European countries. Sea bass is one of the most widely farmed species with a total production of 105.900 tons in Europe in 2008 (FAO, 2008).
This study was aimed to determine the effects of scale, descaling and skinning on chemical changes and sensory attributes of marinated sea bass (Dicentrarchus labrax) in sunflower oil during storage at 4ºC.

Materials and Methods Materials
A total of 50 kg, aquacultured sea bass samples (Dicentrarchus labrax) in the Eagean Sea coast were harvested by Kılıc Fisheries Holding (Mugla, TURKEY) and ice-chilled within 18h. Fish sample used in this study were degutted manually by the workers in the fish processing plant and were transported to the laboratory in polystyrene boxes with flaked ice within 90 minutes of purchasing. The mean weight of fish was 330±10 g.

Methods
Samples were divided into three lots. First lot of 18 kg sea bass was just filleted with no other treatment. Second lot of 18 kg was first descaled then filleted. Remaining lot of 14 kg was skinned and then filleted. Then, all three lots of fillets were dipped in an ice-water solution (1:4 v/v) for 60 minutes to provide physical cleaning and temperature stability of the fish samples. Then marinating solution composed of 2.5% acetic acid and 11% sodium chloride was added to fillets put in 5000 mL glass jars with the lids. Jars filled with sea bass fillets and marinating solution at a weight ratio of 1.5:1 were stored in a refrigerator at +4 (±1)°C for marinating. Marinating and brining processes were completed after 72 h. Fish samples were removed from the solutions and put into glass jars, filled with sunflower oil. During storage in oil, samples from all three groups were analysed at the first day 0 and the following days after 7, 14, 21, 28, 42, 56, 70, 90 days of the marinating.

Sensory analysis
Sensory analysis of marinated fillets was evaluated by 6 experienced panellists and the results were given as described by Sallam et al. (2007). Three representative fish samples were individually presented to each panellist. The judges were not informed about the experimental approach. Panellists were asked to evaluate the overall acceptability with regard to fish-like appearance, colour, texture, odour, rancidity, juiciness, tenderness, sour taste, salty taste and flavour-aftertaste. An eight-point hedonic scoring scale, with 8 = extremely intense/juicy/tender, 7 = very intense/juicy/tender, 6 = moderately intense/juicy/tender, 5 = slightly intense/juicy/ tender, 4 = slightly bland/dry/tough, 3 = moderately bland/dry/tough, 2 = very bland/dry/tough, 1 = extremely bland/dry/tough, was employed for odour and flavour intensity, juiciness, and tenderness, respectively, while a nine-point hedonic scale, ranging from extremely acceptable (9) to extremely unacceptable (1) was utilized for evaluation of the appearance. Sea bass samples receiving overall scores of more than 4 were considered acceptable, while a score between 3 and 4 was considered the borderline of acceptability. The parameters were tested throughout the storage period of 90 days.

Determination of quality changes
pH, Total Volatile Basic Nitrogen (TVB-N), Trimethylamine Nitrogen (TMA-N) and Tiobarbituric Acid (TBA) analyses were performed three times for each group of samples. The pH value was determined by dipping a pH electrode into homogenates of filleted muscle in distilledwater (1/1) (Manthey et al., 1988). All measurements were performed at room temperature using pH-meter (WTW Inolab, Weilheim, Germany). TVB-N was used as indices for the quality of fresh fillets of sea bass and determined according to the Antonocopoulus (1973). Volatile bases were seperated by steam distillation of homogenized samples, collected in 0.1 N HCl and titrated back with 0.1 N NaOH. TMA-N, which is produced by the decomposition of TMAO by bacteria or enzymatic reaction, is a spoilage indicator for fresh fish. TMA-N amount of the homogenized samples were determined according to the Schormüler (1968). Samples were extracted with trichloracetic acid. Bases in the extract were fixed with formaldehyde and after adding picric acid, the absorbance was measured at 410 ŋm. TBA was used as an indicator of lipid oxidation and calculated according to the Tarladgis et al. (1960). HCl were added to the fish samples and processed at condenser. TBA solution prepared with 90% glacial acetic acid was added to the distilled solution and was left in a water bath. The absorbance was determined by a spectrophotometer at 538 ŋm. For chemical analysis, 6 fillets were used for each replicate.

Determination of proximate composition
Crude protein content was calculated by converting the nitrogen content determined by the Kjeldahl method (6.25xN) AOAC (1998). Lipid was determined by using the method described by AOAC (1998).

Statistical analyses
SPSS 14 for Windows was used to test the differences between mean values of the different analysed parameters. Differences between means were analyzed by one-way analysis of variance (ANOVA) followed by the Tukey multiple comparison test, when a significant difference was detected between the days of storage (P<0.05).

Results and Discussion
There were significant increases especially for crude fat, TMA-N and TBA values and also there were considerable decreases in overall acceptability, pH and TVB-N values after marination (Table 1). No significant change was detected in crude protein values.
As shown in Table 2, when sensory score changes of marinated seabass fillets under refrigerator conditions were examined, it was seen that panellists found the scaly fillet group more fit and the skinned fillet group less fit in terms of appearence, color, rancidity, juiciness, acetic acid rancidity and salinity. Also, descaled fillet group was found to be more fit and the skinned group was found to be less fit in terms of odour, texture, softness, taste and flavour. It was detected that according to average acceptability assessment, skinned samples reached consumable limit value at day 70 where scaly and descaled samples still could not at day 90. Only for appearence, descaled samples were detected to be under limit values at day 90, skinned samples were detected to be under limit values at day 70; for colour, texture and softness assessment, skinned group was detected to be under limit values at day 70; for rancidity, scaly and descaled samples at day 70, skinned samples were detected to be under limit values at day 56; for acetic acid rancidity and salinity skinned samples were detected to be under limit values at day 56. A similar sensory analysis results has been reported by Baygar et al. (2002) for marinated sea bass. It was concluded that TBA values depending on the level of fatty oxidation and sensory evaluation determined the shelf life of marinated anchovies under vacuum and MAP conditions during storage at 2±2°C (Gunsen et al., 2011). It was reported that there was a correlation between the TBA value and sensory evaluation in many researches (Ramanathan and Das, 1992).

Protein and lipid contents
In our study, it was determined that as the crude fat content of fresh sea bass increased with the marination and storage in sunflower oil (Figure 1). We suggest that salt amount of the marination solution cause that artificial increase of crude fat. Although there was not an important change in crude fat contents of groups during oil storage, there were little decreases for all three group samples during the oil storage. A similar proximate composition (72.10% moisture, 19.87% protein, 4.84% fat, 1.54% ash) has been reported by Arık et al. (2002) for marinated fish.   Such as the crude fat content, crude protein content also showed an artificial increase during the beginning of oil storage. The protein content decreased during the oil storage and again reached its initial value (Figure 2). Erkan ve Ö zden (2007) determined crude protein level as % 20.35 ± 0.41 for sea bass. This protein results are consistent with initial values in our study.

pH value
An important intrinsic factor related to fish flesh is the very high post-mortem pH (>6.0). Most fish contain only very little carbohydrate (<0.5%) in the muscle tissue and only small amounts of lactic acid are produced post-mortem . pH value in raw fish flesh was 6,35. During marination process, scaly, descaling and skinning pH values dropped down to 4.32, 4.17 and 4.13, respectively. There was a significant difference (p < 0.05) in pH value between samples in sunflower oil. Lower pH values were found skinning samples in sunflower oil (Figure 3). Gokoglu et al. (2004) reported insignificant changes in pH levels of marinated fish.

TVB-N analysis results
TVB-N value is a quality index for unprocessed fishery products indicative of fish spoilage as a result of metabolic activity of fish spoilage bacteria and endogenous enzymes action (Gunsen et al., 2011). In marine fish, TVB-N values of 15-20 mg N/100 g show good quality, whereas TVB-N values of 50 mg N/100 g show poor quality (Connell, 1980). However, upper levels from TVB-N values of 28 mg N/100 g have been reported as "unacceptable" in processed fish, according to the Turkish Manual of Seafood Quality Control Limits (Anon., 2008). There was a significant change in TVB-N value of all three groups during marination then an increase occured after storage in oil ( Figure  4). Although the highest TVB-N value was seen for skinned samples, all groups were under the consumable limit. TVB-N value is useful in assessing the degree of fish deterioration than in evaluating the changes occurring during the first stages of storage (Gokoglu et al., 2009), Ludorf andMeyer (1973) suggested levels of 30-35 mg TVB-N/100 g as the upper limit of acceptable freshness. The samples did not reach such limit value throughout storage.

TMA-N analysis results
TMA-N is also used as an index of quality for assessing the state of freshness of fish and considered as a valuable tool in the evaluation of the quality of fish. At the first 7 days of oil storage, there was an important increase at TMA-N value of all groups and after the decrease at 14 th day, it proceeded as a slight increase ( Figure 5). Higher TMA-N values were found skinning samples. A TMA-N value of 5-10mg/100 g sample was reported as the limit for acceptability of fish (Sikorski et al., 1989). All TMA-N levels for samples in marination conditions of the present study were below the limit level. Similar results were reported in the previous researches (Dokuzlu, 2000;Gokoglu et al., 2004).

TBA analysis results
The initial TBA value of raw fish was found as 0.30 + 0.01. At the end of the 90 days storage period, the TBA values recorded for marinated scaly, descaled and skinned fillets were 7.82, 8.50 and 9.68 mg malonaldehyde/kg fish, respectively. The acceptable TBA limit value of 8 mg malonaldehyde/kg fish muscle was reached on days 70 and 56 for both the marinated skinned fillets and descaled, respectively ( Figure 6). Sallam et al., (2007) found the raw TBA value of pacific saury (Cololabis saira) which they marinated in different acid solutions to be 0.37 mg MA/kg and emphasized that marinades were of good quality although no steady increase had happened during the 90 days of storage period. Baygar et al. (2012) reported after the marinating process, the TBA values recorded for marinated scaly, descaled and skinned fillets were 4.25, 5.74 and 5.95 mg malonaldehyde/kg fish, respectively. Olgunoglu (2007) found out that the TBA value of anchovy (Engraulis encrasicolus) marinades at the beginning of storage period was 1.16 mg MA kg -1 and it steadily increased and reached 4.20 mg MA kg-1 at the end of the 7 months of storage period. Varlik et al. (2007) and Kaya et al. (2010) maintained that the TBA number should not be <3 in a very good material and it should not be >5 in a good material.

Conclusion
After storage in oil for 90 days, scaly and descaled fillets were found to be more suitable than skinned fillets for marinating and oil storage. Skin and scales effect relatively the acid and salt transition, especially at tail parts of thin fish flesh during storage in brine solution and oil. Variability in acid and salt taste in different parts of skinned fillets was not approved by panellists during the oil storage. Likewise changes at juiciness, tenderness and texture of skinned fillets were less acceptable than other samples. These problems of skinned samples affected the overall acceptability and TBA value so 70 days was determined as the maximum storage time of skinned fillets. Both of the scaly or descaled sea bass fillets are suggested to those companies who will do sea bass marination. Also, when scaly fillets will be used for marination, it is advised to clean the scales carefully before taking the fillets into oil.