CHANGES OF SENSORY ATRIBUTES OF CHILLED VACUUM-PACKAGED COLD-SMOKED COMMON CARP ( CYPRINUS CARPIO ) AND COLD-SMOKED BIGHEAD CARP ( HYPOPHTHALMICHTHYS NOBILIS )

: The aim of this research was to monitor changes of selected sensory properties and instrumental colour parameters (L*, a*, b*) of vacuum-packaged cold-smoked common carp ( Cyprinus carpio ) and cold-smoked bighead carp ( Hypophthalmichthys nobilis ) fillets during storage at 3 ± 0.5 °C and to determine the shelf life of the products. Sensory tests and instrumental color determination were performed on days 1, 7, 10, 12, 14, 15, and 16 of storage. All estimated sensory characteristics of common carp samples received significantly lower (P < 0.05) score on day 15. “Musty” odour of common carp samples detected on day 16 caused that odour score was below the acceptability limit of 2. On the last day of experiment, reduced intensity of pink cream colouring of carp muscle was observed together with softened texture and surface slime. A decrease of scores of the sensory attributes of cold-smoked bighead carp fillets was observed throughout the storage period. However, all estimated sensory characteristics were within the acceptability level. Significant increase (p < 0.05) of L* value was noted in both group of samples, while redness (a*) and yellowness (b*) remaid quit stable during the storage. Based on the sensory results, it was concluded that vacuum-packaged cold-smoked common carp samples remained acceptable for up to 15 days of storage, whereas vacuum packaged cold-smoked bighead carp samples remained unchanged until the end of the experiment.


Introduction
Most of the wild fish and fish from aquaculture consumed in Serbia are marketed for human consumption as fresh or frozen. However, smoked products have seen a considerable surge in popularity. Smoking is one of most acceptable fish-processing method, because it does not require expensive equipment, the production period is short, acceptability on the Serbian market is very good, considering that the local population is accustomed to eat mostly smoked pork (Kilibarda et al., 2009). Modern consumers demand high quality food that retains the sensory characteristics and nutritive value of the raw material from which it is produced; also, it is expected to satisfy very demanding safety standards. This requirement is largely met by packaging the products in vacuum or modified atmosphere. The efficiency of vacuum in extending the shelf life of fish depends on several factors, such as the fish species, fat content, initial microbial cell count and most importantly, the storage temperature (Babić Milijašević, 2017). The most common reason for spoilage of smoked fish products is microbial activity. Microbial growth and the creation of products of their metabolic activity can lead to undesirable odour and taste, and the appearance of discoloration (Leroi et al., 2001).
Cyprinid species (common carp, bighead carp, and grass carp) are most commonly bred in Serbia (Statistical Yearbook, 2020). The aim of this research was to monitor changes of selected sensory properties and instrumental colour parameters (L*, a*, b*) of vacuum-packaged cold-smoked common carp (Cyprinus carpio) and cold-smoked bighead carp (Hypophthalmichthys nobilis) fillets during storage at 3 ± 0.5 °C and to determine the shelf life of the products.

Sampling
Samples from eleven common carp and eleven bighead carp of 2.5 ± 0.3 kg and 2.7 ± 0.5 kg live weight, respectively, were obtained from a fishpond where a semi-intensive rearing system was used. Fish were processed at freshwater fish processing plant using a standard procedure (killing by electrocution, descaling, evisceration, and filleting). Two fillets from each carp were made, and each fillet was divided into 2 portions, i.e. a total of 4 portions were obtained from 1 fish. After primary treatment, fish portions were washed and soaked in brine containing 25 g/L NaCl for 24 h at 5 °C and then pressed, laid on the grid in chambers for 1 h at 12 °C. Smoking was performed on an automated smokehouse for 8 h at the temperature of 28 °C.

Sensory evaluation
The sensory evaluation was performed by six trained panelists. Panelists were trained according to international procedure (EN ISO 8586, 2014) and sensory assessment was carry out in test room who meet requirements of EN ISO 8589, 2010. Prior the sensory evaluation, samples were taken out from refrigerator to remain at room temperature for 30 min. For each day of examination, each assessor was provided with portion of fillet. The samples were evaluated for overall acceptability, with regard to odour, flesh colour, and texture using 1-5 intensity scale, with 5 corresponding to the most liked sample, and 1 corresponding to the least liked sample. The product was defined as unacceptable with a score of less than 2 points recorded by at least of 50% of the judges.

Instrumental colour determination
Colour of fish fillets was evaluated using colorimeter (Minolta Chroma Meter RC-400). The CIE system color profile of L*, a*, b* was measured by reflectance colorimeter using illuminant source D65, 8-mm aperture and 10° observation angle. The L* value represents lightness (L*= 0 for black, L*= 100 for white), while a* scale represents the red/green dimension, with positive values for red and negative ones for green. The b* scale represents the yellow/ blue dimension, with positive values for yellow and negative ones for blue (CIE, 1976). The colorimeter was calibrated throughout the experiment using a standard white ceramic tile (Y = 87.2; x = 0.3173; y = 0.3348). At each day of examination three measurement was carried out on surface of two portion.

Statistical analysis
The mean values and standard deviations were calculated by using column statistics with the processing of 6 values for each analyzed group. Significant differences between groups were calculated using one-way ANOVA analysis by Tukey comparative test, Pearson's correlation analyses were performed on means of the values in the program Microsoft Office Excel (2010). Differences were evaluated as significant at P < 0.05.

Results and Discussion
The results of the sensory evaluation of cold-smoked common carp fillets and cold-smoked bighead carp fillets are presented in Table 1 (mean value ± standard deviation). At the beginning of storage period, colour, flesh texture, odour as well as overall acceptability were evaluated with very high scores in both group of samples. First day, color of common carp and bighead carp fillets was graded with high scores (4.2±0.30 and 5.0±0.00 respectively). Average grade of colour acceptability of cold smoked common carp and bighead carp fillets decreased during the storage. On the last day of experiment, reduced intensity of pink cream colouring of carp muscle was observed and it was evaluated as barely acceptable (2.1±0.20). The color of cold-smoked bighead carp muscle remained acceptable throughout the storage period (16. day it was evaluated as "acceptable" 3.4±0.60). In their research, Kolodziejska et al. (2002) and Leroi et al. (2001) found that colour intensity of vacuum packaged cold-smoked fish did not significantly change during three weeks of storage at 4°C. Likewise, Bugueno et al. (2003) did not reveal changes of smoked salmon colour packaged in vacuum and modified atmosphere during the storage at different temperature conditions. Numerous studies in the European Union showed that the colour of smoked food products, specially smoked fish, is a main parameter that influence consumer decision to buy a particular type of food (Espe et al., 2004). Johnston et al. (2000) suggested that undesirable colour, as one of the most important cold-smoked salmon quality parameters, greatly reduces the cost of this product on the market.
In our research, textural changes was detected in common carp muscle during the storage. On day 16, softened texture and surface slime was discovered, and this sensory attribute were evaluated as "barely acceptable" (2.2±0.40). On the other hand, average scores of flesh texture of bighead carp muscle during the storage remained essentially unchanged. It was estimated as "very acceptable" at the beginning (5.0±0.00) and "acceptable" (3.1±0.20) at the end of storage period. The texture of cold-smoked fish has great importance for the quality of fish meat (Lakshmanan et al., 2005). During the storage, autolytic processes can cause changes in fish meat structure and consequently undesirable fish softening, even when the microbiota which usually causes spoilage is still not sufficiently developed to do so (Olafsdottir et al., 2005;Dondero et al., 2004).
During the experiment it was observed decrease of average grade of odour acceptability of cold smoked common carp fillets. On day 16 "musty" odour caused that odour score of common carp samples was below the acceptability limit of 2 (1.2±0.20). However, average scores of odor of bighead carp muscle during the storage period remained within the acceptability limit. It was estimated as "excellent" at the beginning (5.0±0.00) and "acceptable" (2.8±0.50) at the end of experiment. The high average odour score of bighead carp muscle in the present study had a positive impact on the overall acceptability of these fillets. Truelstrup Hansen and Huss (1998) and Leroi et al. (1998) in their examination of vacuum packaged cold-smoked trout, also found that during storage, the odour and taste of smoke intensity decreased, and became milder or almost neutral. Volatile substances that are usually produced by bacteria are the cause of undesirable odours in smoked fish (Olafsdottir et al., 2005;Dondero et al., 2004). These substances include trimethylamine, the volatile sulphur compounds, aldehydes, ketones, esters, hypoxanthine, and other low molecular weight substances. Trimethylamine is responsible for the typical "fishy" odour, indicator of spoilage.
As the results show, all estimated sensory characteristics of common carp samples received significantly lower (P < 0.05) score on day 15. A decrease of scores of the sensory attributes of cold-smoked bighead carp fillets was observed throughout the storage period. However, all estimated sensory characteristics were within the acceptability level.
Changes in fish meat begin at the moment fish dies, or already at the time of the catch and are the result of the activities of their own enzymes, the metabolism of microorganisms and the oxidation of lipids. Changes in the sensory characteristics of the fish usually result from the microbiological development. The decomposition of food ingredients and the growth of microorganisms cause an unpleasant smell and taste as well as the production of visible pigmented or unpigmented colonies. The synthesis of polysaccharide extracellular materials and diffuse pigments results in sensory changes in the form of mucus formation and discoloration (Fletcher et al., 2002). On the other hand, chemical changes such as auto oxidation or enzymatic hydrolysis of fats may cause the rise of unpleasant smell and taste or, in the latter case, the activity of tissue enzymes may lead to unacceptable softening of the fish meat.
Changes in instrumental colour parameters of cold-smoked common carp fillets and cold-smoked bighead carp fillets are presented in Table 2. (mean value ± standard deviation). Group I : cold-smoked common carp samples; Group II: cold-smoked bighead carp samples; Same lowercase letters in a row indicate no significant differences (p˃0.05) In our research, from 1. to 12. day of experiment lightness (L*) of common carp samples did not change significantly (P ˃ 0.05). After that period of time to the end of experiment lightness (L*) increased significantly (P < 0.05) by 24%. Lightness (L*) of bighead carp muscle was quite stable during the first fourteen days of storage. From that on L* value contineusly increase by the end of storage by 14.70%.Value for redness (a*) and yellowness (b*) remaid quit stable during the storage in both group of samples. Our results are in agreement with Choubert et al. (2005) who reported increase in L* value of vacuum packaged sliced smoked rainbow trout stored at chilled temperature. They concluded that during storage slices became paler, while changes in a* and b* parameters was not significant. During 40 days of storage at 4 °C Rizo et al. (2015) detected significantly increase of L*, a*, b* values of smoke-flavoured salmon packaged in water vapour permeable bags. The increase in lightness is explained by the water loss produced during storage which lead to greater water deposits on the fish surface. On the other hand, Bugueno et al. (2003) did not find any significant differences in L*, a*, b* values of smoked salmon packaged under vacuum and in modified atmosphere as a function on storage time. They suggested there was good colour stability of the examined product. In our research moderate negative correlation between lightness (L*) and flesh colour as well as flesh texture and overall acceptability of cold-smoked common carp and cold-smoked bighead carp fillets was detected. Weak negative correlation between lightness (L*) and odour of cold-smoked common carp and bighead carp samples was determined.
Strong positive correlation between overall acceptability and odour, flesh colour and flesh texture of cold-smoked common carp and cold-smoked bighead carp samples was recorded.