Study on Textural, Microbial and Sensory Quality of Blast Frozen Pangasius Fish Fillets Under Frozen Storage Studi Kualitas Tekstur, Mikroba dan Sensori Fillet Ikan Pangasius Blast Frozen dalam Penyimpanan Beku

Pangasius fish is one of the rapid growing fresh water species in aquaculture. This study was aimed to assess the microbial number, physical quality and sensory evaluation of the Pangasius fish fillets from Nepal. Pangasius fish were collected from Chitwan and Nawalparasi districts, fillets were prepared and blast frozen simultaneously. The microbial number decreased rapidly during storage in frozen condition while Staphlylococcus aureus and Escherichia coli (which were present initially in fillets) disappeared during storage. The sensory panellists preferred air blast frozen fillets stored for one month than stored for two months. Different parts of the fish body showed different textural properties. Hardness was increased during second month of storage while other textural properties were found satisfactory in first month of storage in frozen condition. This study showed that sensory qualities of air blast frozen Pangasius fish fillets changes with time even in frozen storage.


INTRODUCTION
Pangasius (Pangasius hypothalamus) is a type of catfish belong to the family Pangasiidae and is also known as Sutchi catfish, striped catfish or Tra fish. Though, it is considered an endemic to the waters of Mekong basin in south-east Asia (Guimarães et al., 2016), its farming is considered as one of the fastest growing type of aquaculture in the world (Gurung et al., 2016). The fish rapidly attains body weight of 1.2 to 1.3 kg within a short period of 6 months. Usually, it is harvested after 8 months, depending on marketability (Gurung et al., 2016). Rao, Murthy, & Prasad (2013) reported that the consumers preferred the fish due to its tender and white flesh, absence of fishy odour, firm cooked texture with high nutritive value with excellent sensory attributes.
Nepal being a landlocked country, aquaculture was an exotic concept and was only introduced in the 1940s with the introduction of several exotic carp species (FAO, 2016). Furthermore, fisheries has been considered as one of the major sub-sector for poverty reduction by the Government of Nepal in its 13 th Plan (National Planning Commission, 2013) and had further em-Study on Textural, Microbial... phasized to make Nepal a self-sufficient country in fish production by increasing its productivity from its current status 4.9 mt/hectare to 6.0 mt/hectare in its 15 th Plan (National Planning Commission, 2017). The annual fish production of Nepal was 77,000 mt in 2017, where 55,500 mt was contributed by aquaculture (Ministry of Agricultural Development, 2017).
It is stated that the total production of Pangasius in the world is 1.6 million metric ton and it is dominated by the leading producer Vietnam, which solely produces approximately 81% of it. It has the high market in Europe. Nepal imports Pangasius from Vietnam, India and Bangladesh (FAO, 2012;Ramadhan, Suwandi, & Trilaksani, 2016). Because of having high correction factor, fast growth and adaptability to a wide range of culture conditions, Pangasius has the highest productivity in culture system. Initially, Pangasius was introduced by the Nepali Farmers themselves from India. Following its introduction, the Government of Nepal initiated research on breeding and rearing to fulfil the demand of fries at the local level (Sah et al., 2018). Following its farming and availability in the market, the demand for Pangasius is growing in Nepal due to its lower market price and presence of fewer spines in the flesh. These indicate that Pangasius has a potential market and if can be processed into value added products such as frozen fillets, can substitute the import.
Among different products of Pangasius, frozen fillets are the highly preferred product in the Nepali market. Fish fillets are simply the fleshy sides of the fish, cut lengthwise along the backbone, and either can be boneless or may contain pins (small bones). It may or may not have skin intact on the outside. These fillets are then frozen for marketing purpose. The flat shape and frozen state of fillets make packaging, storing, transporting and marketing much easier compared to fresh Pangasius.
At current, Pangasius farming is increasing yet, there is no industry which produces frozen Pangasius fillets in Nepal. Thus, this study was conducted with the main focus on the processing of frozen Pangasius fillets in the context of Nepal with the objectives to study the changes in textural properties, microbial number and sensorial quality during frozen storage.

Pangasius fishes were collected from
Chitwan and Nawalparasi districts of Nepal. They were cleaned, degutted, deskinned, deboned and then filleted. Then the fillets were blast frozen and stored at -18C. The microbial number and textural properties along with sensory quality were studied on for two months.

Study of the Microbial Number of the Frozen Fillets during Storage
The microbial number of the fresh fillets in the beginning and frozen fillets at an interval of one month were analysed.  (FAO, 1992).

Study of Texture
The textural properties of the fresh fish and frozen fillets were studied using texture analyser (TA.XT Plus, Stable Micro Systems Ltd., UK). The texture of frozen fillets was studied at an interval of one month. For the study of texture profile, separate pieces having dimensions of L*B*T = 30 mm*30 mm*5 mm were taken from head, body and tail parts ( Figure 1). The fish pieces were compressed to 20% of their original height at a constant speed of 0.5 mm/s. A diameter platen of 50 mm was used as probe for the study. Each analysis was done in duplicate. Texture profile analysis (TPA) was performed on forcetime curves to calculate hardness, cohesiveness, springiness, resilience and chewiness.

Study of Sensory Properties of Frozen Fillets
The sensory properties along with texture profile were studied using untrained panellists every month for two months.

Microbial Analysis of Fish Fillet Samples
The microbial number of the whole Pangasius fish, fish fillets and stored fillets were studied ( Figure 2). Both the total plate count and coliform count were highest for the fillets which showed there remains the possibility of contamination during filleting. But both the total plate count and coliform count of the fillets decreased during frozen storage for two months. Ikasari & Suryaningrum (2015) found the total plate count of 10 5 CFU/ml on the 18 th day of storage in ice (0 -4 C). Lavanya et al. (2016) while studying antioxidant treated Pangasius fillets in frozen storage (-18 C) for four months, reported an initial TPC of 10 3 CFU/g which gradually decreased during frozen storage. This research outcome is in accordance to her study as it showed that frozen storage of fillets lead to decrease in total coliform and total plate counts.  Similarly, Escherichia coli was present initially in whole Pangasius fish but was neither detected on fish fillets nor on stored frozen fillets. Salmonella species were absent in all fish samples, whereas Staphylococcus aureus was detected in fresh fillets showing contamination while preparing fillets, however disappeared during frozen storage (Table 1).

Texture Analysis of Fish Fillet Samples
The changes in average hardness of the different body parts of Pangasius fish fillets during storage (in frozen condition) are presented in Figure 3a. The average hardness of the head andtail increased gradually during frozen storage for two months. But for body part, the average hardness increased in the first month while decreased in the second month. The reason for the increase of hardness in fish muscle can be the outcome of the polymer-polymer associations caused by partial dehydration and pressurizing effect of ice crystals production during frozen storage, which resulted in new aggregation promoting stronger super-junctions. Furthermore, it has been claimed that ice formation during frozen storage can lead to increased ionic strength in the unfrozen matrix forming more rigid network (Solo-de-Zaldívar et al., 2014). The decrease in the hardness of the body part in the second month of frozen storage can be the result of enzymes acting on proteins and forming breakdown metabolites, causing softness in fish proteins (Barraza, León, & Álvarez, 2015). The hardness of head and tail part of the fillet did not follow the same trend as the body part. The difference can be because of less amount of muscle being contained by the head and tail part as compared to body part which in turn will have less active enzymes to act upon the proteins.
The changes in the average cohesiveness of different body parts of Pangasius fish during storage (freezing) are shown in Figure 3b. The average cohesiveness of head and tail of Pangasius fish fillets remain unchanged during first month but decreased during the second month of storage in frozen condition. The average cohesiveness of body decreased gradually during two months of storage in frozen condition than infresh fillets (Figure 3b). As cohesiveness represents the strength of the internal bonds of the fish fillets, the result showed that with the increase in hardness, cohesiveness decreased. Thus, these results are aligned with the result expressed by Barraza et al. (2015) as they also reported that the cohesiveness of Atlantic salmon under frozen storage decreased significantly with time.
The springiness of the different body parts of Pangasius fish fillets increased during the first month of storage in frozen condition while it decreased during the second month of storage (Figure 3c). Except for the tail part, highest springiness was observed during first month of storage under frozen condition. As springiness relates to the hardness of the fillets during the second bite, the results showed that the head and body parts of the fish fillets follow the same trend, while the tail part differed. Furthermore, the springiness of body and tail parts followed the same trend, decrease in springiness of Atlantic salmon under frozen storage as reported by Barraza et al. (2015). The difference in the trend of springiness of tail part can be due to lower amount of muscle present as compared to head and body parts. The average chewiness of head and body part of Pangasius fish during storage (freezing) increased in the first month of storage but decreased with increase in storage time. However, the average chewiness of tail part increased gradually throughout the frozen storage. The highest chewiness was observed in head part of the fish during first month of storage while lowest was observed in the body part of the fish during second month of storage (Figure 3d). Though chewiness is the product of hardness, cohesiveness and springiness, the trend was found to be aligned more with the trend of springiness rather than hardness and cohesiveness.
The mean resilience of different body parts of Pangasius fish fillets during storage (freezing) is shown in Figure 3e. The average resilience of head part of the fish decreased during first month of storage but was reported to be increased during the second month of storage while that of body and tail increased during the storage. The highest resilience was observed in body part of the fish at the fresh stage while lowest was reported in tail part of the fish during second month of storage. As resilience shows how well a product regains its original position, the trend was found to be very much aligned with that of springiness.

Sensory Analysis of Fish Fillet Samples
When paired comparison test was carried out for fresh Pangasius fillet and frozen Pangasius fillet (stored for one month) among fifteen number of untrained panellists, thirteen people (87%) reported differences between the two fish fillets (Figure 4a). Interestingly, on the basis of texture, higher number (67%) of panellists preferred frozen fish fillet to fresh fish fillet. Similarly, when similar test was carried out for after two months of frozen storage (Figure 4b), among twenty untrained panellists, nineteen people (95%) reported differences between them which was higher than for the one month frozen stored fillets. Yet, in the second month, fourteen panelists (70%) preferred fresh fillet rather than frozen stored fillets.
This sensory evaluation showed that the texture of frozen Pangasius fillet was acceptable or rather preferred more than fresh fillet during the first month of frozen storage. Yet, the preferences of panellists drastically decreased with longer frozen storage of fillets. According to Ikasari & Suryaningrum (2015), the sensory evaluation of the fillets preserved in ice (0 -4 °C), showed the fillets were acceptable only up to the 15 th day. The fillets were rejected on the 18 th day as change in colour, off-odour and mushy texture were detected. A study done by Lavanya et al. (2016) showed frozen storage gradually decreased the acceptability of the product based on the sensory score.

CONCLUSION
Longer frozen storage of Pangasius fillets led to changes of textural properties along with sensory preferences. The textural properties analysis on hardness, cohesiveness, springiness and chewiness were in agreement with the sensory analysis result obtained from untrained panellists. These physical textural properties changed with the length of frozen storage, leading to increase in hardness and resilience but decrease in cohesiveness, springiness and chewiness over time. These results can be a promising guideline for interested entrepreneurs to process frozen fillets from Pangasius in Nepal. Furthermore, from the microbial study, it can be concluded that if taken proper precautions during the processing, post contamination with pathogenic micro-organisms can be avoided to produce safe fillets. Also, this result provides the basis for the acceptance of Pangasius fillets under frozen storage based on the textural properties.