INFLUENCE OF METHOD OF PACKING ON PHYSICOCHEMICAL AND TEXTURAL CHANGES IN RAINBOW TROUT (ONCORHYNCHUS MYKISS L.) DURING FROZEN STORAGE

Grzegorz Szczepanik, Piotr Mielcarek, Krzysztof Kryża
Department of Dairy Technology and Food Storage, West Pomeranian University of Technology, Szczecin, Poland

ABSTRACT

During the four-month frozen storage, mass loss from sublimation occurred in the air packed rainbow trout, but was not observed in the vacuum stored fish, which were packed in steam-tight packages. After venting, the wrapping fitted close to the surface of the fish, which almost completely eliminated mass loss from sublimation. The percentage moisture content in the air-packed  fish was found to be on average by 4% lower comparing to the vacuum packed fish. The use of vacuum packaging resulted in a slight texture deterioration in the frozen stored fish comparing to the fresh ones, which indicates that prolongation of the fish shelf life has been obtained without compromising their quality. However, greater changes in texture parameters were observed in the air packed fish, comparing to the fresh material.
In the frozen stored carcasses, greater variability in texture parameters was observed for sections A and B, comparing to section C. In the frozen stored fillets, a reverse tendency was found. Fillets were distinguished by greater variability in texture parameters comparing to carcasses, similarly as the vacuum packed samples comparing to the air packed ones.

Key words: fish, rainbow trout, storage, texture, vacuum.

INTRODUCTION

Both raw fish and fish products are an important food group. This applies to both nutritional and medicinal properties [21]. Fish are most of all an excellent source of complete and easy available proteins. The richest in protein (over 20%) are lean fish. Among proteins in fish meat, only 3-5% are sub-standard connective tissue proteins. Especially healthful properties are exhibited by fish fat. It is distinguished by high availability (92%) and high content of polyene fatty acids. Monoene and polyene acids predominate (over 70%) in the fish muscle lipid composition. Of special nutritional significance are long-chain polyene Ω-3 acids [8].

Fish are products of short expiry date, and they easily undergo adverse microbiological and enzymatic changes [18]. Post-mortem spoilage of fish and marine invertebrates can be fast and effectively prevented by freezing [20]. Temperature reduction during freezing is accompanied by numerous changes in physical properties, like viscosity, density, thermal and electrical conduction, etc. However, the most significant change is water transition from liquid to ice, which results in the product solidification and essential change in numerous physical features of food [16]. Simultaneously, ice crystals forming in extracellular spaces of the frozen tissues can damage histological structures. Mechanical damage contributes to the release of some enzymes from the sub-cellular structures, especially ATP-ase, phospholipase and cathepsins [20]. Disruptive changes in the frozen flesh occur mainly at temperatures between -1.5°C and -10°C. Therefore freezing technology requires fast temperature lowering below this range to minimize structure damage [6].

At -30ºC, which is the temperature currently recommended for long-term frozen storage of fish, only slight hydrolysis of proteins occurs, and it does not significantly affects the frozen products quality. In contrast, important are denaturation changes. Proteins of fish flesh stored for several months at insufficiently low temperature lose partly their solubility, water absorbability, enzymatic activity and functional properties. Flesh of such fish is tough after thermal processing, which is a consequence of denaturation of muscular proteins (mainly myofibrillar). There are several causes of this denaturation that affect fish flesh with various intensity in different fish species. One of these causes is disruption of hydrophobic interactions and hydrogen bonds that participate in maintaining natural protein conformation in the environment [20]. Nevertheless, freezing is regarded as the most suitable method of food preservation. This process allows for maximum prolongation of food products shelf life [17].

The basic task of food technology is to provide high quality food and guarantee maintaining this quality for a long as possible. This concerns all aspects of food shelf life: both sensory quality and health safety of final products [1]. Texture is one of the most important properties that influence the quality and consumers' acceptance of foodstuffs. It is perceived usually subconsciously, when the feature falls short of one's expectations, when is associated with inedible materials and when contributes to unpleasant mouthfeel, leading further to fully conscious perception. Food texture is a complex concept that can be described in both physical and sensory terms. In the physical sense, it is a rheological property of product, which means it encompasses relationships among tension, deformation and time [15].

In the instrumental texture profile analysis (TPA), the product is compressed to half of its original thickness, and parameters such as hardness, cohesiveness, adhesiveness, springiness and tenderness are read from the force-distance curve. Other parameters, such as chewiness and gumminess, are mathematically calculated [4].

The range of qualitative requirements and the values of applied criteria differ significantly depending on fish species and product type. The quality assessment is based, above all, on odour and flavour desirability and on texture. Odour and flavour should be natural, typical for the particular fish species. Unacceptable are odours or flavours that are foreign, rancid, or indicating bacterial spoilage of the product. The lowest quality grade accepts reduced perceptibility of natural odour and flavour of meat or its reduced desirability caused by frozen storage of the product [13].

According to Casas et al. [5], instrumental measurements are preferred over sensory evaluations since instruments may reduce variation among measurements due to human factors and are more precise.

Sigurgisladottir et al. [19] investigated microstructure and texture changes during smoking of fresh and frozen/thawed Atlantic salmon from three different origins. They concluded that freezing affected the muscle structure of smoked salmon fillets, as well as their texture. The muscle fibres from the frozen and thawed fish shrank, and extracellular space increased, which can lead to liquid leakage (highly undesirable) from the smoked fillets.

Casas et al. [5] analysed textural properties of raw Atlantic salmon measured at three different points along the filet using different instrumental methods employing blade, sphere and cylinder probes. The variables examined were the force, energy and the slope of the force-deformation curve. They concluded that if a standard sample is used to measure texture, it should always be taken from the same location in the fillet. Back or belly regions should be used instead of the tail location since the latter shows a wider range of textures. They indicated the compression test using a cylindrical probe as the most appropriate method for differentiating among locations of raw salmon fillets.

Jain et al. [10] studied texture parameters in Indian major carp during the eight-day long iced storage. Textural parameters such as skin hardness, toughness and stiffness were evaluated using a mechanical texture analyzer in each day of storage. A reduction in skin hardness and toughness was observed after fifth day of storage. Simultaneously, an increase in pH of fish flesh (from 6.10 to 6.90) during the storage period occurred. Total changes undergoing in fish deteriorated its rheological parameters comparing to the raw fish.

Regarding the above, we examined changes in textural properties of muscle tissue in carcasses and fillets of rainbow trout during 4 months of frozen storage in air-tight packs or in air-containing packs.

MATERIALS AND METHODS

The studied material was rainbow trout (Oncorhynchus mykiss L.). The raw material (50 specimens) was bought at retail, as fresh fish stored on ice. The study was conducted in the Department of Dairy Technology and Food Storage, West Pomeranian University of Technology in Szczecin.

Preliminary measurements of raw material were based on evaluation of:

Parameter	Average
Fish weight [g]	364.0 (44.57)
Total length of fish[cm]	32.5 (1.80)
Fish thickness [cm]	3.5  (0.12)
Carcass weight [g]	288.0 (32.88)
Carcass length (excluding tail) [cm]	21.7 (0.93)
Carcass length (including tail) [cm]	25.5 (0.50)
Skinned fillet thickness [cm]	0.83 (0.08)

Standard deviation presented in parentheses

Raw fish intended for further examination were wrapped in polyethylene (PE) bags. Samples were packed without deaeration or by vacuum packing using a TURBOVAC SB 420 vacuum packer.
Operating conditions of vacuum packing: suction 15 millibar (mbar), additional vacuum OFF, degassing 100 mbar, sealing 2 seconds (s), soft air OFF.

The packed fish were frozen and stored at -25°C for 1, 2, 3 or 4 months. Each month, samples for analysis were thawed in a freezer (at 4°C) till attained internal temperature of 4°C, and next processed to carcasses and fillets. So prepared material was further analysed.

Determination of sublimation loss
After 1, 2, 3 and 4 months of frozen storage, the samples were taken out, unpacked, and weighed electronically, to an accuracy of 0.01 g, to determine differences in weight resulting from storage (sublimation). Then the samples were defrosted by air at 4°C until the product reached 4°C. Until the following texture test, the samples were stored in a freezer at 4°C.
Mass loss resulting from sublimation (B) was calculated according to the formula:

B  =  raw material weight – frozen material weight after frozen storage

Lipid extraction from muscle tissue
Lipids were extracted by a chloroform-methanol mixture (2 : 1) according to Linko [14]. Quantification results are expressed as g total lipids kg^-1 wet muscle.

Determination of lipid content in trout muscle tissue
From the flask containing the chloroform extract, subsamples of 10 cm³ were transferred to previously dried and weighed to the nearest 0.0001 g conical flasks of 100 cm³ capacity. The solvent was distilled off on a water bath, under reduced pressure, and at temperature of about 40°C. The residues left in the flasks were dried in a dryer at 80°C for 1 h, next cooled down in a desiccator for 20 min and weighed. All samples were analysed in triplicate.
Percentage lipid content in muscle tissue was calculated according to the following formula:

x = (b – a) · 10 · 4  [%]

where:
x – percentage lipid content
a – flask weight, [g]
b – weight of flask with lipid residue after drying, [g]
10 – pipette volume
4 – numerical coefficient converting the amount of homogenized muscle sample into 100 g.

Determination of moisture content
An amount of 1.5 g of minced material was placed into a glass weighing bottle filled with sand and with a short rod inserted, previously dried at 105°C for 30 min, cooled down in a dessicator and weighed to the nearest 0.0001 g. The whole was weighed again with the same accuracy. The sample added was carefully mixed with the sand using the glass rod. So prepared weighing bottles were placed in a dryer and kept in 105°C for 3 h. Next the samples were cooled down in a desiccator and weighed to the nearest 0.0001 g.

Percentage moisture content was calculated according to the following formula:

where:
x – percentage moisture content
a – weight of the bottle with sand and rod, [g]
b – weight of the bottle with sand, rod and sample prior to drying, [g]
c – weight of the bottle with sand, rod and sample after drying, [g].

Determination of pH
Measurements of pH were conducted using electronic pH-meter IQ240 from Scientific Instruments Inc.

Rheological analysis
Rheological tests were performed using a TA.XT texture analyzer (Stable Micro Systems Ltd., England) using a cylinder shaped aluminum tenon with a diameter of 6 mm (part code P/6). The test parameters were as follows: pre-test speed – 1.0 mm·s^-1; test speed –2.0 m·s^-1; post-test speed – 5.0 mm·s^-1; distance – 5 mm; time – 15 s; trigger force – 5 g. The trout carcasses measured about 20 cm. This length was divided into 10 sections, and the first section was excluded from examination. Two subsequent sections of 4 cm total length constituted first measured part of the fish (A), next two subsequent sections (4 cm) formed part B, and next two sections (4 cm) – part C (Fig. 1).

Fig. 1. Diagram of distribution in rainbow trout of the three studied sections subjected to the TPA assay

In each section, 4 measurements were made in 0.5 cm intervals, always about 0.5 cm above the lateral line.

Statistical analysis
The obtained data were statistically analyzed using the Excel and Statistica 8.0 software. Significant differences were determined with the Tukey's test at the significance level p ≤ 0.05.

RESULTS AND DISCUSSION

During frozen storage surface sublimation occurs, which results in mass losses. The scale of natural losses is the outcome of multiple factors, therefore setting precise standards is very difficult in industrial conditions. Physical processes occurring in fish muscle during cool processing result from changes affecting water contained in the muscle. Surface character of sublimation together with practically unchangeable water distribution in frozen products prevent compensation of moisture losses through water migration from the inside of the product. This way, an external layer of porous structure and deprived of moisture is formed, which promotes oxidation processes and absorption of foreign odours [2]. This explains the sublimation-derived  mass loss occurring in freeze stored fish. Relatively high mass loss from sublimation, increasing along with storage duration (from 4 g after first month of storage to 8.5 g after fourth month), distinguished the air-packed rainbow trout. Whereas in the vacuum-packed fish, mass loss from sublimation was at a low level during the whole frozen storage period (Table 1). These results are in agreement with the thesis formulated by Błoński [2] that the most important technological solution for mass loss reduction is wrapping meat in appropriate plastic films.

Table 1. Changes in sublimation mass loss and pH levels in muscle tissue of rainbow trout, packaged in air and in vacuum, during frozen storage at -25°C

Month of frozen storage	Sample	Sublimation mass loss [g]	Moisture content [%]	pH	Water activity
0	fresh	–	78.81 (4.63)	6.19 (0.04)	0.910 (0.003)
1	air	4 g	74.49 (1.37)	6.27 (0.05)	0.990 (0.006)
	vacuum	1 g	71.53 (3.12)	6.42 (0.09)	1.000
2	air	5.5 g	75.44 (2.22)	6.44 (0.10)	0.868 (0.013)
	vacuum	0.5 g	71.26 (2.67)	6.49 (0.05)	0.788 (0.027)
3	air	7.0 g	75.37 (1.55)	5.77 (0.02)	0.832 (0.002)
	vacuum	1.7 g	72.10 (2.10)	6.14 (0.07)	0.934 (0.002)
4	air	8.5 g	71.77 (0.17)	6.11 (0.38)	0.831 (0.003)
	vacuum	1.0 g	71.61 (0.98)	6.28 (0.04)	0.919 (0.005)

Standard deviation presented in parentheses

A suitable packaging keeps moisture inside and prevents oxygen penetration from outside of the package. Steam-tight packages that fit closely to the product eliminate freezer burn almost entirely. While in steam-tight packages not fitting closely to the product, an internal freeze burn occurs resulting from temperature fluctuations in the spaces between the product and the packaging. This has been confirmed by the present results for moisture content in rainbow trout muscle tissue, that significantly correlated with mass loss from sublimation. Percentage moisture content in the vacuum-packed fish was on average by 4% higher comparing to the air-packed fish (Table 1). Vacuum packing reduces drying, beneficially limiting products' surface colour changes, oxidation in surface layers of raw meat and final meat products, as well as flavour and odour loss [3]. Kaćeńak et al. [12] observed that vacuum storage reduced mass losses and eliminated external atmospheric influences (moisture, oxygen, etc.). Their observation was confirmed by the present findings for water activity (A_w) which equalled 0.910 for the raw material (Table 1). In the air-packed samples, A_w tended to decrease to as low as 0.831 in the last month of storage. Such a substantial A_w reduction might have been caused by mass loss from sublimation that gradually increased during storage. In the vacuum-packed trout, only insignificant decrease in A_w was observed, and A_w values were within the limits of statistical error. This indicates that mass loss in the vacuum-packed fish during frozen storage was reduced to minimum.

Błoński [2] observed that muscles, initially tender and flexible, gradually harden up to total stiffness. This condition, called rigor mortis, at room temperature commences after several hours after death, and gradually dissipates after further several hours. In freezing temperatures, the rigor mortis duration can be prolonged even up to several weeks, so it could overlap with the thawing process, which would result in significant quality loss. In the present study, the rigor mortis in fish intended for frozen storage had dissipated before freezing, and pH of fresh raw material equalled 6.19. During the initial two months of frozen storage, pH was increasing regardless of the packaging method used (Table 1). While in the following months, pH showed fluctuations with no clear trend. An explanation for pH rise after second and third month of frozen storage was given by Chwastowska and Kondratowicz [6], who indicated that thawing of meat or meat products is accompanied by protein denaturation, as well as by enzymatic and biochemical transformation of lipids and lipoproteins. Intensification of biochemical processes during thawing, bound up with water freezing out in meat, releases and initiates numerous enzymes concentrated in lysosomes and mitochondria. The ongoing proteolytic processes increase pH in the thawed meat.

The strongest influence on meat texture is exerted by two basic systems of protein structures: myofibrils and connective tissue proteins. The effects of cytoskeleton proteins and intracellular water are also of significance. Moreover, especially during heating, meat product texture is also determined by changes in sarcomere structure [7]. 

The present study has revealed that fillets after four months of storage were distinguished by higher hardness compared to carcasses, regardless of the packaging method used (Fig. 2).

Fig. 2. Changes in hardness of rainbow trout fillets and carcasses [parts: A, B, C], fresh [0] or stored at -25°C [months: 1, 2, 3, 4] in air or in vacuum

What is more, in both fillets and carcasses, regardless of the packaging method used, section C was the firmest. Similar results were reported by Casas et al. [5], who examined textural parameters of Atlantic salmon. The investigators divided salmon fillets into three parts (tail, belly and back regions) and analysed texture parameters, including hardness, using the TPA assay. They showed the tail to be firmer than the rest of the salmon fillet, and  explained hardness changes along the salmon fillet to result from the distribution of fat, pigments and collagen. They also concluded that higher tail firmness was related to higher density of collagen fibrils in the tail connective tissue comparing to the other fillet regions. In the present study, hardness was not following any clear trend, and fluctuations in hardness were observed for all experimental variants. Hardness changes during storage were slight comparing to the fresh raw material. Similar observations were made by Cierach and Stasiewicz [7], who found that hardness of coarsely minced sausage and finely minced sausage fluctuated during storage without any clear tendency. Those fluctuations were relatively small and hardness was not significantly changed during the whole storage period. This confirms that vacuum packaging prevents changes in firmness of frozen fish during storage.

Springiness and cohesiveness are texture parameters used for describing the ability of muscles to recover their original form after deformation force is removed, as well as their resistance to subsequent deformation [5]. In the present samples, both springiness (Fig. 3) and cohesiveness were lower for fillets than for carcasses, regardless of the packaging method used. In all cases, lower values of these parameters were obtained for sections C comparing to sections A and B. Casas et al. [5] made similar observations examining Atlantic salmon.  This is in accordance with the findings of earlier research by Jonsson et al. [11], who observed that the muscle fibres near the tail section of the fillet were more sensitive to rupture than the anterior part of the fillet. The authors explained this by smaller diameter and higher number of muscle fibres near the tail region, than in the remaining parts of fish fillets. Herrero et al. [9] reported that fresh fish muscle is tender, elastic and pale. These texture related features undergo changes during chilled and frozen storage. The present results show that springiness and cohesiveness remained on a similar level during the whole storage period in both fillets and carcasses, regardless of the packaging method used. However in the air-packed fillets and carcasses higher fluctuations in these parameters occurred during the four-months frozen storage, comparing to the vacuum-packed samples. This indicates that vacuum packing is a more effective method of fish preservation. According to Casas et al. [5], if cohesiveness is <1, the deformation of the first compression is partly irrecoverable. This has been confirmed by the present study, where deformation of all samples was <1, although the tail (section C) values were lower comparing to sections A and B. Casas et al. [5] suggested that this region is therefore less elastic and more rigid.

Fig. 3. Changes in springiness of rainbow trout fillets and carcasses [parts: A, B, C] , fresh [0] or stored at -25°C [months: 1, 2, 3, 4] in air or in vacuum

Casas et al. [5] obtained greater gumminess values for the tail than either the back or belly regions. Similarly in the present samples, gumminess after four-month frozen storage was the greatest for section C (Fig. 4).

In the present study, lower values of chewiness were obtained for vacuum packed fillets and carcasses, compared to the air packed fish. In addition, the course of changes in chewiness along with storage duration was found to be slightly different for each section (A, B, C). However, no considerable change in chewiness during four-month storage was observed for both fillets and carcasses, regardless of the packaging method used. According to Cierach and Stasiewicz [7], no considerable changes in chewiness occurred also during storage of vacuum packed sausages, both finely minced and coarsely minced.

Fig. 4. Changes in gumminess of rainbow trout fillets and carcasses [parts: A, B, C] from rainbow trout, fresh [0] or stored at 25°C [months: 1, 2, 3, 4] in air or in vacuum

Casas et al. [5] indicated hardness, springiness and cohesiveness to be primary mechanical variables for characterizing the texture properties of food.  In the present study, merely slight fluctuations in the texture parameters in the vacuum packed fish were observed. According to Cierach and Stasiewicz [7], if a particular type of packing allows for only slight texture deterioration, it enables prolongation of products' shelf life without compromising their quality.

Statistical analysis of the present results has revealed that fillets were distinguished by greater variations in the examined texture parameters comparing to carcasses (Fig. 5).

Fig. 5. The results of statistical analysis of all TPA test parameters in rainbow trout fillets and carcasses in the subsequent months of frozen storage

Fig. 6. The results of statistical analysis of all TPA test parameters in different sections of rainbow trout fillets and carcasses throughout the period of frozen storage

Fig. 7. The results of statistical analysis of all TPA test parameters of rainbow trout fillets and carcasses throughout the period of frozen storage

The greatest variability of all the tested texture parameters was observed for the section C in fillets, while the smallest for the section C in carcasses (Fig. 6).

Texture analysis during the four-month storage period has revealed that the greatest variability of all tested texture parameters occurred in the fillets stored under vacuum packaging (Fig. 7).

CONCLUSION

During the four-month frozen storage, mass loss from sublimation occurred in the air packed rainbow trout, but was not observed in the vacuum packed fish. The percentage moisture content in the air-packed  fish was found to be on average by 4% lower comparing to the vacuum packed fish.

Vacuum packaging has stabilized A_w in the stored trout comparing to the air packed fish. In the latter, A_w showed a clear downward tendency during the frozen storage period.

Slight variations in primary variables that characterize texture (hardness, springiness and cohesiveness) observed in the vacuum packed fish confirm better protective effectiveness of vacuum packaging comparing to packaging in air.

In the frozen stored carcasses, higher variability of texture parameters was obtained for sections A and B comparing to section C. In the frozen stored fillets, a reverse tendency was revealed. Fillets were distinguished by greater variability in texture parameters comparing to carcasses, similarly as the vacuum packed samples comparing to the air packed ones.

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Accepted for print: 31.05.2011

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Grzegorz Szczepanik
Department of Dairy Technology and Food Storage,
West Pomeranian University of Technology, Szczecin, Poland
Papieża Pawła IV/3, 71-459 Szczecin, Poland
email: gszczepanik@zut.edu.pl

Piotr Mielcarek
Department of Dairy Technology and Food Storage,
West Pomeranian University of Technology, Szczecin, Poland
Papieża Pawła IV/3, 71-459 Szczecin, Poland

Krzysztof Kryża
Department of Dairy Technology and Food Storage,
West Pomeranian University of Technology, Szczecin, Poland
Papieża Pawa VI 3, 71-459 Szczecin, Poland

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