Impact Of Long-Term Storage On The Instrumental Textural Properties Of Frozen Common Carp (Cyprinus Carpio, L.) Flesh

Abstract

The aim of the work was to measure and confirm changes in the qualitative properties of frozen carp flesh (Cyprinus carpio L.) during 84 days of storage at -20°C, using instrumental texture profile analysis focused on hardness, cohesiveness, and springiness. Hardness in fresh fish was 2.908 N, in fish stored 28 days 1.238 N, after 56 days 0.505 N, and after 84 days 0.491 N. Cohesiveness (dimensionless unit) in fresh fish was 0.838, after 28 days 0.740, after 56 days 0.720, and after 84 days 0.712. Springiness (dimensionless unit) in fresh fish was 0.682, after 28 days 0.625, after 56 days 0.577, and at 84 days of storage 0.534. The texture profile analysis effect was determined by several regression and linear models. It is possible to calculate a certain time period in which changes develop up to the level of 50% of deterioration—in hardness 17.5 days and in cohesiveness 12.9 days, as this was proven to a defined significance level (P > 0.01). Only changes in springiness showed a linear regression in the level of 0.002 (rounded) for each day. Studies showed that the intensity of the most important changes in instrumental textural properties occurred during the initial 56 days after freezing and after up to 84 days of storage the changes continued at a lesser rate.

Keywords:

• Freezing
• Storage
• Textural properties
• Hardness
• Carp
• TPA

INTRODUCTION

Due to the amount of fish consumed and processed in Europe, an enhancement of quality provides a competitive edge over periods of distribution chain and household storage. A defined steady temperature had been selected to eliminate highly different temperature changes to which the product is exposed, case by case, during its storage in retail, transport, and the consumer household. There is comparatively little information on what happens following purchase by the consumer during storage in a domestic freezer and the effect of storage conditions on food quality.[1] Fish processing spans a wider geographical area and the carp fish species chosen for the study are mainly representative of Asia and Central and Eastern European regions. An understanding of the changes leads to more efficient utilization of fish, and knowledge of the textural parameters in fish flesh is an important item for evaluation of safe and proper distribution methods from processing through the market to a consumer.[2]

The advantages of food preservation by freezing with minimum change of organoleptic properties are, to a certain extent, counterbalanced by the textural changes within the tissue. It is well known that the crystallisation of ice has two steps: the first is the formation of nuclei and the second is the later growth of the nuclei to a specific crystal size. Rapid freezing leads to a superior food quality compared to that obtained by slow freezing, which may cause severely damaging intracellular ice crystallisation.[3– 5] Freezing preservation offers great logistical advantages, which are required for the handling of fish fillets[6] and storage of fish in household facilities. Some authors[7– 9] mentioned three deterioration phenomena that are often closely related: mechanical damage, denaturation of muscle proteins, and loss of water-holding capacity. These effects have been widely demonstrated in many fish species, such as cod, haddock, hake, Alaska pollock and tilapia.[10– 13] It is known that post-mortem textural changes are caused directly or indirectly by physicochemical changes in myofibrillar proteins and changes in extracellular structure, such as loss of fibre compaction and increase of extracellular space between fibres.[14– 16] Rheological properties and other parameters commonly cited are white stripes (connective tissue), bloodstains, and marbling[17– 19] as the fish muscle is made up of millions of fibres arranged in short segments or blocks.[20]

The texture of fish flesh may be measured by an instrumental method, such as texture profile analysis, which is effective in food texture assessment.^[21,22] Double compression makes it possible to perform a texture profile analysis (TPA) from a plot of force-time curves.[23, 24] Many attempts have been made to correlate physical measurements with sensory evaluation of texture.[25– 27] The reproducibility of texture measurements may be affected by sampling technique because of the heterogeneity of the fish fillets,[19, 25, 28] especially in carp for its content of inter-muscular bones within muscle tissue. Despite substantial research accomplished,[5, 6, 29, 30] there is little satisfactory or coherent explanation for qualitative alteration during the longer term storage in frozen carp flesh. The aim of this study was to identify, through a more detailed knowledge of texture changes, the chronological succession of hardness, cohesiveness, and springiness in stored fish flesh at households during a commonly recommended term of 3 months.

MATERIAL AND METHODS

Experimental Material

The fish—common carp (Cyprinus carpio, L.)—were cultured in one collective earth pond at the farming facilities of the Fisheries Třeboň Ltd., Czech Republic, and sampled in September 2009. The fish were reared on the basis of natural food (plankton and benthos in the pond) and supplemented by cereal feeding (rye). The fish were of 2500 ± 100 g in live weight, 450 ± 10 mm in length, 140 ± 10 mm in width, and the age was 3 years. Twenty carps were caught, immediately stunned by head blow, bled, eviscerated, cleaned, and manually filleted. Each fillet was cut into 4 parts (locality 1–4, Fig. 1). The fresh fillets were placed in moisture-impermeable polyethylene bags. One group of fresh samples (A) was used immediately for measuring and other samples were frozen in a Sabroe compartment (Johnson Controls, Hoejlojerg, Denmark) on horizontal steel plates. The temperature

Table 1 Temperature and pH in analyzed fish

	Fresh fish		28 days		56 days		84 days
Samples	Temperature (°C)	pH	Temperature (°C)	pH	Temperature (°C)	pH	Temperature (°C)	pH
1	16.5	7.14	16.6	6.98	16.6	6.68	16.9	6.58
2	16.6	7.21	16.7	7.05	16.8	6.76	17.0	6.81
3	16.8	7.31	16.8	6.89	16.9	6.58	16.8	6.64
4	16.9	7.25	16.9	6.89	16.6	6.71	16.9	6.75
5	16.8	7.12	16.5	6.77	16.8	6.85	16.8	6.82
6	16.5	7.24	16.8	6.95	16.9	6.75	16.7	6.71
7	16.7	7.12	16.7	6.87	16.7	6.64	16.6	6.69
8	16.6	7.09	16.9	6.81	16.6	6.69	17.1	6.79
9	16.8	7.18	16.8	6.84	16.9	6.72	16.9	6.63
10	16.5	7.23	16.7	7.02	17.0	6.83	16.7	6.74
Mean	16.67	7.18	16.74	6.90	16.78	6.72	16.84	6.71
± SD^a	0.149	0.07	0.126	0.09	0.147	0.08	0.15	0.00
^aValues are standard deviation.

decreased instantly to the target freezing temperature of −40°C, a temperature of −10°C being reached in about 8 min and in 45 min the temperature reached −40°C (35 mm·h⁻¹). Then the fish were stored in a freezing box at −20°C for specific times of 28 days (B), 56 days (C), and 84 days (D). The temperature in the freezer and freezing box was monitored by a temperature logger type S0110, range −40 to +80°C (Comet System Ltd., Roznov pod Radhostem, Czech Republic), and the data was collected with an acquisition rate of one measurement per 5 s. The flesh pH was monitored using a pH Tester 20, range 1.00 to 14.00 (Chromservis Ltd., Prague, Czech Republic). Changes in pH values are not discussed in this article. For texture measurements, the samples were kept at around 16.5°C (Table 1) for 12 h to allow them to unify and integrate well before measurements. The temperature of the fish fillets while outside of the freezing box was monitored by a thermometer Testo 106, range −50 to +275°C (Testo Ltd., Prague, Czech Republic). Texture profile analyses measurements were performed on the flesh located above the lateral line (Fig. 1).

Basic Chemical Analyses

For basic chemical composition, three descriptors were determined: content of dry matter in flesh—DM, crude proteins—CP, crude fat—CF. The DM content was determined gravimetrically following the reference method (Czech Standard No. 57 6021) for determination of moisture content in flesh after drying the sample with sand down to a constant weight at 103 ± 2°C. The CP content was determined as the amount of organically bonded nitrogen (recalculating coefficient f ₁ = 6.25) using a semiautomatic analyzer Kjeltec System (Tecator, Horanas, Sweden) with the method following the recommendations of the producer. The content of CF was determined quantitatively by extraction in diluent (petrolether) using Soxtec semiautomatic system (Tecator) with the method following the recommendations of the producer. Results were based on application of the TA.XTPlus texture analyzer (Stable Micro Systems, Godalming, England) with a load cell of 50 kg. This instrument provides a rigid framework for tension, compression cycling, and texture tests to generate a true 3-dimensional product analysis of force, distance, and time.

Methods

The texture of carp flesh was characterised by an instrumental method—texture profile analysis (TPA)—for hardness, cohesiveness, and springiness. Textural parameters were calculated using a software TPA application (TA.XTPlus, Stable Micro Systems, Godalming, England). Hardness was defined as maximum force detected during first compression, expressed in N. Cohesiveness was measured as the ratio of the positive force area during the second compression to the positive area during the first compression. Springiness was defined as the ratio of the time from the start of the second area up to the second probe reversal to that between the start of the first area and the first probe reversal. Cohesiveness and springiness are dimensionless. Flesh samples of 42 mm in diameter were prepared by calibrated tube blade equipment, measured at a temperature of 16.5°C. No inter-muscular bones were removed. Distal body parts were placed skin side on the base of the analyser facing intramedially to the compression plate.

Compression Plate

A round compression plate of 75 mm in diameter was used. Probes approached the sample at the speed of 2 mm/s; target mode strain was 50%. A penetration depth of 7 mm into the sample was selected as the maximum distance that could be applied without breaking the muscle fibres and affecting the muscle structure by disrupting it and leaving a mark on the sample. The sample was allowed to rebound for 15 s with the compression plate just touching the surface. Double compression was applied to construct the TPA parameters. The compression plate was then pressed on the sample a second time and TPA was obtained by analysing the force time curve as in Godavari Bai et al.[31] Data collection and calculations were carried out using the Texture Expert program, version 1.11 (Stable Micro Systems Ltd., Godalming, England).

Statistical Analysis

Several regression models were considered (linear regression, polynomial regression, and non-linear regression), with the aim of modelling the textural properties of the fillets.

As the most useful model with some theoretical and empirical support for the particular texture properties, a non-linear regression model was chosen of the form:

It can be pointed out that such reparametrization of a more familiar form of negative exponential decay model can facilitate the interpretation of regression parameters. In such a case, three parameters, , can be interpreted directly as follows: is the ultimate value of a considered property, more formally the asymptote. The parameter is the total amount to be lost, and, finally, the regression parameter could be interpreted as the time taken to lose half the amount remaining to be lost. For estimation of the non-linear regression coefficients included in these models, a numerical optimization method—specifically the Gauss-Newton algorithm method—was used.[32] The numerical evaluation was carried out through the marked programming environment R 2.6.2.

RESULTS AND DISCUSSION

The work was focused on evaluation of the fish storage (in domestic freezers for three months as storage recommended time at households) on the change in the textural properties of the fish flesh (hardness, springiness, cohesiveness) in common carp.

Flesh Characteristics

The composition of nutrients and other materials analyzed from the sample fish flesh is given in Table 2. The pH of the flesh when analyzed ranged from 6.58 to 7.31 and the temperature of the flesh ranged from 16.5–17.1°C as shown in Table 1. The progress and changes in fish flesh (fresh and during the subsequent period of 84 days) are stated in Table 3.

Table 2 Composition of fresh carp flesh

Specification	Content (g kg^−1)^a
Dry matter	234 ± 49.6
Crude protein (N × 6.25)	140.3 ± 31.7
Crude fat	69.5 ± 21.2
Ash	25 ± 10.4
^aValues are mean of 20 fish ± standard deviation.

Table 3 Confidence intervals for texture properties with respect to time from freezing

Property	Units	Fresh fish	28 days	56 days	84 days
Hardness	N^a	2.908 ± 0.419	1.238 ± 0.193	0.505 ± 0.069	0.491 ± 0.006
Springiness	Dimensionless^a	0.682 ± 0.039	0.625 ± 0.036	0.577 ± 0.025	0.534 ± 0.011
Cohesiveness	Dimensionless^a	0.838 ± 0.055	0.740 ± 0.031	0.720 ± 0.031	0.712 ± 0.008
^aValues are mean of 10 samples ± standard deviation.

Model for Hardness, Cohesiveness, and Springiness Changes

In the study, significant changes in texture profile during the first month of carp flesh storage have been proven. In those properties that were evaluated, the course of exponential changes was proven. In Table 4, the estimated values of regression coefficients for particular models with their standard errors, correlations, and significance were provided. The estimated negative exponential decay model for hardness could be written as with mean square error (MSE) = 0.0539, for cohesiveness as with MSE = 0.00096. The graphs for these negative exponential models are shown in Fig. 2. It is possible to calculate a certain time period in which further changes will develop up to the level of 50% of deterioration in hardness 17.5 days and cohesiveness 12.9 days—as it was proven to a defined significance level (P > 0.01).

Table 4 Parameters summary for reparameterized negative exponential decay models.

					Correlation matrix
Texture properties	Parameter	Estimate	Standard error	p-value
Hardness		0.331	0.102	0.0025	1
		2.584	0.122	<0.0001	−0.78	1
		17.565	2.304	<0.0001	−0.85	0.56	1
Springiness		0.257	0.469	0.549	1
		0.424	0.465	0.912	−1	1
		136.663	185.710	0.466	−1	1	1
Springiness		0.678	0.0078	<0.0001	1
Linear model		−0.00175	0.000149	<0.0001	−0.8	1
Cohesiveness		0.712	0.010	0.0025	1
		0.125	0.014	<0.0001	−0.69	1
		12.942	4.568	0.00742	−0.77	0.43	1
is the ultimate value of a considered property, more formally the asymptote; is the total amount to be lost; could be interpreted as the time taken to lose half the amount remaining to be lost.

Only changes in springiness (Fig. 2) showed a linear progress in the level of 0.00175 for each day. It is apparent from the results shown in Table 4 that the exponential decay model chosen for springiness is not the best one. In this case, a simple linear regression was used. These results are presented in the same table. A better estimated model for springiness could be written as with MSE = 0.000825 and a coefficient of determination of 0. 7842.

DISCUSSION

It is supposed that changes of TPA properties are particularly influenced by two main factors: the formation of ice crystals and protein denaturation. In the above mentioned method of freezing technology, ice crystals grow and this amplifies any mechanical disruption of the tissue. Increased formation and growth of ice crystals are directly connected with the damage of cells caused by changes in the structure of frozen raw material. There is also an increase in dehydration and salt concentration in the tissue. Ice crystals bind water from the proteins, resulting in the disruption and weakening of the protein binding system. This distortion leads to the break-up of the three-dimensional structure of proteins and their aggregation. Subsequently, the proteins cannot bind the water molecules, which are retained by means of capillary forces only.[33, 34] It is assumed that the freezing temperature of −10 to −20°C leads to the increase in concentration of dissolved solids by ten times.[35] The most susceptible proteins for denaturation are myofibrillar proteins. It is particularly myosin, as stated in,[36, 37] which, during the freezing of cod, reach an 80% deterioration compared to the natural myosin form, whereas under the same conditions, the deterioration of the natural form of actin was negligible. Ganesh et al.[13] presents that, during 180 days of carp fillets being frozen at a temperature of −18°C, the protein solubility decreased from initial values of 77.48 to 61.42%. This decrease in solubility may be explained by denaturation of the proteins, in particular regarding the actinomyozin. Another factor that could contribute to the change of textural properties of frozen carp fillets is lipid oxidation. Sharp and Offer[35] stated that the free fatty acids arising from non-enzymatic and enzymatic hydrolytic lipids are found in cell membranes and have an unfavourable effect on the texture quality of fish. Also, as it has been reported in the carp fillets study,[38, 39] the toughness of fish flesh increases during frozen storage.

It is assumed that the carp fillets are less susceptible to degradation during freezing than marine fish, probably due to the absence of the trimethylamine system responsible for the formation of formaldehyde, which promotes protein denaturation and deterioration of the texture of fish flesh during the period of storage. It can be understood that the composition of fish flesh has a substantial effect on the desirable storage period of frozen fish maintained in proper conditions during their shelf life, which lasted from 2 to 3 months at −18°C.

Based on several studies, it is recommended for fish species more susceptible to oxidative rancidity, that they should be stored at very low temperatures (at least −29°C), while species less susceptible to rancidity might be stored at temperatures from −18 to −23°C.[40] In the course of food defrosting, water from small ice crystals is reabsorbed into the tissue at the same position as its original location and this does not cause heavy damage to the cells. In contrast, slowly frozen foodstuffs contain large ice crystals that damage the tissue and, during thawing, the water leaks away. The damaged tissues lose their desirable appearance and other important characteristics of a foodstuff.[41, 42] Investigations of Puchala et al.[43] on chilled and frozen carp meat mention changes in fatty acids composition. Also, in the investigations of Hallier et al.,[44] it appeared that fillets of European catfish became less juicy after a freezing-thawing cycle, perhaps due to a loss of water-holding capacity. They also stressed that an increase in hardness and a decrease in resilience highlighted after a freezing-thawing cycle. On the other hand, Sequeira-Munoz et al.[45] mention that freezing procedure did not have a significant effect on the texture of carp fillets at −25°C for 75 days of storage. The above investigations indicate gaps in knowledge of the cause of stability variation in different species.

CONCLUSION

The study was concerned with the influence of freezing and storage time of common carp flesh at −20°C on the quality of resulting textural properties in the thawed foodstuff. Studies have shown that important changes, especially in hardness and cohesiveness, occur in a more rapid way during the initial 28 days after freezing. Up to 84 days of storage, the changes continue at a lesser rate. The TPA effect was determined by several regression models and linear models (P < 0.01). Thus, the experimental protocols already developed can help in finding more detailed solutions to the processing and texture profile analysis of the properties of the carp fillets during/after frozen storage.

ACKNOWLEDGMENTS

This study was supported by the projects QH71011, OC09042, MSM 600 7665 806, 047/2010/z, and CENAKVA CZ 1.05/2.1.00/01.0024.