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Slaughter stress, postmortem muscle pH and rigor development in farmed Atlantic cod ( Gadus morhua L.)

2006; Wiley; Volume: 41; Issue: 7 Linguagem: Inglês

10.1111/j.1365-2621.2005.01149.x

1365-2621

Silje Kristoffersen, Torbjørn Tobiassen, Victoria Steinsund, Ragnar L. Olsen,

Protein Hydrolysis and Bioactive Peptides

In intensive aquaculture fish are normally fed to satiation, giving them a high growth rate, but also depositing large amounts of lipids and increasing the glycogen content in the liver and muscle (Ang & Haard, 1985; Rustad, 1992; Einen et al., 1999). Increased amount of glycogen is important for the muscle quality since it results in a high glycolytic potential and consequently a low ultimate pH postmortem. A low ultimate pH is often associated with increased fillet gaping (Lavéty et al., 1988; Haard, 1992) and higher liquid loss and textural changes (Love, 1988; Ofstad et al., 1996; Einen & Thomassen, 1998; Ingólfsdóttir et al., 1998; Einen et al., 1999). There is increasing evidence that exercise, activity or stress prior to slaughter also contribute to fillet softening and gaping (Sigholt et al., 1997; Robb et al., 2000; Roth et al., 2002; Kiessling et al., 2004). This is often explained by the vigorous ante mortem muscle activity and the accelerated onset and earlier resolution of rigor (Nakayama et al., 1992; Ando, 1999; Bremner, 1999; Robb, 2001). Since energy rich compounds in the muscle are consumed during the anaerobic preslaughter activity, the muscle pH is lower at slaughter and the ultimate pH is reached faster than in fish not exposed to exercise, activity or stress before death. Atlantic cod (Gadus morhua L.) has for centuries been the most important commercial species in the North Atlantic fisheries and is now regarded as a very promising species in coldwater fish farming. Although well fed cod seems to be especially prone to quality deterioration such as fillet gaping and muscle softening postmortem (Ang & Haard, 1985; Love, 1988; Rustad, 1992), the effects of slaughtering conditions on muscle quality of farmed cod has to our knowledge not been reported. We are currently investigating how slaughtering and processing of farmed cod should be carried out in order to obtain fillets of highest quality. The aim of this initial study has been to determine the effects of moderate preslaughter activity on the development of postmortem muscle pH and rigor mortis in farmed cod. Gaping and texture of the fillets were also evaluated. Wild Atlantic cod which had been caught alive by Danish seine and then fed capelin twice a week in a net pen for 9 months were used in the experiment. The seawater temperature at slaughter in February was around 4 °C. The fish which were not starved before slaughter were divided in two groups. In the control group (low stress), the fish (n = 12) were netted carefully from the cage and killed immediately with a blow to the head. Full loss of consciousness was assumed since fish showed no movements after the blow. In the stressed group, the fish (n = 12) were netted from the cage and transferred to a 500 L tank filled with seawater. The tank was transported for 1 h from the aquaculture station to the institute. The fish were then exposed to CO2 until they were fully anaesthetised. Then the fish were killed with a blow to the head. All fish were tagged and the average weight and length were determined to 6105 ± 1320 g and 80.2 ± 6.7 cm. After gutting and heading, the fish were washed in seawater and stored iced in boxes. The condition factor (CF) and the hepatosomatic index (HSI) were calculated by using the equations [total body weight (g)/fork length (cm)3] × 100 and [liver weight (g)/total body weight (g)] × 100, respectively. Four fish from each group were used for measuring the muscle pH, four for determining the rigor mortis index and the last four used for the evaluation of gaping and texture. We realise that the number of fish in each group is very few, but since commercial cod is of large size the numbers available were limited. The muscle pH was measured above the lateral line in the loin part of the fillet by using a hand-held WTW 330/Set-1 pH-meter (Wissenschaftliche-Technische Werkstätten, Weilheim, Germany) equipped with a Hamilton double pore glass electrode (Hamilton Bonaduz AG, Bonaduz, Switzerland). After making an incision through the skin, the electrode was inserted into the muscle tissue. The subsequent readings were performed by making new incisions in the skin c. 1 cm away from the previous one. The zero time measurement was made within 20 s after killing the fish. The development of rigor mortis was assessed by using the rigor index method described by Iwamoto et al. (1987). Rigor was determined 10 times on each fish during the 6 days ice storage period. A rigor index of 0% indicated no rigor while 100% indicated full rigor. The final four gutted and headed fish in each group were stored in ice for 7 days postmortem before filleting. Four persons experienced in determining quality of cod, evaluated the fillets with regard to gaping and softness. Gaping was determined by visual inspection while carefully lifting the fillet while softness was assessed simply by touching and pressing fingers onto the cut side of the fillet. A scale from 0 to 3 was used. A score of 0 denotes no gaping or a firm and natural texture while 3 denotes extreme gaping or very soft texture. Analyses of significant differences among sample means were determined by the Tukey's test. The farmed cod used were of good market size and of approximately similar weight and length. A calculated CF of 1.18 ± 0.14 and HSI of 11.22 ± 1.76 showed that the fish were of very good biological condition and consequently have had access to ample amounts of feed. Such high HSI may even be classified as abnormal and suggests that the cod have had a high growth rate (Jobling, 1988). The results showed that the cod killed with as little preslaughter activity or stress as possible, had a muscle-pH of 7.9 immediately postmortem (Fig. 1). It then decreased rapidly during the first hour before levelling off at pH around 7.2. After about 8 h, it started to decline again reaching an ultimate pH of 6.2 after about 24 h postmortem. The cod exposed to stress prior to slaughter had a stable muscle-pH of c. 7.0 during the first 6–8 h period. The ultimate pH was very similar to what was found in the control fish and comparable with previously published values for net-pen fed cod (Rustad, 1992; Ofstad et al., 1996). Our finding that the ultimate muscle pH of cod is not affected by preslaughter activity is similar to the results reported for other species (Korhonen et al., 1990; Izquierdo-Pulido et al., 1992). Others have, however, published a slightly lower ultimate muscle pH due to preslaughter activity (Sigholt et al., 1997; Thomas et al., 1999; Robb et al., 2000; Ruff et al., 2002; Kiessling et al., 2004). Decline in muscle pH in farmed Atlantic cod stored in ice for (a) 25 h; and (b) 120 h postmortem. Fish exposed to slaughter stress (•) and control fish (○) (mean values ± SE, n = 4). Robb and co-workers reported a corresponding development in postmortem muscle pH in rainbow trout anaesthetised prior to slaughter (Robb et al., 2000). Most other reports have, however, not been able to detect the high muscle-pH immediately after death and the initial rapid rate of decline in fish not exposed to preslaughter stress or activity (Izquierdo-Pulido et al., 1992; Nakayama et al., 1992; Jerrett & Holland, 1998; Skjervold et al., 2001). The reason for this could be that in these reports the first measurement took place too long time after death or that the fish had been exposed to ante mortem activity or stress. In addition, the buffering capacity of the muscle may be dependent on the species or on biological condition of the fish. It is often said that it is the formation of lactic acid which causes the decrease in muscle-pH under anaerobic conditions such as occurring postmortem (Love, 1980; Lawrie, 1998; Robb, 2001; Skjervold et al., 2001). This concept of lactic acidosis has been challenged by several and it appears that although the drop in pH correlates to a large degree with the amount of lactate produced, the main source of protons is the hydrolysis of ATP and the formation of reduced nicotinamide adenine dinucleotide during glycolysis (Foegeding et al., 1996; reviewed by Robergs et al., 2004). The findings by us and others (Nakayama et al., 1992; Robb, 2001) that muscle-pH is relatively stable for several hours postmortem could at least partly, be explained by a buffering capacity of the tissue and that lactate production from pyruvate actually consumes protons. Since muscle pH is an important factor for muscle quality the results in our study suggest that a more thorough investigation into the biochemistry of postmortem muscle acidosis is necessary. As expected from studies on other species (Nakayama et al., 1996; Sigholt et al., 1997; Morzel et al., 2002; Roth et al., 2002; Ruff et al., 2002), the preslaughter stressful handling of cod accelerates the onset and the resolution of rigor mortis (Fig. 2). The maximum average rigor was reached after 20–24 hours for the fish exposed to preslaughter handling stress while this was not reached before approximately 48 h postmortem for the control fish. The fillets from the stressed cod were judged to have more gaping and softer texture 10 days postmortem than the fillets from the controls (Table 1). The number of fish in each group in this study, however, was few and this experiment should be repeated on a larger scale. The mechanisms behind the increased gaping and softening are not completely known, but it is known that the strength of the muscle connective tissue at least in cod is profoundly weakened by a low pH (Love et al., 1972). A consequence could be that relatively rough handling such as machine filleting of cod with a low muscle pH, will result in a fillet product with tears and gaps. Avoiding preslaughter activity or stress may, therefore, be of particular importance in farmed cod so that filleting can be carried out prerigor. Assessments of rigor mortis development in farmed Atlantic cod exposed to slaughter stress (•) and control fish (○) stored in ice for 120 h postmortem (mean values ± SE, n = 4). This study was supported by the Research Council of Norway through grant no. 134988/140.