Ž .
for rainbow trout Keene et al., 1998 , without causing swimming reactions. A model muscle high activity group was generated using electro-stimulation post-mortem. The
Ž .
fish used were rainbow trout Oncorhynchus mykiss , as they are good models for the more valuable salmon flesh, having the same muscle pigments and similar constraints of
commercial flesh quality. They were used preferentially to salmon for this experiment as they were easier to obtain and hold at the experimental facilities at Langford.
2. Materials and methods
Forty fish were selected at random from a population of one thousand rainbow trout of average weight 1.5 kg. The fish had been raised together from hatching. None of the
Ž fish selected were maturing and all had been fed a commercial trout diet Vextra Classic,
. EWOS, U.K. containing the pigment astaxanthin at 50 mgrkg of diet. All fish were
placed in a 3-m diameter tank with a 1-m depth of water, fed by spring water at 16 8C
and oxygenated to a level of 85 saturation. The fish were left to recover, unfed, from the handling for 3 days.
The water level in the tank was slowly reduced and the water supply shut off. The
w
Ž .
anaesthetic AQUI-S AQUI-S New Zealand, New Zealand was slowly added to
achieve a concentration of 17 ppm. This resulted in deep anaesthesia of the fish after about 30 min. The fish were then netted out by hand and killed by a blow to the head.
Ž Twenty fish were chosen at random and were electro-stimulated immediately within
. 1 min after being killed in order to model a high level of muscle activity. This was
carried out by inserting a needle dorso-ventrally through the flesh just behind the head, passing close to the backbone. The needle was attached to an electrode connected to a
Ž .
control box J.S. Engineering, U.K. . A second electrode from the box was attached by a crocodile clip to the tip of the caudal peduncle, just at the end of the spine. Each fish
received 94 V pulsed direct current at 14.3 Hz, causing muscular contractions along the length of the fish. After 2 min of stimulation, the fish were disconnected from the
power. The remaining twenty fish were left unstimulated, resulting in the ‘rested’ group.
Ten fish from each treatment group were filleted, individually tagged and a series of measurements were made on the area between the dorsal fin and the midline. The
surface of the fillet was wiped with absorbent paper and then the core temperature was Ž
. determined using a temperature probe Whatman, U.K. . The pH was measured using a
Ž .
PHM8 Portable pH Meter Radiometer, Denmark
with a spear combination pH Ž
. electrode Mettler Toledo . The colour of the fillets was determined using a CR-200
Ž .
Chromameter Minolta, U.K. with a pulsed xenon arc lamp giving diffuse illumination, Ž
. with 0
8 viewing angle over an 8-mm diameter area. This resulted in CIE 1976 values Ž
U
. Ž
y1
Ž
U U
.. Ž Ž
U 2 U 2
.. for lightness L , the angle of hue tan
b ra and chroma 6 a
q b .
After measuring, the fillets were placed one layer deep on a sheet of plastic over a layer of ice in polystyrene boxes. A second layer of plastic and ice was placed above the
fillets and the boxes stored in a dark chiller at 4 8C. Further measurements were carried
out on the fillets after approximately 1, 2, 6, 8, 22, 33, 45, 57, 69 and 75 h. At the end of the storage period the fillets were measured a final time and were
Ž scored subjectively for colour using the Roche colour card F. Hoffmann-La Roche,
. Switzerland by four assessors. The fillets were then drawn, skin down, over a right
angle using a constant force. This simulated rough handling and potentially caused the appearance of gaping along the fillet, which was then scored subjectively by four judges,
Ž .
Ž .
Ž using a four point scale: 0 no gaping , 1 one or two breaks , 2 four or more small
. Ž
. breaks , or 3 large breaks .
The remaining 10 fish from each group were used to follow the progression of rigor. Ž
. A modified version of the apparatus used by Azam et al. 1990 was set up. The fish
were placed on a flat table so that the body behind the posterior end of the dorsal fin was hanging over the edge, unsupported. The angle between the horizontal and the line
from the corner of the table to the tip of the tail was then measured to the nearest 5 8
Ž .
termed the angle of droop . The fish were then turned over and the measurement repeated for the other side of the fish. The fish were measured at 0, 4, 8, 12, 20, 24, 34,
48, 56, 72 and 96 h after slaughter. In between measurements the fish were placed on ice in polystyrene boxes which were stored in a chiller at 4
8C. The significance of the differences between the groups in temperature, pH, lightness,
angle of hue, chroma and rigor development were tested at each time post-slaughter Ž
. using a repeated measures analysis of variance ANOVA . In order to determine the
rigor changes, the mean angle of droop for each fish was calculated from the two angles measured and then the ANOVA performed. This compensated for the fish setting at an
angle during rigor. The mean Roche colour card scores for each fish were compared using Student’s t-test. A Mann–Whitney U-test was applied to the non-parametric data
from the gaping scores.
3. Results
