Quality Issues in Commercial Processing of Tilapia (Oreochromis
niloticus) in Zimbabwe
Simon Jarding (1), Lars Windmar (1), Rob Paterson, Ph.D. (2),
Jógvan Páll Fjallsbak (3)
(1) Danish Institute for Fisheries Technology and Aquaculture (DIFTA)
The North Sea Centre, DK-9850 Hirtshals, Denmark
Jh@difta.dk
(2) Elanne Farm
P.O. Box 32, Chirundu, Zimbabwe
(3) Danish Technological Institute, (DTI)
The North Sea Centre, DK-9850 Hirtshals, Denmark
ABSTRACT
In order to gain a high quality product in fish processing, it is necessary to obtain knowledge
of a variety of aspects that will influence the final product. This paper reports from trials
regarding rigor mortis of tilapia (Oreochromis niloticus) conducted at a commercial
processor in Zimbabwe. Rigor mortis onset occurred more rapidly for fish stored on ice than
for fish at ambient temperatures. The effect from cold shortening is believed to cause the fast
stiffening for fish stored on ice. It is recommended to process fish on ice within 60 minutes
after death, and fish at ambient temperatures within 120 minutes after death. Quick
processing is therefore of paramount importance.
INTRODUCTION
It is widely acknowledged that fish supplies from traditional marine and inland capture
fisheries are stagnating, and that the projected shortfalls in fish supply will probably be met
mainly from expansion within the aquaculture sector (FAO 1997). For cultured finfish and
shellfish, the annual contribution to total finfish and shellfish production rose linearly from
11.7% in 1989 to 18.5% in 1995 and presently more than a quarter of total world supply of
food fish is derived from aquaculture (FAO 1997). One of the more promising aquaculture
species is the freshwater tilapia. Tilapias do best at temperatures between 25°C and 30C,
making it suitable for culture in tropical environments (Muir and Roberts 1988; Balarin
1988). Tilapia is a white fish, and thereby represents a good substitute for the declining
supply of other white fish such as cod. However, in the recent past, consumers have become
more refined and demanding regarding safety and quality of fish (Brown and Sylvia 1994).
This in turn has placed restrictions on the third world fish processing sector and strict
regulations and requirements have been imposed by U.S. and E.U. authorities (FDA 1997;
EEC 1991) with respect to imports of seafood products.
To ensure that product quality is retained during production and storage, it is important to
have insight into specific issues such as rigor mortis. Bleeding fish leads to death followed
by post-mortem changes such as rigor mortis (Huss 1995). Fish enter rigor mortis when
ATP levels in the muscles reach a minimum after death, and myosin and actin are
interconnected irreversibly. The mechanisms underlying resolution of rigor mortis are still
not fully understood (Huss 1995).
Some of the earliest descriptions of rigor mortis in fish were given by Ewart (1887) and
Benson (1928). These studies included observations of the effects of several variables of the
time of rigor mortis. However, the earlier publications did not connect the process of rigor
mortis with quality and processing issues of fish. The fish industry had not focused on rigor
mortis, since there had been a natural time delay from catch to processing for marine species,
which traditionally was done on shore. Development in processing technologies which lead
to the processing of fish on board trawlers stimulated interest in the rigor mortis stage
(TemaNord 1995). Today it is well recognised that the phenomenon of rigor mortis is an
important quality aspect that may influence appearance and structure of fish muscle (Ando et
al. 1991; Berg et al. 1997; Buttkus 1963; Love et al. 1969).
Although several studies have examined the rigor process, there have been few investigations
in the way of method standardisation for fish. Buttkus (1963) described a method for
measuring the rigor stage of fish muscle, however no index was actually mentioned in his
study. A quantitative rigor mortis index was described in TemaNord (1995) where the
degree of rigidity at selected time intervals was measured. In this example the whole body of
a fish was placed on a horizontal table with half the body (tail) left overlying the edge.
Another method was developed to follow rigor mortis in which the progress of stiffening was
measured by observing the sag of the head when the fish was clamped in a vertical position
by the tail (Korhonen et al. 1990).
The increasing and demanding market of tilapia in the western world leads to the necessity of
optimising the processing of tilapia and the urgency to gain increased knowledge regarding
parameters affecting product quality. In the present study onset of rigor mortis using tilapia
(O. niloticus) was examined at a commercial Zimbabwean producer.
MATERIALS AND METHODS
The experiments were made on site at the Elanne (Pvt.) Ltd. fish farm (Chirundu, Zimbabwe)
and tilapia (O. niloticus) reared and grown on the farm were used during experimentation.
Fish were harvested and transported (app. 10 minutes in a live transport tank) from the growout channel to a holding tank. Animals were taken after 10 minutes in the holding tank and
were transferred to a 40 L plastic container.
Whole fish
The experiment involved four groups each consisting of five fish. Animals were sacrificed
using two methods: two groups were instantly killed by cranial fracture, and a further two
groups were bled for 10 minutes after severance of the gills and or ventral aorta. Fish were
weighed and placed on the rigor measurement apparatus (Figure 1) where they were left
throughout the measurement period. Animals were left either at an ambient temperature
(20C 3C) or iced (2°-5C internal temperature, using a mercury thermometer). Average
weight and standard deviation of fish in the four groups are listed in Table 1. In the described
configuration the posterior part of a fish was held down by a vice, while the anterior part was
positioned on an adjustable board. During rigor measurements the board was lowered.
Measurements were made at the point when the tail lifted from the adjustable board. Both
horizontal and vertical (X,Y) parameters were taken using a millimetre ruler (Figure 1).
(x)
Table
(xi,yi)
(y)
Figure 1. Sketch of Co-ordinate System Used When Measuring Rigor Mortis Development. (Xi, Yi) Is the
Tail End Point of the Fish.
Preliminary experiments were undertaken regarding the portion of the fish hanging over the
side. Two different approaches were investigated:
1. Body anterior to the gill cover, positioned over the side.
2. Half of the body length, positioned over the side.
It was concluded that fish with the part anterior to the gill-cover positioned over the side gave
results with greatest spectra, which would also give larger accuracy during measurements.
This method was therefore used in all further experiments.
Table 1. The Four Groups Employed During Wwhole Fish Rigor Experiments. No
Significant Differences in Weight Were Recorded Between Groups (P<0.05).
Temperature
Ambient
2-5C
Killing method
Bled for 10 minutes
585,0  91,0 g
480,6  97,3 g
Cranial fracture
509,3  57,8 g
485,1  63,8 g
Result Treatment
For whole fish the horizontal and vertical measurements were converted into an angle
measurement, using the formula: Angle  ARCTAN
( Xn  X0 )
(Yn  Y0 )
;
n = 1,2,3,...,n i
(X0,Y0) was set to be (0,0) and (Xn,Yn) in the following measurements.
Statistical Analyses
For all pairwise comparisons Student-Newman-Keuls t-test was used (P < 0.05) to evaluate
the data from the experiment. One Way Repeated Measures ANOVA was used when more
than two groups were compared (P < 0.05) with all pairwise multiple comparison procedures
(Student-Newman-Keuls Method). All pairwise comparisons compared all possible pairs of
treatment. The computer package SigmaStat for Windows Version 2.0 (Jandel Corporation)
was used for this purpose.
RESULTS
Determining Placement of Fish on Rigor Apparatus
90
80
70
Degrees
60
50
1
2
40
30
20
10
0
0
30
60
90
120
150
180
210
Time, min
Figure 2. Determining Placement of Fish on Rigor Apparatus.
.
From Figure 2 it is seen that fish with the part anterior to the gill-cover over the side (1) gave
results with greatest diversity compared to fish with half the body length over the side (2).
The difference in the means at 0 min. was app. at an angle of 15. Due to a spectrum ranging
from app. 45 to 90 for (1) compared to app. 60 to 90 for (2), the first method was
therefore applied in the following experiments.
Rigor Mortis, Whole Fish
Definitions used during the rigor mortis stage are listed in Table 2 and defined from the
results gained from the experiment.
Table 2. Definitions of the rigor mortis stage.
Not in rigor
Angle
<40
Onset of rigor
40-83
Full rigor
>83
Onset was found when an angle was significantly different from the initial angle (P < 0.05).
Full rigor was found when the angle was equal to the maximum angle (P < 0.05). Onset of
rigor mortis was significantly faster for fish stored on ice than fish stored at ambient
temperatures (Figure 3 upper part). However, there were no differences between the two
killing methods applied in the experiment. All groups reached full rigor at app. 83.
Animals stored on ice reached this point after 210 minutes, whereas animals stored at
ambient temperatures reached full rigor after 1200 minutes (Figure 3 lower part). All groups
also entered onset of rigor after time 0, though at different speeds, where fish stored on ice
had reached a significantly (P < 0.05) higher level than fish stored at ambient temperatures
after 30 minutes, and this result was observed until full rigor. After 60 minutes fish stored on
ice had reached angles of app. 70, whereas fish stored at ambient temperatures only achieved
50 angles. The experiment for groups stored at ambient temperatures were terminated after
30 hours since fish were spoiled at this point. For groups stored on ice the experiment was
terminated after 6 days, however fish were kept on ice in an insulated box for another 6 days
after which fish were still deemed to be in full rigor (see Table 3).
Table 3. Rigor Development for Whole Fish Stored at Ambient Temperatures and at
0C.
Time
Non-bleed
Bleed
Non-bleed
Bleed
stored at ambient
stored at ambient
stored on ice
stored on ice
a
a
a
0
36,4 1,7
34,3 1,4
33,9 2,2
32,5 2,8 a
30
41,1 3,9 a
39,0 4,4 a
54,0 11,7 b
63,0 11,7 b
60
51,0 7,6 a
51,0 10,5 a
69,2 3,8 b
72,9 9,3 b
90
55,6 4,9 a
56,4 9,7 a
77,4 2,3 b
76,6 6,4 b
a
a
120
62,3 4,9
63,2 6,3
210
72,8 6,4 a
72,7 3,7 a
84,3 2,8 b
83,1 1,9 b
420
77,8 4,4 a
75,7 2,7 a
1200
82,0 2,9 a
82,0 2,5 a
83,2 2,3 a
82,4 2,1 a
1740
84,5 1,8 a
84,3 2,5 a
2880
84,8 2,5 a
84,3 1,6 a
4500
83,3 2,2 a
82,9 2,2 a
5430
83,6 2,6 a
82,3 2,3 a
8310
82,2 2,4 a
81,7 1,2 a
90
b
70
b
b
Degrees
60
a
a
b
50
40
a
30
b
b
a
a
b
b
80
a
a
a
a
Non-bleed, stored at ambient
Bleed, stored at ambient
Non-bleed, stored on ice
Bleed, stored on ice
20
10
0
0
30
60
90
120
150
180
210
240
270
300
Time, min.
90,0
80,0
70,0
60,0
Degrees
Non-bleed stored at ambient
50,0
Bleed and stored at ambient
Non-bleed stored on ice
40,0
Bleed and stored on ice
30,0
20,0
10,0
0,0
0
20
40
60
80
100
120
140
Time, hours.
Figure 3. Whole Fish Stored at Ambient Temperatures (23C) and 5C for 5 hours. Two different killing
methods were applied. The figure above shows the onset of rigor mortis until 300 minutes and letters
denote significant differences (P < 0.05) at its respective point of time. The other figure shows the rigor
mortis stage over app. 140 hours (6 days).
DISCUSSION
Onset of rigor mortis was faster for fish stored on ice than for fish stored at ambient
temperatures. Abe and Okuma (1991) gained similar results with carp and suggested that
rigor mortis proceeds faster with increasing difference between live acclimation temperatures
and storage temperatures. There appears to be a difference in rigor mortis onset between
temperate and tropical fish species. Temperate freshwater fish are reported to go faster into
rigor with increasing temperatures (Tomlinson et al. 1961). Carp in the study by Abe and
Okuma (1991) achieved full rigor mortis after 24 h on ice when acclimatised to 30C. In
contrast, fish stored on ice in the present study reached full rigor mortis after 2½ h. The
phenomenon that tropical fish become stiff shortly after death when stored on ice has in the
literature been referred to as cold shortening (Curran et al. 1986 I+II). Cold shortening
occurs when the fish muscle is chilled prior to the use of ATP post mortem. The chilling has
an inhibiting effect on Ca2+-pump and this creates a contraction of the muscle. The level of
ATP is still high enough to exclude the fish from being in rigor mortis but nevertheless a
contraction occurs due to the cold shortening. Cold shortening in tilapia was already in 1986
by Curran et al. (1986, I) pinpointed to have major implications in terms of handling fish in
tropics post harvesting.
Similar results were found for tilapia with respect to the onset of rigor mortis or cold
shortening but, in contrast to studies by Curran et al. (1986 I+II), in this study it was not
found that it would be possible to fillet the fish after they had entered the stiffening phase.
This was due to the fact that the fish muscle never appeared in a relaxed stage after rigor
mortis. It is obvious that the ATP level must have been conversed at this stage and that the
effect from the cold shock had ceased its impact. The reason why the fish in this study stayed
in a very stiff phase is still unclear. However, further experiments must be conducted in a
more controlled environment and with access to laboratory facilities. From a practical point
of view the most interesting thing is that the fish remained stiff and not suitable for filleting.
No difference was found in time duration until onset or strength of rigor mortis between the
two killing methods. It is generally accepted that stressed fish will go into rigor mortis more
rapidly than unstressed animals (Tomlinson et al. 1961; Nakayama et al. 1992), and stressed
fish are also reported to generate greater tension in fish muscle post mortem (Nakayama et al.
1992). In trout, a cranial fracture is shown to delay onset of full rigor mortis, compared to
other killing methods believed to involve stress (Azam et al., 1990). If this was the case, then
fish in the present study must be assumed to be stressed at the start of the experiment. Under
commercial conditions the bleeding step will therefore have no significance on the stress
level or the onset of rigor mortis, if fish are harvested under stressful conditions during catch,
transport and holding.
Rigor mortis strength is reported to be linked to temperature in carp (Abe and Okuma 1992),
giving stronger rigor mortis tension with larger differences between acclimated and storage
temperatures. The current findings, however, showed similar rigor mortis strength for tilapia
kept on ice and at ambient temperatures. Again, stress factors may explain these results. A
possible solution to avoid the fast onset of rigor mortis problem is the application of
modified harvesting methods. A critical limit of when to stop processing a whole fish for
fillets was set to be the mean of onset of rigor mortis interval, being (40+83)/2, 61.5C.
Thus fish stored on ice should not be processed after 60 minutes on ice, whereas the critical
limit is reached after app. 120 minutes for fish left at ambient temperatures.
The rigor mortis stage of fish kept on ice was followed cautiously for 6 days and even after
an additional 6 days on ice no resolution was observed. This is inconsistent with other
findings which show relaxation after 8.5 and 24 h for exhausted rainbow trout stored without
and in ice, respectively (Tomlinson et al 1961). Unstressed tilapia (Tilapia mossambica) are
reported to enter onset of rigor mortis after 2 h, reach full rigor mortis after 7½ h and resolve
after 11½ h. In the same study, freshwater Mrigal (Cirrhina mrigala) were shown to enter
onset of rigor mortis after 5½ h, reach full rigor mortis after 13 h and resolve after 56 h
(Pawar and Magar 1965) when kept in crushed ice at app. 2C. In a study by Ando et al.
(1990) no resolution of rigor mortis was observed within 72 h for carp and rainbow trout
stored at 5C after which the experiment was terminated. These fish were obtained from a
city market and were therefore likely to be in an exercised or exhausted state. Findings
regarding duration of rigor mortis are thus variable but duration of rigor mortis recorded by
the present study, >12 days, represents one of the longest on record.
The method for measuring rigor mortis in whole fish used in this study differs from other
studies (TemaNord 1995; Korhonen et al. 1990) where a percentage rigor index was used.
Here an angle of the bending degree was calculated which is believed to ease the
understanding and interpretation in a written text. The angle can be understood independent
of a graph. The placement of fish when measuring the rigor mortis stage also differs from
other studies (TemaNord 1995; Korhonen et al. 1990) but this must be dependent on size and
shape of the fish species used. TemaNord (1995) also approves this.
It is generally accepted that fish should not be filleted in rigor mortis, since fillet quality will
be reduced. Fish should therefore be filleted either prior to or after the onset of rigor mortis.
The long rigor mortis period of tilapia points to the need for processing pre-rigor mortis. It
is also important that fish are frozen prior to rigor mortis since rigor mortis of animals will
develop gaping when frozen (Love et al. 1969).
CONCLUSION
Rapid processing of tilapia (O. niloticus) after death is essential since onset of rigor mortis
will occur immediately and within 1-1½ hours the level of rigor mortis will be too high for
filleting in a commercial harvest situation. Cold shortening is an aspect, which must be
considered in handling practices post harvest. Tilapia will stay in rigor mortis for at least one
week, leading to decreased shelf life compared to fish processed pre-rigor, or even spoilage
while waiting for resolution of the rigor mortis stage.
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