The texture of Atlantic salmon ( Salmo salar)muscle as measured instrumentally using TPAand Warner–Brazler shear testJon Olav Veland1* and Ole J Torrissen2

1Mjatveitvegen, N-5918 Frekhaug, Norway2Institute of Marine Research, Department of Aquaculture, N-5392 Storebø, Norway

Abstract: Muscle texture measurements were performed on Atlantic salmon (Salmo salar) using two

different instrumental methods; The Texture Pro®le Analysis, which is a uniaxial compression test,

and the Warner±Brazler shear test. The performances of the two tests were evaluated as to their ability

to differentiate between recently killed salmon and salmon stored on ice for up to 24 days. Both tests

performed well, but the shear test was slightly more sensitive than the compression test. Further,

salmon were either starved or fed for two weeks prior to slaughter. The muscle from fed salmon lost its

strength slightly faster than that from starved salmon, but this difference was only detectable during

the ®rst two days of chilled storage.

The effects of temperature, ®sh size and degree and mode of deformation on the instrumental test

results were studied and were found to be signi®cant. Also, the sample geometry, ie the thickness of the

®llet was found to have a very signi®cant effect on the TPA-test results.

# 1999 Society of Chemical Industry

Keywords: Atlantic salmon; muscle texture; chilled storage; post-mortem degradation

INTRODUCTIONAtlantic salmon (Salmo salar) is often smoked or

marinated, then cut into thin slices and eaten without

any heat treatment. The ®rmness of the raw or

processed muscle is a critical quality parameter

determining the acceptability of the product. Chilled

stored salmon and trout may have acceptable taste,

odour and colour, but may occasionally be too soft and

`mushy', falling apart when cut into slices or offering

too little resistance to mastication.

The post mortem tenderisation of ®sh muscle has

been demonstrated in several microscopy studies to be

closely related to the degradation of collagen ®brils of

the endomysium and perimysium. In the case of severe

collagen breakdown, the myotomes are separated from

the myocommata and gaping occurs.1±4 Although little

evidence exists on the exact mechanism of this

degradation, these same studies suggest that endo-

genous proteinases are involved. Such a mechanism

has also been suggested in studies on the solubility of

collagen from ®sh muscle,5±7 and in studies on

sexually maturing salmon.8±12

The proteolytic degradation may be facilitated by

the post mortem accumulation of lactic acid and

reduced pH, as these conditions can induce leakage

of proteolytic enzymes from lysosomes (Whiting et al1975) and break collagen crosslinks, thus making the

collagen more available as a substrate for the proteo-

lytic enzymes.5,14

In order to develop optimal farming conditions,

feeding regimes and slaughter procedures for the

production of consumer-desired quality there is a

need for a reliable instrumental test for evaluation of

salmon muscle texture. This test should be robust and

simple to perform, and ideally correlate well with the

sensory perception of soft or ®rm texture. Segars etal,15 using a tensile test, demonstrated that ®sh muscle

generally responds to deformation in two phases. Up

to about 15% tensile deformation ®sh muscle behaves

as a soft rubbery material with a low deformability

modulus. At higher deformations the deformability

modulus increases. In other words, a small increase in

deformation results in an increase in the resisting

force. The change in deformability modulus occurs as

the deformation increases from recoverable to irre-

coverable deformation. When evaluating the textural

properties of salmon muscle it seems reasonable to

suggest that the deformation applied in the testing

should be in the range of irrecoverable deformation.

This is because the resistance the muscle offers to such

deformation is comparable to the resistance during

mastication and also to the stress or deformation the

muscle tolerates without falling apart when cut into

slices.

Journal of the Science of Food and Agriculture J Sci Food Agric 79:1737±1746 (1999)

* Correspondence to: Jon Olav Veland, Kringsjaveien 28, N-5163 Laksevag, Norway(Received 19 June 1998; revised version received 5 May 1999; accepted 18 May 1999)

# 1999 Society of Chemical Industry. J Sci Food Agric 0022±5142/99/$17.50 1737

This view is also supported by Szczesniak16 who

reported that instrumental tests that apply small

recoverable deformations generally correlate well with

sensory assessment performed outside the mouth,

such as the `®nger test'. Large strain, failure-type tests

on the other hand correlate better with oral evalua-

tions. Similarly, Ando et al17 reported that in a

puncture test the maximum resistance of the sample

to penetration correlated well with sensory perception

of ®rmness as evaluated by chewing slices of the

muscle.

In the present study both small and large deforma-

tion compression tests and a shear test were applied,

and the suitability of the tests were evaluated as their

ability to differentiate between recently killed salmon

and salmon stored on ice for up to 24 days.

Furthermore, the salmon were either starved or fed

in excess for two weeks prior to slaughter, in order to

investigate if the common practice of starving salmon

before slaughter could have an effect on the postmortem development of muscle texture. Initially, the

effects of ®sh size/sample geometry, mode and degree

of deformation, and temperature at the time of

measurement were investigated in two independent

studies.

METHODSTexture measurementAll texture measurements were performed using the

TA-XT2 Texture Analyser from Stable Micro Sys-

tems, Surrey, England. The instrument was controlled

by a computer using the Texture Expert v 1.0

software. Unless otherwise stated, all texture measure-

ments were performed in the epaxial muscle of the

Norwegian Quality Cut (NQC), which is the part of

the ®llet con®ned by vertical lines at the posterior end

of the dorsal ®n and the anterior end of the anal ®n.

Texture Pro®le Analysis (TPA)

A spherical probe with a diameter of 25mm was used

for all tests in this study. The ®llet was placed under

the probe and the probe moved downwards at a

constant speed of 5.0mm sÿ1 in experiments 1 and 2,

and 2.0mm sÿ1 in experiment 3. When the probe ®rst

came in contact with the ®llet, the thickness of the ®llet

was automatically recorded by the software. The probe

continued downwards a ®xed distance or a ®xed

percentage of the ®llet thickness, returned to the initial

point of contact with the ®llet and stopped for a set

period of time (5.0s in experiments 1 and 2, 15s in

experiment 3) before the compression was repeated.

During the test run, the resistance of the ®llet to

compression was recorded every 0.01s and plotted in a

force±time plot as illustrated schematically in Fig 1.

The vertical lines labelled `zero' or `max' mark the

moments of zero and maximum compression. A con-

stant compression speed was used in all tests and the

areas under the force±time curve are therefore directly

proportional to the work performed by the probe

during the downstroke and by the ®llet during the

upstroke.

From the force±time plot the TPA parameters were

read or calculated as described by Bourne,18 except

resilience, which was calculated as described in the

TA-XT2 manual. A brief description of these calcula-

tions is included below.

Because the spherical surface of the probe used in

the present study at all times was lubricated by water

and oil from the ®llet, all forces applied by the probe

on the ®llet could be assumed to be perpendicular to

the surface of the probe, as there would be only

negligible friction. As a result of this, the force

distribution on the surface of the probe would include

forces with a greater angle to the direction of the

compression as the probe was forced deeper into the

®llet. The TPA parameters should therefore give a

measure of the response of the muscle when subjected

to tensile stress as well as compression and shear stress,

and were read/calculated as follows:

Hardness. The resistance at maximum compression

during the ®rst compression. The hardness of the

sample at the ®rst bite.

Hardness (2). The resistance at maximum compression

during the second compression. The hardness of the

sample at the second bite.

Fracturability. The force at which the ®llet fractures

during the ®rst downstroke. Gives a measure of the

breaking strength of the muscle when subjected to

tensile, shear and compression stress. The force

required to bite through the surface of the ®llet.

Area 1. The area of the curve during the ®rst down-

stroke, which is proportional to the work performed by

the probe on the ®llet during the ®rst compression.

The work performed during the ®rst bite.

Area 2. The area under the curve during the ®rst

Figure 1. Schematic illustration of the force-time plot from a TPA test. Themoments of zero or maximum compression are marked with vertical lines.

1738 J Sci Food Agric 79:1737±1746 (1999)

JO Veland, OJ Torrissen

upstroke, which is proportional to the work performed

by the ®llet on the probe during the ®rst decompres-

sion. The work performed during relaxation after the

®rst bite.

Area 1�2. The area under the curve during the ®rst

compression cycle. The total work performed during

the ®rst compression cycle. Total work performed

during the ®rst bite.

Area 3. The area under the curve during the second

compression cycle. The total work performed during

the second compression cycle. Total work performed

during the second bite.

Cohesiveness. The area of the second compression cycle

(area 3) relative to the area of the ®rst compression

cycle (area (1�2)), Cohesiveness gives a relative and

dimensionless measure of how much of the muscle's

strength is retained after the deformation of the ®rst

compression. If cohesiveness=1, the muscle has

maintained its strength and regained its structure

completely during the pause between the compres-

sions, and offers the same resistance to the second

compression as to the ®rst. If cohesiveness is <1, the

deformation of the ®rst compression has been partly

irrecoverable.

Resilience. The area of the ®rst upstroke (area 2) relative

to the area of the ®rst downstroke (area 1). Resilience

gives a measure of the elasticity of the muscle, and

considers not only the distance, but also the force and

speed with which the ®llet bounces back after the

initial deformation. If resilience=1, all the work

performed by the probe during the downstroke is

returned by the ®llet during the upstroke. If resilience

<1, the ®llet has not recovered completely to its

original thickness, or has recovered with less force or

speed than it was compressed with.

Warner±Bratzler shear test

A borer with internal diameter 25mm was used to cut

out cylindrical samples, perpendicular to the plane of

the ®llet. The test cell consisted of a 3-mm thick steel

blade which had a 73°V cut into it, and was ®tted

through a 4-mm wide slit in a small table (like a

guillotine with aV cut into the blade). The blade was

not sharpened and ®tted loosely into the slit in the

table. For testing, the sample was placed on the table,

under the V of the blade, and was cut through as the

blade moved down with a constant speed through the

slit of the table (5.0mm sÿ1 in experiments 1 and 2,

and 2.0mm sÿ1 in experiment 3). The resistance of the

sample to shearing was recorded every 0.01s and

plotted by a computer in a force-deformation plot as

illustrated schematically in Fig 2.

From the force-deformation plot the shear par-

ameters were read/calculated as follows:

Fitting area. The area under the curve before shearing,

when the cylindrical sample is ®tted into the V of the

blade. This area attempts to measure how much work

is required before the sample fractures.

Breaking strength. The peak resisting force at the time

when shearing begins. Breaking strength should be

comparable to fracturability in the TPA tests, but the

shear test should develop more shear stress and less

compression and tensile stress in the sample.

Shear area. The area under the curve during shearing.

This area attempts to measure the work performed

during plastic (irrecoverable) deformation.

Total area. Fitting area�shear area. The total amount

of work required to cut through the sample.

Maximum shear force. The highest peak of the curve,

which is the maximum resistance of the sample to

shearing.

EXPERIMENTALThe experiments described in this article have been

approved by the local responsible laboratory animal

specialist under surveillance of the Norwegian Animal

Research Authority (NARA) and registered by the

Authority. The experiments have thus been conducted

in accordance with the laws and regulations control-

ling experiments in live animals in Norway, ie the

Animal Protection Act of20 December 20 1974, No

73, chapter VI, sections 20±22 and the Animal

Protection Ordinance concerning Biological Experi-

ments in Animals of 15 January 1996.

The Atlantic salmon (Salmo salar) used in this

experiment were of local breed and were produced by

the Institute of Marine Research, Matre Aquaculture

Research Station under semi-commercial conditions.

During the sea-water production they were reared in

5.5�5.5�6.0m sea cages and fed a commercial

extruded salmon diet produced by T Skretting A/S

(Stavanger, Norway).

Figure 2. Schematic illustration of a force-deformation plot from a sheartest.

J Sci Food Agric 79:1737±1746 (1999) 1739

Instrumental texture of Atlantic salmon

Experiment 1Thirty salmon with an average weight of 1.2kg were

netted, 1±3 salmon at a time, and stunned by a blow to

the head. The ®sh were killed by cutting off all gill

arches on one side and bled in seawater (6±7°C) for

5±10min. The salmon were then gutted, packed on ice

in Styrofoam boxes and stored in a refrigerated room

(2°C) for 48h before they were ®lleted (total of 60

®llets). Twenty ®llets were used to cut out samples for

the shear test. Samples were not taken from the left

and right ®llet of the same ®sh. Four samples were cut

out from each ®llet, and incubated at 0, 4, 10 and

20°C (total of 20 samples at each temperature). The

40 remaining ®llets were then incubated at 0, 4, 10 and

20°C, 10 ®llets at each temperature. Left and right

®llets from the same ®sh were not incubated at the

same temperature. When a stable core temperature

was reached, the shear and TPA tests (10mm

compression) were performed.

Experiment 2Twenty-two salmon weighing from 0.51 to 8.2kg

gutted weight were slaughtered, gutted, stored on ice

for 48h and ®lleted as described above. The salmon

were taken from three different age groups; 15 salmon

weighing from 0.51kg to 1.0kg, ®ve salmon weighing

from 1.9kg to 2.7kg, and two salmon weighing 5.8kg

and 8.2kg. TPA tests were then performed on the

®llets with compression set to 6mm (17 tests), 10mm

(17 tests), 20% of ®llet thickness (17 tests), 30% of

®llet thickness (17 tests) and 40% of ®llet thickness

(12 tests). In order to avoid testing on the same

location more than once, tests were not performed

with all compression modes on the smaller salmon.

Because of this, fewer than 22 tests were performed

with each compression mode. A total of 102 shear tests

was also performed on samples from these ®llets (one

to ®ve samples from each ®llet). In order to obtain

shear and TPA tests from the same ®llets, the shear

samples were cut out of the epaxial muscle anterior to

the NQC.

Experiment 3Two hundred salmon weighing from 1.7 to 2.6kg wet

weight (average gutted weight 1.7kg) were transferred,

100 to each of two sea cages (5.5�5.5�6.0m). In one

of the cages the salmon were fed in excess (Royal AB

9mm pellets from T Skretting AS, Stavanger), while

the other group was starved. After 14 days all the

salmon were slaughtered, gutted and packed on ice as

described above. At 0 (3h after slaughter), 1, 2, 4, 8,

12 and 24 days of storage on ice, the length and gutted

weight of 12 ®sh from each group were recorded and

the ®sh ®lleted. On each ®sh two shear tests and two

TPA tests with compression set to 6.0 and 12.5mm

were performed. The force±time curves of the ®rst

downstroke of the TPA tests were also transformed to

show the normalised stress (resisting force/compressed

area) of the muscle as a function of the strain (ie

relative deformation=deformation/®llet thickness).

The stress-strain curves thus obtained were used to

compare the moduli of deformability (the slope of the

curve) of muscle from salmon stored on ice for

different lengths of time. Furthermore the curves were

used to determine the strain at which a shift from

recoverable to irrecoverable deformation occurred.

Statistical analysisAll statistical analysis was performed with the Statis-

tica v 5.0 software (Statsoft Inc, Tulsa OK 74104,

USA).

Multiple linear regression was used to estimate the

effects of incubation temperature and ®llet thickness

on TPA parameters. One-way ANOVA was used to

estimate the effect of incubation temperature on shear

parameters.

Simple linear regression and non-linear regressions

were used to estimate the effect of ®llet thickness or

gutted weight on TPA and shear parameters respec-

tively.

Multiple linear regressions were used to estimate the

effects of storage time on ice, treatment before

slaughter and ®llet thickness (TPA) or gutted weight

(shear) on the texture parameters. Storage time and

the texture parameter were both ln-transformed in

these regressions in order to achieve more homoge-

nous variances and improved linearity. Also, simple

linear regressions were ®tted to each texture parameter

and treatment group using storage time on ice as the

independent variable, and the slopes of the regression

lines were compared between treatment groups using a

t-test. Storage time and the texture parameter were

both ln-transformed in these regressions in order to

achieve more homogenous variances and improved

linearity.

RESULTSExperiment 1Incubation at 20°C resulted in a 15±25% decrease in

both TPA and shear parameters, as can be seen in Fig

3 for area 1 from the TPA tests and total area from the

shear tests.

Figure 3. Area 1 (TPA) and total area (shear) measured after incubation at0, 4, 10 and 20°C. Mean�SEM (box)�SD (bars).

1740 J Sci Food Agric 79:1737±1746 (1999)

JO Veland, OJ Torrissen

For the TPA parameters the effect of temperature

was found to be statistically signi®cant (p<0.05) for

hardness, hardness(2), area 1, area 2 and area (1�2).

The temperature effect was also close to signi®cant for

fracturability (p =0.055). For the shear parameters,

the effect of temperature was not statistically signi®-

cant, although very close for the total area (p =0.053).

Experiment 2The results clearly demonstrated that both the size of

Figure 4. Scatterplots of hardness, area 1, cohesiveness and resilience (TPA parameters) against fillet thickness with compression set to 6.0 (n =17) and10.0mm (n =17) or to 20 (n =17), 30 (n =17) and 40% (n =12) of fillet thickness. Scatterplot of total area (shear parameter) against gutted weight (72 samplesfrom 22 salmon).

J Sci Food Agric 79:1737±1746 (1999) 1741

Instrumental texture of Atlantic salmon

the ®sh and the geometry of the sample have

signi®cant effects on the instrumental measurement

of texture. The variation in TPA parameters was best

explained by the ®llet thickness, but with different

explanatory models for compression set to a ®xed

distance or to a percentage of ®llet thickness. The

variation in shear test parameters was best related to

®sh weight. Figure 4 shows how the peak forces, areas,

cohesiveness and resilience depended on ®llet thick-

ness with the different compressions, and how the

shear parameters depended on gutted weight.

When compression was set to a ®xed distance, the

peak forces and areas of the force±time plot decreased

with increasing ®llet thickness. For both 6- and 10-

mm compression tests this decrease levelled off for the

thicker ®llets (at less than 40±50% compression).

In the same tests cohesiveness and resilience

increased with increasing ®llet thickness. With com-

pression set to 10mm this increase was linear

(cohesiveness) or slightly progressive (resilience), but

for the 6-mm compression tests the increase levelled

off for the thicker ®llets. This occurred at approxi-

mately 20mm ®llet thickness, which corresponds to

30% compression.

When compression was set to a ®xed percentage of

®llet thickness, an almost opposite dependency on

®llet thickness was found. Increasing ®llet thickness

resulted in a linear increase in the peak forces

(hardness, hardness(2)) and a progressive increase in

the areas (area 1, 2, 3) of the force±time plot. A slight

but signi®cant linear decrease was found for resilience,

but cohesiveness was unaffected by ®llet thickness.

Fracturability was also unaffected by ®llet thickness,

but a clear fracture was only found in the 10-mm

compression tests.

Sample geometry was not a source of variation in the

shear tests, because a standard size sample was cut out.

However, a slight linear increase was found in all shear

parameters with increasing ®sh size. There was also a

very great variation within samples from the same ®sh,

especially in the two largest ®sh (the two clusters of

points at 5.8 and 8.2kg in Fig 4).

Experiment 3The development of rigor mortis was not objectively

measured, but was observed manually during ®lleting.

On day 0 no obvious signs of rigor mortis were present

during ®lleting. On day 1 all ®sh had entered rigor, but

a stronger contraction was seen on day 2. On day 4

rigor mortis had started to resolve and on day 8 all ®sh

were ¯exible post rigor.In all TPA parameters except cohesiveness a similar

trend was seen when compression was set to 12.5mm,

as there was a rapid initial decrease during the ®rst four

days of storage. After the fourth day the decrease

levelled off, but a slight decrease could still be

detected. The decrease in the TPA parameters was

also accompanied by a corresponding decrease in the

variation of each parameter. Cohesiveness differed

from this pattern, as it did not decrease throughout the

storage period.

From the 6-mm compression tests apparently

different results were obtained, as there was only a

very slight decrease in all parameters, except cohe-

siveness and resilience. Resilience decreased rapidly

during the ®rst four days of storage and then levelled

off, as described for the 12.5-mm parameters. Cohe-

siveness differed from the other parameters by

decreasing from day 0 to day 1 and then increasing

slightly from day 1 to day 2, before decreasing further

on day 4 and onwards.

The shear test parameters also developed similarly

to the 12.5-mm compression TPA parameters, but the

decrease in the shear parameters was slightly stronger

after the fourth day of storage. Figure 5 shows this

development for cohesiveness, resilience (6-mm com-

pression), hardness (12.5-mm compression) and

maximum shear force.

Figure 5. The development during 24days of storage on ice of cohesiveness,resilience (6.0mm compression),hardness (12.5mm compression) andshear area. Mean�SEM (box)�SD(bars).

1742 J Sci Food Agric 79:1737±1746 (1999)

JO Veland, OJ Torrissen

During the storage period there was also a strong

decrease in the deformability modulus of the salmon

muscle. This can be seen in Fig 6 as a decrease in the

slope of the curves of the normalised stress (resisting

force/compressed area) against strain (deformation/

®llet thickness). The major decrease in deformability

modulus occurred during the ®rst four days of storage.

Furthermore, the stress developed during compression

was almost unchanged throughout the storage period

for the ®rst 25±35% deformation (5±7mm compres-

sion). At deeper compressions, however, the stress-

strain curve has a much steeper slope for the recently

killed salmon than for those stored on ice for some

time.

The multiple linear regressions did not reveal any

differences between the starved and fed group, but

con®rmed that ®llet thickness and gutted weight had a

signi®cant effect on the TPA and shear parameters

respectively.

The fed group gave higher readings for most texture

parameters on days 0 and 1, but also showed a slightly

faster decrease, so that this difference was equalled or

reversed on day 2. On the fourth day and later in the

storage there was very little difference between the two

groups. This difference in the rate of decrease was also

seen as greater (negative) slopes for the fed group

when simple linear regressions were ®tted to each

group for each texture parameter. The fed group had a

faster decrease for all TPA parameters (when a

signi®cant decrease was found), and the differences

was signi®cant at the 5% level for hardness(2) and area

3 (12.5-mm compression). The fed group also had a

signi®cantly faster decrease in maximum shear force,

and the same trend was present in shear area and total

area, but not in ®tting area and breaking strength.

DISCUSSIONThe temperature study demonstrated that when

temperature was increased from 0 to 20°C, there

was a 20±25% decrease in the force (hardness,

hardness (2)) and work (areas 1 and 2) required to

achieve a deformation in salmon muscle. A similar

decrease in strength has also been demonstrated by

Love et al19 on isolated myocommata from cod. In the

same study it was also demonstrated that much of the

strength of the myocommata was recovered upon

cooling. This evidence suggests that the observed

decreases in the TPA and shear parameters are caused

by a weakening of the connective tissue of the muscle.

Regardless of the underlying mechanism, the results

demonstrate that temperature is an important factor to

control in texture studies. Also, the results demon-

strate that salmon should always be chilled before

®lleting, slicing or handling that can cause damage

such as gaping in the muscle.

The studies with varying ®sh size demonstrated that

®llet thickness must be included as a source of

variation when a compression test is applied directly

to the ®llet. Only cohesiveness and resilience with

compression set to a ®xed percentage of ®llet thickness

were unaffected or relatively little affected by ®llet

thickness. Cohesiveness and resilience are both mea-

sures of the elasticity of the muscle, as they describe

the ability of the muscle to recover from deformation

and offer resistance to a subsequent deformation.

However, neither parameter describes the force

required to achieve these deformations. Consequently,

these parameters cannot describe the hardness and

strength of the muscle, and are not alone suf®cient to

describe the texture of the muscle.

When compression was set to a ®xed distance both

cohesiveness and resilience increased with increasing

®llet thickness, demonstrating that the thicker ®llets

were able to absorb more of the deformation as elastic

deformation. For the 6-mm tests this increase levelled

off at approximately 20mm ®llet thickness, which

corresponds to 30% deformation. This indicates that

the shift from recoverable to irrecoverable deformation

which was demonstrated by Segars et al15 as occur at

approximately 15% deformation in a tensile test,

occurred at approximately 30% deformation in the

compression tests applied in this study.

This was also indicated in the starvation and storage

study as an increase in the modulus of deformability

when compression was continued beyond 5±7mm,

which again corresponds to a deformation of approxi-

mately 30%. Although this shift could be recognised,

at least in the ®llets from freshly killed salmon, it was

much less obvious and occurred at a greater deforma-

tion than that described by Segars et al15 for tensile

tests. This is because the compression test only

gradually developed tensile stress in the sample, and

at the same time developed more compression and

shear stress. Consequently, the muscle ®bres affected

by the deformation would not all be stretched at the

same rate and would not all reach the limit of elastic

(recoverable) deformation at the same time.

The starvation and storage study demonstrated that

the most dramatic changes in texture occurred during

the ®rst four days of storage. During this period there

Figure 6. The normalised stress of fillets during the first compression of theTPA test as a function of the relative deformation. Starved and fed groupsare pooled.

J Sci Food Agric 79:1737±1746 (1999) 1743

Instrumental texture of Atlantic salmon

was a strong decrease in all 12.5-mm TPA parameters

except cohesiveness, in all shear parameters and in

cohesiveness and resilience from the 6-mm TPA tests.

After the fourth day the decrease levelled off, but could

still be recognised, particularly in the shear par-

ameters.

Cohesiveness was only successful in detecting postmortem changes when 6-mm compression was used.

This suggested that cohesiveness re¯ected changes in

the response of the intact/undamaged muscle ®bres to

forced stretching and relaxation. When 12.5-mm

compression was applied, irrecoverable deformation

occurred and the muscle ®bres were unable to contract

to their original shape during the 15-s pause between

compressions. The peak in cohesiveness on day 2 can

perhaps be explained by a peak in rigor contraction

occurring at the same time. At the time of maximum

rigor contraction it seems likely that the muscle would

contract strongly after forced stretching and thus

develop a peak in cohesiveness.

Resilience developed similarly for both 6- and

12.5-mm compression tests. This suggested that

resilience was less dependent on the functioning of

the myo®brillar system and re¯ected more the

elasticity of the connective tissue and the viscoelasti-

city of the muscle.

Area 1 and hardness decreased clearly only when

12.5mm compression was used. This can also be seen

from the force-deformation curves, where there was a

clear decrease in the deformability modulus only in the

range of plastic deformation. (>30% or 5±7mm). It

can thus be concluded that during chilled storage two

major changes occurred in the response of salmon

muscle to applied forces: When the deformation was

within the elastic range of the muscle (recoverable

deformation) there was little decrease in the resistance

of the muscle to initial compression, but the muscle

became less resilient and offered less resistance to

repeated compression (cohesiveness). When the de-

formation was continued into the plastic range

(irrecoverable deformation) the muscle offered less

resistance to the initial deformation. This was seen as a

decrease in hardness, fracturability and area 1 from the

12.5-mm tests and as a decrease in the shear par-

ameters. The decrease in these parameters was caused

by a decrease in the modulus of deformability, as could

be seen from the stress-strain curves from the TPA

tests.

A small difference in the development of the texture

parameters during storage was found between the

starved and fed group. The fed group had higher

values for most texture parameters on the day the ®sh

were slaughtered, but this difference was eliminated by

a faster decrease in the same parameters during the

®rst two days of storage. Although this difference in

the rate of decrease was found to be statistically

signi®cant, it did not result in differences between the

groups after the second day of storage, when the

decrease in the texture parameters levelled off for both

groups.

Several studies have demonstrated that the physio-

logical condition of the ®sh at slaughter affects the postmortem development of the muscle. Fish exposed to

exhausting slaughtering procedures will go through

rigor mortis faster and with a stronger contraction, have

a faster accumulation of lactic acid and decrease in

muscle pH and consequently a faster tenderisation and

loss of breaking strength of the muscle.20±27 These

observations have been explained by a reduced

content of energy-rich phosphates (ATP, PCr) in the

muscle of exhausted ®sh, and accordingly an earlier

onset of anaerobic glycolysis, accumulation of lactic

acid and failure to maintain homeostasis.

On this background one could assume that the

muscle of starved ®sh would have a lower energy status

and consequently go through the post mortem changes

more rapidly, leading to tenderisation.

On the other hand, several studies have demon-

strated that during starvation, ®sh reduce the metab-

olism of the white muscle by decreasing both protein

synthesis rate,28,29 activity of glycolytic pathway30,31

and the rate of muscle protein degradation.32 From

this information, the opposite hypothesis may be

assumed, as starved ®sh could be expected to exhaust

their energy reserves more slowly post mortem, and thus

preserve their muscle texture.

A combination of these apparently contradiction

theories can perhaps explain the differences observed

between the starved and fed groups in our study. If the

assumption is made that the texture parameters

re¯ected the post mortem changes described above,

then the lower readings of the starved group immedi-

ately after slaughter could be explained by a lower

energy status of the starved ®sh, giving these ®sh a

`head start' in the series of post mortem changes.

Furthermore, the faster initial decrease in texture

parameters observed in the fed group could be

explained by a higher metabolic capacity of the muscle

tissue, enabling these ®sh to `catch up' with the starved

group.

If the ability of each test to differentiate between

recently killed and stored ®sh is taken as a measure of

how well the test describes the texture of the ®llet, the

shear test is the most appropriate. The shear test is

perhaps also the more appropriate as an imitation of

mastication, as it applies large deformation with semi-

sharp edges. Because a standardised sample was used

the shear test was less affected by varying ®sh size, and

the observed dependency can be assumed to be caused

by a true difference in the shear strength of smaller and

larger ®sh. The relatively greater variation in the shear

test results in the study with varying ®sh size can

probably be explained by the fact that the samples

were not all taken from the same location in the ®llet.

Such variation has also been reported by Sigurgisla-

dottir et al.33

The shear test applies only one deformation to the

sample and thus gives no measure of how much of the

applied work is absorbed as elastic deformation, or of

the work required in successive chewings. Such a

1744 J Sci Food Agric 79:1737±1746 (1999)

JO Veland, OJ Torrissen

measure is attempted by the TPA test in the

calculation of resilience and cohesiveness, but cohe-

siveness was only successful in describing post mortemchanges when the applied deformation was small (30%

or less). Furthermore, the TPA test was successful in

describing the loss of strength and decrease in the

modulus of deformability only when the deformation

was continued beyond the elastic range.

Neither of the tests applied in this study was

designed speci®cally for testing on ®sh muscle. The

Warner±Bratzler shear test was originally designed for

testing on meat, which generally has a much tougher

texture than ®sh muscle. The test was designed as a

universal test for use on a wide variety of solid and

semi-solid foods. Several test condition parameters

need to be considered in order to design an optimal

test for ®sh muscle.

The temperature of the ®sh muscle was demon-

strated to have an effect on the texture parameters, and

must be kept constant during testing.

The geometry of the sample strongly affects the

instrumental texture parameters. A standard sample is

preferable to testing directly on ®llets of various

thickness. If this is not possible, the sample geometry

must be included as an explanatory factor when the

test results are analysed. In this case an approximately

linear relationship between ®llet thickness and the

texture parameters can be assumed if the variation in

®llet thickness is small. With more variation in ®llet

thickness, this correlation will not be linear and

comparison becomes more dif®cult and uncertain. If

a standard sample is used, it should always be taken

from the same location in the ®llet in order to eliminate

variation within individuals.

Using a standard size sample will also eliminate the

question of whether a constant deformation (mm

compression) or a constant strain (% compression)

should be used. Regardless of the mode of the com-

pression, the degree of compression must be kept well

within the range of either elastic or plastic deformation

for all samples, in order to get comparable results. This

means that the shift in the modulus of deformability

for the speci®c test should be found from stress-strain

curves before the degree of deformation is decided.

Instrumental tests in the elastic range can be expected

to correlate well with sensory evaluation performed

outside the mouth (the ®nger test), while tests in the

plastic range will correlate better with sensory evalua-

tion performed in the mouth.16

The geometry of the probe or testing device will also

affect the test results. In this study, the shear test

differentiated better between recently slaughtered

salmon and ice-stored salmon than the TPA test with

a spherical probe. The performance of the TPA test in

the range of plastic deformation could perhaps be

improved by using a cylindrical probe, which will

produce more shear stress in the sample. When a

spherical probe with and deformation in the elastic

range is used, the TPA test is non-destructive. When

larger deformation is applied, the test becomes more

destructive, and a cylindrical probe will be more

destructive than a spherical probe.33

Finally, it should be noted that other factors, not

evaluated in this study can probably also affect the

results of instrumental texture measurements. Because

®sh muscle is a viscoelastic material, the speed of the

applied deformation is probably an important factor.

Furthermore, when more than one deformation is

applied, as in the TPA test, the duration of the pause

between deformations will probably affect the results.

Another factor that may be of importance is the

direction of the deformation relative to the direction of

the muscle ®bres.

ACKNOWLEDGEMENTSThe present study was a part of the ful®lment of the

Cand Scient thesis of Jon Olav Veland at the University

of Bergen, Institute of Fisheries- and Marine Biology.

Special thanks to professor Dag Mùller for his help and

support in this process. The study was conducted at

Matre Aquaculture Research Station, N5198

Matredal, Norway.

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