3.3. Effect of length of frozen storage on eating quality Thirty of the 32 eating quality attributes showed no significant change over 34 weeks
of frozen storage. The two attributes affected by length of frozen storage refer to the Ž
. appearance of the cooked salmon Table 8 . There were no significant effects of length
of frozen storage on odour, flavour, aftertaste or texture.
4. Discussion
4.1. Freezing and frozen storage For logistical reasons, all of the sensory analyses reported in this paper were
conducted on salmon that had been frozen and stored for up to 24 weeks. For this reason, sensory studies were conducted to evaluate the effect of freezing and length of
frozen storage on the eating quality of salmon, using profiling methods. The results Ž
. Tables 7 and 8 showed that a number of attributes relating to appearance and texture
Ž were affected by freezing separation, juicy and moist appearance, tenderness and moist,
. Ž
light and springy texture and by length of frozen storage
separation and moist .
appearance . The loss of juiciness and increase in toughness of fish observed on freezing Ž
. Table 7 is well known and is believed to be due to changes in the muscle proteins
Ž .
Mackie, 1993 . The nature and causes of these changes have been reviewed by Ž
. Ž
. Shewfelt 1981 and Mackie 1993 . The decreases in juicy and moist appearance, and
in separation, also observed on freezing, are likely to be a consequence of a reduction in water holding capacity arising from protein damage. Likewise, the further changes in
moist appearance and separation detected during prolonged frozen storage would have
Ž .
been caused by continued denaturation of the surface proteins Mackie, 1993 , but this Ž
. was not sufficient to cause further textural changes Table 8 . Thus, the results reported
in this paper for the texture and appearance of the frozen fish do not necessarily reflect those which would have been obtained for fresh salmon.
In contrast, the only aspect of flavour that was affected by freezing was oily flavour, which was found to receive higher scores for the fresh fish; length of frozen storage had
no significant effect on flavour. Therefore, it is probable that the results for the profiling of odourrflavour attributes for frozen salmon are closely related to those which might
have been obtained for fresh fish.
The salmon used to examine the effect of length of frozen storage were harvested during a period from August to February. As the eating quality of salmon may be
Ž .
affected by the season of the year at which the fish are harvested Haard, 1992 , the effect of frozen storage was, to some extent, confounded by the effect of season.
However, as few significant effects were observed overall, it is unlikely that either time of year or duration of storage was affecting eating quality. The probability of these two
factors exactly compensating for each other is likely to be small. It would, therefore, appear that, once frozen, vacuum-packed salmon steaks could be stored for up to 34
weeks without further effects on the perceived organoleptic quality.
4.2. Principle components analysis The main effects of source and environment on the eating quality of salmon are
Ž .
illustrated by the results of PCA on the profiling data Figs. 1 and 2 . Although the variance explained by the first two principle components is rather small, the results for
the salmon harvested during the two seasons show many similarities. In both cases, the mean values for salmon from each source tend to occupy the same region of the PCA
Ž .
Ž plot Figs. 1a and 2a . Furthermore, fish from similar types of salmon source i.e.
. river-wild, seawild and sea-farmed are grouped together. For example, the river fish
Ž .
solid shapes all tend to be positive with respect to PC1, the wild sea-caught salmon Ž
. open shapes are often negative on PC1, but positive on PC2, while the sea-farmed fish
Ž .
star shapes tend to be negative on both axes. Thus, sensory differences between source types are greater than those within them. Comparison of Figs. 1a and 2a with 1b and 2b,
respectively, illustrates the relationship between salmon types and sensory profiling attributes.
For both 1993 and 1994, PC1 largely comprises descriptors relating to earthy descriptors and other aroma and flavour attributes and tends to differentiate between
Ž .
river and sea fish whether wild or farmed . During both seasons, PC2 relates mainly to Ž
aspects of texture although appearance, time of aftertaste and the earthy attributes are .
also included and partially differentiates between wild and farmed sea-caught salmon, especially those captured during 1994. PC3 discriminates between salmon on the basis
Ž .
of colour peach, orange colour versus paleness and beige , but does not differentiate Ž
. between source types results not illustrated . Thus, the attributes causing the greatest
sensory differences between salmon were consistent over the two seasons studied. The inclusion of length of frozen storage for individual fish in a PCA did not indicate
any relationship between this factor and sensory profiling scores, supporting the above Ž
. results Table 8 , that extended frozen storage had little effect on sensory quality, and
suggesting that this factor was not important for salmon eating quality in this study. The trends illustrated in Figs. 1 and 2 are supported by the sensory data for specific
Ž .
eating quality attributes Tables 3 and 4 . The effects of salmon source on the sensory scores are discussed under the headings of aroma, flavour and aftertaste, texture and
appearance.
4.3. Aroma, flaÕour and aftertaste Ž
. As illustrated by the results of PCA Figs. 1 and 2 , the main differences in aroma
and flavour attributes were not between wild and farmed salmon, but between river and Ž
. sea-caught fish whether wild or farmed . Earthy odour, flavour and aftertaste were
Ž .
higher in the river-caught fish than in the sea-caught fish Tables 3 and 4 . During the 1993 trial, River 1 salmon, in particular, received higher scores for these earthy
attributes than salmon from Rivers 2 and 3, which were, in turn, more earthy than the other four sources. During 1994, similar scores for these attributes were obtained for
salmon from all three rivers. Earthy and muddy off-flavours in drinking water and in fish, especially those from freshwater, are commonly caused by the compounds,
geosmin and 2-methylisoborneol. Their occurrence and origin have been extensively
Ž .
studied and reviewed for example: Persson, 1980 . These compounds may be formed
Ž .
by actinomycetes and various species of blue-green algae Slater and Blok, 1983 . Ž
. Analytical studies Mitchell and Farmer, unpubl. data found geosmin to be present in
river-caught salmon at up to 1.26 mg kg
y1
, while 2-methylisoborneol was not detected Ž
y1
. -
0.2 mg kg . These concentrations of geosmin are higher than the odour threshold
Ž .
for this compound in fish Persson, 1980 and, therefore, geosmin is likely to be the primary cause of the earthy attributes detected in these salmon.
The scores for other undesirable aroma and flavour attributes were elevated for salmon from some river sources. The scores for stagnant odour were significantly higher
in salmon from River 1 than all other sources in 1993, and for salmon from River 3 in 1994. The River 1 fish were also subject to a short-lived off-flavour incident during one
week in July 1993; a manure-like, farmyard odour was detected which rendered the fish
Ž unpalatable. This was found to be caused by elevated levels of skatole Farmer et al.,
. 1995 . Exclusion of the four affected fish from the statistical analyses did not materially
affect the overall results and these data are included in Table 3 and Fig. 1. Farmyard odour and flavour were monitored during the 1994 trial; however, although scores for
these attributes were elevated for salmon from all the rivers, no specific incident occurred such as that experienced during 1993.
Salmon-like odour and flavour were most intense in the farmed salmon, with the lowest scores being received for the river-wild fish. Indeed, scores were significantly
higher in salmon from some farmed sources than some sea-wild fish. It is possible that the panellists were more familiar with the flavour of farmed salmon and that this
affected their definition of this attribute. However, the panellists were subjected to a long period of training to familiarise them with the characteristics of salmon from all
sources. It is more likely that, in the river-fish, the salmon-like aroma and flavour were, to some extent, masked by the relatively high earthy odour and flavour as observed by
Ž .
Bett and Johnsen 1996 . It has been shown that the aroma and flavour of farmed Ž
Atlantic salmon become less intense as maturation progresses Asknes et al., 1986; .
Blokhus, 1986 . However, these changes occur after the skin colour has changed from Ž
silver to brown and at the same time as the underjaw changes shape Asknes et al., .
1986 . The salmon used in this study did not show any visible signs of maturation in terms of altered shape or colour of the whole fish. It has also been suggested that the
aroma and flavour of river salmon may be reduced due to the depuration of important Ž
. flavour compounds on entering freshwater Boyle et al., 1992 and that the balance of
odour compounds in raw freshwater fish is altered due to metabolic changes and their Ž
. Ž
. effect on lipid breakdown Josephson et al., 1984 . In contrast, Ostrander et al. 1976
did not detect any consistent differences in flavour between saltwater and freshwater farmed Pacific salmon. Further studies on the compounds responsible for the flavour of
salmon are required to establish which of these mechanisms is the main cause of the flavour differences between sea and freshwater salmon in this region.
Ž .
Laverty 1993 advises that the bleeding of fish is generally recommended as blood can give a strong metallic taste, although no quality differences were detected between
bled and unbled farmed trout. In this study, only salmon from Farm 1 were bled on Ž
. slaughter. The mean scores for metallic aftertaste Tables 3 and 4 are not reduced for
salmon from this source. Instead, metallic aftertaste is highest in the river fish and is Ž
. roughly correlated with earthy aftertaste. It was also suggested Laverty, 1993 that the
removal of iron resulting from bleeding reduces lipid oxidation. Again fish from Farm 1 did not show any differences in, for example, oily or fishy aroma or flavour which could
Ž .
be attributed to this practice. These results agree with those of Laroche et al. 1995 who found that bleeding did not affect the odour or flavour of carp fillets.
The data from the 1994 season show that oily aroma, flavour and aftertaste were consistently highest in the farmed fish and lowest in the river salmon, with the sea-wild
sources receiving intermediate scores. However, there were no significant differences between the sources of wild and farmed sea-caught salmon in these attributes. These
data do not reflect the lipid content of the fish; some of the sea-wild fish tended to have the highest lipid content, with farmed and river-wild containing lesser amounts of lipid
Ž
. Ž
. Farmer et al., 1997 . As oily flavour was reduced by freezing Table 7 , it is possible
that a similar comparison using fresh fish may emphasize any differences in oily characteristics between wild and farmed sea-caught salmon.
No significant differences were observed in fishy flavour in 1993, but in salmon caught during the 1994 season, some differences were observed; the highest scores
tended to be received by the farmed salmon and the lowest by the river-wild fish. Fishy odours can develop when fish is stored for long periods or under inadequate conditions.
Such odours may be due to trimethylamine or dimethylamine, formed in marine fish from trimethylamine oxide by the action of microbial or endogenous enzymes, respec-
Ž .
tively Lindsay, 1988 . In addition, the compounds trans,cis,cis- and trans,trans,cis- 2,4,7-decatrienal cause unpleasant fishy odours which are formed by autoxidation of
Ž .
Ž .
long-chain n–3 fatty acids Lindsay, 1991 . Skonberg et al. 1993 found that coho Ž
salmon and trout fed a diet including herring oil had a more fishy aroma and flavour in .
the salmon than those where the oil was sunflower oil enriched in oleic acid. It was proposed that an increase in monounsaturated fatty acids decreases the risk of oxidative
Ž .
rancidity, although the fatty acids in the fish were not analysed Skonberg et al., 1993 . This is possible as a higher concentration of the long chain n–3 fatty acids derived from
the herring oil could act as precursors for the 2,4,7-decatrienals discussed above. However, in the study described herein, no correlation was observed between scores for
Ž fishy attributes and the overall content of any fatty acid or group of fatty acids Farmer
. et al., 1997 . Indeed, overall fatty acid composition offered no explanation for any of the
differences in perceived flavours or odours for the salmon harvested during 1994 Ž
. Farmer et al., 1997 , despite the fact that many important flavour compounds are
derived from fatty acid precursors. Furthermore, there was no evidence of any relation- ship between frozen storage time and flavour or odour attributes, by either correlation or
Ž .
PCA not shown . It is, therefore, likely that flavour development in salmon depends on other factors, such as the fatty acid composition of the membrane phospholipids, the
formation of free fatty acids, the amount and nature of antioxidants present or differ- ences in enzyme activity.
Hedonic scaling experiments on the acceptability of 1993 salmon, using an untrained panel, showed that the river fish tended to be less acceptable than sea-caught fish in
Ž .
terms of flavour, aftertaste and overall acceptability Table 5 . However, the flavour, odour and aftertaste of farmed salmon is at least as acceptable as that of the sea-wild
fish. This agrees with the results of the profiling studies, which indicated that there were few differences in flavour between the wild and farmed salmon when both were
Ž .
harvested from the sea Tables 3 and 4 and that salmonlike flavour was even slightly higher in salmon from farmed sources than in some of the sea-wild fish. These results
counter the widespread belief that wild salmon have a superior flavour to that of farmed Ž
. fish. The data reported herein contrast with those of Sylvia et al. 1995 , who reported
Ž .
that wild Pacific chinook salmon possess a more ‘delicate, fresh fish flavour’ than their Ž
farmed counterparts; but these data agree with other studies Bartos, 1989; Higgs et al., .
1989 that the flavour of farmed salmon is at least as good as that of the wild fish. Ž
. Similar results were obtained for lagoon-reared wild and farmed sea bream: there were
Ž .
no differences in the odour or flavour of the cooked fish Orban et al., 1997 . Ž
. Correlation coefficients Table 6 indicate that acceptability of flavour was related to
salmon-like flavour and lack of earthy flavour. An examination of the data showed that these correlations were due to the high earthy flavour and low salmon-like flavour of
Ž .
one treatment River 1 ; no correlation was observed for the other treatments. In contrast, the acceptability of aftertaste showed consistent correlation with the mean
profiling scores for strength of aftertaste, time when aftertaste starts and metallic aftertaste. No significant correlation was observed for aroma attributes. It has been
suggested that the relationship between acceptability scores and profiling scores is
Ž .
curvilinear rather than linear Moskowitz, 1981 . However, none of the correlations tested in this study showed any evidence of curvilinear relationships. Overall acceptabil-
ity of the cooked salmon was significantly correlated with the scores for acceptability of Ž
. aftertaste and flavour Table 6 , suggesting that the above mentioned attributes were
particularly important for the overall acceptability. The acceptability of salmon from different sources may reflect the previous experience of the panellists. For instance, it is
possible that, for anglers and others accustomed to eating river-caught salmon, a slightly earthy flavour is a desirable and characteristic attribute of these fish. However, cus-
tomers used to eating sea-caught wild or farmed salmon may be less likely to accept these attributes.
4.4. Texture In contrast to flavour, the texture of the cooked salmon differed between wild and
Ž .
Ž .
farmed salmon Tables 3 and 4 . Two of the farmed sources Farms 2 and 3 , and to a Ž
. lesser extent the third Farm 1 consistently received the highest scores for moist, light
and tender textures, whereas the wild fish received the highest scores for firmness and Ž
. chewiness Tables 3 and 4 . These effects were significant in comparison with at least
some of the other sources and were particularly pronounced during the 1994 season. The study conducted on the acceptability of salmon harvested during 1993 did not
demonstrate any significant differences in the acceptability of texture, although salmon from Farms 2 and 3 received the most acceptable scores. If a similar study on
acceptability had been conducted on the 1994 salmon, when the differences in texture were greater, it is possible that significant differences may have been observed. Given
Ž .
the differences in texture between fresh and frozen salmon Table 7 , it would also be desirable to repeat the acceptability study using fresh salmon. Nevertheless, the accept-
ability of texture was found to be significantly correlated with the mean scores for light Ž
. and moist texture Table 6 suggesting that panellists preferred salmon with these
attributes.
Ž In contrast to the above profiling results, various authors e.g. Higgs et al., 1989;
. Sylvia et al., 1995 have reported that there is no significant difference in texture
Ž .
between wild and farmed salmon. One of these studies Sylvia et al., 1995 used fresh, unfrozen fish, unlike the work reported herein, but the work reported by Higgs et al.
Ž .
1989 was also performed on frozen fish. Studies on other fish species have shown differences in texture between wild and farmed fish. In red sea bream the muscle of wild
Ž .
Ž .
fish was found to be harder Aoki et al., 1991 and preferred Hatae et al., 1989 to that Ž
. of the cultured fish, although this was not true for all species Aoki et al., 1991 . Orban
Ž .
et al. 1997 found that the meat from farmed sea bream was more juicy, greasy and less fibrous than that of the wild fish; this was attributed, in part, to the much higher lipid
content of the farmed fish. In contrast to the results for flavour, there were no significant differences between
wild fish captured in the sea and river for texture attributes. These results agree with Ž
. those of Ostrander et al.
1976 , who found that salt versus fresh water had no significant effect on the texture or moisture of farmed Pacific salmon species, as
measured by hedonic scaling methods. A difference in texture may be caused by a range of factors. Activity has been found
to affect the softening of the flesh after slaughter: softening occurred more slowly in red sea bream subjected to additional swimming exercise compared with non-exercised fish
Ž .
and in wild bream compared with cultured fish Tachibana et al., 1988 . Other studies have shown that fish allowed to struggle prior to death gave flesh with a softer texture
Ž during the first few days of chilled storage than that of anaesthetized fish Izquierdo-
. Ž
. Pulido et al., 1992 . Asknes et al. 1986 has reported that the texture of farmed Atlantic
salmon deteriorates on maturation, becoming watery and tough. However, as for flavour, these changes were only significant in October, 2 months after the first visible signs of
maturation were observed.
A possible cause of the texture differences observed in this study between wild and Ž
. farmed salmon may be suggested by the PCA results Figs. 1 and 2 . Sea-wild and
sea-farmed salmon are partially differentiated by PC2, which is composed primarily of texture attributes. It may be observed that this differentiation is greater in 1994 than
Ž 1993, and that this is due to a ‘movement’ of farmed salmon especially those from
. Farm 1 from the upper to the lower left quartile of the plot. Discussion with the farms
concerned indicated that, while Farms 2 and 3 had used the same feed during 1993 and 1994, Farm 1 had increased the oil content of their feed in 1994. This suggests that the
light, moist texture of the farmed fish may be influenced by dietary factors. Further studies are needed to determine which texture attributes are most acceptable and to
establish how rearing conditions affect them.
4.5. Appearance The amount of juice seeping out of the cooked salmon was termed ‘juicy appearance’
by the panellists, while the visible oil in these juices was termed ‘separation’. ‘Moist appearance’ referred to the cut surface of the cooked salmon. Separation, moist and
Ž .
juicy appearance were significantly affected by source Tables 3 and 4 . During both 1993 and 1994 trials, salmon from Farm 3, and to a lesser extent the other farmed
sources, received higher scores for separation than most of the river salmon. Farm 3 salmon also received higher scores for moist appearance in 1993; while, in 1994, salmon
from River 3 were perceived as appearing significantly less moist than all other sources. Farmed salmon received significantly higher scores for juicy appearance than river
salmon in the 1994 study. These effects may have been caused by the generally higher lipid content of the farmed fish. However, Sea 1 salmon contained the most lipid of all
the sources and Sea 1 and Sea 2 salmon, which contained widely differing amounts of
Ž
y1
. total lipid 136 and 68 mg g
, respectively; Farmer et al., 1997 received similar scores for these attributes. It is possible that the type of lipid used in the diets for farmed fish
may affect these measures of appearance. The flesh colour of the cooked salmon also differed widely between sources. Farmed
salmon from Farm 1 exhibited a significantly more intense peach colour than four of the remaining five sources examined in 1993 and a more orange colour in 1994. Pink colour
also differed between sources in the 1994 trial. Lower scores for pink, orange andror Ž
. peach colour were observed for some wild sources Tables 3 and 4 . These differences
may be attributed to the amount and type of pigmentation in the diet. Wild salmon obtain the pigment astaxanthin from marine crustaceans, while astaxanthin, or the
synthetic pigment, canthaxanthin, is incorporated in the diet of farmed salmon to give the desired flesh colour. Thus, the differences in colour between the farmed sources is
likely to be caused by the diets used. The differences in colour between the wild sources must be determined by the availability of crustacea in coastal waters.
Ž .
Laroche et al. 1995 found that the appearance of cooked carp was affected by whether or not the fish were bled at slaughter: bled fish were darker, more pink and less
homogeneous in colour. Salmon from Farm 1 had the lowest scores for pink colour, and Ž
. the highest scores for peach and orange colours Tables 3 and 4 . It is not possible to
judge whether the bleeding of salmon from Farm 1 contributed to these differences in appearance.
The salmon from Farm 1 received the most favourable scores for acceptability of Ž
. appearance, as determined by hedonic scaling studies Table 5 , and acceptability of
appearance was significantly correlated with the mean scores for peach and orange Ž
. Ž
. colour
Table 6 . These data support the view Sigurgisladottir et al., 1994
that additional pigmentation improves acceptability.
5. Conclusion
