Flavor Characteristics and Lipid Composition of Atlantic Salmon

Chapter 10

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Flavor Characteristics and Lipid Composition of Atlantic Salmon 1,2

1,3

Linda J. Farmer , Janice M . McConnell , and William D. Graham

2

1

Department of Food Science, The Queen's University of Belfast, and Department of Agriculture for Northern Ireland, Newforge Lane, Belfast BT9 5PX, United Kingdom 2

Wild and farmed Atlantic salmon (Salmo salar) were harvested from eight sources around the shores of Northern Ireland. Sensory evaluation of the flavor characteristics of the salmon was conducted using a sensory profiling method. The total lipids and fatty acid compositions of the salmon muscle were also determined. The results showed that the main differences in flavor occurred between wild, river and sea-caught salmon (whether farmed or wild). There was little difference in flavor between wild and farmed salmon when both were from the sea. In contrast, the main differences in fatty acid composition occurred between farmed and wild salmon. While both contained a similar proportion of n-3 fatty acids, farmed salmon contained higher concentrations of n-6 fatty acids and, therefore, a lower n-3/n-6 ratio than wild salmon. No direct correlation was found between fatty acid precursors and flavor. The Atlantic salmon (Salmo salar) has long been valued as a luxury food and over the last 25 years has become widely available as a farmed product. However, it is frequently asserted that wild salmon possess more flavor than their farmed equivalents. It is, therefore, of considerable interest to both consumers and the salmon farming industry to determine whether this perception is based on fact. Farmed salmon will inevitably be compared with its wild equivalent and must duplicate or improve on the quality of the wild fish if it is to compete favourably on the market (1). The possible quality differences between wild and farmed salmon have been the subject of research for both the Atlantic salmon and the various species of Pacific salmon (Oncorhynchus sp.); these studies have yielded conflicting results, with some authors reporting that the flavor of farmed salmon was similar or preferred to that of wild (2,3) while another study found that the wild salmon had more 'delicate, fresh fish flavor' (4). 3

Current address: Evron Foods Limited, Portadown, Co. Antrim, Northern Ireland, United Kingdom

© 1997 American Chemical Society Shahidi and Cadwallader; Flavor and Lipid Chemistry of Seafoods ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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FLAVOR AND LIPID CHEMISTRY OF SEAFOODS

Many of the volatile compounds contributing to the odour and flavor of salmon and other fish are derived from lipid oxidation (5-8). In addition, the oily fatty acids which, when incorporated into the human diet, have been shown to exert a protective effect against cardiovascular disease (9). For this reason considerable interest has focused on the fatty acid composition of edible species of fish (e.g. 10). The effect of fish farming on fatty acid composition has also received attention; in general, wild salmon appear to have a higher n-3 to n-6 fatty acid ratio than their farmed equivalents (e.g., 2,11). Thus, the fatty acids present in Atlantic salmon are of interest both as potential flavor precursors and as nutritional food components. Most of the above studies compare only one or two sources of farmed and wild salmon of any species. In order to reduce the impact of the sensory differences which occur between fish from separate farms, this paper presents some of the results of an investigation involving eight sources of Atlantic salmon. The full sensory results for salmon from 1993 and 1994, including those for texture and appearance, will be reported elsewhere. This paper reports the results of sensory profiling of flavor-related characteristics and the analyses of lipid composition for salmon harvested during 1994 and examines evidence for any correlation between the concentrations of lipid and fatty acid precursors and sensory scores for flavor attributes. Materials and Methods Salmon Comparison of Salmon from Eight Sources. Farmed and wild Atlantic salmon from eight sources in the U K and Republic of Ireland (ROI) were obtained during the wild salmon season (June to September) of 1994. These sources are listed in Table I. A l l salmon were held on ice until transported to the laboratories of the Food Science Department, within 4 days of harvest. Salmon were eviscerated after the resolution of rigor mortis and ten or more 25mm steaks were immediately cut from the main body between the pectoral fins and the cloaca (labelled A to ca. J, as shown in Figure 1). The steaks were washed under running water and individually vacuum-packed in Table I. Sources of salmon Name

Source type

Location

Farm 1 Farm 2 Farm 3 Seal Sea 2 River 1 River 2 River 3

Sea-farmed (SF) Sea-farmed (SF) Sea-farmed (SF) Sea-wild (SW) Sea-wild (SW) River-wild (RW) River-wild (RW) River-wild (RW)

Pens in enclosed sea-water inlet. Pens in off-shore location Onshore tanks with pumped sea-water Saltwater location near mouth of River 1 Saltwater location near mouth of Rivers 2&3 3 km from river mouth 24 km from river mouth 4 km from river mouth

Shahidi and Cadwallader; Flavor and Lipid Chemistry of Seafoods ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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Flavors & Lipid Composition of Salmon

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laminated vacuum pouches (nylon 20um, polythene 60um; Brow Packaging, Belfast) within 15 mins of cutting, frozen and stored at -24°C ± 3°C until required. Effect of freezing. To compare fresh and frozen salmon, farmed salmon from Farms 2 and 3 were processed and packaged as above. Salmon to be analysed fresh were refrigerated at 4 °C for one to two days prior to analysis. The remainder were frozen at -24°C ± 3°C for 3-14 days prior to sensory analysis. To examine the effect of length of frozen storage, salmon from Farm 2 were harvested, processed and frozen 33, 15, 8 and 4 weeks before the commencement of sensory analysis. Panels were conducted on two consecutive weeks. Sensory Evaluation. Salmon steaks were thawed (at 4°C, overnight), washed and cooked using a bain marie method, as follows: The steaks were wrapped in grease proof paper, fold uppermost and placed individually on inverted, perforated aluminium trays (180mm x 120mm) which were placed in stainless steel baking trays. Tap water was added to a depth of 10mm and the whole tray covered in aluminium foil. The steaks for the comparison of fish from different sources were cooked for 20 min at 200°C. A cooking temperature of 180°C was used for the studies on the effect of frozen storage due to the smaller number of steaks in the oven. The ultimate internal temperature ranged from 76°C to 85°C depending on the size of steaks. Each panellist received one half steak served on a heated porcelain plate. A small portion from the ventral region was served separately in a heated, lidded porcelain dish (80 mm diameter) for odour assessment. Tap-water, filtered through a domestic water filter (Boots the Chemist, U K ) to remove any extraneous flavors, and water biscuits were supplied as palate cleansers. The eating quality of salmon from the eight sources was compared using quantitative descriptive analysis (sensory profiling). The same method was used to compare fresh and frozen salmon and fish stored at -24°C for four to 33 weeks. A list of 37 attributes describing the appearance, aroma, flavor, texture and aftertaste of cooked salmon was agreed by a trained panel of 8 people. Of these, 19 described aroma, flavor or aftertaste. Table II lists the agreed definitions for these attributes.

Figure 1. Allocation of salmon steaks


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Table II. Descriptors used for sensory profiling of salmon aroma and flavor GROUPINGS

ATTRIBUTES

DEFINITIONS

AROMA

Salmon-like

FLAVOR

Oily Salty Earthy Stagnant Farmyard Salmon-like flavor

Intensity of distinctive salmon-like odour. Intensity of fish oils odour. Intensity of sea-like/salty odours. Intensity of any earthy or peaty odours. Intensity of stagnant water odour. Intensity of manure/cow-dung odour. Intensity of distinctive salmon-like flavor. Intensity of salt-like flavor. Intensity of fish oil flavor. Intensity of any other fish-like flavors. Intensity of non-salmon fishy flavor near the skin of the salmon steak. Intensity of earthy or peaty flavor. Intensity of manure/cow-dung flavor. Time when aftertaste starts. Intensity of aftertaste Intensity of chicken-like aftertaste. Intensity of fish oil aftertaste Intensity of earthy aftertaste. Intensity of metallic aftertaste.

Salty flavor Oily Flavor Fishy flavor Fishy flavor (skin)

AFTERTASTE

Earthy flavor Farmyard flavor Time Overall aftertaste Chicken-like a'taste Oily aftertaste Earthy aftertaste Metallic aftertaste

Panellists examined a total of ten salmon from each source and scored each attribute using a 100 mm linescale. A l l panels were conducted in ventilated booths under N-sky lighting and the data collected using PSA 1.64 data collection software (Oliemans, Punter & Partners, Utrecht, The Netherlands). Analysis of Total Lipids and Fatty Acids. Five salmon from each source were analysed for total lipids and for fatty acid composition. Steak A (Figure 1) was used in all cases except two; in these cases, steak C was used and was shown to have a similar lipid content to steak A . The two halves of the steak were homogenised separately to give duplicate samples. Analyses were conducted to determine the nature of any changes in lipid composition over the length of a salmon. The total lipid was extracted from the salmon homogenate (5g) using choroform:methanol 2:1 (25ml x 2) by the method of Folch et al. (12). After filtration (Whatman No. 1) and washing with NaCl solution (0.37%) the extract was allowed to separate overnight, the organic layer removed and made up to 100 ml. Aliquots (10 ml) were taken for gravimetric analysis of the total lipids, while a volume (containing 150-200 mg lipid) was taken for fatty acid analysis.


10. FARMER ET AL.

Saponification of the lipids, by refluxing with methanolic sodium hydroxide (0.5M) and methylation with boron trifluoride methanol (14%), was conducted by the method of Metcalf et al. (13) Heptane (5ml) containing butylated hydroxytoluene (0.1 mg ml" ) and internal standard (fatty acid methyl ester 22:0, 1 mg ml" ) was added and the samples refluxed for a further 1 minute. The heptane layer was dried with N a S 0 (anhydrous) and analysed by gas chromatography. Gas chromatography was conducted using a Hewlett Packard 5890A gas chromatograph equipped with a HP7673A automatic injector, a HP3396A integrator and a fused silica capillary column coated with CP SIL 88 (50m x 0.25mm i.d. x 0.2um film thickness; Chrompak U K Ltd., London). Quantification of the fatty acids was achieved by comparison with the known concentration of internal standard and the results analysed by analysis of variance. The fatty acids were identified by comparison with authentic fatty acid methyl ester standards from marine and animal sources (Supelco Ltd). Identities were confirmed by analysis of selected samples by chemical ionisation GC-MS using a Hewlett Packard 5890A gas chromatograph connected to a HP5971 mass selective detector operating in full scanning mode over mass range 100 to 500 atomic mass units. The reagent gas was methane. 1

2


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1

4

Results and Discussion Effect of Freezing and Storage on Flavor Quality. The sensory investigations reported in this paper were conducted on frozen stored salmon. This was necessary due to the relatively short season over which the salmon were collected and because the supply of wild fish was dependent on river flows and was, therefore, sporadic. It was, therefore, essential to evaluate the effect of freezing and of frozen storage on eating quality characteristics. Profiling studies comparing fresh and frozen salmon indicated that, of the odour and flavor attributes, only 'oily flavor' was significantly altered; the mean score for 'oily flavor' was reduced by freezing (PO.001). The period of frozen storage, from eight to 33 weeks, had no significant effect on the odour or flavor of the cooked salmon. However, appearance and/or texture were affected by freezing or frozen storage (data not shown). Effect of Origin of Salmon on Flavor Quality. Table III shows the results for the flavor-related attributes of sensory profiling for salmon from the eight sources. The river-caught fish tended to receive lower scores for 'salmon-like odour' and 'salmon-like flavor' and higher scores for 'earthy odour', 'earthy flavor' and 'earthy aftertaste' than the sea-caught fish, whether farmed or wild. These results may be due to the masking of the 'salmon-like' attributes by earthy notes derived from freshwater microflora, by depuration of flavor compounds on entering freshwater (16) or due to the onset of maturation. Sexual maturation has been reported to cause a reduction in odour and flavor of the boiled salmon (17). While no visual changes associated with maturation were observed in the river fish, it is possible that the metabolic changes associated with maturation had commenced. In contrast, a comparison of the sensory scores for sea-caught salmon from the three farmed and two wild sources show little difference between them in odour or flavor attributes. These results broadly agree with those reported for Pacific coho



***

b

bc

ef

30.55 27.99" 19.52*° 23.48°" 30.31 29.39* 21.84*

d

b

25.08 26.62° 21.74 25.80* 23.47* 20.23

Farm 2

f

b

31.05 27.74 19.15*° 24.33°" 33.19" 32.00*° 23.67*

24.74* 27.06° 21.93 26.77* 25.79* 19.20*

Farm 3

1x5

26.56°* 25.76* 17.59* 21.44*° 30.06 27.48* 19.98*

22.87* 25.66"° 19.34 27.15* 23.55* 20-53*

Seal b

26.02"°" 26.29* 20.49"° 22.64"°" 31.60°" 28.81* 22.61 *

25.18 26.39° 21.09 27.65* 24.39* 23.40*

Sea 2

18.64* 24.95* 38.21" 32.25"

18.59* 24.36* 19 (j

— 4 —

0.9 1.1 1.3 18:3 (% total fatty acids)

1.5

1.7

Figure 4. Relationships between n-3 fatty acids

As the fatty acid compositions of the farmed salmon differed considerably from those of the wild fish, as well as varying from farm to farm, the data for the wild salmon were considered separately. The main positive correlations were between 18:4 or 20:5 and 14:0 (R = 0.80, 0.79), 18:0 and 16:0 (R = 0.74), 20:5 and 18:4 (R = 0.77) and between 22:1 and 20:1 (R = 0.78). The main negative correlations were between 20:1 or 22:1 and 16:0 (R = -0.90, -0.84), between the same two fatty acids and 18:0 (R = -0.72, -0.77), and between 18:4 or 20:5 and 18:1 (R = -0.74, -0.82). A l l these correlations were significant (PO.001) when compared with critical values for the Pearson product-moment correlation coefficient (26). Is there any relationship between fatty acid precursors and flavor? Of particular interest was the possibility of a relationship between the amounts of n-3 fatty acids and any of the flavor attributes. Only weak correlations were observed, and these occurred solely between the sensory scores for salmon flavor and some fatty acids. Salmon flavor appeared to be positively correlated with 18:3 (R = 0.58) and negatively correlated with 20:5 (R = -0.56) and 22:5 (R = -0.63). Although these correlations are significant (PO.01 or PO.001), i f compared with critical values for the Pearson product-moment correlation coefficient, they do not obey the assumptions required for the use of this significance test (26); examination of the results indicates that the data are grouped according to whether the source of the salmon was river or sea. Figure 5 shows that any apparent correlation between salmon flavor and 18:3 was due entirely o the different scores for the three types of salmon; within each source type there is no correlation. As mentioned earlier, the low scores for 'salmon flavor' in river fish was probably due to other factors than the lack of availability of precursors. Given the likely importance of the polyunsaturated n-3 fatty acids for the formation of some of the important flavor compounds it is


10. FARMER ET AL.


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surprising that salmon flavor was inversely correlated with 20:5 and 22:5. However, again, these relationships appear largely due to the influence of the river fish; i f the river fish are omitted the correlation is very low. 40 35

8 g 30 iDownloaded by NANYANG TECHNOLOGICAL UNIV on September 20, 2017 | http://pubs.acs.org Publication Date: July 1, 1997 | doi: 10.1021/bk-1997-0674.ch010

CO

•

S~ 25

M

20 +

TO

15

2

A

>

•

A D AA A AO

a

#

10

SF

H

0.5

0.7

0.9 1.1 1.3 18:3 (% total fatty acids)

A

S W • RW 1

1

1.5

1.7

Figure 5. Plot of salmon favour against fatty acid 18:3 (%total fatty acids)

Thus, such direct correlations between lipid or fatty acid concentrations and flavor attributes cannot explain the differences in flavor between individual salmon; the amounts of these precursors have little effect on flavor. Other factors must be critical for flavor development. These may include the fatty acid composition of a particular lipid component, such as the membrane phospholipids. Studies on meat flavor have shown that phospholipids play a more important role than the triglycerides in the generation of aroma compounds (27,28). The amount and nature of antioxidants present or differences in enzyme activity may also play a critical role. Further studies are required to elucidate the prerequisites for desirable salmon flavor. Conclusions Assertions that wild Atlantic salmon have more flavor than farmed are not supported by the results of this study. The main difference in flavor was observed between the sea-caught and river-caught salmon; the latter had less salmon flavor and odour and higher scores for earthy attributes. There was no significant difference in flavor between the wild and farmed sea-caught salmon. In contrast, the fatty acid composition differed most between wild and farmed salmon. Fatty acid analyses showed that although wild salmon had a higher percentage of n-3 fatty acids, the generally higher lipid contents of the farmed salmon made them good sources of n-3 fatty acids. However, the wild fish had much lower amounts of n-6 fatty acids and higher n-3/n-6 ratio than farmed fish. A n examination of the correlations between measures of lipid composition and sensory scores indicated that there is no direct relationship between overall concentrations of fatty acid precursors and the perceived flavor.


108



Acknowledgments Funding from the E U Interreg Initiative and the Department of the Environment for Northern Ireland - Environment Service is gratefully acknowledged. The authors also wish to thank Miss E. Flanagan and Miss J. Sinton for conducting the lipid analyses, Mr T. Hagan and Mrs D. Rea for assistance with the preparation and sensory analysis of the salmon, M r J. Hamilton and M r C. McRoberts for the mass spectrometric analyses, Dr D. Kilpatrick and Mr A . Gordon for the statistical analyses and the numerous people whose co-operation facilitated the procurement of the salmon. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

20. 21. 22.

Moe, H.N. Proc. Nutr. Soc. NZ 1990, 15, 16-22. Higgs, D.A.; Skura, B.J.; Dosanjh, B.S.; Yan, D.; Powrie, W.D.; Donaldson, E.M. Can. Aquacult. 1989, 5, 51-53. Bartos, G.E. MSc Thesis, The Pennsylvania State University, College Station, PA. 1989. Sylvia, G.; Morrissey, M.T.; Graham, T.; Garcia, S. J. Aquat. Food Prod. Technol.. 1995, 4, 51-64. Josephson, D.B.; Lindsay, R.C.; Stuiber, D.A. J. Agric. Food Chem. 1984, 32, 1344-1347. Lindsay, R.C. In A Partnership of Marine Interests. IROC Oceans '88. 1988, pp 61-65. Milo, C.; Grosch, W. J. Agric. Food Chem. 1993, 41, 2076-2081. Milo, C.; Grosch, W. J. Agric. Food Chem. 1996, 44, 2366-2371. Herold, P.M.; Kinsella, J.E. Am. J. Clin. Nutr. 1986, 43, 566-598. Hearn, T.L.; Sgoutas, S.A.; Hearn, J.A.; Sgoutas, D.S. J. Food Sci. 1987, 52, 1209-1211. Cronin, D.A.; Powell, R.; Gormley, R. J. Food Sci. Technol. 1991, 15, 53-62. Folch, J.; Lees, M.; Sloane Stanley, G.H. J. Biol. Chem. 1957, 226, 497-509. Metcalf, L.D.; Schmitz, A.A. Anal. Chem. 1961, 33, 363. Yu, T.C.; Sinnhuber, R.O.; Crawford, D.L. J. Food Sci. 1973, 38, 1197-1199. Andersen, H.J.; Bertelsen, G.; Christophersen, A.G.; Ohlen, A.; Skibsted, L.H. Z. Lebensm. unters Forsch. 1990, 191, 119-122. Boyle, J.L.; Lindsay, R.C.; Stuiber, D.A. J. Food Sci. 1992, 57, 918-922. Aksnes, A.; Gjerde, B.; Roald, S.O. Aquaculture 1986, 53, 7-20. Farmer, L.J.; McConnell, J.M.; Hagan, T.D.J.; Harper, D.B. Water Sci. Technol. 1995, 31, 259-264. Josephson, D.B.; Lindsay, R.C. In Biogeneration of Aromas; Parliment, T.H.; Croteau, R., Eds.; American Chemical Society: Washington, DC, 1986; pp 201219. Lindsay, R.C. Food Rev. Int. 1990, 6, 437-455. Porter, P.J.; Kramer, D.E.; Kennish, J.M. Int. J. Food Sci. Technol. 1992, 27, 365-369. Hornstra, G. In The Role of Fats in Human Nutrition; Vergroesen, A.J.; Crawford, M., Eds.; Academic Press, 1989; pp 151-235.


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23. Simopoulos, A. J. Nutr. 1989, 119, 521-528. 24. Aursand, M.; Bleivik, B.; Rainuzzo, J.R.; Jorgensen, L.; Mohr, V. J. Sci. Food Agric. 1994, 64, 239-248. 25. van Vliet, T.; Kahan, M.B. Am. J. Clin. Nutr. 1990, 51, 1-2. 26. O'Mahoney, M. Sensory Evaluation of Foods: Statistical Methods and Procedures. Marcel Dekker, NewYork, 1986. 27. Mottram, D.S.; Edwards, R.A. J Sci. Food Agric. 1983, 34, 517-522. 28. Farmer, L.J.; Mottram, D.S. J Sci. Food Agric. 1990, 53, 505-525.


Flavor Characteristics and Lipid Composition of Atlantic Salmon

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