Chapter 15
Omega-3 Fatty Acids Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/28/18. For personal use only.
Farmed Atlantic Salmon as Dietary Sources of LongChain Omega-3 Fatty Acids: Effect of High-Energy (High-Fat) Feeds on This Functional Food Robert G. Ackman, Xueliang X u , and Catherine A. McLeod Canadian Institute of Fisheries Technology, DalTech, Dalhousie University, P.O. Box 1000, Halifax, Nova Scotia B3J 2X4, Canada
High energy diets are popular in today's salmon farming, but the lipid deposition in Atlantic salmon Salmo salar, particularly in steaks and fillets, and finally the effect of these diets on product quality, have not been examined. Over 12 months three commercial diets were fed, these being similar in all nutrients except the dietary fat levels (25, 29 and 31%). The muscle protein content was not modified by the three dietary treatments. Texture measurements and sensory panels were conducted on two specific sites in dorsal muscle steak cuts. A significant difference in muscle texture was found. There was a positive correlation between dietary fat and lipid deposition in the fish muscles. The total lipids in white muscle were, respectively, 1.27, 1.41and 2.25 %, in the trimmed steaks, 2.97, 4.58 and 5.38%, and in the whole steaks 5.45, 7.64 and 9.76 %. The fatty acid composition analyses showed only a slight effect of diet on phospholipids rich in DHA. In the triacylglycerols the fatty acids reflected the dietary fatty acids with DHA and EPA at around 10% each. However, the DHA content slightly exceeded that of the EPA.
Aquaculture is now one of the world's largest fishing operations and functions on a world wide basis (i). Among the fish types farmed are salmonids, and among the various species the Atlantic salmon Salmo salar is now farmed in salt water in
© 2001 American Chemical Society
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192 various oceans with suitable cool water temperatures. Among these are sites in Norway, Scotland, Iceland and the Faroe Islands, Chile, New Zealand, the USA and Canada. Taste is now recognized as the most significant factor in food selection (2), closely followed by aroma, a factor not necessarily totally separated (5). However not far behind these variables is texture, a co-determinant since consumers evaluate food with the same systems although texture is primarily based on mastication. This releases non-volatileflavorand also aromas, and facilitates their physiological evaluation. Texture may be a slower phase of the evaluation than appearance, odor, or flavor, but is important with each class of food types (2). Although, fish muscle is usually regarded as very tender, except when toughened by extended frozen storage (4,5), texture has become of acute interest to the salmon farming industry of Atlantic Canada as competition has increased in the last decade. The most recognizable reason for this concern has its roots in the steady increase in the fat content of farmed salmon diets shown in Figure 1. The concepts behind this change reflected the increasing world price offishmeal and the relatively low price of fish oil. The latter is a traditional ingredient in the diets of salmonids since for them dietary long-chain omega-3 fatty acids are "essential" (6-9). Some reports of soft texture in Atlantic salmon were felt to be due to the extra fat deposited in the muscle from the use of these so-called "high energy" diets. Accordingly our project was designed to evaluate salmon texture on an instrumental food science basis as well as by sensory panels. Since fats were intimately involved this review will separate into two parts, the salmon growth and texture and the exploration of the long-chain polyunsaturated omega-3 fatty acids available to consumers from salmon both as a food and as an oil source.
Experimental All fish were smolts obtained from the Department of Fisheries and Oceans, Halifax for the pilot study, or from a commercial operation for the second study. They were held in 2m tanks provided with running sand-filtered and aerated seawater. Feeding was ad libitum on a daily basis. A 12 h photoperiod was provided. Weighing was done by netting a dozen fish at random from each tank for representative weights. Diets (Table I) were distributed to pairs of tanks so that the pilot study had two diets fed in triplicate and the subsequent final study had three diets fed in duplicate. Fish for research were placed in an ice-seawater mix for 30 min and after cutting the gill were then bled for 20 min and gutted before transport to the laboratory for analysis. Steaks were cut from just infrontof the dorsal fin, the foremost for texture measurement and for protein and lipid analysis. The adjacent steak under the fin was used for sensory panel evaluations.
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Figure 1.
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Percentages of major components in Atlantic salmon diets prepared by one firm in recent years. Data courtesy of W.D. Robertson (Connors Bros. Limited) and S.P. Lall (Institute for Marine Biosciences, National Research Council Canada)
194 Before texture measurement the steaks were held in ice for 24 h to allow the muscle to pass through rigor and were then allowed to warm to 17°C. At a selected central site in each loin a physical measurement of depression, rebound and recovery time was conducted with a TA-XT2 Texture Analyser (Texture Technologies Corp. Scarsdale, NY) following the principles set forth by Gill et al. (4) and Botta (10). The texture index (dr/di) is derived from deformation distance (di) and rebound distance (dr). Sensory panel evaluation was conducted by a trained panel using the triangle test (11). Loin white muscle of steaks was cut into small cubes, mixed and oven baked at 250°C in a covered -lass petri dish. Panelists were especially instructed in evaluation of mastication qualities (12). A l l lipid extractions, including feeds, were conducted by the method of Bligh and Dyer (13). Fish muscle lipids were separated by streaking on thin-layer chromatography plates coated with silica gel, with development in hexane.-diethyl ethenacetie acid (85:15:1, v/v/v). Two individual lipid classes (phospholipids, PL, and triacylclycerols, TAG) were recovered from the silica get by CHCI -MeOH extraction. The fatty acids of these lipids were converted to methyl esters with BF -MeOH (14). This analysis was conducted on an Omegawax-320 capillary column operated in a Perkin Elmer 8420 gas chromatograph. FED peak areas were converted to weight percent fatty acid (15). 3
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Results and Discussion The pilot study was run with two levels of fat in commercial fish diets (25% and 30%, Table 1). The fish muscle (whole steaks) differed in fat content (6.3 ± 0.6% and 8.3 ± 0.34%) after 12 months on diets, but there was no difference in either a mechanically measured texture index or sensory panel evaluation of texture (16). Accordingly the subsequent definitive study was modified to include three diets. Table I also gives the proximate composition of the second set of diets. The growth of these fish over one year on diets is given in Figure 2. The inflections in the growth lines presumably reflect water temperature changes. Although the supplies of commercialfishdiets were purchased at different times over the feeding period, lipid analyses on each lot showed the fat contents to fluctuate only slightly, usually within ±1 % of the expectedfigure,and were not thought to be responsible for the change in relative growth curves where lines for the intermediate group fed diet Β and the high fat diet group fed diet C finally converged. This is in fact thought to result from a reduction in growth rate in the high fat group for reasons discussed below, although water temperature fluctuations affecting, feed consumption could be applicable to all groups. The necessity of aging steaks (Figure 3) to avoid an excessive fat figure from inclusion of the belly flaps and/or the fat deposit under the dorsal fin is described in detail by Sigurgisladottir et al. (17) and Bell et al. (8). Table II provides the lipid contents for whole steaks, aged steaks, and the white muscle loin area where the
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Duration of feeding experiment (Months) Figure 2.
Actual weights of fish on the three diets for 12 months.
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Figure 3.
Parts of Atlantic salmon steak used in this study. W M is the white muscle including area for mechanical texture measurement. D M = dark muscle, SF = skin fat, MYO=myocommata (connective tissue). Texture measurements were made at the site TM, with minimal myocommata, shown for one side of W M by the square box. "Trimmed" portion of steak is between the two lines.
197 texture measurements were made, as well as of other body parts and the actual texture index results. The basis of fat distribution in Atlantic salmon has recently been clarified by this laboratory (18,19) and by Aursand et al. (20). In brief there is in white muscle a basic muscle cell functional membrane composition of 0.6-0.8% lipid, primarily phospholipids (21,22). Bell et al. (8) show this in the form of the proportions of triacylglycerol and polar lipid plotted for a large number of Scottish farmed salmon. A l l other fat is in the form of adipocytes, literally bags of fat within a thin membrane and with the nucleus on the outside. The adipocytes are distributed along the connective tissue bands (myocommata in Figure 3) in the white muscle, but since the adipocytes are not very pigmented these appear to be the white streaks usually seen in raw salmon steaks, running through the "white" muscle, which is in fact usually well pigmented. The term "white" muscle is used to distinguish this type from the darker lateral line muscle common in all fish. This "dark" or "red" muscle has different lipid characteristics (23), but is relatively small in Atlantic salmon (Figure 3). 3
Table I. Proximate Compositions of diets used in salmon feeding studies by actual analyses Pilot study
Crude protein Crude fat Crude fibre Ash Digestible energy (Mj/kg) a
Final study
Low
High
A
Β
C
44 25 2 8 17.8
43 30 1 8 19.3
46 25 2 10 19.7
45 29 2 9 20.5
45 31 2 9 21.1
Two different suppliers
In this definitive study there were significant differences(p
