NutraFacts Volume 1, Number 2

Any discussion of carotenoids and salmonid flesh color will ultimately come down to astaxanthin. Although scientists have identified 19 other carotenoids in salmonids as well, astaxanthin remains the benchmark in comparisons simply because it accounts for more than 90 percent of the carotenoid found in wild salmonids.

In the wild, salmonids ingest astaxanthin either directly from vegetative sources or indirectly through lower members of the food chain. Absorbed in the intestine, astaxanthin is then transported by lipoproteins in the blood to the skin, flesh, liver, gonads and ovaries, appearing in either its free form or various esters.

Economically, it is the deposition of free astaxanthin in the fish's flesh that matters most to the aquaculture industry. In wild salmonids, astaxanthin occurs in the flesh at levels ranging from about 4 to 34 parts per million (ppm), with deposition rates varying by species. But visible pigmentation increases only to concentrations of 8 to 10 ppm, when it plateaus.

To produce flesh pigmentation similar to that found in wild salmonids, FDA now permits fish farms to include up to 80 ppm of supplemental astaxanthin (72 g/ton) in finished feed. Dr. Doug Yungblut of Hoffmann-La Roche notes that optimal levels of supplementation will vary by operation.

One key determinant is the target animal--not just its species, but its genotype, sex, maturity, weight and health. With sexual maturity, for example, astaxanthin stores in the flesh are depleted as more astaxanthin goes to the reproductive organs.

Another determinant is the feeding regimen. Although the deposition rate for astaxanthin depends mainly on dietary use levels, it is also affected by the type and composition of feed, the amount and quality of fat in the feed, the length of feeding period, and water quality, temperature and salinity.

Astaxanthin uptake increases significantly as the dietary fat content and feeding period increase. In one study (Torrissen, 1985), rainbow trout were fed 20 ppm astaxanthin in diets with various fat levels for 37 days. After 21 days of supplementation, the flesh of fish whose diet contained 3.5 percent fat had 0.63 ppm astaxanthin, while the flesh of fish fed a 35.9 percent fat diet had 0.92 ppm. After 37 days of supplemental astaxanthin, fish on a 3.5 percent fat diet had a tissue concentration of 1.12 ppm, and those fed 35.9 percent fat had an average concentration of 1.9 ppm astaxanthin.

In an unpublished 1992 study, March and McMillan noted typical flesh deposition in two salmonid species for two different dietary levels fed for various lengths of time (Figure 1). But Yungblut emphasizes that such data provide only rules of thumb when estimating inclusion rates; optimal levels must be adapted to the operation.

A vitamin A precursor, astaxanthin is used by salmonids to synthesize that important vitamin and help maintain their health.

Miki (1991) reported that astaxanthin had five times the cellular antioxidant activity of beta-carotene and 10 times that of vitamin E. It also functions in reproduction, egg quality, embryonic livability, photo- response and behavior. For example, Craik (1985) found that astaxanthin helps protect eggs against harmful light, elevated temperature, metabolic products of adult fish and low oxygen tension, thus helping reduce mortality during embryonic development.

"Nature-identical" pigmentation of salmonids is only the first of astaxanthin's benefits. Others are greater efficiency in supplementation and flesh color which stands up better to the rigors of processing.

The advantages begin at the cellular level, where salmonids deposit astaxanthin more efficiently. Various studies have found that canthaxanthin has only 70 percent of astaxanthin's deposition rate in salmon and 60 percent in rainbow trout.

Astaxanthin also binds more tightly to the actomyosin complex in muscle tissue, forming two bonds compared to one with canthaxanthin. The result is greater intramuscular binding strength. According to Henmi et al. (1991), this binding strength is 0.78 mg/g of protein for astaxanthin and 0.47 mg/g for canthaxanthin.

In color itself, astaxanthin allows salmonids to more closely duplicate the appearance of wild fish. Color quality in salmonids--an important aspect of consumer appeal--is measured in redness, yellowness and lightness, with the ideal color being a rich pink color.

Skrede et al. (1990) used instrumental color analysis and assessment by trained panelists to compare the flesh color of astaxanthin--and canthaxanthin--fed fish. The raw flesh of the astaxanthin- fed fish had less lightness and more red hue than canthaxanthin-fed fish when flesh depositions were equal. This was true whether color was measured by the eye or an instrument (Figure 2). Processing caused the flesh coloring to fade in fish fed either carotenoid, but the astaxanthin--pigmented flesh maintained more red color.

In DSM Nutritional Products studies using spectrophotography, astaxanthin--fed salmonids had darker, redder flesh than canthaxanthin--fed salmonids at the 8 to 10 ppm deposition rates that are considered optimal. Astaxanthin-fed fish also exhibited less of a yellow tinge in raw or processed flesh.

References:

• Craik, J.C.A. 1985. Egg Quality and Egg Pigment Content in Salmonid Fishes. Aquaculture. 47:61.
• Henmi  et al., 1991. Studies on the Carotenoids in the Muscle of Salmon. Comp. Biochem. Biophysiol. 99B(3):609.
• March, B.E. and C. McMillan, 1992. Unpublished data.
• Miki, W. 1991. Biological Functions and Activities of Animal Carotenoids. Applied Chem. 63(1):141 Pub. Index No. 1874.
• Skrede, G., T. Storebakken, and T. Ness, 1989. Color Evaluation in Raw, Baked and Smoked Flesh of Rainbow Trout Fed Astaxanthin or Canthaxanthin. J. Food Sci. 55(6):1574.
• Storebakken, T. and H.K. No, 1992. Pigmentation of Rainbow Trout. Aquaculture 100:209.
• Torrissen, O.J. 1985. Pigmentation of Salmonids: Factors Affecting Carotenoid Deposition in Rainbow Trout. Aquaculture. 46:133.