A strong red colour is considered an important feature in terms of quality
Recently, I witnessed a strange incident while visiting the fish counter of my favourite supermarket when a customer was loudly complaining about the intense vermilion colour of a salmon fillet on display: “The salmon farmer has once again gone way too far and used far too much colour! The fillet looks completely unnatural!” The sales staff were overwhelmed by this situation. It would have been easy to take the wind out of the angry lady’s sails, as the fillet in question came from sockeye salmon, a wild Pacific salmon species that is not produced in aquaculture and is certainly not artificially coloured, but develops that deep red fillet colour solely through its natural diet.
However, this grotesque scene draws attention to a topic that is widely discussed and unfortunately often misunderstood by large parts of the public. Fuelled by social media and self-proclaimed experts, new claims and myths about the red colouring of salmon meat are constantly emerging. It is not uncommon for this to involve deliberate manipulation with food colouring and consumer deception. In the US, packaged farmed salmon must even be officially labelled with the phrase ‘added colour’ under FDA disclaimer guidelines, leading many consumers to believe that farmed salmon is somehow artificially coloured. In fact, both wild and farmed salmon get their colour in the same manner, i.e. exclusively from their diet. The red colouring of muscle meat is not caused by artificial colouring, but by carotenoid molecules, which also occur in nature. Of the more than twenty carotenoids found in salmon fillets, astaxanthin is by far the most important, as it accounts for over 70 per cent of the total pigment content in Atlantic salmon. Astaxanthin (the name is derived from the former Latin name of the lobster Astacus gammarus) is actually a plant pigment that also finds its way into numerous animals through the food chain. It is found in yeasts, microalgae and other phytoplankton, carrots, many crustaceans such as krill, shrimp, lobsters and crabs, and even in the plumage of flamingos.
Colour sells when buying salmon
The red colouring of the salmon fillet is a typical visual feature of this fish, which is perceived by many consumers as a criterion of value and quality and is preferred when shopping. Intense red is often even considered an indication of naturalness. The preference for strong red is particularly strong in Asia, where it influences both the acceptance and the price of salmon products. The average astaxanthin content of Atlantic farmed salmon varies between 6 and 8 mg/kg of fillet meat. The Japanese market tends to favour wild Pacific coho and sockeye salmon, which have astaxanthin contents of up to 25 mg per kg of fillet.
Salmon and other salmonidae are among the few fish species that can store astaxanthin in its pure form. They store the pigments not only in their muscle meat, but also in their skin and in their eggs, which then acquire a yellowish to orange-red colour and protect the embryos from overly intense UV light. The second important colouring pigment besides astaxanthin is canthaxanthin, which is also fat-soluble and comes either from natural sources (e.g. plants, fungi and crustaceans) or is produced synthetically, for example, as food additive E 161g. Canthaxanthin is a controversial substance that is suspected of having pathogenic effects and is therefore subject to strict limits. In natural products such as crustaceans, however, the amounts are so small that there is no risk to health. Furthermore, canthaxanthin may continue to be used in animal feed albeit in limited quantities. The Scientific Committee on Animal Nutrition of the European Commission has set the maximum concentration at 25 mg canthaxanthin/kg feed for salmonidae.
Pigments have important biological functions
Astaxanthin and other pigments not only give salmon its red colour, but are also essential nutrients that ensure health and growth. For example, astaxanthin acts as a powerful antioxidant that protects the fats in muscle meat from breaking down, stimulates the immune system, and protects body tissue from oxidative damage. The antioxidant effect of pure astaxanthin is said to be 100 times stronger than that of vitamin E, which is contained in some human food supplements. Part of the ingested astaxanthin is converted in the body into vitamin A, which supports cellular respiration and has antimicrobial and anti-inflammatory effects. For example, tests on Atlantic salmon have shown that intestinal inflammation, which often occurs when soy bean meal is fed, can be prevented by adding the astaxanthin-containing microalgae Chlorella vulgaris to the feed. Astaxanthin is also believed to have a significant effect on the reproductive performance of many fish species, such as egg production and sperm quality, as well as the fertilization and survival rates of the eggs. Pigments are therefore much more than just simple colouring agents because they also have important biological functions.
The salmon cannot produce the red pigments themselves, only absorbing them through their food. In aquaculture today, most fish are fed formulated feed, which is a mixture of various components tailored to the needs of the fish. Many of these contain insufficient or partially destroyed carotenoids, as these pigments are extremely sensitive. They can lose their functionality due to high temperatures and pressure during the preparation of the raw materials or incorrect storage of the feed pellets and must therefore be added from suitable sources when mixing the feed. This can perhaps be compared to the intake of nutritional supplements that some people use to supplement their diet. Astaxanthin, which is available over-the-counter at the chemists and elsewhere, brings health benefits not only to salmon, but also to humans. It has an anti-inflammatory effect, increases the production of antibodies and the proliferation of immune cells. It is fairly well established that astaxanthin protects against stress-related cardiovascular diseases, certain types of cancer and diabetes and helps prevent obesity. Preclinical studies suggest that astaxanthin can also regulate the gut microbiome. However, anyone who regularly eats salmon or other red-fleshed salmonidae can easily do without dietary supplements containing astaxanthin, as the fillet of these fish contains enough of these pigments. In addition to omega-3 fatty acids and highly digestible protein with all essential amino acids, this is an additional argument in favour of regularly eating these fish.
Origin of pigments in salmon feed
In principle, the carotenoids used in salmon feed can be produced from natural sources such as bacteria, fungi, algae and plants or from shrimp shell waste. However, this is complex and expensive (naturally-sourced astaxanthin is four times more expensive than the synthetically produced pigment). In addition, it would be very difficult to meet the immense demand of aquaculture from natural sources alone. This is why feed producers mainly rely on synthetically produced astaxanthin, which is nature-identical, fully biologically effective and very easy to dose. Carophyll Pink is particularly often used in salmon aquaculture as a colour-enhancing feed additive, which has been on the market since 1988. It contains antioxidants and a colour-enhancing canthaxanthin carotenoid supplement that significantly increases the colour of the fillets and fish skin. At a dosage of 50 grams per ton of feed, a noticeable improvement in colour can be seen after just two weeks.
In recent years, the desire of many consumers for salmon to be raised in a more natural, quasi-organic way has led to an increased search for natural sources of astaxanthin. The focus here is on microalgae, which produce pigments in addition to polysaccharides, sulfolipids and polyunsaturated fatty acids. Green algae such as Chlorella sp. and Tetraselmis sp., for example, are rich in chlorophylls and carotenoids. Although relatively little is currently known about how microalgae, many of which have high antioxidant capacity, affect salmon health, some species are already being used with good success in salmonidae feed. For example, the green freshwater microalgae Haematococcus pluvialis (blood rain algae), which is considered an excellent source of astaxanthin for salmon and trout.
In the organic salmon sector, yeasts of the genus Phaffia have been used as natural sources of astaxanthin for some time. The species Phaffia rhodozyma and Xanthophyllomyces dendrorhous are particularly noteworthy. Both produce astaxanthin as the main pigment, although only in relatively small quantities, which makes the natural Phaffia pigment still quite expensive. Therefore, biotechnological methods are being used to increase the yield in order to make the production process cheaper. Several yeast mutants that are involved in carotenogenesis and give the colonies a strong red pigmentation have already been isolated.
Some types of bacteria are also potential sources of natural pigments. Species of the genus Paracoccus from the Rhodobacter group, which are eaten by some crustaceans and contribute to their red colouring, appear to be particularly productive. These micro-organisms play a central role in the production of the industrial product Panaferd, which contains not only natural astaxanthin but also other carotenoids such as adonirubin and canthaxanthin. According to the manufacturer, Panaferd contributes to a more natural diet for salmon because it replicates carotenoid sources that these fish would also consume in the wild.
Colour differences are unavoidable
Despite the same pigment content in the feed, salmon often show differences in the colour of their flesh. The red colour is not only influenced by pigments, but also by a variety of other factors. For example, environmental influences, the activities of the fish, the nutrient composition of the feed, its genetic constitution and more. Not all salmon can process and store the pigment molecules equally well, which means that some animals even develop white or slightly grey flesh. In the wild, one in 20 chinook salmon (also known as king salmon) in the Northwest Pacific has white or white-marbled flesh. This is due to a recessive genetic trait (presumably the beta-carotene oxygenase 1 gene), which plays an important role in the carotenoid metabolism of this type of salmon. In the past, such colour aberrations were usually discarded after the fish were caught, but today white king salmon are considered a special attraction and delicacy.
The flesh of Atlantic salmon (Salmo salar), which is naturally lighter than that of the Pacific Oncorhynchus species, often shows individual differences in colour. The reasons for this usually remain unknown because the colouration processes are complex. We know little about how external factors and biological mechanisms interact to influence the pigmentation of salmon fillets. Science has difficulty tracking carotenoid metabolism because various non-pigmented metabolites occur in its course. It is undeniable that some salmon are simply better at processing the pigments and developing intensely red flesh. It appears that breeding conditions and stress can affect astaxanthin levels. It is also known that high-fat feed generally promotes the red colouration of the fillet (fish with a lower fat content tend to have white flesh). With rapid growth, the colouration is usually weaker. Even within a fillet, noticeable colour differences can be seen. The explanation is often given as discrepancies between the role of astaxanthin in protecting against oxidative stress and its deposition in the muscle cells. Whether this is really true is as difficult to prove as it is to disprove. In aquaculture, salmon are indeed exposed to numerous stress factors such as sorting, handling, transport, high stocking densities, diseases, vaccinations, occasional food deprivation and intraspecific aggression, which lead to acute or chronic stress, which in turn increases the release of hormones such as adrenaline, noradrenaline and dopamine into the bloodstream and can potentially cause colour changes.
Changes in the feed could also affect pigmentation. Less fish meal and fish oil reduce the salmon’s appetite and promote fat accumulation in the intestine. However, phospholipids are particularly important for the transport of nutrients through the intestine and affect the utilization of pigments. The amount of phospholipids in the feed influenced the salmon’s astaxanthin and fat digestibility. In experiments, salmon feed enriched with phospholipids from soy beans led to redder-coloured fillets. Such studies suggest that intestinal absorption could be a bottleneck in the utilization of astaxanthin, which ultimately determines how much pigment actually ends up in the salmon muscle meat.
Research needs for the future
Over the last decade, farmed salmon fillets have tended to become paler, despite more pigments being added to the feed. To clarify the causes and extent of this development, the Norwegian Seafood Research Fund has funded the Knowledge Mapping Pigmentation project. As part of the study, data will be collected through questionnaires, interviews and seminars with salmon farmers and feed manufacturers to provide information on changes in production methods and feed composition. Another focus will be on diseases and increasing stress, especially due to frequent delousing, which have a negative impact on astaxanthin levels. Land-based production in RAS with its usually high stocking densities, the accumulation of waste products in the water and its constant disinfection will also be examined in the course of the study. These are all exciting scientific questions that also have great practical significance and create added value because fillet pigmentation influences salmon sales.
Pigmentation problems could even increase in the future, because it is fairly certain that climate change with rising temperatures contributes to the increased breakdown of pigments in the fillet in summer. This can lead to the formation of stripes or, in extreme cases, even to a complete loss of pigment in the fillet. In the future, pigmentation and astaxanthin utilization could also become selection criteria in breeding programs. Near-infrared spectroscopy (NIRS) is a fast and reliable screening tool for predicting the expected amounts of fat and pigment in the meat. In addition, some researchers are already seriously considering specifically switching off those genes in the salmon’s genetic material that may have a negative effect on pigmentation. The biological mechanisms involved in the storage of astaxanthin in muscle cells have not yet been sufficiently explained, but the first ‘candidate genes’ have already been identified. With new, powerful gene editing methods (CRISPR/Cas9), it should not take too long to clarify the role of these genes in the absorption, conversion and storage of astaxanthin in various tissues and organs.
Of course, with all these studies and projects, it is important to keep consumers and their wishes in mind. Perhaps some customer groups will soon prefer fillets that are less coloured but still look natural. Consumer preferences may change faster than salmon change their colour.
Manfred Klinkhardt