Creating the perfect pink fish fillet: The strategy behind pigmentation

Pigments deposit in fish tissue 

The characteristic colour to many plants and animals, like for instance to carrots and tomatoes and to flamingos and salmon or trout, originate from carotenoids, organic pigments that consist of biologically active molecules that have many important functions. 

In nature, salmonids extensively feed on crustaceans containing the carotenoid pigment astaxanthin that is deposited in the muscles of the fish. This explains why, to most consumers, salmonid fish like salmon and big trout always have stand out for their attractive orange, pink or even red fillet colour. Moreover, the pigment is important in skin coloration for camouflage and sexual attraction. 

Pigmentation of the fillet 

Salmon and trout farmers strive to enhance the value of their produce by catering for market demands by obtaining the most advantageous fillet colour. Pigment is being added to the feed in specific doses and to be fed over specific periods of time, with the aim of being retained in the fish fillet. 

Pigmentation strategy 

EU regulation allows feed for all salmon and trout species to be enriched with up to 100 ppm astaxanthin (100 mg astaxanthin per kg). However, various circumstances make such large doses unattractive as a general strategy. The main reason is that the ability of the fish to absorb the pigment decreases significantly when the pigment content in the feed increases. 

A research project investigating how much of the astaxanthin content in the feed is retained in the trout fillet showed that increasing the astaxanthin content in the feed from 25 to 100 ppm reduces the retention by more than half. The quantity of astaxanthin retained in the fillet does indeed increase at 100 ppm, but proportionally seen definitely not as much as one probably would expect given the increased astaxanthin content in the feed. 

Astaxanthin can be quite expensive compared to other feed ingredients. Financially, it is therefore advantageous to add a more moderate amount of pigment to the feed and instead one should obtain a satisfactory colour by using pigment over a longer period. Another advantage of such defensive strategy is that the fish reach acceptable pigmentation level considerably earlier. This may be significant for instance in connection with emergency slaughter. 

A rule of thumb says that the pigmentation of trout will reach a satisfactory level if they double their weight with the use of a feed with 50 ppm (50 mg astaxanthin per kg feed). Whether this rule applies is, however, highly dependent on the desired results.  

Nevertheless, it is indisputable that the result basically depends on two parameters: 1. the pigment content in the feed and 2. the proportion of the fish’s growth which is based on pigment-enriched feed: 

ppm astaxanthin in feed   X   growth   =   fillet colour 

BioMar recommends starting the pigmentation early in order to avoid high doses. Feed with 100 ppm astaxanthin should be regarded as an emergency solution. 50 ppm should be adequate for fish that are not also farmed for roe as part of the slaughter product given the fact that astaxanthin stored in the flesh is mobilised to the eggs during maturation. 

Remember, the fish can always become redder! However, it is a mistake to think that the fish must be as red as possible. That will merely result in added expense. Instead, the aim should be even pigmentation. This is what shows the real quality of pigmented fish. 

Assessment of fillet colour 

The colouration of salmonids is an important parameter, but difficult to quantify/measure. Various methods exist: 

• Assessment by the human eye by means of a fan, a ruler, or a handy chart, with scales from 20 or 25 to 34. Using these requires quite a lot of training. As the assessment is dependent on the surrounding light, it is very important that this is either daylight or artificial light with a colour temperature corresponding to daylight. The difference in colour assessment of a fillet in light from ordinary fluorescent tubes and in daylight can be considerable.  

• Assessment by instrumental colour measurement by means of advanced equipment capable of describing a colour electronically/optically. The advantage of this technology is that it is independent of the surrounding light and that the colour is described by a figure which is independent of human assessment. 

• Chemical analyses can provide information about the absolute pigment content of the fillet. However, the connection with a visual assessment is not always clear, for instance due to varying fat content in the fillet. 

 

Risks in connection with pigmentation 

There are some common pitfalls when working with pigmented fish: 

The fat content of trout fillets can vary significantly. Very fast-growing fish are normally much fatter than fish that grow more slowly. Fillet pigmentation is caused by the pigment being deposited in the musculature rather than in the fat, which remains largely white. Despite identical pigment content, a fat fillet will therefore always look paler than a lean fillet. 

• Sexual maturity results in the development of roe and darker skin – the fish assumes mating colours. Both roe and darker skin require pigment, which is derived partly from the feed and partly from the pigment stored in the fillet. Once the weight of the roe reaches three to five per cent of the weight of the fish the fillet pigmentation begins to decrease. When the fish is ready for stripping, its fillet pigmentation will be at the very bottom of the targeted scale, perhaps even outside it. 

• Some fish farmers pursue a strategy of re-pigmenting the fish after stripping with a view to slaughter. In this context, the fish cannot be expected to achieve the same pigmentation quality and evenness as from pigmentation before it reaches sexual maturity. 

• Illness can result in poorer and especially more uneven pigmentation. 

• A low feed conversion rate means that the fish does not eat very much feed in relation to its growth. As a result, very efficient feed cannot be expected to produce the same pigmentation result as less efficient feed types with the same declared pigment content. The fish simply eats less pigment per kg of growth.