Variable quality has prompted physical chemists to take a closer look.
WHAT MAKES SEA TROUT TASTE SO GOOD? Is the pink color of its flesh a sign of quality? How should it be cooked to satisfy gourmets? And how should it be raised to suit processing requirements, particularly in connection with smoking, a technique in which French companies have long specialized?
Although France is among the world leaders in trout production, the study of trout aquaculture is almost thirty years behind the study of beef and other kinds of meat. One of the chief problems is inadequate genetic selection, which causes unwanted variability in the quality of farm-raised fish. Benoît Fauconneau, Michel Laroche, and their colleagues at the Institut National de la Recherche Agronomique stations in Rennes and Nantes, in collaboration with the Institut Français de Recherches sur la Mer and several private companies, have been exploring the physicochemical characteristics and sensory properties of trout filets in an attempt to determine the causes and effects of differences in quality.
It was known that the gustatory qualities of the large salmonids and their susceptibility to treatment by means of techniques such as smoking and marinating depend principally on the concentration of lipids—molecules that dissolve aromatic compounds and that also can communicate tastes when they are heated or oxidized—in the flesh of the fish. Preliminary analysis of fario trout (Salmo trutta) raised in the sea by Elsamer S.A., in collaboration with food technologists in Camaret, revealed that the filets obtained from these fish contained 65–70% water, 20–24% proteins, and 2–12% lipids.
The pink color of trout filets, which results from the presence of carotenoid molecules (astaxanthin and canthaxanthin) contributed by food, seems not to be a reliable indicator of quality. Despite some genetic variability in the fixing of these pigments, the color varies mainly according to whether the diet of the fish contains these carotenoids (which cause their flesh to turn from yellow to pink).
Why, then, does the shade of color vary between fish born of the same parents? The answer undoubtedly has to do with the fact that young fish mature at different rates. Indeed, studies have shown that their muscles gain in lipid content in the course of growth: The bigger (and therefore more rapidly developed) the fish, the fattier it is. Because the red component of their characteristic color increases with lipid concentration, it also increases with the rate of growth. This means that a trout that eats a great deal, which is to say one that fattens rapidly, absorbs more pigments than a small fish. In other words, the quality of smoked trout filets would be uniform if the rate of growth could be controlled.
The physical characteristics and sensory qualities of fish depend on the macroscopic organization of their muscle cells, which is very different from that of meats. Muscle cells in animals consist of very long fibers sheathed in collagen, a protective protein, and collected in bundles, which are themselves sheathed in collagen, and so on. The cooking of meat therefore involves a delicate compromise between hardening, which results from the coagulation of the proteins contained in these cells, and tenderizing, a consequence of separation and dissociation of the collagen molecules.
Tender to Cook
In the case of fish, by contrast, the cooking time must be short because their flesh contains little collagen. The muscles are not individually sheathed but are grouped together in sheets of which only the surface is supported by collagen. It is the lipids, localized within the muscle sheets in anatomically distinct adipose tissues, that play a predominant role in holding the sheets together and therefore in determining the texture of the flesh. Tests of resistance to compression have shown that the flesh of trout is firmer than that of other freshwater fish such as carp and catfish.
To understand how trout filets are modified during cooking, the Nantes researchers began by comparing two filets obtained from the same fish: One was characterized at once in its raw state, and the other was analyzed after poaching, at temperatures ranging from 10°C (50°F) to 90°C (194°F), depending on the experiment (the filet was placed in a sealed bag and the temperature increased by 1°C per minute, then rapidly lowered by immersion in ice water).
These studies showed that although cooking did little to change the chemical composition of the filets, it did increase their mechanical resistance as a direct result of the temperatures to which they were heated, which caused the muscle proteins to coagulate. The overall composition of the filets was largely unaffected because their constituent elements were lost in the same proportions in the cooking juices. Because the juices contained a substantial quantity of lipids, the red color contributed by dissolved pigments diminished—all the more so because the luminosity of trout flesh increases as a result of protein coagulation. All told, the loss of matter in the form of liquid, amounting to 10-20% of the total mass, increased as the poaching temperature rose.
Research continues into the effects of cooking (one preliminary result is that trout filets seem not to become softer the longer they are cooked) and the role of diet, which seems to affect the chemical composition of the flesh without much affecting its gustatory qualities. Why should trout not be like fowl, whose taste varies greatly depending on their diet? We do not yet know.