Because consumers tend to have no idea how long ago a piece of fish has been harvested, it’s important to recognize good-quality fish to maximize its shelf life. Like a potential mate, it’s more than looks and smell, there are some factors that only a reliable fish supplier can share. They’re passionate about food too, so it doesn’t hurt to befriend that guy behind the counter!
If you’re purchasing whole-round fish:
The belly of the fish should be clean, the skin should be firmed, and bones should not bend like rubber bands. Signs of bacterial presence (and enzyme degradation) usually include swollen or broken flesh near the stomach.
The eyes should be bright, black, and the lens of the eye should convex outward (imagine a dome-roof). With time, the proteins in the eye begin to break apart and the eye lens begins to flatten out rather than extending outward.
[Source: http://media.photobucket.com/image/fish%20eyes/carolinadancer/FishEyes.jpg]
The gills should be clean and follow the naturally color of the fish.
The outer mucosal lining (The slime) should be transparent and sheen light. With time, the proteins coagulate and dry out creating a milky or off-color appearance. This slime may be difficult to find because most of it is washed off during cleaning.
The natural smell should either resemble oceanic water, or of crushed leaves. Beyond the fact that fish tend to live in the sea, the oceanic aroma (bromophenols) are synthesized by the algae that are consumed by sea life. However, farmed fish are supplemented with bromophenols in their artificial feed. And smell like leaves… weird right? Well, it’s because fish contain many highly unsaturated fatty acids that break down to smaller aromatic-carbon fragments. These carbon-rings have a heavy green, geranium-leafy smell that plants also emit.
I love walking through the seafood section of an Asian supermarket. If you can spend a few hours touring the bins, playing with lobsters and geoducks, and have the dexterity to lift a crab, then my heart's almost won! The only problem is that this dating destination would stink…and it’s largey due a chemical called TMA (TriMethylAmine). Remember last week how I mentioned sealife structurally contain extra amino acids to combat exerted osmotic pressure? Well there are extra chemicals such as this tasteless compound called TMAO (Trimethylamine Oxide) which breaks downs to stinky TMA once the fish is dispatched. The TMA then converts to ammonia which attributes to the nasty smell.
Unless you buy your fish as soon as it’s caught at Tsukiji fish market in Tokyo, there's going to be some trace odors.
[Source: http://www.nationalgeographic.com/healthyliving/gallery/japan/pic_08.jpg]
However, many cultures have combated the “fishiness” by preemptively reacting with TMAO preventing the conversion to nasty TMA. In addition, the usage of acidity has been very successful to counter TMA. Because acids contribute a hydrogen proton to stinky TMA, this causes TMA to become positively charge allowing water and other electric negative molecules to bond well. Our nose never picks up the odor because TMA+ never escapes as a vapor from the surface of the fish. The mutualistic inclusion of lemon wedges became widely popular because of this and deep frying fish with a layer of buttermilk (which is acidic) is widely popular in many fish-frys. Vinegars also work in the same manner and I’d like to think this is why malt vinegar is served with my fish and chips.
By the way, some freshwater fish carry an unpleasant “muddy” aroma which is present in bottom-feeding fish such as catfish and carp. The chemical behind this is geosmin which too, breaks down in acidic conditions. Because of this, many recipes utilizing these bottom-dwellers include vinegar, lemon/lime juice and other acidic ingredients.
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