I was interested to note recently that many people do not realize that there are several species and dozens of sub-varieties of oyster. Most seem to just ask for oysters and leave it to chance. There are huge differences between oyster species in regards to flavour, availability and texture, and that is not taking into account for regional variations. Growing conditions and locale have a marked impact on the final flavour of oysters.
Identification
There are three main species of edible oyster, the Pacific, belon and rock oysters.
The Pacific oyster (Crassostrea gigas) is also commonly known as the Japanese oyster, indicating the origin of this particular species. This oyster is widely cultivated in the Pacific region and Australia, but due to its fast growth habits it has been introduced to many areas around the globe. In some areas the species is considered a marine pest.
Pacifics are a medium sized oyster and are harvested when they are between 50mm and 100mm. They possess a thin, roughly textured shell, which is cup shaped and flinty grey in colour. Pacific oysters have a forthright briny, iodine flavour. If you march into a fishmonger's shop and simply request oysters, this is the species you will most likely receive.
The belon oyster (Ostrea angasi) is also marketed as the flat, mud and angasi oyster. This species is native to the Mediterranean, with a sub-variety native to Australia. This is the highly esteemed Belon oyster of France.
The belon is a large oyster with some reaching 150mm in size. It has a round and flat, flaky shell, with yellow tinges to the regular gunmetal oyster shell colour.
Belon flesh has golden highlights to the edges and possesses a very clean and tannic seawater flavour. It is a very meaty oyster, with regards to not just its size, but big flavour as well.
The rock oyster (Saccostrea commercialis or occaisionally S. cucullata) is native to Southeast Asia and parts of Australia. It is also sold under the monikers Sydney rock and coral oyster.
This is the smallest oyster of the three, but it possesses the biggest flavour. Rock oysters have a strong briny, oily flavour with a long aftertaste. They are harvested at a size between 40 and 80mm.
These oysters have a thick smooth shell that is elongated and cup shaped. The flesh is creamy gold in colour, with occasional greenish highlights. In Australia if you simply order oysters, this is the species that you will (mostly) get.
History
There is a long history of the human consumption of oysters, with the Celts, Romans and Greeks all being noted oyster lovers. The last leave evidence of cultivating oysters in beds. According to Larousse Gastronomique, Louis XIV's doctor recommended the consumption of oysters, albeit always cooked in their shells. I have personally visited shell middens in the Sydney basin where Kooris feasting on oysters over thousands of years have left a shell speckled imprint in the shoreline strata.
Habitat and cultivation
Oysters are bi-valve filter feeders and hence are susceptible to contamination through pollutants. In their native habitat, on rocks or in muddy mangroves this can be a serious problem, as it can be hard to gauge what has flowed past a wild oyster. These days almost all oysters are farmed on tiered or trellised beds that reside in tidal brackish waterways. This, in addition to the use of filtering tanks after harvest have greatly reduced the chance of eating contaminated oysters. However, there is always a minimal risk, so if you are in a high-risk group, pregnant, aged or infirm, it may serve you well to avoid raw oysters.
Spawning and flavour
Spawning has a marked impact on the flavour of all oyster varieties, as well as a reduction in size. Instead of tasting briny and zingy with seawater, spawning oysters will taste dull and flabby. Although oysters are considered a warm weather treat, it is mainly during the summer months that oysters spawn. Mid-summer can be the worst time to buy oysters, so it pays to ask your fishmonger for advice.
Shell, shucked or bottled?
Raw oysters are sold in three forms. Whole, or unshucked; on the half-shell, or shucked; and shucked, then bottled in brine, totally removed from the shell.
I cannot stress enough the superior flavour of a freshly shucked oyster. If this means you have to shuck your own, then I recommend you go to the extra effort.
Oysters attach themselves to the inside of the shell with the aid of a muscle, or foot as it is often called. Once an oyster is shucked and this muscle is severed, the oyster dies. You now have between 6 and 12 hours to enjoy a fresh oyster, as they deteriorate very rapidly once killed. In addition, all pre-shucked oysters are washed, then flipped in the shell to present the more attractive side of the oyster. This travesty robs you of the full pleasure of a fresh oyster; its iron laden, seawater juices.
The type of oyster you buy will determine the best way to store them. Bottled oysters must be kept refrigerated and consumed well before the date on the bottle. Half shell oysters are best removed from their packaging, if any, then transferred to a plastic container that has been lined with a damp kitchen towel. Keep well wrapped and in the refrigerator. Try to use within 24 hours. Full shell live oysters are best stored in a damp hessian or burlap sack. Store them in a cool area, such as a cellar if you live in a cool climate for up to 4 days. If you live in a warm climate, store in the refrigerator. The cold will make the oysters open and eventually die, so they are best used within 2-3 days if you are using this method of storage.
Serving oysters
As far as cooking oysters goes, I have one word on the matter; don't. Oysters cook very rapidly and dry out to an unpalatable mess when over cooked. There are a couple of notable exceptions. Oysters quickly fried in a tempura batter, served with a ponzu and ginger sauce are sublime. Oysters Kilpatrick are a fun 70's nostalgia trip, but I would recommend using a hot grill or broiler, rather than an oven as the latter will overcook the oysters.
You could try tea smoking oysters, and simonc mentions that placing whole, unshucked oysters into an open fire BBQ is a great method. I have never tried this, but as long as you removed the oysters just as they open, I imagine it would impart a delicious smoky element
If a recipe asks for cream or cheese, avoid it. You will simply mask the oyster's flavour and may as well be eating something else.
The best way to serve oysters is simply with lemon, freshly ground pepper and perhaps a few drops of Tabasco. Some French recipes ask for a dressing made from wine vinegar and enlivened with chopped shallots. There is a quirky, regional French dish that serves fresh oysters with baby chipolata sausages, this is offbeat, but delicious. I have also been topping freshly shucked oysters with a Thai nam jim dressing, and they are sensational.
Oysters and wine
To accompany oysters, the most traditional drink is sparkling wine or Champagne. Good quality sparkling wine has a real affinity with oysters and is a simple way to give festive feel to a dinner party. Another way of cutting the rich, oily taste of oysters is a young dry Riesling, as flinty as possible. Try any from Australia's Clare Valley, or a very dry, premium German Riesling.
In Ireland, and around the world on St. Patrick's Day, it is considered de rigueur to wash down a plate of oysters with a pint of Guinness. Maybe try a black velvet for a celebratory variation.