Oysters

Evidence of a taste for oysters (and other molluscs, including terrestrial ones – snails) goes back to the Paleolithic (earliest stone tool era). Middens of oyster shells have been dated to at least around 20,000 years ago (Australian aborigines). Once food for the poor, and eaten to near extinction by the time of the 17th century, they made a comeback with modern farming methods. Wild oysters are not harvested.

Oyster flavour comes about because oysters fill their cells with amino acids as a buffer against the salinity of their environment (and also as an energy store). Seawater is ~3% salt, whereas seafood needs to maintain an internal concentration of no more than 1%. Higher water salinity means higher amino acid levels.

Finfish have the same salinity-control problem as molluscs. While fish also make use of tasty amino acids, they rely more on a flavourless amine – trimethylamine oxide (TMAO) – to achieve their goal. Once a fish is killed, the natural enzymes (freed of control mechanisms) begin to break this amine down and release ammonia, hence the telltale smell of old fish.

Strictly speaking, oysters are at their best (and healthiest) in winter months, although with modern transport options, they can be harvested in cold regions and flow to warmer ones for sale. In warmer weather, they spawn (males releasing sperm and the females eggs – tens of millions), and in doing so they deplete their amino acid energy stores and lose flavour. There is also a loss of texture. Some oysters are bred not to spawn.

Amino acids have a complex range of flavour profiles, some (e.g. glycine) are sweet, others (glutamate – yes, MSG) are savoury. Put these in an oyster with its natural saltiness and the result is deliciousness. Cooking an oyster gels its proteins and locks away amino acids, making them less available to our taste receptors. Hence, oysters are most delicious when eaten raw (with some associated health risk accepted).

Vibrio vulnificus is the main bacterium that can infect oysters. While rare, it is a natural part of the ecosystem. With sous-vide, it is possible to use a version of heat-cool pasteurisation (HCP) to reduce V. vulnificus. Place whole oysters in their shells directly into 60C water, and hold for 8m (Update 2018: I have had a failure with this because the oysters opened slightly – probably better to bag them and use the water-displacement method). Transfer to an ice-water bath (50:50) and cool. There is no loss of flavour, and the oyster still presents as raw. There’s a video here.

Some commercial operators may already perform HCP, or use irradiation, individual quick freezing (IQF) or high pressurization processing (HPP) to destroy the bacterium. It is not usually clear at point of sale though.

The process has two further benefits; it nicely plumps the oyster, and it weakens the ligament, making it theoretically easier to shuck. Some suggest 65C for 7m as a further option. The commercial version of HCP holds at 53C for 24m. The higher temperatures are for the culinary purpose of plumping the oyster.  I’m not absolutely sure the domestic version achieves full pasteurisation – it will depend on the time taken for the interior of the oyster to come up to temperature. It’s a good bet though, as 10m at 50C has been shown to reduce the bacterium to undetectable levels in shell-stock oysters.

OystersThe left oyster is raw, the middle 60C and the right 65C. There was not a lot of difference in taste between 60 and 65, but I thought the 60 prevailed. Notice the plumpness. In the absence of sous-vide control, and given the similarity in both heated versions, this process should work with a large pot of water (to minimise temperature fluctuation), an accurate thermometer, and 7-8 minutes of vigilance holding temperature in the 60-65C range.