A "true" oyster sauce of good quality should be made by condensing oyster extracts, which is made by cooking oysters in water until a white broth is produced. The opaque broth is then cooked until a desired viscosity has been reached and the liquid has caramelised to a brown colour. No other additives, not even salt, should be added to the sauce, since the oysters should provide all the savoury flavour. Many oyster sauces are actually diluted solutions thickened with starch, coloured with caramel colouring (E150), with oyster extracts and synthetic preservatives. In some countries, including the UK, the oyster content in some sauces is lower than its Asian counterparts of the same brand due to laws regulating the import of seafood.
Keep refrigerated once opened.