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Fermentation involves the hydrolysis or breakdown of proteins into their constituents peptides and amino acids. The development of the characteristic odours and flavours of putrefaction is prevented by the addition of salt in varying but usually large amounts, similar to those used for pickled fish. The products are not dried after salting. In some circumstances, carbohydrates may be added which result in the formation of acids which further help to impart a characteristic flavour and odour as well as providing a further degree of preservation.

In general, three types of fermented fish products (Subba Rao, 1961) can be distinguished:

(i) Products in which the fish retain substantially their original form or in which large chunks are preserved (e.g. pedah siam (Thailand), makassar (Indonesia) and buro (Philippines));

(ii) Products in which the fish are reduced to a paste (e.g. ngapi (Burma), pra-hoc (Cambodia), belachan/trassi (Malaysia/Indonesia) and bagoong (Philippines));

(iii) Products in which the fish are reduced to a liquid (e.g. budu (Malaysia), patis (Philippines), nuoc-mam (Viet Nam) and nampla (Thailand)).

Few of these products and processes are found outside South-East Asia and, as with other methods of fish curing, traditional practices developed over many years predominate. These vary considerably from place to place, depending on local taste, raw materials available and care taken during processing. Many traditional products are of excellent quality and often rely on traditional skills which are difficult to emulate with modern processing methods. It is unclear whether the fermented fish products could be introduced successfully into other areas due to problems of consumer acceptability. The fermented products of South-East Asia are many and varied and, for the purposes of this review, it is only possible to cover a few of the more important products and processes.