In recent years, the flavour market has undergone a tremendous ‘back-to-nature’ demand, which is illustrated by the consumers’ preference for ‘natural’ flavouring substances instead of synthetic ‘artificial’ compounds. These natural products can be obtained by extraction from plant material, but they can also be produced by biotechnological processes using micro-organisms. De novo synthesis of ‘bioflavours`, such as volatile esters, and biotransformation of monoterpenes are fields of investigation that gain a growing interest. Terpenes are obtained from the essential oil of many plants and are relatively cheap. They are usually isolated from the oils by rectification. This renders abundant monoterpenes, such as α-pinene and limonene, inexpensive starting materials for chemical and biochemical transformations. The fungal biotransformation of these natural precursors to more valuable aroma compounds offers a very interesting alternative source of natural flavours.

This review article deals with the biotransformation of some monoterpenoids by both fungi and bacteria. As substrates myrcene, ocimene, geraniol, nerol, citronellol, citral, citronellal, linalool, limonene (and related compounds), pinenes, menthol, camphor, pulegone, ionones, etc... are discussed.