Alcohol Oxidases and Dehydrogenases

In general, aldehydes are more potent flavor compounds than their alcoholic counterparts. Hence, alcohol oxidases are interesting enzymes for the in vitro production of flavoring preparations which, among others, can be applied in beverages. Typical examples include the use of methanol oxidase from Pichia, Hansenula, and Candida (17) for the production of natural acetaldehyde from ethanol. This enzyme is induced during growth on methanol. At the end of the logarithmic growth phase cells are harvested and incubated with ethanol. In this way concentrations of

1.5% natural acetaldehyde can be achieved, which can be concentrated further to the desired application level. From yeast to yeast the substrate specificity of the alcohol oxidase is different. Hence, this procedure can also be used to convert other alcohols, such as hexenol and other long-chain alcohols, to their corresponding aldehyde (10, 18).

As alternatives to alcohol oxidases the corresponding dehydrogenases could in principle be used (19). A severe drawback, however, is that these dehydro-genases require the expensive cofactor NAD(P)+ instead of (cheap) oxygen as an electron acceptor. Although various sophisticated NAD(P)+ cofactor regenerating systems have been designed and substantial cost reductions have been realized in this way, it is evident that in commercial applications oxidases are preferred over their dehydrogenase counterparts. The use of dehydrogenases for food purposes seems to be restricted to whole-cell conversions.

Vanillyl alcohol oxidase (VAO) from Penicillium simplicissimum is a special type of alcohol oxidase. Recently, it has been shown that this stable, flavin-containing enzyme has a very broad substrate specificity and readily converts para-substituted phenols into interesting flavor precursors or flavoring compounds (20-22). Apart from natural vanillin and coniferyl alcohol, different vinylphenols (e.g., para-vinylguaia-col) and allylphenols can be produced from cheap raw materials and oxygen as an electron acceptor. VAO can also be used for generation of flavor building blocks. To that end a natural mix of phenolic compounds could be treated with VAO to enrich foods or flavor preparations with a range of vinylic/allylic and aldehydic substances.

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