Rcho

Alcohol dehydrogenase

NADH NAD*

R-CH,OH

Fig. 3.13 Generalised scheme for higher alcohol synthesis.

The relative contribution made by the anabolic and catabolic routes for higher alcohol synthesis is influenced by a number of factors. In the case of n-propanol, the anabolic route is the only one possible since there is no corresponding amino acid. Chen (1978) reported that with increased chain-length of the alcohol the catabolic route was progressively used. The wort amino acid concentration is important. Thus, where only low levels of assimilable amino nitrogen are available the anabolic route is predominant. Conversely, high concentrations of wort amino nitrogen favour the catabolic pathway due to feedback inhibition of the amino acid synthetic pathways by their respective products. In brewing fermentations Schulthess and Ettlinger (1978) concluded that yields of higher alcohols were approximately equal from both the anabolic and catabolic pathways. However, the catabolic pathway is of greater importance during the early phase of fermentation when wort amino acids are plentiful whereas the anabolic pathway is increasingly used as wort amino acids are assimilated (Inoue, 1975).

The metabolic explanation for the formation of higher alcohols is obscure.

Acetaldehyde

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