Analysis of microvolatiles in brandy: relationship between methylketone concentration and Cognac age

Headspace solid phase microextraction and gas chromatography/mass spectrometry were used to identify and quantify four odd-numbered methylketones in commercial Cognac brandies. These ketones are in part responsible for the desirable and complex characteristic called rancio charentais or Cognac rancio which is found in grape brandies aged in oak barrels for several decades. The ketones 2-heptanone, 2-nonanone, 2undecanone and 2-tridecanone form through (beta)-oxidation and decarboxylation of long-chain fatty acids originating from yeast metabolism. The concentrations of these ketones increased with Cognac age classification in the 42 brandies analysed, and 2-heptanone was present at the highest concentration in most samples. The average concentrations and rates of formation decreased with increasing chain length. Total concentrations ranged from 21 to 328 (mu)g l^-1. The esters propyl octanoate and ethyl octanoate followed the same trend as the methylketones and appear to play an additional role in the formation of the rancio character.