The effects of size, temperature and air contact on the outcome of heating oak fragments

The concentrations of the important oak aroma volatiles guaiacol, 4-methylguaiacol cis- and trans-oak lactone and vanillin in extracts of French and American oak heated under various conditions were measured using stable isotope dilution analyses coupled with gas chromatography-mass spectrometry. Heating resulted in marked increases in the concentrations of guaiacol, 4-methylguaiacol and vanillin, with more formed at higher temperature. Approximately twice as much guaiacol and 4-methylguaiacol and two to five times as much vanillin were formed by heating in the presence of air compared to heating under argon. Oak lactone concentration was less affected by heating. The effects of heating different sized oak pieces were investigated for French and American oak samples. Compared to heating larger oak pieces, heating smaller fragments of oak generated up to twice as much guaiacol and 4-methylguaiacol and two to four times as much vanillin at 235 degrees C for both French and American oak. This effect is ascribed to the exposure of a greater surface area of oak to air when smaller fragments are heated. Variable effects were observed for cis- and trans-oak lactone. Variation in chip size, as well as heating time and temperature, is clearly one way of obtaining different aroma profiles from oak products.