The Influence of Oak Seasoning and Toasting Parameters on the Composition and Quality of Wine

Abstract

During toasting of American oak (Quercus alba) for wine maturation, the extent of thermal breakdown of the major constituents became greater as seasoning time increased, even though the temperature of the wood was lower during toasting. Under the same toasting conditions, the temperature difference between staves that had been seasoned for 24 months compared to kiln -dried material was nearly 20°C after toasting. A concentration gradient of extractives existed across the thickness of the stave. The greatest differential was in polyose (holocellulose) caramelization products such as 5-hydroxymethylfurfural, furfural, and 5-methyl furfural. These compounds were highest at the toasted surface and decreased rapidly into the depth of the stave. Vanillin and related lignin breakdown products decreased more slowly. Tannins were lowest at the toasted surface and ellagic acid was formed at the expense of ellagitannins. Oak lactones occurred in their highest concentration halfway into the depth of the stave. Wine that was matured in contact with only the surface layer of toasted wood had a dominant toasted character, high total intensity but low complexity. Wine from the subsurface layer gave the most complex wine, while wine matured in contact with only wood from the middle of the stave was astringent and green and showed poor integration of fruit flavor. The most obvious microscopic feature of toasting was the production of transverse cracks in the wood surfaces in heavy-toast samples, occasionally to a depth of 700 µm. These cracks were particularly straight, even when they crossed cell walls. At the toasted surface, cells showed thinning and possible volatilization of material from the ray cell lumina. There appeared to be some corresponding condensation in the surrounding cells.