Abstract: It is important for winemakers to understand the chemical constituents of oak heartwood used for barrels as different grain types and toasting regimes influence wine flavor. Changes induced by different heating regimes in low-molecular-weight (LMW) phenolics and volatile compounds of oak heartwood of different grain types were evaluated. The results showed that a higher temperature during the toasting process changed the chemical compositions of the heartwood and increased the amounts of several key soluble and volatile compounds as a result of various heat-induced reactions. Moreover, independent of toasting, the fine-grained oak samples had higher concentrations of volatile compounds, e.g., cis/trans-oak-lactone, cis/trans-isoeugenol, furanic aldehydes, syringol, and LMW phenolics, particularly sinapaldehyde, coniferaldehyde, vanillin, and syringaldehyde, while the medium- and coarse-grained oak samples showed higher levels of total phenol, tannins, ellagic acid and gallic acid. Principal component analysis (PCA) showed that the moderately and heavily toasted samples of medium- and fine-grained oak displayed higher positive correlations with the level of volatile compounds, while the untoasted and lightly toasted heartwood of coarse-grained oak were positively correlated with total tannin and ferulic acid contents.

Key words: oak heartwood, grain type, toasting level, phenolic compounds, volatile compounds