Influence of Toasting Levels on the Chemical Components of Eucalyptus camaldulensis Dehnh. Wood

Barrel-making commonly involves wood toasting processes that improve the sensory profile of alcoholic beverages. This study aimed to determine the effect of toasting levels on the wood chemical components of Eucalyptus camaldulensis Dehnh., known as river red gum (RRG) using TAPPI and ASTM methods. Total phenolic and sugar contents and volatile components of this wood aged in a model spirit solution were measured using UV-Vis spectrophotometry and gas chromatography-mass spectrometry (GC-MS), respectively. Results showed that heavy toasting significantly reduced the lignin content of RRG wood from 31.91% to 28.40%. The GC-MS analysis showed that the thermal degradation of lignin formed new volatile compounds such as 3,5-dimethoxy-4-hydroxycinnamaldehyde, coniferyl aldehyde, syringaldehyde, syringylacetone and vanillin in the heavily toasted and charred RRG wood extracts. Furthermore, charring of RRG wood led to a decrease in total phenolic content from 398.49 mg GAE/L to 374.26 mg GAE/L. Hemicellulose was significantly decreased from 48.24% to 44.94% (heavy toasted) and 34.15% (charred), which was further confirmed with the increase in total sugar content of RRG wood extract from 186.29 mg Glu/L to 230.44 mg Glu/L (heavy toasted) and 342.87 mg Glu/L (charred). Additionally, results in Fourier transform infrared spectroscopy confirmed the thermal degradation of hemicellulose and lignin. Overall, the toasting treatment had a significant effect on the RRG wood chemical components, particularly on lignin and hemicellulose, leading to the formation of new sensory compounds that could add complexity and a unique sensory profile to alcoholic beverages likes distilled spirits.