Of the structural components of wood, cellulose and hemicellulose are giant chains of repeating glucose molecules, and the heat of coopering breaks those into sugars—glucose, hexose, and pentose. But the third major component, lignin, is different. It’s a massive molecule, too, but with nonrepeating subunits. About half of them are vanillin (vanilla flavored), and the rest is barbecue-flavored guaiacyl, clove-flavored eugenol, and syringaldehyde. At high heat, the spicy aromatic aldehydes in the lignin undergo Maillard reactions and yield the same flavors as browned meat.
