In a French study of the solids content of various spirits, cognacs and other brandies turned out to be more than 1% by weight polyphenols, carbohydrates, and caramel materials, all of which the spirits extract from the wood barrels in which they're aged. Armagnacs and rums contained about half the solids of cognacs, bourbons about a fifth, and whiskies a tenth. From 4 to 51 samples of each spirit were tested. These are large differences, and indicate that brandies should often seem more astringent, sweet, and full-bodied than their distilled cousins.
Picque, D. et al., J. Agric. Food Chem. 2006, 54 (15) 5220