Which of the following contributes to the caramel and butterscotch flavors in wood-aged beer?

The toasting of the barrel is a critical factor in developing caramel and butterscotch flavors in wood-aged beer. When barrels are toasted, the heat causes chemical reactions in the wood, leading to the breakdown of hemicellulose and the production of various compounds, including lactones and vanillin. These compounds impart rich flavors reminiscent of caramel and butterscotch into the beer as it interacts with the wood during aging.

In contrast, while the cask material is important for overall flavor due to the properties of the wood itself, it is the toasting process that specifically enhances those desirable sweet notes. Yeast strain contributes to flavor profiles primarily through fermentation characteristics, but it doesn't produce caramel or butterscotch flavors in the same direct manner as barrel toasting. Similarly, grain type plays a role in the base flavors of the beer but isn’t responsible for the caramel and butterscotch notes that emerge during wood aging. Thus, the toasting of the barrel directly facilitates the creation of those specific flavors.