The incomparable flavor of simple wheat bread has three sources: the flavor of wheat flour, the products of yeast and bacterial fermentation, and the reactions caused by oven heat during baking. The aroma of low-extraction white flour is dominated by vanilla, spicy, metallic, and fatty notes (from vanillin, a furanone, and fatty aldehydes), while whole-meal flour is richer in most of these and in addition has cucumber, fried, “sweaty,” and honey notes (from other fatty aldehydes and alcohols and phenylacetic acid). Yeast fermentation generates a “yeasty” character, a large part of which comes from fruity esters and eggy sulfur compounds. Baking contributes the toasty products of browning reactions. Starters add general complexity and a distinctive sour note from acetic and other organic acids.