Right off the bat, our tasters noticed big differences in flavor in the plain tasting, calling our preferred tahinis “toasty” and “nutty” and lower-scoring tahinis “bitter” and “sour.” Flavor discrepancies were less evident in hummus. Although tahini can be made with whole sesame seeds, most manufacturers use seeds that have their outer coatings (hulls) removed before grinding. The hulls contain bitter compounds called oxalates that can be the source of those unpleasant flavors (seeds with the hulls intact are also harder to grind to a smooth consistency). Overroasting the seeds is another factor that can lead to bitterness. Off-flavors can also be the result of rancidity from poor handling or storage of the seeds.