Barley contains gluten. It contains around 5 to 8 percent gluten, so it shouldn't be consumed by people with celiac disease or non-celiac gluten sensitivity. Gluten is found in many whole grains, including wheat and rye.
That pinch of roasted barley (about 10 percent of the total grain bill) gives Guinness its color and its burnt, coffeelike flavor with an acidic note in the finish. Draft Guinness is a subtle beer.