Irish soda bread is known as a quick bread because the recipe calls for buttermilk and baking soda instead of yeast. The acid present in buttermilk acts on the baking soda immediately, so there is no wait time for the dough to rise like there is with a yeast bread. If you use regular milk or a milk alternative, you will get a dense bread. By acidulating the milk with 1 tbsp of vinegar or lemon juice, you not only get a great subtle flavour, but the acid interacts with the baking soda and puffs up the bread beautifully in the oven.