Bread made from 100% rye flour is popular in the northern half of Europe. It is usually leavened with sourdough. Baker's yeast is not useful as a leavening agent for rye bread, because rye does not contain enough gluten. The structure of rye bread is based primarily on the starch in the flour, as well as other carbohydrates known as pentosans. however, rye amylase is active at much higher temperatures than wheat amylase, causing the structure of the bread to disintegrate as the starches are broken down during cooking. The lowered pH of a sourdough starter, therefore, inactivates the amylases when heat cannot, allowing the carbohydrates in the bread to gel and set properly. In the southern part of Europe, where panettone was originally made with sourdough, sourdough has become less common in recent times. It has been replaced by the faster-growing baker's yeast. Sometimes longer fermentation rests are used to allow for some bacterial activity to build flavor.