What ingredient, when added to flour mixtures, can decrease the volume of the end product?

Bran is an ingredient that, when added to flour mixtures, can decrease the volume of the end product. This is primarily due to the high fiber content in bran, which can interfere with gluten development in the dough. Gluten is essential for providing structure and support to baked goods, allowing them to rise. When bran is included, it takes up space and absorbs water, leading to a stiffer dough that doesn't rise as effectively as it would without the bran.

The presence of bran results in a denser texture and a smaller volume in the finished product because it does not contribute to the elasticity and extensibility of the gluten network in the same manner as the starches in the flour. Consequently, this can be particularly noticeable in items like bread or cakes where volume is a key characteristic.

While sugar, salt, and yeast are also important ingredients in bread and baked products, they generally serve different purposes, such as enhancing flavor, aiding fermentation, or strengthening gluten. Sugar often contributes to a higher volume by aiding in yeast fermentation, salt can help stabilize gluten structure, and yeast is responsible for creating carbon dioxide that causes dough to rise. Thus, bran's unique properties lead to a decreased volume when incorporated into flour mixtures.