Gluten (pronounced GLOO-tim) is a protein that forms when wheat flour mixes with water. Gluten (along with starch granules and, in some cases, egg proteins) is the structural framework of most baked goods, especially breads and many pastries. When wheat flour and water are mixed in doughs or batters, gluten strands form and intertwine, weaving themselves into strong, stretchy sheets. This is what baker Rose Levy Beranbaum means when she talks about gluten development in Making Lattice-Top Fruit Pies. Too little gluten makes pie pastry crumble; too much makes it tough.