By Harold McGee
Whether or not a protein is soluble in water depends on the strength of the bonds between molecules, and on whether water can separate the molecules from each other by hydrogen bonding. The wheat proteins that form gluten when flour is mixed with water are a kind of protein that absorbs considerable amounts of water but doesn’t dissolve, because many fat-like groups along their molecules bond with each other, hold the proteins together, and exclude water. Similarly, the proteins that make up the contracting muscle fibers in meat are held together by ionic and other bonds. On the other hand, many of the proteins in milk and eggs are quite soluble.