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Proteins in Water
Appears in
By Harold McGee
Published 2004
In living systems and in most foods, protein molecules are surrounded by water. Because all proteins are capable to some extent of hydrogen bonding, they absorb and hold at least some water, although the amounts vary greatly according to the kinds of side groups present and the overall structure of the molecule. Water molecules can be held “inside” the protein, along the backbone, and “outside,” on polar side groups.
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.