Proteins from the grape remain in solution in all white wines but in some (notably those vinified from muscat, gewürztraminer, sauvignon blanc, and sémillon grapes), the concentration is often so high that the proteins coagulate to form an unsightly haze or cloud. Such haziness, which can be initiated when the wine is warmed, is irreversible. To avoid this happening after bottling, which renders the wine unstable, the heat-unstable proteins are removed by bentonite fining as part of normal winemaking stabilization procedures. Research has shown that the troublesome, heat-unstable proteins of white wines belong to a particular group from the grape known as pathogenesis-related or PR proteins. The unique properties of PR proteins—their stability in acid conditions and resistance to degradation by proteolytic enzymes—means that fermentation and the other processes of winemaking selectively eliminate other proteins of the grape leaving the PR proteins as virtually the sole survivors.