Chill Haze occurs when a beer is chilled below approximately 1.6°C (about 35°F) and constituents can aggregate to form relatively large colloidal (gel-like) particles. These become visible to the naked eye as a cloudiness or haze. This haze is temporary, however, and completely redissolves when the temperature of the beer rises. Measures taken to prevent chill haze are referred to as “chillproofing.” Chill haze is caused by low-molecular-weight polyphenols and larger proteins and polypeptides when cross-linked through weak interactions such as hydrogen bonds. Hydrogen bonds are disrupted when the beer warms up, so the particles are only large enough to be visible when beer is cold. Particles range in size from 0.1 to 1 μm. Minor components of chill haze may also include carbohydrates and metals. The particles may eventually become large enough to precipitate and settle to the bottom of the package, forming a dusty sediment or deposit that, when poured into a glass, will result in a cloudy appearance. Chill haze will usually develop into a permanent condition over time.

The major components of these aggregates are certain classes of proteins and polyphenols that are derived from brewing raw materials, namely malt and hops. Excessive levels of protein/nitrogen, namely soluble protein, in malt will present problems with colloidal stability. As little as 2 mg/l of protein is sufficient to induce a haze of 1 European Brewery Convention unit. Haze-active proteins have been identified as deriving from hordeins that are relatively rich in proline. The haze-sensitive proteins make up only a small percentage of the total protein found in beer. Polyphenols also impart certain flavor characteristics and act as natural antioxidants, preserving the original taste of beer so their complete removal is not always desirable.

See also chillproofing, clarity, haze, and pvpp.