Chill haze is a common form of protein haze. As the beer is chilled, excessive protein will come out of solution and throw a haze. The lower the temperature the greater the haze. As the beer is warmed up again the protein can go back into solution and the haze disappears. Chill haze is caused by proteins reacting with, and combining with, oxidised polyphenols. Polyphenols are naturally present in both malt and hops. Proteins on their own, or polyphenols on their own, are freely soluble and do not cause a haze. However the two bonded together have a low solubility, and is known as a partially soluble substance. The saturation point, or the solubility, of these protein-polyphenol complexes is fairly low; that is that the amount of this complexed protein that can go into solution before saturation level is reached, is low. Quantities above the saturation level will end up in suspension and throw a haze. Also, the solubility is temperature dependant. Chill the beer and less can remain in solution and the excess will throw a haze.

The combination of proteins and oxidised polyphenols into an insoluble substance is a chemical reaction, and a very slow reaction at that. This reaction continues during the life of the beer, slowly converting soluble protein into its partially soluble equivalent, probably by gradual auto-oxidation of the polyphenols. This means that the level of the partially soluble protein in the beer gradually increases with time. The temperature at which a chill haze forms increases in tune with it, until eventually the amount of the partially soluble protein exceeds the saturation level, even at room temperature, and the haze becomes permanent. Alongside this a secondary mechanism takes place, again a slow one, in that the complexed protein particles grow in size, probably by further oxidation and coalescence, until they are too large to remain in solution but not large enough to drop out of suspension unaided, thereby producing a permanent haze. The particle size of chill haze is up to about one micrometre, whereas the particle size of a permanent haze is between one and ten micrometres. These reactions can take days, weeks, or even months before a haze becomes permanent, depending upon circumstances. Therefore the importance of taking extraordinary precautions against protein haze depends upon the length of time you intend to store the beer, and whether or not you intend to chill it.

Haze Stabilisation

As the haze is formed by proteins combining with polyphenols, if there is not enough protein to combine with the polyphenols, or not enough polyphenols to combine with the protein, a haze cannot form. Therefore chill-haze prevention measures are targeted at reducing one or both of these components.

The last runnings of the mash is rich in polyphenols and nitrogenous components (proteins) of high molecular weight; both of which are potentially haze forming. Excessive sparging can cause clarity problems for this reason. One of the reasons for stopping sparging before the gravity drops below, say, 1008 is to reduce the possibility of leaching an excessive amount of these components, particularly polyphenols from the mash. Indeed, one reference gives the polyphenol level of the majority of a mash run-off at less than 200ppm, but the last runnings exceed 1,500ppm. If the pH of the goods rises significantly during sparging, excessive polyphenols will be extracted, particularly towards the end of the sparge when the buffering power of the mash is lost. Keeping the sparge water below about pH6 goes a long way to reducing the risk of this happening. Some polyphenol, albeit at a low level, is desirable in the mash run off, because it assists in cold break formation later.

Likewise, good wort-boiling practices are necessary. The often-stated dictum of a "good rolling boil" for 90-minutes or more is stated for very good reason. The long boil gives time for sufficient protein to be denatured, which a relatively slow process. The rolling boil is to provide the mechanical action necessary to throw the denatured protein particles into collision with others so they stick together and grow in size forming the hot break. The vigour of the boil is critical, the more vigorous the better. A mere simmer is not good enough. The use of copper finings, described later, is fairly standard practice to secure a good cold break.

Hot break and cold break are not the same thing. Hot break is a cooking process, like egg-white cooking. Cold break is a chemical reaction, a reaction between protein and polyphenol, that only takes place below 80°C - the colder the better. The use of copper finings has no effect on hot break, but will help to get the maximum amount of the complexed protein out of suspension as cold break. The presence of calcium is essential for the coagulation of protein to take place, so ensuring that sufficient of this exists is also important. The longer the boil the more protein will be precipitated. Skimping on boil time will give rise to poor haze stability. Getting the wort off the trub as soon as possible is also a good idea, because the protein can be reabsorbed, particularly as the pH of the beer falls during fermentation. This is the reasoning behind the adoption of the dropping system of fermentation so widely used by commercial brewers in the past.

A good, long, hard boil along with the use of copper finings is generally adequate for cask ales and 'proper' beers. For beers that are likely to be highly chilled, further preventative measures may be needed. The mega-keggeries that produce keggyflade, nitrokegs, smooothflows, lagers and the like, chill the beer to close to freezing point to force a chill haze, then pump the stuff through micro-pore filters which takes out the haze, the yeast, bacteria and much of the flavour too. Lager malt, even British lager malt, has a higher nitrogen / protein level than pale ale malt and has a higher haze potential. The protein rest period at 50°C, traditionally employed at the start of the mash for lagers, breaks down high molecular weight proteins into simpler substances that are less likely to haze. The traditional lagering period of several weeks at temperatures close to freezing is to encourage protein / polyphenol complexes to form and then settle out. Not many lagers are genuinely "lagered" these days, but are chilled and filtered. It should be pointed out that just about all of our big breweries that produce mass-market stuff, and a fair few of our medium-sized ale breweries, employ a 50°C protein rest prior to mashing. It would be nice to be able to say that some of our well-respected ale brewers use a 50° rest to produce a beer with less haze potential, but regretfully its motivation is because it enables them to get away with malt made from cheaper, feed-grade (high protein) barley without incurring a haze penalty.

Home brewers do not usually have chilling and filtering facilities, but other methods can be resorted to. Again these centre around either removing the protein-polyphenol complexes, or removing either protein or polyphenols, or both, so that the complexes cannot form in the first place. Protein can be precipitated using tannic acid. This is a winemaking fining agent that is sometimes used in beer. More typical for beer is adsorption of protein by silica gel or the adsorption of polyphenol with PVPP (trade name Polyclar). Polyclar Plus contains both PVPP and silica gel in combination, giving the best of both worlds. The most common treatment for conventional brewers is the use of silicated auxiliary finings, which is very effective at removing protein. Silicated in this sense is not the same as silica gel, but refers to acidified sodium silicate.