I did a pretty detailed write-up on chill haze for my Irish brewing buddies back in 2008 while I was studying for the IBD brewing cert exam that I'll repost here; it covers prevention and removal and how it's viewed at the commercial level.
What chill haze is, how it forms and preventing and getting rid of chill haze:
What chill haze is/is not & how it forms:
Not all "hazes" in beer are chill haze; carbohydrates in solution such as unconverted starch (will fail an "iodine test") and Beta Glucan "gums" from too much poor quality malt or adjuncts like oats can also cause haze. Too much calcium can lead to the formation of calcium oxalate crystals which will also form a haze that's not technically "chill haze", but this source is rare.
Assuming that you don't have starch or too many beta glucans and your beer only gets cloudy when it's chilled, you're dealing with chill haze.
Chill haze is defined as a reversible bonding of proteins with polyphenols. The main proteins involved are polypeptides which come from the malt protein hordein, and may of the polyphenols are "tannins"; in this instance "polyphenols" and "tannins" can be used interchangeably.
The presence of dissolved oxygen is necessary to help bond the polyphenols and proteins, as is a very small quantity of iron or copper which helps the reaction out. The reaction occurs faster with larger quantities of oxygen and at higher temperatures.
Proteins come from the grain/wort and are really unavoidable and polyphenols come both from grain husks (especially if you over-sparge) and hop material; polyphenols are particularly high in concentration in the "braun hefe" or brown crud on the top of your krausen during an active fermentation, leading credence to the traditional German practice of removing braun hefe to prevent harsh bitterness. (Note: "Braun hefe"/ dark brown yeast crud on krausen also contains non-isomerised hop alpha acids; it's not JUST polyphenols.)
The reaction starts via electrostatic forces, the polyphenols and proteins are oppositely charged and attract each other but are not yet bonded; then loose hydrogen bonds are formed between the proteins and polyphenols; this colloid of proteins and tannins/polyphenols is insoluable at low temperatures in beer but is soluable at higher temperatures so the "chill haze" forms when the beer is chilled and is dissolved into solution when the beer is warmed up.
If the beer goes through several heating/cooling cycles or (and I don't understand this bit) as you get more and more "chill haze" particles in suspension, stronger covalent bonds are formed between the proteins and polyphenols; these covalently-bonded haze molecules are insoluable at both high and low temperatures; these insoluable haze molecules are referred to as "permanent haze".
Preventing/ Removing Chill Haze
Preventing:
Prevention basically involves preventing the introduction of oxygen, reducing the amount of polyphenols in the beer, reducing the amount of copper/iron in the beer, and reducing the extraneous amounts of protein in the beer.
- ensure heavy metals (iron, copper, etc..) are removed from your water source
- use low protein malt (heavily modified ale malt vs. undermodified lager malt (although most pilsner malt is actually low protein); use European malt varieties vs American varieties)- CHECK YOUR MALT SPEC SHEET!
- ensure a vigorous boil to precipitate excess proteins via "hot break"
- use kettle finings to assist in forming a good hot break and cold break (reduce protein loading)
- chill wort quickly to ensure a good hot break (reduce protein loading)
- skim off "braun hefe" from the top of krausen to remove excess polyphenols
- don't over-sparge, which introduces excess polyphenols
- ensure a strong, healthy fermentation as yeast will absorb protein to obtain the nitrogen in it during their growth phase
- Modify your processes so you don't introduce oxygen post-fermentation
- use a small quantity of "polyclar" (pvpp) as a kettle fining to drop polyphenol levels
Removing Chill Haze:
Removing chill haze generally involves using post fermentation finings which will precipitate or absorb proteins or polyphenols and drop them out of solution or even to break down proteins into smaller proteins or amino acids using enzymes.
Protein Reduction:
Adsorbants- these chemicals will act as a "sponge" and "suck up" excess protein and hold it within its molecular structure and then drop them to the bottom of the fermenter
-Silica Gels (acidified sodium silicate)
Big breweries will also use hydrogels or xerogels immediately prior to filtration but they really need to be filtered out and probably not useful for most homebrewers.
Protein precipitants- bentonite used to be used in beer but is now really only used in winemaking; I can't remember why but it's not an ideal option
tannic acid can be used, but it creates a thick layer of "fluffy" protein precipitate and causes a lot of beer loss if you're not filtering
Protein breakdown- big breweries often use an enzyme called "papian" which breaks down the proteins into smaller proteins or amino acids; they'll add it prior to running the bottled beer through a pasteurizer because it needs higher temperataures. The product remains in the bottled beer and if it works too well it will impact beer foam formation and stability.
Polyphenol reduction:
adsorbants- polyvinyl polypyrrolidone (PVPP)AKA "PolyClar" -traps polyphenol molecules AND whole chill haze molecules and drops them out of solution (must be filtered in the US to adhere to FDA food processing laws).
Note: From experience, if used as a kettle addition PolyClar used with other kettle finings also results in extra fluffy cold break and increased wort losses. Polyclar used after fermentation may strip some flavor and color from the beer if used in too high of quantities and may lead to other flavor profile changes.
Oxygen Scrubbing:
Another solution that should help not only chill haze formation but also other oxygenation during ageing is to use "oxygen scrubbers" such as live yeast or vitamin c (ascorbic acid) at bottling so that any oxygen that does get introduced during packaging is automatically taken up by the yeast or ascorbic acid before it can react to form chill haze molecules.
Note: Only 0.5ppm of oxygen can create oxidation-related off flavors post fermentation; that's one thimble full of oxygen in 20,000 liters of beer; ascorbic acid & oxygen scrubbing caps probably aren't "good enough" to fully prevent oxidation reactions during extended storage/aging.
If you want to go crazy you could tackle it in 5 parts (for breweries outside of the US because this method doesn't filter polyclar):
1. Limit the introduction of oxygen in all steps of the process
2. Chill beer at -1C for 4-7 days so that chill haze is formed
3. Douse beer with silica hydrogel to absorb protein on the way to filtering (1 micron filters will get most chill haze; especially what's trapped in the silica hydrogel gel matrix; but 0.5 micron is required to get all free floating chill haze molecules)
4. Douse with PVPP after filtration to remove any remaining polyphenols or chill haze molecules (and leave long enough for PVPP to settle)
5. Add oxygen scrubbers (yeast, vitamin c, silicon dioxide (YUCK; I'm sure not RECOMMENDING this))
-IBD says that just chilling for 4-7 days, dousing with silica hydrogel, filtering, and then using PVPP can result in a beer that will resist the formation of chill haze for up to 18 months.
Adam
Original thread:
http://www.beoir.org/community/viewtopic.php?f=13&t=6389