The role of polyphenols in beer haze formation

The role of polyphenols in beer haze formation
By Moritz Kallmeyer
Master Brewer of Drayman’s Brewery & Distillery, Silverton Pretoria, February 2003

Abstract:

Phenolic compounds are present in all vegetable foods. Polyphenols are extracts of various plants which have the ability to react with protein in animal skins to produce leather. Tannins are polyphenols and nowadays the term tannin has come to describe all polyphenols in a plant extract regardless of their ability to tan leather. Beer has a complex mixture of phenolic compounds from 150mg/L to 330mg/L in concentration. The majority (about 2/3) are malt derived especially from the husk material of the malt. The remainder (about 1/3) comes from the hops extracted during the copper boil.

Polyphenol –are they essential?

to their specific properties, brewers find polyphenols interesting technologically (foam maintenance, physical and chemical stability and shelf life). Health researchers find them interesting because phenolic compounds can act as antioxidants in the human body, for example as protective agents against the oxidation of ascorbic acid and unsaturated fatty acids. Studies proved that the intake of beer significantly increased plasma antioxidant capacity. Today we know that polyphenols are important molecules in brewing. Positively speaking they have the ability to react with proteins during wort boiling to form the hot break; during cooling to form the cold break; during post fermentation when they are involved in the formation of chill haze and permanent hazes – which can then be removed by filtration. Negatively speaking they have the ability to react with proteins to form haze in the final pakage after a period of time (expiry date). Beer remains an unstable product.

Simplistically, polyphenol is one of the two chemical entities which control the colloidal stability of beer, the other being proteins. Proteins are derived from malt and are extracted from the malt during the mashing process. The fate of proteins is subsequently controlled by other brewing processes (not the scope of this article). The generation of beer haze is due to the extremely complex bio-chemical interaction between these two entities.The majority of modern treatments used to improve the final beer stability are aimed at the removal of either half of the protein fraction, or half of the polyphenol fraction. In some breweries both fractions are treated.

Polyphenols may be conveniently divided into three classes:

1. Simple (phe)nols which are derivations of hydroxyl benzoic or cinnamic acid (mostly from malt);
2. (Flavo)nols with more complex structures (mostly from hops);
3. Proanthocyanidins, anthocyanogens, catechins and leucoanthocyanins (arises equally from malt and hops).

All of the above single molecules of the various compounds are the building blocks of larger molecules, the polyphenols. The single units are referred to as monomers. Two monomers, which not necessarily are identical, can join to form a dimer. A dimer plus a monomer form a trimer and so on.

Various studies have shown that monomeric phenols have little effect on haze formation but that dimers and trimers strongly accentuate haze formation. Polyphenols on their own contribute little to haze formation. Haze is composed fundamentally of complexes between condensed polyphenols and proteins.

In hot water extraction of hops such as occurs when making a hop tea for dry hopping also leads to the extraction of polyphenols into the hot water. This can have an adverse effect on colloidal stability. When dry hopping with whole hops or hop pellets, this does not occur.

Oxidizable Polyphenols:

Amongst the larger condensed phenols it is not only polyphenols that are problematic, but oxidizable phenols. There are not many of these oxidizable phenols in hops. It is mainly a malt problem. The simpler phenolic molecules are more polar, i.e. their relatively simple structure has more pronounced spots of unbalanced charges, and they are thus electrically dipolar. Without going into detail, polar molecules are very soluble and in fact, the vast majority of all phenolic compounds are extracted with the first runnings! Over time simple phenols will complex into polyphenols under acid conditions and oxidise and complex with protein into haze. However, good beer doesn’t last long enough for this to happen! The real problem with haze and astringency is the existing small fraction of less polar, i.e. more complex and large polyphenols. Many fingers have been pointed at Catechines (flavan-3-ols) and Anthocyanogens (flavan-3,4 diols), and especially their polymers, in the formation of haze.

As you would have gathered by now, polyphenols are not so soluble and when they are dissolved by higher Ph water (>5.5), they are repulsed by the polar medium that they are in. They thus tend to floc together (hydrophobic force) and with catalysts like metal ions and oxygen, complex with other large (and less soluble) molecules fairly quickly. This is the unsightly but essentially tasteless so called “tannin-protein haze” (it actually contains many other components of the wort). The protein neutralizes the tanning power of tannoids by forming essentially “tanned” bonds. Tannoids, on their own, have a definite dry astringent taste!

The oxidized polyphenols with tanning power (MWt 700-1000) are called “tannoids” (or tannigens) and they try to turn your tastebuds into leather. They do this by covalently cross linking proteins in your tastebuds just as they do in tanning leather and in forming haze. They were not actually intended to do this, they seem to be located in the husk fraction of malt mainly as an astringent inhibitor against fungal and bacterial attack on barley corn. Nowadays certain sorghum grain varieties are bred with high astringency as a “put off” to birds.

The oxidized polyphenols in sweet wort will “readily complex out” as hot break. Despite their size, they are a first and middle runnings extraction problem. These are best controlled by recycling wort through the grain bed, or vigorous boiling to form hot break during the boil phase.

The unoxidized, oxidizable polyphenols are less soluble and typically a late runnings problem. A large portion can survive (uncomplexed and unprecipitated) into the hopped wort, waiting for oxygen so as to cause haze and astringency problems by becoming tannoids. These are best controlled by terminating the sparge early at SG 1.010, keeping sparge water pH<5.5, keeping sparge temperature below 75ºC (as measured at the strike point on top of the grain bed and not as a reading on the hot liquor tank gauge) and crushing your malt coarse.

Factors affecting haze formation from polyphenol extraction:

Barley variety:
Levels of anthocyanogens were found to be higher in 6-row barleys than in 2-row barleys. Beers from 2-row malts thus had better colloidal stability.

Malt modification:
Well modified malts of high proteolytic ability have a greater degree of solubilisation of tannins than less well modified malts. Well modified malts tend to form less chill hazes.

Polyphenol content:
A barley variety ANT-13 which was bred with low polyphenol levels resulted in a beer with higher colloidal stability.

Milling of Malt:
The “husk fraction” in brewing literature includes the true leafy husk and the bits of fused-on pericarp/testa & aleurone layer. Many of the problematic polyphenols in the true leafy husk of malt have been leached out during repeated steeping during malting. However there are also high concentrations of problematic polyphenols in the pericarp/ testa and aleurone layers. These are some of the least modified (enzymically broken down) parts of the malt kernel and contain the least extract. As least modified, they are more likely to remain as big bits in a coarse crush. The sparge process removes extract from between and from within the kibble of the grain bed. The osmotic leaching process of removing extract (of anything soluble) from within bits is slowed down if the bits are big. The solvent (water) simply has further to penetrate. If the big bits are the “husk fraction”, which is low in fermentable extract and high in oxidizable polyhenols, then a coarse crush is beneficial for reduced polyphenol extraction. If the big bits happen to be starchy endosperm bits, then advanced gelatinization of starch is beneficial for better extract of sugar.

Brewing Liquor:
CaCl2 and CaSO4 reduce the mash pH. Calcium will precipitate oxalate and proteins responsible for haze.

Mash pH:
Measure your mash pH from a hot sample taken from inside the mashtun about 10 minutes after mashing in. (Make sure your pH probe is accurate and can handle the temperature.) Aim for 5.3 to 5.4, no higher. Remember it is the pH of the mash that matters, not the pH of the water before it enters the mash! You can alter the mash pH by addition of lactic acid to the brewing water or straight into the mashtun.

Sparging:
Stop sparge when SG drops to 1008 to 1010 and pH drops to 5.5. or you run the risk of extracting tannins that will give your beer an astringent flavour and which will increase the risk of hazes.

Hops:
The higher the anthocyanogens content of hops, the higher the “haze risk index”

Good Rolling Boil:
Boil for at least 70 minutes. Don’t boil for more than 2 hours since you run the risk of the hot break re-dissolving.

Carragheenan:
Use Irish Moss for the last 30 minutes of the boil. Many home brewers omit it because they see no difference, in many cases the reason is that they use too little. I recommend 5g per 25L boiler volume.

Protein Rest:
For well modified malt some studies recommend a rest at 40°C for 20-30 minutes followed directly by a conversion rest. I personally have good results with a 55°C rest for 20 minutes.

Chill Quickly:
Home brewers should spend more money on their chiller design efficiency than on state of the art filtration systems. Maximum cold break formation is essential to improve beer quality and stability. Even standard ales should be heat-exchanged to 14°C.

Acknowlegements:

I am greatly indebted to the authors below who gave me a better understanding of polyphenols and their role in brewing. I have unashamedly copied whole paragraphs from their articles for this write-up – simply because I could not explain it better than they have had.

1. Controlling Phenol Extraction, Charlie Scandrett, April 1997. Brisbane Australia.
2. Advances in Beer Stabilization, Mike O’Neill, February 1996.
3. Beer Hazes, Gillian Grafton, October 1995.
4. Basic Brewing Science, Dr. Trevor Wainwright, January 1998.