Understanding Polyphenols

Understanding Polyphenols 

Polyphenols present a classic dilemma for the brewer. On one hand, they taste bad. These bitter, astringent compounds— tannin is probably the best-known form of polyphenol — can ruin the flavor of beer.

On the other hand, they readily bind with proteins and, under the right circumstances, make them drop out of solution. This plays a significant role in preventing haze. So what’s a brewer to do: Encourage tannins and risk astringency, or minimize them and risk haze? And even then the choice isn’t so clear cut. Too many tannins cause chill haze, just as too much protein does, and many beer stabilization methods are aimed at limiting tannins. The answer, as it often is in brewing matters, is to find balance.

Polyphenols come from plant matter, among other sources. They are responsible for the browning in cut apples, pears, bananas, potatoes, and other foods. They have been used in the tanning of leather for many centuries. The polyphenols present in brewing come from the grain and, to a lesser extent, the hops.

Polyphenols are active during mashing, lautering, and boiling, and while they help proteins drop out of solution, they are also responsible for some interactions causing chill haze.

Finding Togetherness

During the mashing process some of the tannin material from the grain is released into the mash solution. These polyphenols adhere to protein matter and its smaller parts, such as polypeptides and amino acids.

This joining is a result of electrical charges at the molecular level. If the protein has the proper charge showing on its outside surface, then the polyphenols join to the protein material. If enough of these compounds join, they will clump together. If enough clumping occurs, the product will become visible and, more important, it will fall out of solution.

This is part of what makes up the teig material during the lautering process. Teig also is frequently called top dough. If enough of this material lies on top of the lauter bed, it will clog the channels through the grain bed. This can result in a stuck lauter or a very slow lauter (although this is by no means the most common cause of a stuck lauter). However, if there were no top dough, the lauter could be excessively fast, leaving behind much residual sugar. If you do get a stuck lauter from teig settlement, a simple fix is to take a knife or fork and cut new channels in the top of the lauter bed.

Beating the Heat
The higher the sparge temperature you use, the more tannins will leach out of the lauter bed. This can become a problem if you sparge above 180° F. High pH in the lauter, above pH 8, also causes tannins to leach out at a high rate.

To prevent this problem, use a large proportion of fresh, healthy barley malt, and don’t rush your mash. When the malt is in good condition and the brewer is not under time constraints during the mashing process, a large amount of protein, polypeptides, and amino acids are released into the mash and lauter. The tannins help to scavenge off these protein products and thus keep them out of the finished beer. And too much protein in the finished beer can lead to problems. The bonus is that the tannins will drop out as well.

Hot Break
During the boil polyphenol interaction increases significantly. When you boil, a sort of sludge forms on the top of the boiling wort. This sludge will be most pronounced if the boil is not very vigorous, which is usually the case if you boil on the kitchen stove. What you are seeing is called hot break.

Hot break is the interaction between polyphenols and protein byproducts that occurs during the boiling of the wort. Proteins and polyphenols are removed during lautering when their opposite charges attract. But there is a fair amount of protein and protein byproduct that does not have the proper interactive charges to react with the polyphenols.

This is where the boiling process comes into play. Boiling is a violent physical reaction. A tremendous amount of energy is released when water is boiled. Boiling the wort denatures the protein (changes its structure). A similar effect occurs when eggs are fried and meat is cooked.

This energy is unleashed on the proteins that were not taken up by polyphenols during the lautering and mashing. The boil causes the proteins to change their structure; what was inside might now be on the outside. As a result many proteins, once boiled, change their outside electrostatic charge. This change makes them susceptible to polyphenol interactions, and thus more tannin material and protein is removed in the boil. If the boil is not  vigorous enough, these clumps of protein and tannin tend to get large and form a scum on the top of the boiling wort. If the boil is vigorous, which it should be for many reasons, then this top layer breaks into smaller particles.

Also, there’s a new source of polyphenols during the boil. The addition of hops increases the polyphenol content in the wort by roughly 20 percent to 30 percent. This figure is based on the use of about two ounces of hops for a five-gallon batch. While there is a loose correlation between alpha acid content and polyphenol content, the total weight of hops used is also an important factor.

Cold Break
Once the boil has been stopped, there is another break that occurs. The term generally used is cold break. However, cold break has also been used to describe the sediment left behind after chilling a conditioning tank. When the boiling stops and the wort starts to cool, once again some of the protein products change their outside charge and more protein-tannin interactions occur. This is why your cooling wort develops that disturbing cloudiness. At the end of the boil, the wort is crystal clear. You think to yourself, “This is going to be a totally clear beer.” Then to your horror, as the wort cools it gets more and more cloudy.

Do not fret; this is normal. The clumps of polyphenol and protein in this break are much smaller than the hot break. Therefore, they tend to stay in suspension longer, but given time they will sink. This is why some breweries let the wort stand cold for some period of time before transferring it to a new tank to pitch the yeast. This is a risky endeavor to try at home, however, because the chance of contamination is very high.

By the time the fermentation stage arrives, much of the polyphenolic material should have been removed. The key is to make sure you haven’t left enough behind to cause problems.

Polyphenolic material does have some affinity to yeast. Of course different strains of yeast react differently to polyphenols. This is because the outside coats of yeast strains differ slightly.

In general if you have an abundance of polyphenolic material in your fermenting wort, you will tend to guege (pronounced gooj) up your yeast. In other words the yeast will pick off a coating of polyphenols, which in turn will inhibit the ability of the yeast to take up nutrients, reproduce, and stay afloat in the fermenting wort. This adsorbance of polyphenolic material also causes a dramatic reduction in the number of times you can repitch a yeast, an issue that is much more significant for professional brewers than homebrewers.

By the time the wort finishes fermenting, a large amount of the polyphenolic material and protein has been removed by adhesion and coagulation, and it has then sunk to the bottom of the fermenter. However, what remains of these two compounds is what can cause chill haze.
Proteins and polyphenols zip around in the beer when it gets cold, and each seeks a partner. But as soon as the beer warms up, the partners separate. As the temperature cools again, the proteins and tannins pick new partners. Over time some of the partners form permanent bonds with each other, and the chill haze becomes permanent haze.

Cleaning Tips
Many brewers avoid chlorinated cleaning products. If residual chlorine is left behind, the chlorine and tannins will react very aggressively. Chlorinated polyphenolic compounds have some mighty potent off-flavors. The easiest way to solve this problem is not to use chlorinated cleaners.

Iodophor works well as a sanitizer. Hot water and elbow grease work quite effectively as a cleaning agent. Polyphenolic compounds are very reactive molecules that play a major role in the flavor and clarity of your finished beer. Although they are bitter compounds, they also act as a protein cleaner during the brewing process. They coagulate protein in the mash; the hot break is made up of conglomerates of tannin and protein, and the cold break is likewise a conglomerate of polyphenols and protein parts.

Proteins and polyphenols work with each other in the brewing process. It is beneficial to have some amount of polyphenols in the brew, because at certain points of the process they eliminate a large amount of the protein content. Likewise, it is beneficial to have some amount of protein and its derivatives, polypeptides and amino acids, in the brewing process because there always will be some amount of polyphenols in the
brewing process. The trick is to get the right amount of each at the right time.

You should have some amount of amino acids at the point you add the yeast. This will help the yeast stay healthy. However, at the end of fermentation there should be very little protein or protein byproducts and only a small amount of available polyphenols.

Issue: April 1999