Polyphenols in your Beer?
Polyphenols are complex organic compounds partially responsible for chill haze and staling in beer. They are present in the husk and cell walls of barley. Tannins are a subset of polyphenols. Tannins are often perceived as astringency in beer. They combine with proteins when beer is chilled creating a chill haze.
Reducing the amount of polyphenols being extracted from grain can be done by paying attention to steep or mash time, temperature and pH.
Steeping and mashing should be complete in 60 minutes. When steeping grains, 30-60 minutes is sufficient. If mashing grains and the starch to sugar conversion is incomplete in 90 minutes, mashing longer will not do the job. Enzymes were probably killed off with water too hot. In other words, longer isn't better. The steep or mash temperature should never exceed 170°. Mashing at 152° is the sweet spot between alpha and beta amylase enzyme activity. When the temperature gets too high enzymes are killed and tannins are leached from the grain.
All grain brewers typically use 5.2 pH stabilizer or work with water chemistry. This discussion is primarily for extract brewers who steep specialty grains or do mini-mashes.
After adding grains to the mash or steep water the pH should be between 5 and 5.5. Typically extract brewers steep a pound or two of grain in a couple gallons of water. This generally works just fine. As brewers learn that boiling a larger volume will increase the quality of the beer, the tendency is to add the steeping grains to the full volume (4-6 gallons) of water assuming the same results as steeping in a gallon or two. Here's where the astringency comes from. Rather than the addition of the grain reducing the pH of the water to around 5.5, the increased water volume results in a pH of over 7. As a rule of thumb the ratio of water to grain for steeping should not exceed one gallon of water per pound of grain. If the water is highly alkaline, one pound of grain per gallon of water still may be too much water.
Check the pH of the water in which grains are being steeped after the grains have been added. If the pH is higher than 5.5, add a little Lactic Acid to reduce the pH. That's just a little, it goes a long way. Or on the next batch reduce the volume of water in which the grains are being steeped.