(flavan-3,4-diols), also known as proanthocyanidins or leucoanthocyanidins, are a group of polyphenolic compounds that provide most of the tannins that are important in terms of beer stability. Both malt and hops are sources of these compounds (also named “pro-tannins”), in roughly equal amount, with “aroma” hop varieties containing higher levels than “bitter” ones. When it comes to barley-based tannins, six-row varieties tend to contain higher levels than two-row varieties, and crops grown under maritime conditions usually have lower levels than those grown far inland.

Anthocyanogens are readily able to polymerize (both with themselves and with other polyphenols, notably catechins—which are not anthocyanogens per se), although free forms are invariably present as well. This polymerization is a first step on the path to haze formation. It has been known for more than 50 years that the colloidal stability of beer is inversely related to the concentration of anthocyanogens in wort. Anthocyanogens can pass through the brewing process and end up in finished beer, where they may be converted into tannins and then interact with proteins to give unwanted hazes. The importance of wort polyphenols to the colloidal stability of beer has led to there being many analytical methods available for their detection. In the past, some brewers would add formaldehyde to the mash-tun; this would reduce anthocyanogen in the resultant wort (and enhance beer shelf life), but it would also reduce wort fermentability. This is no longer practiced. Oxygenating the mash also achieves a similar result.

A number of post-fermentation treatments are available for improving the colloidal stability of beer, including activated carbon, bentonite, silica gels, and polyamide resins. Thanks primarily to extensive work by plant breeders at Carlsberg, there are now a number of proanthocyanidin-free (PA-free) malting barleys that can produce haze-free beers.