New insight into the mechanism of achieving colloidal stability.

McMurrough, I. and O'Rourke, T.

Abstract
An investigation into the composition and properties of the polyphenolic constituents removed from beer by colloidal stabilization with PVPP and their relationship to haze formation in untreated beers is described. Non biological beer haze is composed of complexes of proteins with oxidized polyphenols, and its formation is prevented by the removal of the polyphenolic constituents. The authors have found that analyses for simple flavanoid polyphenols, total flavanols or total polyphenols provide reasonably accurate predictions of the colloidal stability of beer, as does the alcohol cooling haze measurement test (which is thought to indicate the relative levels of protein/polyphenol complexes), but, surprisingly, the "tannoid assay", which is supposed to measure those polyphenols which coprecipitate with soluble PVP (and therefore have a high affinity for PVPP), was found to be quite unreliable due to its insensitivity to the residual polyphenols in treated beer, some of which are capable of forming haze. As the polyphenols adsorbed onto PVPP cannot be separated from it in quantities sufficient for experimental purposes, two polyphenolic fractions obtained from beer by solvent extraction were analysed. The first contained mainly high molecular weight material, while the second contained significant amounts of simple flavanoids. While the first fraction gave a much stronger reaction in the tannoid assay than the second, when added to beer, both produced significant increases in the amount of haze formed by forced aging. The results of an investigation into the changes in polyphenol composition during forced aging are interpreted by the authors as indicating that the simple flavanoids and partially oxidized complex flavanoids (tannoids) undergo further oxidation and polymerization to form tannins while also forming complexes with proteins. Both simple flavanoids and tannoids therefore contribute to haze formation. It is recommended that the PVPP dosage rate should be adjusted so that the treated beer has a simple flavanoids content (determined by HPLC with electrochemical detection) of 4 to 5 mg/litre and gives a negative tannoid assay result.
Keywords : beer colloidal composition flavonoid haze polyphenol PVPP stabilisation stability tannin