Polyphenols protein precipitation

A similar problem occurs with beer stabilization. A serious problem in the brewing industry is the tendency of some beers to develop hazes during long-term storage due to protein precipitation that is usually stimulated by small quantities of naturally occurring proanthocyanidin polyphenols. In the same way as observed for wine, the excess polyphenols are traditionally removed by treatment with insoluble PVPP, with the same resulting problems. To resolve the problems, several authors have proposed the use of laccase, which forms polyphenol complexes that may be removed by filtration or other separation means. 

Even higher proline contents have been reported in salivary PRPs these can contain 40-45 mol% proline and also have a substantial amount of glutamine. This protein binds ingested polyphenols, which precipitates the PRPs and removes the lubrication these normally provide. The result is the sensation of astringency (Green, 1993 Haslam and Lilley, 1988). 

Butler et al. [43] found that the extraction of protein precipitable polyphenols from Sorghum seeds was reduced by moistening the seeds before grinding and extraction. The effect was shown to be reversible. In... 

To preserve beer colloidal stability, brewers usually remove haze-active materials [34]. To get rid of haze-active proteins, precipitation with tannic acid, hydrolysis with papain and adsorption to bentonite [35] or silica gel [36, 37] are very effective, but unfortunately in some cases, such procedures also remove foam proteins. To remove haze-active polyphenols, the most usual way is adsorption to polyvinylpolypyrrolidone-PVPP. Because of the structural analogy between these compounds and proline [38], pyrrolidone rings bind polymerized flavanoids through hydrogen and ionic bonds. 

Polyphenols are ubiquitous in all plant organs where they are found as monomers or in polymerised forms (Schofield et al, 2001). In addition to the beneficial effect of poljq)henols, they also bind minerals and precipitate proteins and carbohydrates, in effect reducing the nutritive value of foods. Polyphenols have been classified for nutritional purposes into extractable and non-extractable types (Bravo, 1998). Extractable polyphenols are low-and intermediate-weight phenolics while non-extractable polyphenols have high molecular weight and are insoluble in normal solvents. 

When appreciable amounts of pectin, proteins, lipids, unwanted polyphenols, or other compounds are suspected to be present in anthocyanin-containing extracts, some of them can be precipitated or the anthocyanins may be crystalhzed and separated from the others. Pectin and proteins can be removed by organic solvents such as methanol and acetone in order to reduce their solubility, then precipitated and separated by centrifugation. Gelatin was used to remove proanthocyanidin due to its high molecular weight. Anthocyanins were reported to be precipitated early by lead acetate to achieve isolation from other materials. ... 

Tannins are one of several antinutritional factors present in dry beans. Any polyphenolic compound that precipitates proteins from an aqueous solution can be regarded as a tannin (11). Tannins precipitate proteins due to functional groups that complex strongly with two or more protein molecules, building up a large cross-linked protein-tannin complex (12). 

A number of approaches have been used to determine the amount of HA protein in a sample. The most successful of these is based on adding a fixed amount of TA to a sample (Thompson and Forward, 1969) after incubation, the turbidity is measured and the increase in turbidity observed is presumed to be proportional to the amount of HA protein in the sample. This method has the advantage that only substances able to form haze with polyphenols respond. The saturated ammonium sulfate precipitation limit (SAPL) method has also been widely used, but is far inferior in providing useful information (Berg et al., 2007 Siebert et al., 2005). 

Baxter, N.J. et ak. Multiple interactions between polyphenols and a salivary proline-rich protein repeat result in complexation and precipitation. Biochemistry 36, 5566, 1997. 

Complex plant polyphenols readily and reversibly associate with proteins and they can precipitate them from dilute solution. This property is however a direct extrapolation of the characteristics of simple phenols themselves. The structural device represented by the plant polyphenols to a great extent obviates the need for a high molal concentration of phenol and it embodies the added feature that cross-linking between different molecular aggregates may be readily achieved. 

Tannins are polyphenolic compounds of high molecular weight that occur naturally and react with proteins. They can form insoluble complexes with proteins, which are responsible for the taste known as astringency that is caused by precipitation of mouth proteins. Tannins are typically classified into three groups (1) the condensed tannins, (2) the hydrolyzable tannins, and (3) the phlorotannins (Figure 11.3.10). 

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