Proteins and polyphenols

A response surface model of the effects of HA protein concentration (gliadin, the wheat prolamin), HA polyphenol concentration (tannic acid, TA), alcohol, and pH on the amount of haze formed was constructed using a buffer model system (Siebert et al., 1996a). Figure 2.12 shows the effects of protein and polyphenol on haze predicted by the model at fixed levels of pH and alcohol. The model indicates that as protein increases at fixed polyphenol levels, the haze rises to a point and then starts to decline. Similarly, when polyphenol increases at a fixed protein level, the haze increases to a maximum and then declines. 

A more detailed study was carried out with many more levels of protein and polyphenol than were used to construct the initial response surface model (Siebert and Lynn, 2000). The results (see Fig. 2.15) indicated that... 

The effect of pH on haze intensity was striking (Siebert et ah, 1996a) (see Fig. 2.16). When the pH rose from near 3 to slightly above 4, the haze intensity increased by a factor of 7 with the same amounts of protein and polyphenol. At higher pH, the haze intensity declined. While proteins... 

FIGURE 2.16 Response surface model predictions of the effects of pH and ethanol on haze intensity in a model system at fixed levels of protein and polyphenol. Reprinted with permission from Siebert et al (1996a). Copyright 1996 American Chemical Society. 

Siebert, K. J. and Lynn, P. Y. (2008). On the mechanisms of adsorbent interactions with haze-active protein and polyphenol. J. Am. Soc. Brew. Chem. 66, 46-54. 

Studies of the association of polyphenols with proteins have a long history (27). Loomis (28) has succinctly summarised the conclusions of this earlier work. The principal means whereby proteins and polyphenols are thought to reversibly complex with one another are (i) hydrogen bonding, (ii) ionic interactions and (iii) hydrophobic interactions. Whilst the major thrust in earlier work was to emphasize the part played by intermolecular hydrogen bonding in the complexation, Hoff (29) has drawn attention to the possibility that hydrophobic effects may dominate the association between the two species. 

The contribution of lipophilic antioxidants is small. Escobar et al. (E5) found that the TAC of lipophilic antoxidants in blood plasma was 16.5 1.5 pM and corresponded almost exclusively to a-tocopherol the concentration of this compound in the blood plasma, analyzed independently, was 17.6 0.3 pM. Popov and Lewin (PI9) found TAC of lipid-soluble antioxidants in blood plasma to be 28.0 8.1 /u.M, a value comparable with the concentration of a-tocopherol (20.5 6.6 /U.M). These (and other) results confirm that a-tocopherol is the main lipid-soluble antioxidant of blood plasma (II) and indicates that the contribution of the lipid-soluble antioxidants to TAC of blood plasma is in fact negligible, taking into account that TAC of human blood plasma is of the order of 1 mM (see later). The contribution of ascorbic acid is also low. This situation may differ considerably in other biological fluids and tissue homogenates. In seminal plasma, the concentration ratio of ascorbate to urate is about 1 (G3). Ascorbate and urate contribute 29% of the fast TRAP of human seminal plasma the share of proteins and polyphenolic compounds is 57%, whereas tyrosine contributes 15% of the slow TRAP (R14) (Table 7). Ascorbate and uric acid account for about half of TAC of human tears (K3). TAC of urine is determined mainly by urate and proteins (K5). 

Haze formation is mostly attributed to proteins, polyphenols, and their interactions. It is also possible that there are also other factors that inbuence haze formation in beer, but their effect has not been yet clearly debned [ 15]. The amount of haze formed depends both on the concentration of proteins and polyphenols, and on their ratio. Polyphenols can combine with proteins to form colloidal suspensions that scatter light, which creates the cloudy appearance of beer. Beer polyphenols originate partly from barley and partly from hops. The beer polyphenols most closely associated with haze formation are the proanthocyanidins, which are dimers and trimers of catechin, epicatechin, and gaUocatechin. These have been shown to interact strongly with haze-active proteins [13,15-17] and their concentration in beer was directly related to the rate of haze formation [18]. Ahrenst-Larsen and Erdal [19] have demonstrated that anthocyanogen-free barley produces beer that is extremely resistant to haze formation, without any stabilizing treatment, provided that hops do not contribute polyphenols either. Not all proteins are equally involved in haze formation. It has been shown that haze-active proteins contain signibcant amounts of proline and that proteins that lack proline form little or no haze in the presence of polyphenols [13,15-17]. In beer, the source of the haze-active protein has been shown to be the barley hordein, an alcohol-soluble protein rich in proUne [16]. 

The release of simple substances such as sugars, and amino acids by healthy cells probably occurs chiefly by diffusion through the cell plas-malemma. The rate of such release will, therefore, depend on the concentration gradient of the substance across the membrane, and the permeability constant of the membrane for the substance. Large molecules, such as polysaccharides, proteins and polyphenolic substances, are probably excreted by more complex processes but these are quantitatively unimportant (Hellebust, 1974 Fogg, 1975). 

In seaweed, iodine species vary widely with the species of seaweed. In brown seaweed, most iodine exists as iodide while in green seaweed, iodine is mainly bound to organic molecules, such as proteins and polyphenol. 

In order to speed withdrawal of clear wort, there are usually draw-off pipes at various heights. It is normal to run several brews through the whirlpool tank before removing the solids. The consistency of the solids changes with time and eventually the mass will set like concrete due to oxidative copolymerization of proteins and polyphenols. Two methods of removing the solids are the use of a high pressure water spray jet lowered to 2 m (6 ft) from... 

Time-dependent Changes in Both Extracellular Total Protein and Polyphenol Oxidase Activity for Coriolus versicolor Cultures Grown in the Absence of Catechol... 

Trub Precipitated flocks of haze-forming protein and polyphenols produced during hops addition and beer filtration. 

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