Metals, complexes with polyphenols

Tea stains are dry films of tea scum adhering strongly onto the surface of porcelain cups. They are complex mixtures of organic material from the oxidation of soluble tea components interspersed with calcium carbonate crystals [196]. Other metal ions, either from tap water or from tea leaves, are also present, and may form complexes with polyphenolic tea components. Strong complexing agents such as phosphates or phosphonates are capable of extracting the earth alkali metal ions to loosen the polymeric framework and support the action of the bleach systems. 

In some cases, however, the formation of salts and complexes is an unwanted phenomenon, as protein salts and complexes with polyphenols form hazes and sediments in preserved fruit, fruit juices and beer. Protein hazes can also form in white wines after bottling, where the main factor is inorganic sulfate ions. Protein complexes with transition metals are often coloured, and their formation in processed foods is generally undesirable. An example is the protein conalbumin occurring in egg white, which readily forms coloured complexes with metal ions (pink with Fe +, yellow with Cu + and Mn +) in media of pH > 6 through tyrosine and histidine residues of the polypeptide chain. Complexes with iron ions often cause discoloration of egg products, but in media with pH < 4, these complexes dissociate to the original colourless compounds. 

Karadjova and coworkers [90] in a detailed and comprehensive investigation established a scheme for fractionation of wine components and Cu, Fe, and Zn determination in the different fractions. Like Fe, the other two metals may analogously exist in wines as free ions, as complexes with organic acids and as complexes with proteins, polyphenols and polysaccharides. The resin XAD-8 was used for the separation of wine polyphenols. Dowex ion exchange resins were used for the separation of cationic and anionic species of metals that were subsequently quantified off-line in Bulgarian and Macedonian wines by FAAS or ET-AAS (depending on their concentration levels). 

The Cu ion is used as an example of a metal that complexes with a high degree of selectivity on polyphenolic groups of soil organic matter. One of these groups is the biphenol (catechol) type, which dissociates to form the ligand... 

In this case, will be used to exemplify metals strongly predisposed toward precipitation as well as complexation with humus. Like Cu ", AP complexes energetically with catechol-type and other polyphenolic ligands. The stability constant, ATail for the solution reaction ... 

Scheme 18 Complexation of polyphenols with metal ions...

Cobalt (II) complexes with Schiff base ligands are well known to activate dioxygen (16-18) and are frequently used to catalyze the oxidation of organic substrates (19) and to mimick mono and dioxygenases (20). This suggests that the Co-catalyzed oxidation of polyphenols could be a good alternative to the incineration of wastewaters rich in polyphenols. Moreover, the toxicity of Co(II) is much lower than that of other metal ions usually employed as oxidation catalysts. 

The o-quinones formed from phenolics further enhance the intensity of browning by oxidation of other substrates, complexing with amino acids and protein, and polymerization. Non-enzymatic discoloration is believed to involve metal-polyphenol complexing as reported in the processed potato (Bate-Smith et al., 1958), cauliflower (Donath, 1962), and asparagus (DeEds and Couch, 1948), conversion of leucoanthocyanidins to pink anthocyanidins in the processed broad bean (Dikinson et al., 1957), green bean puree (Roseman et al., 1957), and canned Bartlett pear (Luh et al., 1960), and protein-polyphenol complexing in chilled or stored beer (Schuster and Raab, 1961). 

On storage, beer can become cloudy and form a sediment. Proteins and polypeptides make up 40-75% of the turbidity-causing solids. They become insoluble due to the formation of inter-molecular disulfide bonds, complex formation with polyphenols, or reactions with heavy metals ions (Cu, Fe, Sn). Other components of the sediment are carbohydrates (2-25%), mainly a- and P-glucans. For measures used to prevent cloudiness, see 20.1.8.5. Undesirable microorganisms, e. g., thermophilic lactic acid bacteria, acetic acid bacteria Acetobacter, Gluconobacter) and yeasts, can cause disturbances and defects in various process steps (mashing, fermentation, finished product). 

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