Oxidised polyphenols

The oxidised polyphenols give the black colour, its aroma depends on the occurrence of a volatile oil and the tannins of a catechin type give the harsh taste. The caffeine content of a cup of tea is approximately 30—40 mg. 

Methionine can be oxidised by reactive oxygen spedes, fatty acid hydroperoxides and oxidised polyphenols. The primary oxidation product, methionine sulfoxide (free or bound in proteins), is a frequent component of various proteins. The contents of methionine sulfoxide in protein sources used for foods and feeds usually vary from 0 to 30% of the total methionine content (e.g. up to 11% methionine sulfoxide apparently occurs in milk powder, 13% in lean beef, 16-28% in wheat gluten and 50% in orange juice). It is fuUy available as a source of methionine as it can be reduced back to methionine by sulfoxide reductases, but the final oxidation product, methionine sulfone, is unavailable as a source of methionine (Figure 2.34). The oxidation of the methyl group to... 

Total wine polyphenols are oxidised by the Folin-Ciocalteu reagent - composed of a mixture of phosphotungstic and phosphomolybdic acids which are reduced by the oxidation of the phenols, forming a mixture of blue oxides of tungsten and molybdenum. The blue coloration has an absorption maximum at approximately 750 nm, the intensity of which is proportional to the level of phenolic compounds present in the wine. The sequential analyser method is a direct automation of the manual method and results are expressed as a unit-less index. 

The production of black tea starts by withering the leaves for 20-40 hours, depending on temperature and humidity of the air afterwards the leaves are crushed by rolling, when the polyphenols in the cell juice are oxidised and the phlobotannins are converted into phlobaphenes, a process that is terminated by exposure to the air at a temperature of about... 

Another enzyme which will cause loss of ascorbic acid in plant material is phenolase. This is the substance which helps to produce the browning offruit such as apples when polyphenolic species are oxidised by oxygen from the air. The enzyme functions with oxygen and ascorbic acid and reduces or/Ao-quinones back to orfAo-diphenols. This results in the formation of dehydroascorbic acid, which is rapidly converted to 2,3-diketogulonic acid. The process is catalysed by copper(ii) and other transition metal ions, which therefore accelerate the loss of ascorbic acid from vegetables and fruit, when for example they are cooked in copper or iron containers. Of course the major factor in the removal of vitamin C from cooked vegetables is simple dissolution in the cooking water. 

Laccases have very broad substrate specificities. Simple diphenols like hydroquinone and catechol, polyphenols, diamines, and aromatic amines are good substrates for most laccases. The fiill range of compounds oxidised by laccases is not yet known. Due to the discovery of the mediator concept in... 

Certain chemopreventive and growth-inhibiting agents such as selenite (Gopalakrishna et al. 1997), or polyphenolics such as curcumin (Chen et al. 1996), 4-hydroxy-tamoxyfen (Gundimeda et al. 1995), or ellagic acid can inactivate protein kinase C by oxidising the vicinal thiols present within the catalytic domain. 

Chick growth may be adversely affected if inclusion rates exceed 50 kg/t. The eggs of some hens producing brown-shelled eggs are susceptible to the development of fishy taints when rapeseed meals are included in the diet. This is because of the inability of these birds to oxidise trimethylamine produced from the polyphenolic choline ester sinapine. 

A novel synthetic method for polyphenols was described by Inoue and co-workers [172] w-cresol, 3-methylcatechol, 4-methylcatechol and 4-ethylcatechol were polymerised using a redox reaction between phenol derivatives and gold ions. The oxidised phenol derivatives produced by the reduction of gold ions formed water-insoluble polymers with MW of approximately 1,000 g/mol. Alternatively, Wei and co-workers [173] described the electrochemical polymerisation of phenol on 304 stainless steel anodes and the subsequent coating structure analysis. Anodic oxidation was carried out using 304 stainless steel anodes in a neutral 0.1 mol/1 phenol solution with an electrolyte composed of 0.1 mol/1 sodium sulfate. This oxidation generated a yellow brown polyphenol coating on the steel surface of the anode. 

Phenolic compounds known as polyphenols (such as phenohc acids and flavonoids) are easily oxidised by atmospheric oxygen in reactions catalysed by oxidoreductases (o-diphenokO, oxidoreduc-tases). These reactions are known as enzymatic browning reactions (see Section 9.12). Autoxidation of plant phenols catalysed by heavy metal ions (mainly cupric and ferric ions) and oxidation by lipid hydroperoxides lead to similar products. The products of oxidation of the so-called o-diphenok (1,2-dihydroxybenzenes) are o-quinones (1,2-benzoquinones), highly reactive compounds that react with proteins (amino acids) and other food components with the formation of dark-coloured polymeric products resistant to proteolysis. 

Phenolic compounds are present separately from the substrates, predominantly located in the cell vacuoles. Particularly prominent substrates are caffeic acid and its esters, as well as some flavonoid substances, of which monomeric flavan-3-ols (catechins) are the most important. Other groups of phenolic compounds, such as condensed forms of flavan-3-ols and flavan-3,4-diols (tannins), flavonols, flavones, flavans, chalcones, dihydrochalcones and anthocyanins, are only partly oxidised. One of the reasons for this is probably the steric hindrance caused by the corresponding glycosides and substrate specificity of polyphenol oxidases. The content of phenohc compounds depends on genetic factors (on plant species and varieties), the degree of maturity and external (environmental) factors (hght, temperature, nutrients, use of pesticides and so on). The only substrate in animal tissues is the amino acid tyrosine. 

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