Tannin iron complexes

A photoelectric colorimeter was used by Faure and Pallu (1936) for the color of the tannin-iron complex. Kretzdorn (1949) also tested an iron chloride procedure, but citric acid (1 % to 3%) interfered. The hide-powder method was applied to dealoohol-ized wines by Ponte and Gualdi (1931). Feigl and Feigl (1946) proposed use of ,a -ferrous dipyridyl sulfate or a,a -ferrous phenanthroline sulfate for the colorimetric identification of tannin. Synthetic tannins did not react and seven red and white wines gave positive tests. 

The polyphenolic colored substances of cane juice, largely tannins and anthocyanins,34 form even more intensely colored iron complexes. Some of these compounds survive defecation and darken further with prolonged exposure to air and ferric ion at elevated temperatures. 

Black tea has been shown to reduce iron absorption, especially in individuals who have an increased risk of anemia (Disler et al. 1975 Merhav et al. 1985). The postulated mechanism of action is the formation of iron complexes with black tea tannins in the gut (South et al. 1997). However, in a review of clinical trials and epidemiological studies of black... 

Silicate is determined spectrophotometrically with ammonium molybdate and ammonium vanadate. The pH of the sample must be adjusted to 7-8. Potassium cyanide is added to prevent interference of heavy metals. Oxalic acid is added to destroy mo-lybdophosphate and vanadophosphate and to bind aluminum in a complex. As in all spectrophotometric determinations, high and variable optical absorption of the sample (due to color or turbidity) at the wavelengths of investigation causes errors tannin, iron, and sulfide also interfere. To avoid contamination, all contact surfaces should be of polyethene. It is also possible to determine silicate using FAAS, in which case nitrous acid and ethyne must be used as flame gases. As silicates are present in colloidal form, the sample must be introduced into the AAS equipment using an ultrasonic nebulizer. Such a nebulizer is also used when silicate is measured by ICP-AES. 

It is otherwise with cotton, which is almost chemically pure cellulose, and hence is chemically indifferent in a tinctorial sense. Here combination with the dye results from the use of mordants which are adsorbed colloidally on the fibre before dyeing. The mordant can then enter into chemical union with the dye as a complex compound. For an important group of acid dyes (p. 335) the mordants are chiefly metallic hydroxides, namely, those of chromium, aluminium, iron, antimony, tin, etc., whilst for basic dyes tannin is the usual mordant. 

Contradictory opinions have been referred to in the literature particularly on the nature of the iron-tarmate and its interaction with the rusted steel due to the diversity of the material used in different studies. Studies have included the use of tannic acid [7-10], gallic acid [11], oak tannin [12, 13], pine tannin [14] and mimosa tannin [15]. In order to establish the correlation between the ferric-taimate formation and the low inhibition efficiency observed at high pH from the electrochemical studies, phase transformations of pre-rusted steels in the presence of tannins were evaluated. In this work the quantum chemical calculations are conducted to analyse the relationship between the molecular stracture and properties of ferric-taimate complex and its inhibitory mechanism. 

Iron and copper in wines may form complexes with other components to produce deposits or clouds in white wines. Iron clouds generally occur at a pH range from 2.9 to 3.6 and are often controlled by adding citric acid to the wines (2). Copper clouds appear in wines when high levels of copper and sulfur dioxide exist and are a combination of sediments, protein-tannin, copper-protein, and copper-sulfur complexes (169). Further, the browning rate of white wines increases in the presence of copper and iron (143). The results of this study indicate that iron increased the browning rate more than copper. 

The interactions between different foods have sparked some interest. For exam-pie, if rice is consumed with orange juice, the orange juice can enhance the absorption of the iron in the rice. This effect results from the chelation of the iron by the ascorbate in the juice and the increased absorbability of the iron from the complex, On the other hand, if rice is consumed with tea, the tannins in the tea can reduce the absorption of the iron in the rice because the iron in the iron-tannin complex is not readily available. In general, including meat in the diet can increase the availability of iron (iom other foods. The mechanism of this effect is not dear. The availability of heme iron is not much influenced by other components of the food known to influence availability of nonheme iron. These components include ascorbate, tannins, phytate, phosphates, and fibers. 

Tannin is soluble in water and alcohol. It can form colored complexes or precipitates with calcium, iron and lead ions. The tannin that is commonly used as a depressant is called quebracho, which is extracted from apocynaceae, a plant species in South America. It contains agglomerated tannin. 

Wood often contains tannins, and higher levels of tannins are commonly associated with oak. Tannins are water-soluble phenolic and polyphenolic compounds, which can form dark color complexes with iron salts (ferric salts). That is why some WPG deck... 

Gum arable is also at least partially effective as a treatment for ferric casse in red wines. It does not prevent the appearance of a dark, blnish color, due to the formation of colloidal ferric tannate, but it does stop the colloid flocculating. It acts differently from citric acid, which prevents color from changing, as it produces a soluble complex with iron that is no longer capable of reacting with tannins (Section 4.6.2). These two treatments are often complementary (Section 4.6.3). 

Tannins are thought to form insoluble complexes with iron and thus re-duee its absorption. " ... 

Rapeseed meals frequently contain tannins. These are polyphenolic compounds that complex with proteins and carbohydrates to form enzyme-resistant substrates with a consequent lowering of digestibility. This may also result from the combination of the tannins with digestive enzymes, with a consequent loss of activity. Tannins may cause damage to the intestinal mucosa and are known to interfere with iron absorption. 

Taking advantage of the many aromatic hydroxyl functions in conifer bark tannins, Hartmann (83) used ground bark as a polyol for reaction with isocyanates to prepare urethane foams with particularly good flammability resistance. Most uses for conifer bark tannins that involve reactions with the hydroxyl functions center on their complexation with cations. When sulfonated, condensed tannins can also be used as water-soluble heavy metal complexes. One of the more interesting of these applications is the development of water-soluble heavy-metal micronutrient complexes that have been used to correct iron deficiency in citrus... 

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