Colloidal tannins

The current interest in UF is primarily because of its ability to replace several processing steps with a single filtration. The haze in wine is often due to heat-unstable proteins, oxidized/polymerized phenols, gums and pectins. Bentonite and gelatin are added as fining agents to precipitate colloidal proteins so they can be removed with conventional filters. With UF, precipitation is not necessary. It can remove all colloids, tannins, carbohydrates and proteins normally removed by diatomaceous earth and EK pads after fining. In addition, UF can remove the yeast and/or bacteria normally removed by MF. 

In a more recent theory on the behavior of colloidal tannins, hydration of hydrophilic colloids is not considered. When tannin molecules combine to form colloidal particles, the Van der Waals forces between tannins and proteins increase considerably, producing a non-specific adsorption phenomenon (Figure 9.6) (Saucier, 1997). The mechanisms involved are as follows ... 

A final are we should discuss is color removal. This is perhaps the most difficult impurity to remove from waters. In surface waters color is associated with dissolved or colloidal suspensions of decayed vegetation and other colloidal suspensions. The composition of this material is largely tannins and lignins, the components that hold together the cellulose cells in vegetation. In addition to their undesirable appearance in drinking water, these organics can cause serious problems in downstream water purification processes. For examples ... 

In addition, even where foaming is not a specific problem in a boiler, carryover may occur, especially in lower pressure boilers with very high TDS (i.e., over 10,000 to 15,000 ppm TDS) because of the collapse of surface bubbles. This leads to BW aerosol generation and entrainment of the spray in steam. Under these circumstances, antifoam agents such as polyamides are useful in preventing these entrainment problems. Furthermore, the antifoaming action of polyamides is often enhanced by protective colloid materials such as tannins, and consequently, formulations containing polyamide emulsions in an alkaline tannin base are available. 

Colloidal metals are usually prepared by reduction of a salt with a reducing agent, such as phosphorus, acetone, tannin, or carbon monoxide. Platinum metals can also be prepared as finely divided very active blacks by reducing the metal salt in an aqueous solution of sodium or potassium borohydride. 

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. 

Bad appearance. Turbidity is caused by suspended matter, which may contain disease-causing microorganisms. Discoloration that leaves stains can be caused by dissolved or colloidal iron, tannin, or humic acid from decaying vegetation, and excessive salt content. 

Tannins are amorphous substances, which produce colloidal acidic aqueous solutions with astringent taste. With iron salts (FeCls) they form dark blue or greenish black water-soluble compounds. Tannins form insoluble and indigestible compounds with proteins, and this is the basis of their extensive use in the leather industry (tanning process), and for the treatment of diarrhoea, bleeding gums and skin injuries. 

Filtering. Conditioning or lagering gives the beer its desired organoleptic properties, but it still contains yeast, protein-tannin complexes, etc, ie, it has a hazy appearance. A high quality beer must be clear and totally sterile, have colloidal stability, and yeast must be removed to allow the beer to have biological stability. The protein-tannin complexes must also be removed so as not to upset the colloidal stability. 

Even when no additive is used in winemaking, the necessity for small-lot trial before production scale operation is apparent. Because a high percentage of wine is consumed only after chilling, and because chilling may accelerate the precipitation of potassium acid tartrate, ill-defined colloids, anthocyanin-tannin polymers, proteins, etc., simple cold stabilization by refrigeration in the winery may irreversibly alter the product and its eventually-perceived quality level. It often happens, especially in heavy-bodied red varietal wines, that a dark, amorphous precipitate may form in the bottle over several years. Usually tannoid,... 

One hundred milliliters of a 0.001% solution of tetrachloroauric (III) acid (or silver nitrate) in distilled water ia treated with several drops of 1% sodium carbonate 10-hydrate and heated to boiling in a flask. Every 30 seconds one drop of a fresh 1% tannin solution (XJ.S.P.) is added with agitation. When a deep red gold color has formed (or yellow-brown to red-brown for the silver) the colloidal solution is cooled and stabilized by the addition of a few milliliters of chloroform or toluene. 

DIALYSIS. The process of separating compounds or materials by the difference in their rates of diffusion through a colloidal sentipermeahle membrane. Thus, sodium chloride diffuses eleven limes as last as tannin ami twemy-une times as fast as albumin. When the process is conducted under the influence of a difference in electrical potential, as from electrodes on opposite sides of the semipermeable membrane, it is called clectrodialysis. 

The pure substances would be too irritant, but their action is graded by their slow liberation and by the presence of colloid extractives. The most common of these substances are emodin and chrysophanic acid. Numerous isomers of these are possible (15 for emodin alone). The special character of the different drugs is probably due to differences in these isomers, in the stability of their glucosidal combination, and to the presence of other associated substances (tannin in rhubarb). 

The most unstable form of the amorphous variety is colloidal silver, prepared by reduction of an ammoniacal or dilute alcoholic solution of silver nitrate with sodium citrate, sodium potassium tartrate, dextrin, tannin, or formaldehyde, with subsequent addition of ferrous sulphate.13 If forms a chocolate, pale-lilac, blue, or green mass, and dissolves in water to a deep-red solution. The colour depends partly on the mode of... 

A therapeutic preparation of colloidal silver is known as collargol. Crede s ointment also contains this form of silver, and is employed in the treatment of certain types of septic infection. An astringent antiseptic is prepared by the action of an alkaline tannin solution on aqueous solutions of silver salts.11... 

The central vacuole is a relatively simple aqueous phase that can act as a storage reservoir for metabolites or toxic products. For example, the nocturnal storage of organic acids, such as malic acid, takes place in the central vacuoles of Crassulacean acid metabolism plants (mentioned in Chapter 8, Section 8.5A), and certain secondary chemical products, such as phenolics, alkaloids, tannins, glucosides, and flavonoids (e.g., antho-cyanins), often accumulate in central vacuoles. Compared with the central vacuole, the cytoplasm is a more complex phase containing many colloids and membrane-bounded organelles. Because the central vacuole contains few colloidal or other interfaces, any matric pressure in it is... 

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