In vitro studies were made to determine the nature of complexes formed between the condensed tannins (i.e. pro-anthocyanidins or flavolans) of sainfoin (Onobrychis viciifolia) and either the major soluble dietary protein, Fraction 1 protein, of green leaves, or bovine salivary mucoprotein. Fraction 1 protein, uniformly labelled with 14C, formed only insoluble complexes with these tannins. Bovine salivary mucoprotein produced insoluble complexes only at temperatures below 25°C. Fraction 1 protein complexes were stable between pH 3.5 and pH 7.0. At pHs below 3.0 greater than 90% of the protein was solubilised in the presence of PEG but at pH 8.0 only 30% of the protein was released from the complex. In vivo experiments with sheep fed sainfoin, confirmed the stability of the tannin-Fraction 1 complexes in the rumen (pH 6.5) and break-up of the complexes in anterior duodenal samples (pH 2.5) as shown by the extractability of condensed tannin from the complex. The significance of these results with respect to (a) bloat in cattle, (b) nitrogen metabolism of ruminants, is discussed. Polyethylene-glycol, mol.wt. 4000 (PEG) exchanged with protein in the complex releasing protein into solution. The effectiveness of PEG was dependent on the amount of tannin in the complex and the age of the complex at the time of addition of PEG. The results explain the observation that PEG is unsuitable as a marker for rumen volume determination when animals are fed herbage that contains condensed tannins. A method for determining any deleterious effects of tannins on nitrogen metabolism by ruminants, is suggested.

Complexes of the condensed tannins of sainfoin (Onobrychis viciifolia scop.) with fraction 1 leaf protein and with submaxillary mucoprotein, and their reversal by polyethylene glycol and pH

The phenolic constituents of plants and their taxonomic significance. I. Dicotyledons

The term ‘tannin’ is difficult to define precisely, but the essential property of tannins is that they are naturally-occurring plant polyphenols which combine with proteins and other polymers such as cellulose, hemicellulose and pectin, to form stable complexes. In traditional tanning the collagen chains of hides are crosslinked by tannins to give a durable leather resistent to microbial attack. Tannins proper comprize a relatively small part of a large and diverse group of plant phenolics which range from simple C,-C, phenolic acids such as gallic and coumaric acids, through the C,, flavanoids to the highly-polymerized inert lignins. The molecular weight of tannins is usually in the range 500-3000, requirements being that the tannin is soluble and small enough to orientate itself suitably between the protein chains, but has sufficient phenolic groups to crosslink efficiently under suitable conditions of concentration and pH (Goldstein & Swain, 1963). Much confusion exists in the older literature because simple phenolics such as chlorogenic acid which do not have tannin properties in terms of protein precipitation or astringency react with the much-used Folin-Dennis reagent in the same way as true tannins. At the upper end of the molecularweight scale the phenolic polymers become less able to align themselves between protein components as they increase in size. They become less astringent and reactive towards proteins, their solubility decreases and eventually they become bonded to cell-wall components, when they may be measured along with lignin. Although tannins are chemically a diverse and ill-defined group it is usual to divide them into two types (a) the hydrolysable and (b) the condensed tannins. The hydrolysable tannins consist of a carbohydrate moiety in which the hydroxyl groups are esterified to gallic acid

/pdf/nutritional-effects-of-tannins-in-animal-feeds-4fo5q0xre4.pdf

Nutritional Effects of Tannins in Animal Feeds

The subject is reviewed under the headings: Polyphenols involved in browning reactions - catechin, leucoanthocyanidins, anthocyanins, flavonols, cinnamic acid derivatives, simple phenols; Enzymic browning reactions in food - polyphenoloxidase enzyme, general considerations regarding isolation and assay of polyphenoloxidases, cresolase activity, catecholase activity, role of copper, substrate specificity of polyphenoloxidases, secondary reactions of o-quinones, inactivation of polyphenoloxidase, activation of polyphenoloxidase, polyphenols and enzymic browning of individual food materials; Nonenzymic discoloration involving polyphenols - food browning caused by metal-polyphenol complexing, involvement of leucoanthocyanins in discoloration, turbidity in beer; Additional research needs.

Food Browning as a Polyphenol Reaction

Phenolic substances occur primarily in fruits and vegetables and in the seeds of certain pigmented cultivars of sorghum, millets, and legumes. One of the major difficulties encountered in polyphenol research is the lack of a standard quantitative method for the analysis of phenolics that would be suitable for a wide range of seeds, forage crops, and food products and under a variety of experimental conditions. Some methods measure "total phenol", which may not be a true index of the nutritional quality of foods and thus does not distinguish polyphenols of nutritional concern from other low-molecular-weight phenols that also occur naturally in these products. Tannic acid (a hydrolyzable gallotannin) is commonly used as a "reference standard", but this may be a questionable practice since its biological properties differ from those of tannins of flavonoid origin. Polyphenols of cereals and legumes are predominantly of the latter type. Also, commercially available tannic acid has been shown to be a mixture of four phenolic compounds, the relative proportions of which vary with the samples. Thus, the choice of a suitable standard for tannin analysis is also important. The quantitative extraction of the condensed tannins from plant tissue is always difficult, since it may be complexed to a carbohydrate or protein matrix which could be quite insoluble due to a high degree of polymerization. The literature on tannin methodology is diverse and at times conflicting. Currently available methods for tannin analysis range from simple colorimetric, UV spectrophotometric, chromatographic, and enzymic to more sophisticated and expensive nuclear magnetic resonance (NMR) techniques. None of these methods of analyses is completely satisfactory nor can it be applied to different food products with the same degree of success. This review covers physical and chemical methods for tannin analysis of different food products, the problems in analysis and interpretation of data, and future research needs in this area.

E. C. Bate-Smith

Papers

Leuco-anthocyanins. 2. Systematic distribution of leuco-anthocyanins in leaves.