No description available.

No description available.

Tannins have long been thought of as antifeedants owing to their presumed digestibility-reducing properties. In this thesis information, at the molecular level, is presented in a reassessment of this assumption and the apparency theory of plant chemical defence which is dependent upon it. An introductory review provides chemical and opperational descriptions of tannins and a general outline of their ecological impact. Detailed attention is given to (i) tannin-protein complex formation and (ii), an assesment of in vivo evidence concerning the effects of tannins on herbivores. It is concluded that the evidence does not support the hypothesis that tannins uniformly reduce digestion, even though they do generally act as antifeedants. A series of crude tannin-containing plant extracts were prepared and characterised by chemical analyses and by their ability to precipitate protein and inhibit pepsin/cellulase digestion of celulose. Results indicated between-tannin variation but not that the chemical properties of crude tannins might be used to predict their interaction with the other components of a herbivore's diet. Experiments under conditions where soluble tannin-protein complexes formed and which modelled some digestive systems, showed that tannins could under varying circumstances, inhibit or promote the digestion of protein. Soluble tannin-protein complexes were also formed in the presence of bile salts when they would otherwise have occured as precipitates. In these conditions clear relief from digestibility reduction was found. In the light of these results a new model describing the effects of tannins on digestion, consistent with results obtained in vivo, is proposed.

M.Sc., Science Faculty, University of the Witwatersrand, 1991

No description available.

No description available.

Green wattle tannin extracts were separated by lead salt precipitation into a phenolic tannin fraction and a nontannin fraction. The combustion analysis of the tannin fraction corresponded to an empirical formula C₁₅H₁₄O₆. Methoxy and acetyl values showed four oxygens were hydroxy groups while the residual oxygens could either be ether linked or carbonyl groups. Diazomethane methylation produced a white product of high methoxyl value which indicated that the four hydroxyl groups were phenolic. Chromatography of the lead salt purified tannin showed an indistinct non-fluorescent trail. Ether extraction of the tannin removed the associated Phenolic bodies. The ethereal extract unlike black wattle extracts contained no fisetin and evaporation of the ethereal solutions yielded a gummy non-crystalline residue. Alkaline fusion of the purified tannin produced for the first tlme a variety of acidic and phenolic compounds i.e. resorcinol, pyrogaloll, phloroglucinol β-resorcylic acid, gallic acid and protocatechuic acid. From the high yields of resorcinol (6%) and gallic acid (3%), these units appear to predominate. These degradation productions, coupled with the analytical figures, indicate a possible C₁₅ unit with resorcinol and pyrogallol nuclei as a basis. The non-tannin fraction failed to reveal any compound which might cause the excessive redness in green wattle extracts. Chromatography of this non-tannin fraction showed the presence of sucrose.

Les flavan-3-ols appartiennent à la famille des polyphénols retrouvés dans diverses plantes et majoritairement dans le raisin. Ils jouent un rôle primordial dans les mécanismes de défense des plantes, influent sur les propriétés organoleptiques du vin et sont potentiellement bénéfiques au niveau de la santé humaine. La voie de biosynthèse des flavanols monomères est décrite dans la littérature. Cependant, les mécanismes de formation des proanthocyanidines (polymères) sont inconnus à ce jour. Des études ont montré que les flavanols glycosylés sont des intermédiaires potentiels dans la biosynthèse des PA et permettent le transport des unités de flavanols du cytoplasme vers la vacuole de la cellule, où a lieu la polymérisation. Un criblage global des flavanols glycosylés présents dans des raisins à trois stades de développement et dans des vins de différents cépages a été réalisé en analysant séparément les pellicules et les pépins de raisin par une méthode d’UPLC-DAD-ESI-MS. La teneur de ces composés dépend de paramètres de type variété du raisin, tissu biologique étudié et stade de maturité. La présence de dimères de flavanol glycosylés a été montrée pour la première fois dans le raisin. Grâce à l’hémisynthèse de la (+)-catéchine 4’-O--glucoside et 7-O--glucoside, certains monomères ont été caractérisés comme appartenant à la classe des -glucosides. Une étude quantitative a montré l’évolution des flavanols glycosylés au cours du développement de la baie de raisin (pépins et pellicules séparés) provenant de trois cépages différents. Les monomères et dimères d’ (épi) catéchine diglycosylés ont été découverts pour la première fois et uniquement dans les pépins. Une diminution en monomères d’ (épi) catéchine monoglycosylés a été observée dans la pellicule au cours de la maturation du raisin. Les dimères d’ (épi) catéchine monoglycosylés s’accumulent un peu après le stade de véraison et diminuent ensuite à maturité. Les monomères et dimères d’ (épi) catéchine diglycosylés s’accumulent dans les pépins au cours de la maturation. Finalement, l’évolution des teneurs en flavanols mono et diglycosylés au cours de microvinifications a été étudiée. On observe des profils d’extraction similaires pour les deux variétés utilisées (Grenache et Syrah). La quantité totale des différentes familles de composés augmente au cours de la vinification, et ensuite diminue en fin de FA. Certains composés sont dégradés préférentiellement, suggérant la présence d’activités glycosidases spécifiques du raisin ou de la levure. / Flavan-3-ols belong to a group of polyphenols present in a wide variety of plants, and particularly in grapeberries. They play an important role in defense mechanisms in plants, have a significant impact on wine organoleptic properties; and their beneficial effects on human health may help to protect against chronic diseases such as atherosclerosis. The sequence of common flavanol monomer biosynthesis is widely described in the literature, but the formation mechanisms of proanthocyanidins (PA) remain unknown. Studies show that flavanol glycosides are potential intermediates in PA polymerization and have transporter roles of monomeric units from cytoplasm to vacuole in cell, where polymerization takes place. Global screening of grapeberry flavanol glycosides were carried out at three stages of grape development and in wines of different varieties; skin and seeds were measured separately using an UPLC-DAD-ESI-MS method. The composition of the target isomers depends on different parameters such as tissue type or stage of development. The presence epi catechin monoglycoside is reported here for the first time in grapes. Using (+)-catechin 4’-O--glucoside and 7-O--glucoside hemisynthesis, several monomers were shown to -glucosides. Quantitative analysis demonstrates the evolution of flavanol glycosides in both skin and seeds during the development of three grapevine varieties. For the first time monomeric and dimeric (epi) catechin diglycosides were revealed and shown accumulate only in grape seeds during ripening. A reduction in the concentration of monomeric (epi) catechin monoglycoside was observed during grape skin development. Dimeric (epi) catechin monoglycosides accumulate after veraison and then decrease at the end of grape ripening. The extraction profiles of flavanol glycosides during red grape fermentation showed similar evolution patterns for both varieties used. The total concentration of different compound families increases during winemaking, and then decreases at the end of fermentation. Degradation of specific compounds was observed at the end of fermentation which may be explained by the activity of glycosidases from grape extracts released during fermentation and pressing.

No description available.

Pecan kernels from six cultivars were analyzed for phenolic content and antioxidant properties. In addition, kernels from two cultivars were irradiated with 0, 1.5 and 3.0 kGy using E-Beam irradiation and stored in accelerated conditions (40 °C and 55% R.H.). Changes in phytochemical profile and antioxidant properties were monitored for 134 days. Cultivars differed greatly in their phytochemical content. Total extractable phenolic content (TP) ranged from 62 to 106 milligrams of chlorogenic acid equivalents per gram of defatted kernel. Antioxidant capacity (AC) measured by the DPPH free radical had a strong correlation with TP. Shells from each cultivar were 6, 4.5 and 18 times greater for TP, AC and condensed tannin content (CT). Gallic and ellagic acids, epicatechin and catechin were identified in hydrolyzed extracts of all cultivars. Prior to hydrolysis, no compounds were positively identified. Fatty acid profile of kernel oil had a strong inverse correlation between oleic and linoleic oil. Kernels from the same cultivar but different location differed in their fatty acid composition but had similar TP. Irradiation of â Kanzaâ and â Desirableâ kernels with 1.5 and 3.0 kGy had no detrimental effects on AC and TP by the end of experiments. Phenolic profile was similar for all treatments. Tocopherol content decreased with irradiation treatments, but no further degradation was observed throughout storage. Peroxide values increased slightly after 98 and 134 days of storage for â Desirableâ kernels, with slight differences between controls and irradiated samples. Color of kernels decreased in lightness and yellowness and increased in redness with no differences between irradiated samples and controls. For the first time the effect of pecan cultivar and E-Beam irradiation was assessed in phytochemical and antioxidant attributes of pecan kernels. Additionally, irradiation with E-Beam had no significant detrimental effects in phytochemical composition and only a slight increase in peroxide value, indicating potential as pecan kernel sanitization.