Peter T. Gardner

THE RELATIVE CONTRIBUTIONS OF VITAMIN C, CAROTENOIDS AND PHENOLICS TO THE ANTIOXIDANT POTENTIAL OF FRUIT JUICES

Abstract The health benefit of fruit juices have been ascribed, in part, to phenolic antioxidants. The antioxidant potential of a range of fruit juices was assessed by measurement of their ability to reduce a synthetic free radical, potassium nitrosodisulphonate, and also by their ability to reduce Fe(III). Vitamin C was found to account for 65–100% of the antioxidant potential of beverages derived from citrus fruit but less than 5% of apple and pineapple juice. The contribution of carotenoids to antioxidant potential was negligible. Although phenolics appear to be major contributors to the antioxidant potential of the non-citrus juices, their identity and bio-availability requires further investigation.

Anthocyanin-rich extract decreases indices of lipid peroxidation and DNA damage in vitamin E-depleted rats

Anthocyanins are secondary plant metabolites responsible for the blue, purple, and red color of many plant tissues. The phenolic structure of anthocyanins conveys marked antioxidant activity in model systems via donation of electrons or hydrogen atoms from hydroxyl moieties to free radicals. Dietary intakes of anthocyanins may exceed 200 mg/day, however, little is known about their antioxidant potency in vivo. Consequently, the aim of this study was to establish whether anthocyanins could act as putative antioxidant micronutrients. Rats were maintained on vitamin E-deficient diets for 12 weeks in order to enhance susceptibility to oxidative damage and then repleted with rations containing a highly purified anthocyanin-rich extract at a concentration of 1 g/kg diet. The extract consisted of the 3-glucopyranoside forms of delphinidin, cyanidin, petunidin, peonidin, and malvidin. Consumption of the anthocyanin-repleted diet significantly improved (p <.01) plasma antioxidant capacity and decreased (p <.001) the vitamin E deficiency-enhanced hydroperoxides and 8-Oxo-deoxyguanosine concentrations in liver. These compounds are indices of lipid peroxidation and DNA damage, respectively. Dietary consumption of anthocyanin-rich foods may contribute to overall antioxidant status, particularly in areas of habitually low vitamin E intake.

Electron spin resonance spectroscopic assessment of the antioxidant potential of teas in aqueous and organic media

Electron spin resonance methodology has been used to assess the hydrogen-donating (antioxidant) ability of catechins and extracts of green and black tea to Fremys radical and galvinoxyl radical in aqueous and organic solutions, respectively. Catechin flavanols accounted for 77-82% of the total antioxidant activity of green tea but only 47-58% of that of black tea. The radical quenching ability of the green tea was 21-24% more effective than that of the black tea in both aqueous and lipophilic systems. Epigallocatechin gallate was the most effective at reducing the two radical species, whereas epigallocatechin was least effective in the galvinoxyl/ethanol system, and catechin least effective in the Fremys radical/water system. In a model system comprising a mixture of the flavanols in ethanolic or aqueous solution, in the same concentrations as they were present in the tea extracts, the antioxidant potential was a simple summation of the activity of the individual components indicating no synergistic or antagonistic side reactions. Results demonstrate that flavanol components of teas exhibit potent antioxidant activity in both aqueous and organic media. This will be of physiological relevance if such compounds partition into both the aqueous and lipid compartment of cells.

Cell wall porosity and available surface area of wheat straw and wheat grain fractions

Interactions between polymers define many of the physical properties of plant cell walls. The most important of these are available surface area and the related measures of pore size and distribution. Gas adsorption and mercury porosimetry methods for the measurement of these parameters in wheat straw, whole wheat grain and grain fractions were compared. All of the samples examined had a very low surface area regardless of the means of measurement. However mercury porosimetry gave values 10-fold greater than gas adsorption. It is suggested that the assumptions made about pore structure necessary for the use of mercury porosimetry do not hold for plant material and that closed pore systems were distorted by the very high pressures involved. Gas adsorption gave a more realistic assessment of 2–8 m2 g−1 with grain having a lower available surface than vegetative walls. Pore regimes in both grain and straw were very similar and were determined in grain by the bran fraction. Little structure could be found associated with the starchy endosperm. Pores with radii of 1·5–3 nm predominated in wheat cell walls. These are below the size which would allow free penetration of the wall by degrading enzymes. ©1997 SCI

Electron spin resonance (ESR) spectroscopic assessment of the contribution of quercetin and other flavonols to the antioxidant capacity of red wines

Health benefits of red wine have been ascribed in part to the antioxidant properties of quercetin and other flavonols. Red wines, however, contain many other polyphenolic compounds with antioxidant potential. The present study has assessed the antioxidant capacity of seven red wines using electron spin resonance spectrometry to measure their ability to donate hydrogen atoms to a stable Fremys radical. Antioxidant capacity of the wines was strongly related to the total phenol content (r = 0.872, P < 0.01) but less so to total flavonol content (r = 0.651, P < 0.10). The only flavonols detected in the wines by HPLC with post-column derivatisation were the aglycones and conjugated forms of quercetin and myricetin. These accounted for less than 2% of total phenolic content when expressed as gallic acid equivalents. Although these flavonols showed marked ability to quench Fremys radical, they contributed less than 1.5% to the total antioxidant capacity of the wines. Consequently, quercetin is not a major antioxidant in red wine. The antioxidant activity of other polyphenols may be of greater importance in contributing to the reputed health benefits of moderate wine consumption.

Effect of degradation on the porosity and surface area of forage cell walls of differing lignin content

Cell walls, prepared from the stems of wheat, maize, lucerne and rape and from timothy grass, were degraded using a commercial cellulase enzyme preparation. Timothy and lucerne were extensively degraded (60–70% loss of dry matter) while dry matter losses from the more lignified maize, rape and wheat samples were substantially less (30–40%). Residues obtained after 6 and 72 h degradation and original preparations were examined for pore structure and surface area by nitrogen adsorption. The pore regime of all of the samples fell within the range 0.5–5 nm radius with pores of 1–2 nm radius predominating. Degradation had little impact on pore regimes or mean pore radius which were essentially the same as the starting material. The exception was timothy where the mean pore radius fell from 2.80 to 2.36 nm (P<0.001) accompanied by a fall in available surface area (2.49 to 1.39 m2 g −1P<0.001). Wheat showed a similar fall in available surface area after 72 h degradation (3.29 to 2.49 m2 g−1P<0.05) but no significant differences were seen in the remaining samples. In a separate experiment, effect of lignification on pore structure was examined using sequential sections taken from a newly extended maize internode. The phenolic content differed two-fold between the youngest and oldest sections and there was an accompanying steady decrease in electronegativity which weakly correlated with the phenolic content (r2=0.504). No significant differences were seen in pore volume, mean pore radius or pore regimes along the internode, but the available surface area increased significantly with maturity (P<0.01) and was strongly correlated with the phenolic content (r2=0.896). Since porosity is defined by the spacing between polymers of the wall, a considerable commonality of three-dimensional structure evidently existed amongst the cell walls of the plants examined which was little affected by lignification or degradation. This is consistent with exclusion of enzymes by the wall which ensures that bacterial attack in the rumen and elsewhere proceeds by a process of surface erosion. © 1999 Society of Chemical Industry