Power of phenolic compounds

To protect foods against oxidative deterioration, synthetic antioxidants are continuously being replaced by natural antioxidants from plant sources. ML Andersen, R Kragh Lauridsen and Leif H Skibsted report on the latest innovative sources at manufacturers? disposal

Herbs and spices have been used since prehistoric times to preserve foods. Around the Mediterranean area, oregano and rosemary were widely used while in the Nordic countries thyme was added to sausages and lard. The modern development in the use of such plant material for protecting processed food against oxidation started half a century ago. The variety of spices is derived mainly from the plant family Labiatae.

In the mid-1990s, around 10 per cent of the antioxidants used by the European food and feed industry was based on spices.1 Other substances, such as tea, coffee and waste from the manufacture of various vegetable products and beverages, are now being considered as sources for phenolic compounds to prevent or retard oxidation processes in food and to replace synthetic antioxidants. Some of these sources have the obvious advantage that they will not dominate the flavour of the food as do some species such as clove.

For each plant material, extraction conditions need to be optimised with respect to solvent polarity and physical conditions.2,3 In agreement with the high antioxidant activity of the tea catechins found in model systems, green tea is among the more promising sources of natural antioxidants.4 A few examples from recent studies will illustrate some of the developments in the exploitation of various sources of antioxidants for protection of processed foods.

Green tea
Various raw and cooked meat products have been found to have a significantly higher antioxidant capacity when treated with tea catechins than they did when treated with alpha-tocopherol.5,6,7 Part of the antioxidative effect may be due to protection or regeneration of alpha-tocopherol already present in the meat or in the fish muscles. A high affinity of the tea catechins for lipid bilayers of muscles together with the radical scavenging activity was suggested as providing the explanation for the high antioxidant activity.6

For chicken, feed supplemented with tea catechins gave later protection of alpha-tocopherol in the meat during frozen storage and reduced lipid oxidation.8 The tea catechins showed a pro-oxidative effect in corn oil-in-water emulsions, but a significant antioxidant effect in liposomes, a difference that also could be understood on the basis of the high affinity of the catechin to liposome surfaces.4 In another study, green tea extracts have shown a better antioxidant capacity than rosemary extracts in canola oil, pork lard and chicken fat heated to 100?C.9

Tart cherry
Promising results have been obtained by adding tart cherry tissue to ground beef patties.10 Secondary lipid oxidation products for raw patties treated with cherry tissue were reduced by up to 80 per cent after storage for nine days at 4?C. For cooked patties, stored for four days also at 4?C, the reduction was almost 90 per cent compared to control patties without addition of cherry tissue. For the cooked patties, the concentration of cholesterol oxidation products was reduced by up to 92 per cent by the addition of tart cherry tissue prior to cooking, with a comparable reduction in the level of mutagenic amines in the cooked product. This significant reduction in oxidation products seems to be caused by the flavonoids found in tart cherry, consisting mainly of anthocyanidins and flavon-3-ols, although hydroxycinnamic acids derivatives are also present.11

Based on a review of the numerous studies of food products, clove, rosemary and sage were concluded to be among the most potent antioxidative spices.12 In studies comparing different spices and extracts thereof, clove seems to have the largest antioxidant potential in oil-in-water emulsions, while rosemary and sage have the largest antioxidative effect in lard.13,14,15,16,17 Rosemary and sage, in combination with citric acid, seem to improve the oxidative stability of frying oil even more than each of the two spices alone.18

Extraction of antioxidants from spices depends on the polarity of the solvent. Rosemary and sage mainly contain the polar components rosmarinic acid, carnosol and carnosolic acid, of which rosmarinic acid is the most water-soluble, and extracts seem more effective in bulk oil systems compared to oil-in-water emulsions, due to affinity for the lipid-air interface.

Clove contains the monophenols eugenol and gallic acid but, because of the characteristic strong flavour of eugenol, it has limitations as a food additive. Oregano extracts also are effective against lipid oxidation in lard and oils.19 Oregano contains rosmarinic acid and various flavonoids with high antioxidative activity, but with large variation among different sub-species. Dittany, a Cretan mountain herb closely related to oregano, thus has a high content of water-extractable antioxidants.20 Different cultivars of hops with different phenol profiles also yield different protection of beer.21 It is most likely that extracts will continue to be preferred for purified compounds like rosmarinic acid, because the purified compounds will have to go through toxicological testing.

Olive oil
Olive oil is renowned for its high oxidative stability. It contains polyphenols with high antioxidative capacity. Phenolic substances extracted from extra- virgin olive oil have been shown to be effective in protecting minced tuna cooked and stored in brine against oxidation, but to yield less protection of the same product cooked and stored in refined olive oil.22

The higher antioxidative activity in the aqueous system was explained by the greater affinity of the phenol of the extract toward the polar interphase between water and the fish oil. Isolated phenols, especially myricetin, tannic acid and ellagic acid, showed similar effect on lipid oxidation in steam cooked fish. 23,24

Grape pomace
Wines are known to have higher oxygen radical scavenging activity than the juices from the grapes from which the wines are produced.25 Wine also shows antioxidative capacity when tested in corn oil-in-water emulsions.26 There is, however, strong variation in the antioxidative capacity of wines, with red wines having the highest. By-products from wine production, such as grape seed extract, grape skin extract and grape pomace, have great potential as food additives and as dietary supplements.27 Grape pomace in particular seems to be promising as a new source of antioxidants.28

Waste from vegetable products
Evening primrose meal is an example of a by-product with a great potential as a source of natural antioxidants as it has proved effective in meat systems.2 Other waste products like hulls from a variety of grains have also been considered. The large production in China of peeled rhizomes of edible lotus gives large quantities of waste, and the extract of the rhizome knots especially has a high content of polyphenols, giving it great potential as a food additive.29

Plant phenols
Enrichment of processed food with polyphenols protects against oxidation and has better keeping quality because formation of toxic oxidation products, like cholesterol oxides, is being prevented.10 Such enrichment also benefits human health. Both of these benefits, however, hinge on the availability of the phenolic substances.

Polyphenols have been classified into extractable and non-extractable types
Specification and quantification of phenols in plant food and plant extracts depend on development of specific analysis. (See sidebar) The availability in food for protective reactions rather depends on specific assays while the bioavailability depends on human studies. As for the bioavailability of a specific flavonoid, urinary excretion of apigenin was found to be a useful biomarker for absorption of flavonoids from parsley. 30 Catechins from green tea extracts were likewise excreted into urine with a half-life of less than two hours, and only short-term effects on plasma antioxidant capacity were seen. 31

The effect of a 10-week period without fruits and vegetables prior to the intake of meat patties with green tea extracts, however, resulted in a decrease in oxidative damage to DNA, blood proteins and lipids — underlining the general lack of knowledge of the mechanisms behind the beneficial effects of fruits and vegetables.31 While many studies of the content of phenolics in wine have appeared, few data exist on their absorption and metabolism.32 Absorption of catechin and procyanidins from chocolate have been shown to increase serum antioxidative capacity.33

Glycosides vs aglycols
Flavonoids and anthocyanidins occur in plants mainly as glycosides, but seem to be absorbed in the intestines following hydrolysis to the aglycones.34 Individual differences seem, however, to have been detected for absorption of the glycosides.35 Glycosilation of flavonoids at the 3-hydroxy group normally decreases the antioxidative activity due to the reduction of the number of phenolic groups as seen for quercetin/rutin.36 The effect of glycosilation of the 7-hydroxy group seems not to have been investigated. For the anthocyanins and anthocyanidins, high pH as in the intestine will transform the flavylium cation as present under acidic conditions into carbinol pseudo-bases and quinoidal bases, which appear to be the forms being absorbed from the gut into the blood, with an even higher antioxidative capacity.37 Such effects should also be considered when using wines or wine by-products.

Effect of polymerisation
Polyphenols are ubiquitous in all plant organs in either monomer or polymerised forms.38 In addition to the beneficial effect of polyphenols, they also bind minerals and precipitate proteins and carbohydrates, reducing the nutritive value of foods.

Polyphenols have been classified for nutritional purposes into extractable and non-extractable types.39 Extractable polyphenols are low- and intermediate-weight phenolics while non-extractable polyphenols have high molecular weights and are insoluble in normal solvents.

Polymerisation of polyphenols is seen in a variety of beverages. In beer, the transformation of low molecular weight phenols into insoluble polymers is seen as haze.40 Such polymerisation, also observed in alcoholic bitters based on various herbs, is induced by oxidation of soluble phenols and will lower the antioxidative capacity.41 For bitters, the formation of precipitate also decreases the astringency of the product. Enzymatic browning of plant products will lower the phenol content and the antioxidative capacity, notably in green and black teas.

Future trends
Plants modified to have higher content of polyphenols for use by the food industry or for production of dietary supplements will become available together with polyphenol mixtures produced by cell cultures. However, in order to benefit from such production of optimised mixtures for food use, a better understanding of antioxidant synergism will be needed.

In the chart below, antioxidants are classified into four groups and, while the synergistic interaction between the tocopherols and ascorbate is well understood for food systems,42 the interaction between the polyphenols and the carotenoids especially is not.

Antioxidant hierarchies, as known for flavonoids, have also been established for carotenoids with inclusion of the tocopherols.43 In order to combine the hierarchies of the flavonoids and the carotenoids, the thermodynamics and kinetics of their interaction need to be investigated.

The synergism between polyphenols and alpha-tocopherols, now being confirmed in some food systems, can be described as an effect of regeneration of the more-active by the less-active antioxidant. The synergism between ascorbate and the tocopherols rather depends on the phase distribution of the two types of antioxidants.

When a better understanding of the mechanism behind antioxidant synergism is available, protective systems based on such an understanding will probably be developed. The use of green tea extract in meat systems may present a breakthrough in exploitation of synergism between nutrient and non-nutrient antioxidants.8,44

The use of plant material and plant extracts as food ingredients will go beyond antioxidative effects. The ?green revolution? for the food ingredients industry is to modify plants to produce molecules that combine emulsifying and/or thickening effects with antioxidative and antimicrobial effects.

Excerpted from Phytochemical Functional Foods, Johnson I, Williamson G, editors. ISBN 1 85573 672 1. Published by Woodhead Publishing Ltd, Cambridge, England. www.woodheadpublishing.com
Respond: [email protected]

Classification of antioxidants according to solubility and nutritive value









Analysis of phenols in foods
Knowledge of the identity of phenolic compounds in food facilitates the analysis and discussion of potential antioxidant effects. Thus, studies of phenolic compounds should usually be accompanied by the identification and quantification of the phenols.

Reversed-phase HPLC combined with UV-VIS, or electrochemical detection, is the most common method for quantification of individual flavonoids and phenolic acids in foods,1,2 whereas HPLC combined with mass spectrometry has been used for identification of phenolic compounds.3 Normal-phase HPLC combined with mass spectrometry has been used to identify monomeric and dimeric proanthocyanidins.4 Flavonoids are usually quantified as aglycones by HPLC, and samples containing flavonoid glycosides are therefore hydrolysed before analysis.5

The use of HPLC for quantification of phenols is often limited to a single class of phenolics and then often only to low molecular weight compounds that are available as standards. It is, therefore, often necessary to use colorimetric assays such as the Folin-Ciocalteau assay, which rely on the reducing ability of phenols to quantify the amount of total phenolics in a sample.6,7,8 The degree of condensation of polyphenols can be quantified by colorimetric assays such as the acid-butanol assay and the vanillin assay.6,8

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