Formulating a diversity of phenolic compounds with different solubilities, absorption properties and oxidation-reduction potentials is the best way to harness berries? benefits, says Ronald E Wrolstad, PhD
Our laboratory has been conducting research on the composition of fresh and processed fruits for more than 30 years. Much of our work has been directed at quality issues such as the colour quality of strawberry preserves and issues of fruit juice authenticity.1,2 Another topic has been the development of improved anthocyanin pigment-based natural colourants for food use.3 Today, however, our emphasis is on how composition relates to the possible health benefits of fruits and fruit products.
A widely accepted hypothesis is that the antioxidant properties of bioactive compounds in plants are responsible for these health benefits. The category of dietary antioxidants includes several different classes of compounds, many of which contain phenolic substituents.
The term ?polyphenolics? has gained widespread usage. Literally meaning ?many phenolic compounds,? it probably arose because analytical chemists found a multiplicity of phenolic peaks when analyzing fruit and vegetable extracts by high-performance liquid chromatography (HPLC). The polyphenolic HPLC profile of fruits is comprised mostly of flavonoids and free and esterified phenolic acids.
At least 5,000 naturally occurring phenolics have been identified in nature, including more than 2,000 flavonoids. Flavonoid classes include flavonols, flavones, flavanols, procyanidins and anthocyanin pigments. In excess of 600 anthocyanin pigments have been identified in nature. As secondary metabolites, anthocyanins are not essential to the plant and may be completely absent or found in very high concentrations. Anthocyanin pigments are particularly effective at scavenging free radicals and have high antioxidant properties.4
Recently we compared the antioxidant properties, total anthocyanin pigment and total phenolic content in a large sampling of blackberry, black raspberry, blueberry and black currant fruits (See table below).5 Antioxidant values—expressed by both Oxygen Radical Absorbing Capacity (ORAC) and Ferrid Reducing Antioxidant Power (FRAP)—correlated well with both total phenolic and total anthocyanin determinations. The anthocyanin content of highbush blueberries ranged from 73-430mg/100g. It is worth noting that the major commercial cultivars were on the low side of that range while experimental selections were much higher. Thus there is potential for developing varieties of fruits with much higher antioxidant properties through classical plant breeding.
Total phenolics tend to have a higher correlation with antioxidant properties than total anthocyanins. For anthocyanin-rich fruits such as blueberries and blackberries, anthocyanins will be the largest contributor to total phenolics. Different varieties of cherries vary greatly in pigment content, and antioxidant properties are correlated much higher with total phenolics than with total anthocyanins.6 Anthocyanins and polyphenolics are found in highest concentrations in the peel and skin.6,7
Carotenoid pigments are also dietary antioxidants that are present in fruits; however, for fruits such as apples, pears, cherries, blueberries and cranberries, carotenoids contribute less to the antioxidant properties than the anthocyanins and polyphenolics.5,6
Use of berries to treat various ailments is prevalent in the literature. This includes cranberries for urinary tract infections and bacterial anti-adhesion activities;9 bilberries for diabetic retinopathy;10 grape polyphenolics to inhibit platelet aggregation;11 and anthocyanin pigments with antimutagenicity, anti-inflammatory and anticarcinogenic activity.12
The free radical or oxidative stress theory of ageing states that oxygen-derived free radicals or oxidative stress is the underlying cause of ageing and age-related diseases such as cancer and cardiovascular disease.13 While considerable information on the in vitro antioxidant properties of fruits is being documented, less is known on how the dietary phenolics function in vivo. Intact anthocyanin glycosides have been detected in the plasma and urine of humans after consumption of berry extracts; however, the recovered anthocyanins represent less than 1 per cent of the dietary intake.14
Many believe that more attention needs to be directed to the action of gut microflora on anthocyanins and polyphenolics, identification of the metabolites, and determination of their bioactive properties and extent of absorption. Ingested polyphenols that are not absorbed or excreted in the bile can be extensively metabolised by the microflora in the colon to various aromatic acids.15 These can be absorbed through the colon and be further transformed by conjugation with glycine, glucuronic acid or sulfate groups. These microbial metabolites may help explain the health effects of polyphenols.
Variety is the spice of berries
Anthocyanins and polyphenolics are concentrated in the epidermal tissue of fruits where they can function as attractants for seed dispersal, and also be available to serve as antimicrobial agents against invading pathogens. Another important role is to provide protection against the harmful effects of UV-visible irradiation. It is to the plant?s advantage to have a wide range of polyphenolic compounds with different absorption maxima to effectively screen the entire UV-visible spectrum. Perhaps a similar corollary can be made with regard to consumption of dietary antioxidants. It may be advantageous to consume a diverse number of compounds with different solubilities, absorption properties and oxidation-reduction potentials. There is also evidence that polyphenolics may act synergistically.11,16
Perhaps too much attention is being given to the quest for commodities with the highest amounts of antioxidant activity when a diverse combination of dietary antioxidants from different sources will be the healthier alternative.
Ronald E Wrolstad, PhD, is professor at Oregon State University in the department of food science and technology.
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