The possible health benefits of anthocyanin pigments and polyphenolics
Anthocyanin pigments are responsible for the red, purple, and blue colors of many fruits, vegetables, cereal grains, and flowers. They have long been the subject of investigation by botanists and plant physiologists because of their roles as pollination attractants and phytoprotective agents. They have also been very useful in taxonomic studies. Even Linus Pauling was interested in related compounds called anthocyanidins—he discussed their chemical structure to illustrate the use of resonance in understanding structural problems in a 1939 paper. Food scientists and horticulturists continue to study these compounds because of their obvious importance to the color quality of fresh and processed fruits and vegetables. Our laboratory first started studying these compounds because of color degradation of strawberry preserves, fruit juice concentrates, and wine. Subsequent projects were designed to determine their utility as natural food colorants and to authenticate fruit juice.
Today, interest in anthocyanin pigments has intensified because of their possible health benefits as dietary antioxidants. Over 300 structurally distinct anthocyanins have been identified in nature. Anthocyanins are one class of flavonoid compounds, which are widely distributed plant polyphenols. Flavonols, flavan-3-ols, flavones, flavanones, and flavanonols are additional classes of flavonoids that differ in their oxidation state from the anthocyanins. Solutions of these compounds are colorless or pale yellow. Other phenolic compounds that comprise part of our diet include phenolic acids and their esters, such as chlorogenic acid and polymeric tannins. At least 5,000 naturally occurring polyphenolics have been identified, including over 2,000 flavonoids. The term polyphenolics is increasingly being used to describe phenolic-based compounds having similar solubility properties that are analyzed by high-performance liquid chromatography. The polyphenolic profile of fruit juices is likely to include flavonols, free and esterified phenolic acids, and procyanidins. Polyphenolics contribute to food and beverage color by serving as substrates for an enzyme to produce brown pigments. The procyanidins and condensed tannins provide astringency and bitterness in tea and wine.
There is considerable anecdotal and epidemiological evidence that dietary anthocyanin pigments and polyphenolics may have preventive and therapeutic roles in a number of human diseases. Through the much publicized “French paradox”, the public has become aware that certain populations of red-wine drinkers in France and Italy have much lower rates of coronary heart disease (CHD) than their North American and Northern European counterparts. It is widely accepted that red wine phenolics contribute at least partly to this beneficial effect. A number of studies have shown that mortality from CHD is inversely correlated with intake of flavonoids in the diet. Flavonoids may also help prevent strokes. While some flavonoids have been shown to inhibit tumor development, some experts have concluded that compounds other than flavonoids must be responsible for the anticancer effects of dietary fruits and vegetables.
The anthocyanin pigments of Bilberries (Vaccinium myrtillus) have long been used for improving visual acuity and treating circulatory disorders. There is experimental evidence that certain anthocyanins and flavonoids have anti-inflammatory properties, and there are reports that orally administered anthocyanins are beneficial for treating diabetes and ulcers and may have antiviral and antimicrobial activities. The chemical basis for these desirable properties of flavonoids is believed to be related to their antioxidant capacity—their ability to scavenge and trap free radicals that damage biomolecules. Some people believe that eventually we will have a recommended minimum daily requirement for these dietary antioxidants. Much remains to be learned, however, before that occurs. The antioxidant content of many foodstuffs is unknown, making accurate estimates for human consumption of flavonoids and the correlation with disease difficult. Our knowledge of the anthocyanin and polyphenolic composition of many fruits, vegetables and cereals is incomplete, and little is known about the effects of processing and cooking on these substances. It is also uncertain how much of the particular flavonoids are absorbed into the bloodstream and get to various cells.
A pilot project grant from the Linus Pauling Institute permitted us to investigate the antioxidant properties of dietary phenolics. Our lab supplied Dr. Vadim Ivanov of LPI with purified samples of sinapyl derivatives of glutathione that Ling Wen, Ph.D. candidate in Food Science, had identified in pineapple juice. These novel compounds have not been previously found in nature and are present as major compounds in pineapple juice. Their antioxidant capacity, as measured by photo-chemiluminescence methodology, was from 1-4 times greater than Trolox, which is a chemical analog of vitamin E. This initial work has led to several collaborative projects with LPI in which we are investigating the antioxidant capacities of various fruits and vegetables.
The Washington Apple Commission awarded us a grant to determine the polyphenolic composition and antioxidant activities of apples (Red Delicious, Granny Smith, and Fuji varieties), as well as the influence of post-harvest storage on these activities. Bob Durst, Senior Research Assistant, and Maria Widyasari, M.S. student, have supplied Deborah Hobbs of LPI with various apple extracts. Deborah has used two different assays to determine antioxidant activity: the fluorometric based Oxygen Radical Absorbing Capacity (ORAC) and the spectrophotometric Ferric Reducing Antioxidant Potential (FRAP) methods. The ORAC determination is regarded as a fairly direct means of measuring the ability to trap free radicals while the FRAP assay is easier to perform. Preliminary experiments have shown that antioxidant activity of apple flesh and peel extracts is predominantly in the water-soluble part and is attributable to polyphenolics. ORAC values are highest in the peel, with Red Delicious having the highest values, presumably because of the presence of anthocyanin pigments. While ORAC values are lower than those reported for blueberries and strawberries, apples compare favorably when portion size and per capita consumption are considered. The project will also include determination of the biologically relevant antioxidant activity of apple polyphenolics by measuring in vitro effects of individual phenolics on blood plasma antioxidant parameters and oxidative modification of low-density lipoprotein (LDL), also known as the “bad” cholesterol.
The Washington and Oregon Cherry Commissions currently support a project on cherry phytochemicals. Arusa Chaovanalikit, Ph.D. student, is determining the anthocyanin pigment and polyphenolic composition of four cherry cultivars and measuring their distribution in peel, flesh, and pits. This information will be useful for evaluating whether processing waste might be a source for nutraceuticals or antioxidant supplements. Cherries are reasonably high in ORAC activity, which appears to be correlated with anthocyanin pigment content. Cherry polyphenolics and anthocyanins undergo considerable degradation during processing. We have measured over 50% loss of anthocyanins in cherries during 6 months frozen storage at -10ºC. We intend to examine the effects of canning, freezing, and brining on polyphenolic composition and antioxidant activity.
Dr. Richard Moyer, visiting sabbatical professor from King College, Bristol, Tennessee, worked with Kim Hummer of the USDA Germplasm Repository and Chad Finn of the USDA Northwest Center for Small Fruit Research to investigate the levels of dietary antioxidants in various berries. They collected fruits from 107 cultivars and species of Vaccinium (blueberries), Rubus (blackberries, black raspberries), and Ribes (black currants, gooseberries) and determined total anthocyanins, total phenolics, and ORAC and FRAP values. All of these fruits are rich in dietary antioxidants. ORAC values (micromoles Trolox equivalents/gram) in blueberries ranged from 19 to 131, in blackberries and black raspberries from 13 to 146, and in black currants and gooseberries from 17 to 116. Many of the experimental selections were considerably higher in antioxidant levels than common commercial varieties. This illustrates the potential for plant breeders to develop cultivars that are much richer in dietary antioxidants. Antioxidant values correlated well with both total phenolic and total anthocyanin content. A high correlation between ORAC and FRAP for all samples suggests that both assays have validity for determining antioxidant activity.
Charles Brown, USDA Research Geneticist at Prosser, Washington, is working on improving the potato as a functional food. He supplied us with anthocyanin-containing purple and red-fleshed experimental cultivars along with some common potato varieties. We analyzed the anthocyanin pigment content and antioxidant levels and found that potatoes with pigmented flesh displayed antioxidant capacities 2-3 times greater than unpigmented varieties. This preliminary data has enabled us to get a USDA grant, “Developing the potato as a functional food: Breeding, compositional analysis and human nutrition studies”. We will be working collaboratively with Charles Brown, Principal Investigator, and Beverly Clevidence of the USDA Phytonutrient Laboratory in Beltsville, Maryland.
There is much to be learned about the bioavailability of these compounds, how they are metabolized, structure-activity relationships, and the mechanisms by which they may prevent disease. We also need to determine if some of these compounds might act as pro-oxidants at certain dosage levels, leading to harmful effects. It may be advantageous for us to consume a variety of antioxidants in food that have somewhat different oxidation potentials, solubilities, rates of absorption, and mechanisms of action, which mirrors the nutritionist’s recommendation that we eat a wide variety of fruits, vegetables, and cereal grains.
