Antioxidants: Scratching the Surface in Functional Foods

 Heartened by scientific evidence that healthy, nutritious food products can slow down the aging process and help ward off debilitating diseases, scores of consumers and mainstream magazines are singing praises of products that are rich in antioxidants.

Through their chemical composition, antioxidants retard oxidation by scavenging oxygen that starts a chain reaction of chemical reactions that produce dangerous and highly reactive free radicals.

Chain reactions

Elucidation of the mechanism of lipid oxidation is difficult to accomplish. The most widely held theories implicate the involvement of radiant energy, such as light, transition metals or hydroperoxide decomposition. It has been theorized that light may activate food’s natural pigments, such as myoglobin or chlorophyll, which in turn excite ground state, or triplet, oxygen to singlet oxygen. This excited state of oxygen has the ability to react directly with fatty acids to initiate oxidation.

The reactions that describe the stages of lipid oxidation are shown in Figure 1. In these mechanisms, R refers to the unsaturated fatty acid that exists in the free form or as part of a triglyceride, reaction (i) depicts the initiation of lipid oxidation, which involves the conversion of a fatty acid to a free-radical species (R·).

The propagation of lipid oxidation is depicted in reactions (ii) and (iii). The fatty-acid free radical generated during initiation is an unstable and reactive species, and may readily react with ground state (triplet) oxygen from the atmosphere.

This results in the formation of a peroxy radical (ROO·). The peroxyl radical may then abstract a hydrogen atom from another fatty acid, creating a hydroperoxide (ROOH) and another alkyl radical (R·). The species reacts with another molecule of oxygen, and so on.

The hydrogen atoms most susceptible to abstraction during propagation are those adjacent to double bonds. The more double bonds a fatty acid contains, the more rapidly it will oxidize. Hydroperoxides of fatty acids produced during propagation do not have an adverse effect upon the flavor or aroma of foods. However, they readily decompose to aldehydes and ketones that are responsible for the flavors and aromas that are characteristic of oxidative rancidity.

The process by which fats oxidize is characterized as a free-radical mechanism. It involves the generation of unstable free radicals that catalyze the production of more free radicals. Thus, it is a chain reaction that can spread to all susceptible fatty acids. In order for the chain reaction to begin, the first free radical must be produced in a process called initiation.

Essential element

Although oxygen is vital to life, this essential element contributes to human aging and illness. As the body ages, it is less capable of fending off the constant onslaught of oxidative stress. Free radicals invade cells, disrupting the structure of biological molecules, and cause cellular damage in the form of oxidative lesions. Cellular processes can also produce free radicals, adding to the oxidative stress bodies must handle.

The accumulation of oxidative lesions in the body is linked to Parkinson’s disease, certain cancers, late-onset diabetes, obesity, Alzheimer’s disease, rheumatoid arthritis, cardiovascular disease (CVD), and other degenerative illnesses. According to government estimates, coronary heart disease is responsible for 500,000 deaths annually in the United States.

Researchers theorize antioxidants protect key cell components by neutralizing free radicals before they can attack DNA or cause lipids to oxidize. By reducing free-radical attack, antioxidants may prevent chronic diseases and boost immune function. Epidemiological evidence suggests dietderived antioxidants, such as vitamins A, C and E, may be important in maintaining good health.

Among the lesser-known antioxidants, green tea is popular among health-minded consumers. Several industry and academic studies have found it is more effective than both vitamin C and E in protecting cells against cellular damage that is believed to be linked to cancer, heart disease and other debilitating illnesses. As reported in 1998 at the 38th Annual Meeting of the American Society for Cell Biology in San Francisco, researchers from Purdue University, West Lafayette, IN, discovered green tea inhibits an enzyme required for cancer cell growth and can kill cultured cancer cells with no ill effect on healthy cells.

Sometimes lost amid the mounting public chorus and media buzz about health is the important role these diverse chemicals play in food preservation. In the food industry, antioxidants are used in a variety of foods—particularly those that contain fats and oils—to prevent deterioration, rancidity or discoloration caused by oxidation. The oxidation of fats and oils by molecular oxygen is one of the major causes of food-spoilage losses worldwide.

By and large, antioxidants for food protection are more effective when used in combination. This phenomenon is referred to as “synergism.” A good example is the combined antioxidative effect of ascorbic acid and BHA. Ascorbate has the ability to chelate metals and limits their ability to initiate lipid oxidation. BHA, a phenol and the true antioxidant of the two, is much more effective in the presence of ascorbic acid. Many other synergistic mechanisms have been postulated for a number of antioxidant combinations.

Compounds of interest

Literally, thousands of compounds, both natural and synthetic, are classified as antioxidants. Antioxidants are highly abundant in fruits and vegetables (ascorbate or vitamin C, carotenoids and polyphenols), nuts and grains (tocopherols and tocotrienols collectively called vitamin E), meat and dairy (vitamin A), and seafood (selenium). Beta carotene (red, yellow-orange and leafy green vegetables and fruits), lutein (green leafy vegetables), and lycopene (tomatoes) are other well-known antioxidants.

Lesser known antioxidants include epigallocatechin gallate, or EGCG, (green tea), chlorogenic acid (coffee), sulphoraphane (cruciferous vegetables, kale) caffeic acid (apples, pears, citrus fruits, some vegetables, coffee), indoles (cruciferous vegetables such as broccoli, cabbage, cauliflower), and ferulic acid (brown rice, apples, artichokes).

In recent years, food scientists, product developers and health-minded consumers have also demonstrated a growing interest in antioxidants found in aqueous extracts from plant materials, hydrolyzed proteins, amino acids, spices and herbs.

Eat your fruits and veggies

For decades, U.S. health organizations have recognized the beneficial roles fruits and vegetables play in the reduced risk of disease. Accordingly, these groups have sponsored educational initiatives to encourage consumers to eat more antioxidant-rich fruits and vegetables.

Throughout the 1990s, the National Cancer Institute, Bethesda, MD, partnered with the Produce for Better Health Foundation, Wilmington, DE, a nonprofit group, to take its “5 A Day for Better Health” message to grocery stores, classrooms, media outlets, businesses and churches.

Acknowledging the scientific consensus about the health benefits of fruits and vegetables, most notably those that contain vitamins A and C, the FDA released a 2002 health claim for fruits and vegetables and their potential effect on cancer development. The statement, “Diets low in fat and high in fruits and vegetables may reduce the risk of some cancers,” was allowed on food packages that met agency criteria.

In 2005, the Dietary Guidelines Advisory Committee, a 13-member panel formed by the Department of Health and Human Services and USDA, urged consumers to increase their daily intake of fruits and vegetables, whole grains, and nonfat or low-fat dairy products. Within the committee’s recommendations, Americans are advised to consume five to nine servings of fruit and vegetables every day.

Scratching the surface

Functional foods, dietary components that may provide a health benefit beyond basic nutrition, is one of the industry’s fastest-growing sectors. In 2006, functional food and beverage sales reached $25 billion in the United States. According to the Feb. 21, 2007 issue of Marketing Daily, this figure is expected to top $38 billion by 2011.

While new functional products from venerable and upstart companies alike are reaching the marketplace on a daily basis, food technologists and developers agree only the surface has been scratched in this vibrant market. Increasingly, companies are assessing the viability of fortifying foods and beverages with a range of promising ingredients, including soy-based proteins, probiotics and prebiotics, and omega-3 fatty acids.

As functional ingredients, antioxidants hold much appeal for food companies that are seeking to expand product lines and win over consumers. Prior to initiating a formulation change, however, companies must first determine that an antioxidant is GRAS; demonstrates good carry-through properties during food processing; is effective at low concentrations; and does not contribute to any physical or organoleptic changes, such as color, taste, odor and texture.

Recently published literature has substantiated that all antioxidants are not created the same, and to achieve optimal taste, shelf life and bioavailability, processors and manufacturers must take the necessary steps to assure product safety and quality for discerning consumers. These measures may include sensory evaluations, lipidoxidation tests, shelf-life evaluations, process validation studies and other scientific tools.

Moving forward

Through clinical trials, researchers are continuing to learn more about the mechanism of different antioxidants. Also, in a new area of study, scientists are investigating how antioxidants and other common dietary nutrients can affect health through gene-nutrient interactions.

From a regulatory perspective, the FDA is attempting to accommodate functional foods within existing agency regulations in ongoing hearings. It is imperative for functional-food manufacturers and companies that are weighing the possibility of supplementing their products with functional ingredients to remain abreast of these developments.

Pam Coleman is division vice president of chemistry services, Silliker, Inc., Homewood, IL, and has an extensive background in food analytical chemistry, research and development, and quality assurance. John Williams, Jr., is a Silliker senior communications specialist and has written extensively on a wide range of food safety subjects. They can be reached at info@silliker.com.