Antioxidants, Oxygen Scavengers, Free Radicals, and Packaging

Published: April 01, 2002, By Dr. Richard M. Podhajny, Ph.D., Contributing Editor

Elemental oxygen exists in two allotropic forms: as an invisible gas composed of two oxygen atoms; and as a perceptibly blue form composed of three oxygen atoms that we call ozone. Unlike nitrogen or carbon dioxide, oxygen is paramagnetic. This paramagnetic property is explained as two electrons in the molecule that are not paired to each other.

These “free unpaired” electrons can interact with a variety of packaging materials and food products to create “unpaired electron fragments,” which we call “free radicals.” The usual path is through hydrogen abstraction and formation of peroxides, which can decompose to free radicals. The presence of peroxides further degrades packaging materials and food products.

Most of us are aware our aging process can be accelerated by oxygen, and doctors recommend we consume food rich in antioxidants, such as fruit, fresh produce, wines, and vitamins. Nature does a good job in controlling the degradation process by maintaining high levels of antioxidants.

A major role of food packaging is to retard the natural processes that lead to food spoilage by reducing oxygen and moisture. Antioxidants and free radical scavengers are used for this purpose.

Among the oxygen reduction advances in packaging have been the introduction of PVDC-coated films, incorporation of PVOH as an oxygen barrier layer, and the use of vacuum-deposited aluminum to reduce oxygen penetration to packaging products. Also, vacuum packaging and use of inert atmosphere significantly extend the shelf life of many food products.

In addition to reducing oxygen in our packaged products, chemicals are added to arrest oxygen or peroxides that may degrade the package or the package contents. In this role we find antioxidants as well as oxygen and free radical scavengers.

Antioxidants exist in many natural products. For instance, beta-carotene, vitamins C and E, and polyphenols are all potent antioxidants. Red beans, raisins, and blueberries have some of the highest levels of antioxidant activity. However, most antioxidants added to packaging materials are synthetic materials.

Antioxidants used in packaging materials are a diverse group of chemicals that combine with free radicals that would otherwise attack and oxidize packaging materials. Common antioxidant synthetic substances used in packaging are butylated hydroxytoluene [BHT] and butylated hydroxyanisole [BHA]. As a chemical class, these materials are phenolic chemicals that can react with peroxide radicals by hydrogen donation to form hydroperoxides and prevent the process from forming more reactive radicals.

Oxygen scavengers include inorganic materials that can absorb oxygen, as well as organic reactive materials that can consume oxygen through chemical reaction.

Free radical scavengers can include hydroquinones, thiols, hydroxylamines, plus many other materials that will trap free radicals by chemical reaction and prevent the normal degradation process.

It's ironic that, after packaging engineers have worked so hard to keep oxygen out of our food packages, we find oxygen can be beneficial in making some packaged products appear fresher and more desirable to the consumer. For example, oxygen makes red meat appear fresher. Other applications use oxygen to sterilize package contents.

As you can see, the story of oxygen and its role in packaging continues to change as new applications are developed.

Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314; This email address is being protected from spambots. You need JavaScript enabled to view it..