10.2 Antioxidants and Their Mechanisms Antioxidants can be classified as primary or secondary antiox
Antioxidants can be classified as primary or secondary antioxidants according to their antioxidant mechanisms. Multifunctional antioxidants are antioxidants that can exhibit both primary and secondary antioxidant properties.
The primary antioxidants, the so-called chain-breaking antioxidants, are able to react directly with free radicals by transforming them to more stable, nonradical products. Hence, primary antioxidants play an important role in lipid oxidation because they can react with the formed lipid radicals and convert them into nonradicals and thereby hinder further decomposition of the lipids (Decker, 2002).
Phenolic compounds with more than one hydroxyl group (OH) are efficient primary antioxidants due to their ability to donate H-atoms to free radicals, creating relatively unreactive phenoxyl radicals due to resonance stabilization. Synthetic phenolic compounds, like butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), are efficient chain-breaking antioxidants and widely used as food preservatives. Some naturally occurring phenolic compounds such as tocopherol, ascorbic acid, or caffeic acid are also used as chain-breaking antioxidants but are typically less efficient compared with the synthetic ones, but that again depends on the type of food product.
The secondary, or preventive, antioxidants work indirectly on limiting lipid oxidation. Several mechanisms including the chelation of transition metals, singlet-oxygen quenching (in photooxidation), and oxygen scavenging can be exhibited by these secondary antioxidants (Decker, 2002). Furthermore, some secondary antioxidants can work synergistically by regenerating primary antioxidants and thereby restore the antioxidant activity of primary antioxidants to ensure their continuous antioxidant activity. Ascorbic acid is an example hereof. Metal chelating ability of a secondary antioxidant is an important property for antioxidants in food systems because metal-induced lipid oxidation is pronounced in food products due to the presence of, e.g., iron. EDTA (ethylenediaminetetraacetic acid) is an example of an excellent metal chelating antioxidant used in the food industry (Haahr and Jacobsen, 2008).
The synthetic antioxidants, such as EDTA and BHT, are typically cheaper and can be easier to process than natural antioxidants. However, restrictions in the use of synthetic antioxidants have been enforced because of their health risks and toxicity (Branen, 1975; Linderschmidt et al., 1986). Hence there is a significant interest in and demand for replacing synthetic antioxidants with natural plant-based alternatives, not only due to safety issues but also due to increased consumer awareness and interest in natural products and the possible health benefits of natural antioxidants (Halliwell, 1996).