Aromatic plants prevent the oxidation of dietary fats.
BUTTER AND OTHER FOODS CONTAINING FATS turn rancid on contact with air as a result of a chain reaction: the autoxidation of fatty acids. Arresting this degradation, which produces disagreeable tastes and odors and creates free radicals, has long been an elusive goal. Eliminating oxygen from foods and to protecting them from light is not enough. Antioxidant compounds are also needed to combat the precursors of autoxidation that are already present.
Some foods naturally contain antioxidant compounds that protect them from turning rancid, such as the tocopherols (vitamin E) found in virgin olive oil and the ascorbic acid (vitamin C) of lemon. To extend the shelf life of their products, food processing companies began using these natural compounds more than twenty years ago while seeking ways to synthesize substances with greater antioxidant action. However, consumers feared the unknown toxicity of synthesized products, causing research to be confined to natural compounds.
Understanding the autoxidation of fatty acids nonetheless is important for studies of such compounds. This reaction occurs when light breaks the –C–H bonds of a lipid by forming unstable –C• free radicals that react with the oxygen in the air to form other –COO– free radicals. These radicals then react with other –C–H bonds to create a new –C• radical that propagates oxidation.
Phenols, the antioxidants used by the food processing industry, are molecules containing a benzene ring composed of six carbon atoms at the apices of a hexagon, at least one of which is bound to an –OH hydroxyl group. The antioxidant activity of phenol acids and their esters depends on their structure and, more particularly, on the delocalization of the electrons in their aromatic core: Some of their electrons are shared by all the atoms of the benzene ring. When these compounds react with the free radicals formed by the autoxidation of fats, they are transformed into free radicals, but they remain stable because the nonmatching electrons are delocalized, which limits their reactivity. Thus the propagating reaction is blocked, with the result that foods are prevented from turning rancid.
Despite its well-defined purpose, research on antioxidants was a tedious business because the reaction needing to be avoided appears only slowly; fortunately, it takes time for food to go bad. Systematic investigation using traditional tests of rancidity, which lasted several days, was impracticable because it took much too long.
A new test devised in the 1990s by Hubert Richard and his colleagues at the École Nationale Supérieure des Industries Agricoles et Alimentaires in Massy resolved the problem by indicating the antioxidant power of a compound in a matter of only a few hours. They bubbled oxygen into a lipophilic solvent, dodecane, in which methyl linoleate (the fat to be tested) and the candidate antioxidant were dissolved at 110°C (230°F). Gas chromatography revealed that half of the methyl linoleate is oxidized in three hours. The antioxidant power of a compound is measured in terms of its ability to lengthen this half-life.
The new faster method was then used to elucidate the chemical characteristics of antioxidant compounds (in order to predict which ones would be the most effective) and to determine which aromatic plants are the best sources of these compounds. Comparison of several plant phenol acids with four antioxidants commonly used in the food processing industry—butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), 2-tertbutylhydroquinone (TBHQ), and propyl gallate—disclosed the strong antioxidant effect of many compounds. In particular it became apparent that the antioxidant activity of a given molecule seems to depends on how many –OH hydroxyl groups it has and on the degree of stabilization created by the delocalization of electrons. These findings made it easier to predict the antioxidant efficiency of different compounds and to determine in which natural substances the most potent antioxidants are present.
Tests devised at the Massy laboratory for analyzing various extracts from aromatic plants confirmed the long-suspected antioxidant activity of rosemary, sage, cloves, thyme, oregano, ginger, and capsicum, but not of nutmeg. The extracts from rosemary, sage, cloves, and ginger exhibited a degree of activity similar to that of alpha-tocopherol, roughly one-tenth that of BHA and of gamma-tocopherol. Although neither pepper, parsley, celery, Indian celery, nor basil was found to display antioxidant properties, contrary to what earlier studies suggested, benzoin and vanilla act in a way similar to that of alpha-tocopherol. These two plants contain vanillin, which first attracted interest as an antioxidant in 1989.
The most promising plants undoubtedly are rosemary, cloves, sage, ginger, and benzoin. Sage may be a particularly good source, for it has been shown to contain six powerful antioxidant compounds: carnasol, carnosic acid, and isorosmanol, all present in large quantities, as well as rosmadial, rosmanol, and epirosmanol.