Essential oils (EOs) are liquid mixtures of volatile compounds obtained from aromatic plants. Many EOs have antioxidant properties, and the use of EOs as natural antioxidants is a field of growing interest because some synthetic antioxidants such as BHA and BHT are now suspected to be potentially harmful to human health. Addition of EOs to edible products, either by direct mixing or in active packaging and edible coatings, may therefore represent a valid alternative to prevent autoxidation and prolong shelf life. The evaluation of the antioxidant performance of EOs is, however, a crucial issue, because many commonly used "tests" are inappropriate and give contradictory results that may mislead future research. The chemistry explaining EO antioxidant activity is discussed along with an analysis of the potential in food protection. Literature methods to assess EOs' antioxidant performance are critically reviewed.

The kinetic behavior of cinnamic acids, their methyl esters, and two catechols 1-10 (ArOH) in the reaction with DPPH(*) in methanol and ethanol is not compatible with a reaction mechanism that involves hydrogen atom abstraction from the hydroxyl group of 1-10 by DPPH(*). The rate of this reaction at 25 degrees C is, in fact, comparatively fast despite that the phenolic OH group of ArOH is hydrogen bonded to solvent molecules. The observed rate constants (k(1)) relative to DPPH(*) + ArOH are 3-5 times larger for the methyl esters than for the corresponding free acids and, for the latter, decrease as their concentration is increased according to the relation k(1) = B/[ArOH](0)(m), where k(1) is given in units of M(-1) s(-1), m is ca. 0.5, and B ranges from 0.02 (p-coumaric acid) to ca. 3.48 (caffeic acid) in methanol and from 0.04 (p-coumaric acid) to ca. 13 (sinapic acid) in ethanol. Apparently, the reaction mechanism of DPPH(*) + ArOH involves a fast electron-transfer process from the phenoxide anion of 1-10 to DPPH(*). Kinetic analysis of the reaction sequence for the free acids leads to an expression for the observed rate constant, k(1), proportional to [ArOH](0)(-1/2) in excellent agreement with the experimental behavior of these phenols. The experimental results are also interpreted in terms of the influence that adventitious acids or bases present in the solvent may have. These impurities dramatically influence the ionization equilibrium of phenols and cause a reduction or an enhancement, respectively, of the measured rate constants.

The current understanding of the antioxidant properties of phenols (in homogeneous solutions) is reviewed, with particular emphasis on the role of the solvent. Phenols (ArOH) are known to reduce the rates of oxidation of organic matter by transferring a H atom (from their OH groups) to the chain-carrying ROO* radicals, a mechanism that most likely involves a concerted transfer of the hydrogen as a proton and of one electron between the two oxygen atoms, O-H---O* (proton-coupled electron transfer mechanism). The antioxidant capabilities of phenols are strongly reduced by hydrogen-bond accepting solvents since the hydrogen-bonded molecules ArOH---S are virtually unreactive toward ROO* radicals. The magnitude of these kinetic solvent effects is determined by the solute acidity alpha(2)(H) of ArOH (range 0 to 1) and solvent basicity beta(2)(H) (range 0 to 1). Hydroxyl solvents (alcohols) have a double effect on ArOH. On the one hand, they act as hydrogen-bond accepting solvents and reduce the conventional rates of the ArOH + ROO* reaction. On the other hand, these solvents favour the ionization of ArOH into their phenoxide anions ArO(-), which may react with ROO* very rapidly by electron transfer (sequential proton loss electron transfer mechanism). The overall effect is therefore determined by the ionization degree of ArOH. Other aspects of the kinetics and thermodynamics of ArOH + ROO* are also discussed.

The 2,2-diphenyl-1-picrylhydrazyl (DPPH(•)) radical is approaching 100 years from its discovery in 1922 by Goldschmidt and Renn. This radical is colored and remarkably stable, two properties that have made it one of the most popular radicals in a wide range of studies. First, there is the evaluation of the antioxidant abilities of phenols and other natural compounds (A-H) through a "test" that-at a closer look-is utterly inappropriate. In fact, the test-derived EC50, that is, the concentration of A-H able to scavenge 50% of the initial DPPH(•), is not a kinetic parameter and hence its purported correlation with the antioxidant properties of chemicals is not justified. Kinetic measurements, such as the second-order rate constants for H-atom abstraction from A-H by DPPH(•), in apolar media, are the only useful parameters to predict the antioxidant ability of A-H. Other applications of DPPH(•) include kinetic and mechanistic studies, kinetic solvent effects, EPR spectroscopy, polymer chemistry, and many more. In this review these applications are evaluated in detail by showing the usefulness of some and the uselessness of others. The chemistry of DPPH(•) is also briefly reviewed.

The antioxidant activity of selected representatives of flavonoids, coumarins, and cinnamic acids was examined by measuring their protective action toward linoleic acid peroxidation in micelles of sodium dodecyl sulfate in buffer solution, pH 7.4. Results are expressed as relative antioxidant efficiency (RAE), defined as the ratio of the antioxidant efficiency (AE) of the tested compound to that of R-tocopherol. The best RAE values were observed for flavonoids, followed by coumarins and cinnamic acids. From the results, within each class of compounds a structure-activity relationship can be deduced.