Assessing the activity of natural food antioxidants - Oxidation in Foods and Beverages and Antioxidant Applications - page 333

described later in this chapter, a variety of reaction environments can be
employed in certain assays, whereas in others there are limitations. Improper
solubilization of the antioxidant could result in a significant underestimation of
activity. It is therefore necessary to define limits for the applicability of existing
protocols. Such a descriptor is the partition coefficient (log P) values of
compounds to be tested. These values can be either calculated using software or
determined experimentally. Table 14.1 summarizes descriptors for the prioritiza-
tion of compounds with regard to hydro/lipophilicity presented in the last
decade.
Solvent effect is also critical to the ability of antioxidants to scavenge free
radicals. Formation of hydrogen bonds between the solvent and the substituents
of antioxidants may interfere with the reaction mechanism and consequently
affect activity. Other important factors to consider are the physical properties of
the compounds, such as color and volatility. The former may cause interferences
when measuring the efficiency by means of visible spectrometry, while the latter
affects temperature selection in accelerated methods of oxidation. The stability
of some test compounds (e.g., phenols) may also be a factor, as are possible
interactions with other components of the test system. Temperature affects the
rate of reaction, a significant factor bearing in mind that incubation at selected
temperatures is a feature of most methods. Analytical parameters including
monitoring period, result expression, selection of proper reference compounds,
as well as the efficiency of the protocols regarding the number of compounds
that can be tested within a working day with sufficient repeatability of the
measurements, are matters of concern for either the newcomer or the

experienced experimentalist.

14.3 Radical scavenging
As radical scavenging is considered to be the main mechanism of action of
phenolic compounds through hydrogen atom (HAT) or single electron donation
(SET) 24 (R14.1 and R14.2, respectively), assessment of the ability to scavenge
individual oxidizing species is the first goal.
AH ROO · ÿ! A · ROOH

(R14.1)

AH ROO · ÿ! ROO ÿ ±AH · ÿ! ROOH A · (R14.2)

Published review articles cover a series of methods that are classified on the
basis of mechanism of reaction of radical species with antioxidants, in terms of
substrate type (synthetic probe or lipid substrate) or in terms of commonly used
protocols as indicated in Table 14.2.
In the present chapter, the usefulness of theoretical methods is presented
along with experimental approaches. Emphasis is given to protocols assessing
scavenging of biologically relevant radical species.