Assessing the activity of natural food antioxidants - Oxidation in Foods and Beverages and Antioxidant Applications - page 338

reactivity of certain phenolics (catechins, gallic acid derivatives, gentistein,
chalcones) under certain experimental procedures or in the presence of specific
radicals, 49,53±55 the effect of conjugation or of the electronic phenomena induced
by characteristic groups in the side chain of certain hydroxycinnamates. 6,8 It
should be noted that most of the aforementioned published works have been
carried out in the gas phase, since such calculations are less computationally
expensive and, consequently, less time demanding. Yet, gas-phase BDE and IP
values can be valuable primary indices for the antioxidant potential of
compounds. 56 Consideration of solute±solvent interaction is of importance as
reactions take place in solution and may provide better information than those in
the gas phase. 50 A clear limitation at present is extrapolation of such results to
predict activity in heterophasic systems. Nonetheless, the theoretical approach
that can be tailored to the needs and background of experimental researchers can
add to the battery of methods in the field of antioxidants and can be introduced
to SAR studies before conducting experimental work.

14.5 Assays using radicals of biological relevance
Numerous procedures have been developed in order to assess the radical
scavenging activity of individual compounds towards various radical species
(e.g., ROO · , O 2 ·ÿ , NO · ,

·

OH). Characteristic protocols are subsequently

presented.

14.5.1 Peroxyl radicals (ROO · )
Peroxyl radicals (ROO · ) are the most common reactive oxygen species (ROS)
formed in foods containing lipids resulting in nutritional loss and quality
deterioration. 57 In the protocols developed so far, peroxyl radicals are produced
via thermal decomposition of thermolabile azo-compounds (R±N=N±R 0 ) that
provide a constant flux of hydrophilic or lipophilic ROO · . Methods with an
impact in recent literature are the crocin bleaching and the oxygen radical
absorbance capacity assays.

Crocin bleaching assay (CBA)
The protocol introduced by Bors and co-workers 58 and modified by Tubaro et
al. 59 assesses the ability of phenolic antioxidants to protect crocin, a naturally
occurring carotenoid derivative, from bleaching due to competitive reactions
with radicals. Bleaching is observed as a consequence of the attack of the
hydrophilic ROO · generated by 2,2 0 -azobis(2-aminopropane) dihydrochloride
(AAPH) radicals, which abstracts a hydrogen atom resulting in the disruption of
the crocin conjugated system. Scavengers compete with crocin for the reaction
with free radicals. Efficient competitors are consumed preferably so that crocin
bleaching takes place at a lower rate. The reaction is carried out at pH 7.0 at
40 ëC, and the progress is monitored by recording absorbance decrease (A or