There are several different methods of measuring total antioxidant capacity (TAC), and the list is still growing. Until there is international agreement on a definitive method(s) to be applied to this area of research, it is difficult to present a simple and complete picture of the current knowledge of the quantity, overall efficacy (absorption) and potency of the natural antioxidants in the foods we eat. For a full discussion of the analytical enigma,
go to >>>> "Determination of activity of antioxidants in human subjects" By G.G. Duthie
Here, the method of X. Wu et al. J. Food Comp. Anal.
go to >>>> Rich food sources
At this site, data derived from other methods will be collated using conversion factors to build as large a collection as possible from which comparisons can be made.
The state of the art review (Ref 1) discussed the main approaches used in assays developed to measure antioxidant capacities. The reviewers recognised three main classes of assay:-
1. Hydrogen atom transfer reactions: employed by the tocopherols and polyphenols.
a) inhibition of induced low-density lipoprotein autoxidation.
b) oxygen radical absorbance capacity (ORAC).
c) total radical trapping antioxidant parameter (TRAP).
d) crocin bleaching assays.
2. Electron transfer reactions: undertaken by ascorbate.
a) total phenols by Folin-Ciocalteu reagent (FCR).
b) Trolox equivalence antioxidant capacity (TEAC).
c) Ferric ion reducing antioxidant power (FRAP).
d) total antioxidant potential using Cu(II) complex (CUPRAC)
e) 2,2-di(4-tert-octylphenyl)-1-picrylhydrazyl (DPPH)
3. Scavenging capacity assays for:
a) singlet oxygen - (the mechanism used by carotenoids).
b) superoxide anion.
c) peroxynitrite.
d) hydroxl radical.
Source Reference 1 for sections 1-3 above
Reprinted from D. Huang, B. Ou and R.L. Prior, J. Agric. Food Chem., 53 (6), 1841-1856, 2005. With permission Copyright 2005, American Chemical Society
In addition, two valuable rapid survey methods are included to complete the armoury: -
Rapid survey methods
1. Using gas chromatography (GC)
The reaction between various Reactive Oxygen Species (ROS) and alpha-keto-gamma-methiolbutyric acid (KMBA) generates ethylene which can be separated from other constituents and quantitatively measured by gas chromatography (GC). The method was named Total oxyradical Scavenging Capacity (TOSC) assay by Winston and colleagues because it can be used for several ROS such as peroxy, hydroxyl and alkoxyl radicals. The speed and reproducibity of GC analyses makes this a useful survey method.
Reference
G.W. Winston et al., Free Radic. Biol. Med., 24(3) 480-493, 1998
[Permission to reproduce requested from Elsevier]
2. Using single cell gel electrophoresis (SCGE)
The Comet assay (1988) introduced the method of embedding a small number of cells e.g. lymphocytes, in a thin agarose gel on a microscope slide. DNA from lysed cells unwinds and the degree of migration in agarose gel is correlated with the amount of DNA damage. Its application to in vitro andin vivo antioxidants and antioxidant/prooxidant status in relation to oxidative stress has been reviewed recently.
Reference
E. Cemeli et al., Mut. Res./Revs. Mut. Res. 681,(1), 51-67, 2009
The current trend (Feb 2008) is towards optimisation of TAC assays. As with all food assays, the sampling procedure is difficult to standardise, and yet is a major factor in determining the reproducibility of the total analytical method. Improving the sample homogenisation and solvent extraction is being modified by successive generations of analysts. A recent example is the paper by Perez-Jimenez et al., Food Res. Int. Article in Press. However, success in improving the extraction efficiency makes previous results redundant.
Added to these problems, each new growing season introduces changes to the sample for analysis caused by climatic variations, fertilisation, pest infestation, etc.