Assessing the activity of natural food antioxidants - Oxidation in Foods and Beverages and Antioxidant Applications - page 351

Fig. 14.2 Chelating effect of caffeic acid (100M) in 10mM phosphate buffer saline
(pH 7.4) in the absence/presence of CuCl 2 (100M) or Cu(II) and EDTA, EDTA (1 mg/
mL final concentration) (abstracted from Fig. 5 of ref 63).
for the respective compounds. The authors noted that interferences due to buffer
constituents (e.g., phosphates) were possible, however. Furthermore, a ferrozine
based colorimetric procedure has been reported for the same reason. Results
were expressed either as percent chelated iron or in EDTA equivalents. 150
14.8.2 Studies using oxidative enzymes (lipoxygenases)
Lipoxygenases are oxidative enzymes bearing ferrous ions in their active sites,
and can selectively oxidize fatty acids containing cis,cis-1,4-pentadiene moieties,
resulting in off-flavors and off-aromas. For this reason, testing the ability of
antioxidants to inhibit enzymic activity is gaining interest. To date, SAR studies
have been carried out for a number of flavonoids. 151 Assessment of flavonoid
capacity to inhibit lipoxygenase activity is achieved by addition of the test
compound and enzyme to a linolenic acid solution and recording accumulation of
conjugated dienes at 234 nm.
14.9 Assessment of mixtures of compounds
Antioxidant activity can be assessed for mixtures of antioxidants in model
systems or in natural products. Artificial mixtures of individual compounds can
be tested for synergism or antagonism. Such studies can be monitored using
many of the previously mentioned procedures. The molar ratio among tested
compounds varies with regard to the aim of the study, or is carried out at levels
expected in natural products. In such studies, the number of the tested com-
pounds is often limited (e.g., 2±5 compounds). However, natural products,