Assessing the activity of natural food antioxidants - Oxidation in Foods and Beverages and Antioxidant Applications - page 348

emulsions include surface-active proteins, phospholipids, Spans and Tweens. 57
Liposomes are composed of phospholipids which, when dispersed in water, tend
to form closed spherical structures consisting of bilayers organized between
aqueous compartments and a monolayer next to the oil phase. 57
In such models, oxidation is typically carried out at ambient conditions
(T 60 ëC) and under constant agitation at a defined pH value. 57 Addition of
transition metal ions, 3,11,129 thermally labile initiators 130 or oxidative enzymes
(lipoxygenase) 131 combined with heating are also common practices to induce
lipid oxidation. The products of oxidation are monitored using the above-
mentioned assays but with some adaptations. The ferric thiocyanate assay 129 is
preferred when lipid quantities are small. Hydroperoxide accumulation can be
also monitored by measuring the formation of conjugated dienes in the lipid
phase. 132
The importance of the type of substrate and its oxidative status, droplet size,
charge, type and concentration of emulsifier, pH, on the stability of the model
during the course of the experiment has been thoroughly investigated by several
research groups and reviewed by Frankel, 57 and McClements and Decker. 133 It
has been observed that polar antioxidants are more effective in bulk oils,
whereas nonpolar antioxidants are more effective in lipid dispersions. This
finding was initially presented by Porter et al. 134 and is referred to as the
antioxidant polar paradox. Therefore, it is proposed that the effectiveness of a
given antioxidant depends on its physical location, specifically its ability to
partition to air/oil (bulk oils) or water/oil (dispersed systems) interfaces. In the
case of bulk oils, polar antioxidants may also aggregate at reverse micelles,
which result from the stabilization of small amounts of water by relatively
surface active contaminants (e.g., free fatty acids, mono- and diacylglycerols,
phospholipids). Theoretical and experimental procedures designed to assess
partitioning behavior are given in Table 14.1. In the case of liposomes, apart
from polarity, the size and conformation of the molecule affect antioxidant
activity. For this reason, fluorometric techniques that can measure the ability of
AHs to penetrate membranes have been reported. 135
14.7.3 Complex matrices
Complex food matrices are also used as a model to study antioxidant activity of
compounds with the goal of creating model system that more closely resemble
actual foods. Dispersed lipid systems, where AHs are tested in the presence of
other food components, as well as true food matrices, can be used to this end.
There are few studies that report interactions of proteins with AHs. Proteins
can significantly affect the antioxidant activity through, i.e., hydrogen bonding
and ionic interactions with phenols. 57 Studies carried out in the presence or
absence of bovine serum albumin (BSA) have shown that the protein may
increase the efficiency of some antioxidants in an o/w emulsion (epigallo-
catechin gallate, caffeic acid) or liposomes (ferulic acid, malvidin, rutin). 136,137
The improvement in activity has been attributed to the enhanced adsorption of