Understanding antioxidant mechanisms in preventing oxidation in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 240

of -tocopherol regeneration by these antioxidants is low at 1:410 ÿ4 ,
4:310 ÿ4 , and 4:510 ÿ4 mole of -tocoperoxyl radical reduced per mole of
antioxidant, respectively (Pazos, 2007) compared to the regenerating efficiency
by ascorbyl palmitate at 0.93 mole of -tocoperoxyl radical reduced per mole of
antioxidant (Iglesias, 2009).
Since multi-component antioxidant systems can inhibit oxidation at many
different phases of oxidation the resulting antioxidant activity can be more
effective than when using a single antioxidant. This suggests that the most
effective antioxidants systems for foods would contain antioxidants with
different mechanisms of action and/or physical properties. Determining which
antioxidants would be most effective depends on factors such as type of
oxidation catalysts, physical state of lipid (bulk vs. emulsified), and factors
which influence the activity of the antioxidants themselves (e.g., pH,
temperature and ability to interact with other compounds in the foods).
Improved control of lipid oxidation in foods can also been seen by physical
structures that impact the location of the antioxidant. Bulk oil contains numerous
minor components that are amphiphilic, such as mono- and diacylglycerols,
phospholipids, sterols, free fatty acids, and polar products arising from lipid
oxidation, such as lipid hydroperoxides, aldehydes, ketones, and epoxides
(Chaiyasit, 2007a). These surface active components, individually or in the
combination, possess the ability to associate in to physical structures in the bulk
oils in the presence of the small quantities of water (~300 ppm) (Chaiyasit,
2007b). These structures are known as association colloids which includes
reverse micelles and lamellar structures. Reverse micelles are efficient nano-
reactors that allow increased interactions between lipid- and water-soluble
components and can greatly alter chemical reaction rates so it is possible that
association colloids are one of the sites of oxidation reaction in bulk oils. Koga
and Terao (Koga, 1995) observed that the presence of phospholipids enhanced
the antioxidant activity of -tocopherol in model bulk oil systems containing a
trace amount of water (1% v/v). In the presence of 2,2 0 -azobis(2-amidinopropyl)
dihydrochloride (AAPH), a water-soluble peroxyl radical generator, the
presence of phospholipid increased the degradation of -tocopherol more than
in the absence of phospholipids. Degradation of -tocopherol by the water-
soluble free radicals decreased as the phospholipid's hydrocarbon tail group size
was decreased, and thus the ability of the phospholipid to form association
colloids was lost. The investigators suggested that the presence of association
colloids produced by phospholipids increased the accessibility of -tocopherol
to the site where free radical concentrations were greatest. In addition, the
reduction potential of -tocopherol in polar environments have been suggested
to be lower, which could make tocopherol a more effcient free radical scavenger
(Laranjinha, 2001). Koga and Terao (Koga, 1994) also found that the antioxidant
activity of -tocopherol could be increased when it was conjugated to the polar
head group of phosphatidylcholine [1,2-diacyl-sn-glycero-3-phospho-2 0 -
(hydroxyethyl)-2 0 ,5 0 ,7 0 ,8 0 -tetramethyl-6 0 hydroxychroman]. This increase in
activity was again thought to be due to the increased partitioning of the reactive