Chemistry Reference
In-Depth Information
with damage to fruit and vegetables, although it also occurs in seafood. Poly-
phenoloxidase is present in the plastids and chloroplasts of plants whereas
phenolic compounds are present in the cytoplasm. However, tissue damage
brings them into contact together with molecular oxygen. Diphenol oxidase
activity leads to losses of o-diphenols including chlorogenic acid and caffeic
acid. Many polyphenoloxidase enzymes also have monophenol oxidase activity,
and this leads to losses of antioxidants with a monophenol structure including
coumaric acid.
Some proteins such as -lactoglobulin have antioxidant properties. As well as
acting as antioxidant enzymes in some foods such as fruit and vegetables,
proteins may inhibit lipid oxidation by various mechanisms including scaveng-
ing free radicals, inactivation of reactive oxygen species, chelation of prooxidant
transition metals, reduction of hydroperoxides, and changes in the physical
properties of food systems, such as changes in the droplet size or charge of the
disperse phase in food emulsions (Elias et al., 2008; Kellerby et al., 2006).
Some proteins which are relatively weak antioxidants themselves are highly
effective in combination with polyphenols. Synergistic interactions between
albumin and various antioxidants including virgin olive oil phenolic compounds,
green tea catechins, and various water-soluble antioxidants have been reported
(Almajano and Gordon, 2004; Almajano et al., 2007; Bonoli-Carbognin et al.,
2008). The mechanism for this action has not been fully identified, although it
has been noted that reaction of the phenol with the protein occurs during storage
with an increase in the antioxidant capacity of the protein fraction. This is most
likely due to oxidization of the antioxidant to a quinone which subsequently
reacts with a free amine group of a lysine or arginine residue in the protein.
However, it is not clear that the product is the main cause of the synergistic
increase in oxidative stability.
The interactions between Fe(III) and the hydroxycinnamic acids caffeic,
chlorogenic, sinapic and ferulic acids and the flavonoid naringin have been
investigated in aqueous acid solution. The mechanisms for caffeic and chloro-
genic acid are generally consistent with the formation of a 1:1 complex that
subsequently decays through an electron transfer reaction (Hynes and
O'Coinceanainn, 2004). For gallic acid and methyl gallate at pH 1-3, a
protonated complex is in equilibrium with an unprotonated complex (Hynes and
O'Coinceanainn, 2001).
On reaction with iron(III), ferulic and sinapic acids undergo an electron
transfer without the prior formation of any complex, and this confirms that a
mono-hydroxyphenol structure is not sufficient for complex formation under
acid conditions (Hynes and O'Coinceanainn 2004).
13.5 Implications
The use of a wide range of processing techniques to produce foods with varying
textures and compositions necessitates the selection of antioxidants and mixtures
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