Understanding antioxidant mechanisms in preventing oxidation in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 236

to interact with reducing agents or by binding them in a manner that reduces
their redox potential thus making redox cycling easier.
Chelators which exhibit antioxidative properties inhibit metal catalyzed
reactions by one or more of the following properties: prevention of metal redox
cycling; occupation of all metal coordination sites; formation of insoluble metal
complexes; and steric hindrance between metals and lipids or oxidation inter-
mediates (e.g., peroxides) (Graf, 1990). The prooxidative/antioxidative
properties can be dependent on both metal and chelator concentrations. For
instance, ethylenediaminetetraacetic acid (EDTA) is prooxidative when
EDTA : iron ratios are < 1 and antioxidative when EDTA : iron is > 1 (Mahoney,
1986). This is because at low EDTA:iron ratios, iron is still redox active.
Chelators are typically water soluble but some will exhibit solubility in lipids
(e.g., citric acid) thus allowing for inactivation of metals in the lipid phase
(Lindsay, 1996). Chelator activity is dependent on pH since the chelator must be
ionized to be active. Chelator activity can also be decreased by the presence of
other chelatable ions (e.g., calcium) which will compete with prooxidative
metals for binding sites.
10.3.1 EDTA
Ferdinand Munz was the first to describe EDTA in 1935. EDTA preferentially
binds ferric iron. In most food systems, EDTA is the most effective metal
chelator to inhibit lipid oxidation when the EDTA concentration is greater than
the concentration of prooxidant metals. The effectiveness EDTA is due to it high
iron stability constant (1:210 25 ) and the broad range of pK a s of EDTA (1.7,
2.6, 6.3, 10.6) insuring that it is charged and capable of binding iron at the pH
values of most foods. EDTA is generally used a sodium or calcium salt that have
high water solubility. EDTA is also attractive to the food industry due to its low
cost. Many food companies are looking for effective EDTA replacements due to
consumer concerns for synthetic food additives.
10.3.2 Organic acids
Organic acids such as citric and lactic acid can chelate prooxidative metals.
Citric acid is commonly used in foods especially in refined oils where it is added
after the deodorization step. Citric acid is not as effective as EDTA due to its
lower iron stability constant (1:510 11 ) and higher pK a s (3.1, 4.7 and 5.4).
10.3.3 Phosphates
Phosphates can also chelate metals with polyphosphates being stronger chelators
and antioxidants than mono- and diphosphates (Sofos, 1986). Sodium
tripolyphosphate, a commonly used food ingredient, has a stability constant
for iron of 7:210 22 and pK a s of 0.8 and 2.0. However, in some foods
polyphosphates were not found to be effective antioxidants (Hu, 2004). One