Food Lipids: Chemistry, Nutrition, and Biotechnology

of iron from the heme pocket by cooking [129,135] and release of iron from the iron
storage protein, ferritin, by the reducing agents cysteine and ascorbate [136]. These
increases are significant because concentrations of iron (2.2 �M) and copper (1.4
�M) that were found in the low molecular weight fractions of fish muscle were
shown to catalyze lipid oxidation in fish muscle sarcoplasmic reticulum model systems
[137]. In addition, modifications in dietary iron level altered the development
of lipid oxidation in turkey dark muscle and in pork muscles [138–140].
Three general approaches have been taken to decipher the contribution of heme
and nonheme iron to lipid oxidation of muscle foods:
1. Evaluation of the levels of heme and nonheme in the muscle food and
relation of these values to the muscle’s oxidative stability during storage
[54,141];
2. Evaluation of the improvement in oxidative stability upon addition of inhibitor/chelator
that will cancel out the contribution of one of the components
[142–145]; or
3. Evaluation of the level of oxidation induced upon addition of one of the
iron sources to muscle or muscle model systems [142–148].
From these studies, the following conclusions may be drawn:
• In raw red meat and dark muscle fish, heme iron is the major catalyst.
• In both raw and cooked white flesh fish, nonheme iron is the major catalyst.
However, conflicting results make it difficult to draw a conclusion on the role of
heme and nonheme iron in cooked red meat samples. Some factors that may modify
the response are as follows:
1. Concentration of catalysts used in the study.
2. Distribution of catalysts in muscle system. Johns et al. [146] suggested that
conflicting results from model system studies were due in part to the difficulty
of evenly dispersing the catalysts in the system.
3. Concentration of reducing substances in system. Nonheme iron is more
active in the reduced state, whereas heme iron is more active in the oxidized
state [149].
4. pH of system. Heme catalysis is influenced less by increasing pH than
nonheme iron [150]. Therefore, contribution of nonheme iron would increase
with decreasing pH.
5. Amount of heat applied to the cooked system.
6. Presence of H 2O 2 in system.
7. Presence of chelators. Endogenous chelators, such as citrate, phosphate,
and nucleotides, modify the reactivity of low molecular weight iron by
modifying its redox potential to different extents [151].
8. Presence of one or more lipid classes. Oxidative response of chicken muscle
model systems differed depending on the class of lipid(s) present [152].
2. Singlet Oxygen Generation Systems
While many different mechanisms have been proposed for the production of singlet
oxygen, it is believed that the two most common mechanisms involve nonenzymatic
photosensitization of natural pigments [153] or direct enzymatic production of singlet
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.