Reaction of Protein with Intact Lipids
There are different views in the literature on the effect of intact
lipids (i.e., lipids that have not been subjected to partial or total
hydrolysis or oxidation) on fish proteins. On one hand, they
seem to protect proteins; on the other, they form lipoprotein complexes,
which affect protein properties (Shenouda 1980, Mackie
1993). Dyer and Dingle (1961) found that lean fish (fat content
less than 1%) showed a rapid decrease in protein (actomyosin)
extractability when compared with fatty fish species (3–10%
lipids). Therefore, they hypothesized that moderate levels of
lipids may reduce protein denaturation during frozen storage.
In contrast, Shenouda and Piggot (1974) observed a detrimental
effect of intact lipids on protein denaturation in their study of
a model system, which involved incubating lipid and protein
extracted from the same fish at 4◦C overnight. They showed that
when fish actin (G-form)was incubated with fish polar or neutral
lipids, high molecular weight protein aggregates formed. They
suggested that during freezing, lipid and protein components
form lipoprotein complexes, which change the textural quality
of muscle tissue.
Reaction of Proteins with Oxidized Lipids
During frozen storage, lipid oxidation products cause proteins to
become insoluble and harder (Takama 1974). When proteins are
exposed to peroxidized lipids, peroxidized lipid–protein complexes
will form through hydrophobic interactions or hydrogen
bonds (Narayan et al. 1964), thus causing conformational
changes in the protein. The unstable free radical intermediates
of lipid peroxidation remove hydrogen from protein, forming
a protein radical, which could initiate various reactions such
as cross-linking with other proteins or lipids and formation of
protein–protein and protein–lipid aggregates (Karel et al. 1975,
Schaich and Karel 1975, Gardner 1979). Roubal and Tappel
(1966) found that peroxidized protein cross-links into a range
of oligomers, which are associated with protein insolubility.
Careche and Tejada (1994) found that oleic and myristic acid
had a detrimental effect on the ATPase activity, protein solubility,
and viscosity of hake muscle during frozen storage.
Secondary products from lipid oxidation such as aldehydes
react chemically with the amino groups of proteins through
the formation of Shiff base adducts, which fluoresce (Leake
and Karel 1985, Kikugawa et al. 1989). Ang and Hultin (1989)