Food Lipids: Chemistry, Nutrition, and Biotechnology

5. Hydrolysis of Lipids and Associated Effects on Lipid Oxidation
Disintegration of lysosomal membranes in muscle tissue may occur upon mincing
or storage of nonheated muscle foods. As a result, muscle lipids may be exposed to
lipolytic enzymes that are released from these organelles. In support of this statement
is a report that both lipase and phospholipase activities were found in frozen fish
[96]. While phospholipases have been shown to be heat-inactivated more quickly
than lipases [97], responses by these enzymes to frozen storage temperatures have
been variable. When oyster was stored at �35�C, the activity of lipase was suppressed
much more than that of phospholipase [47]. In contrast, in frozen cape hake
mince, phospholipids were hydrolyzed faster than the neutral lipids above �12�C,
whereas neutral lipids were hydrolyzed faster than phospholipids below �12�C [98].
As opposed to short chain free fatty acids in dairy products, long chain free
fatty acids released in muscle foods do not contribute directly to rancid aromas. In
general, further oxidation of these fatty acids is necessary to generate volatile products
that are associated with sensory deterioration of the product. This mode of
deterioration is distinct from the sensory deterioration described by Refsgaard et al.
[99] for salmon. In these samples, hydrolysis of neutral lipids generated free fatty
acids during frozen storage that contributed directly to an increased intensity of
trained oil taste, bitterness, and metal taste by panelists.
The source of the free fatty acids determines whether lipid hydrolysis has an
accelerating or inhibiting effect on subsequent rates of lipid oxidation. Free fatty
acids originating from triacylglycerols accelerate oxidation [100,101], whereas free
fatty acids hydrolyzed from phospholipids have been shown to inhibit oxidation
[58,101,102]. In the latter case, it was suggested that free fatty acids disrupted the
fatty acid alignment that facilitates free radical chain propagation in membranes
[103]. Alternatively, Borowitz and Montgomery [104] concluded that the response
to phospholipase may be dependent on the time of application of phospholipase.
They found that when peroxidation preceded phospholipase A2 (PLA2) activity, the
hydrolysis facilitated propagation of the peroxidative process. In contrast, if PLA2 was activated prior to initiation of oxidation, oxidation was inhibited. This sequence
would account for the acceleration of oxidation during frozen storage when fish had
been held prior to mincing [105].
While membrane lipid hydrolysis modifies the degree of lipid oxidation, the
extent of hydrolytic activity may depend on the extent of membrane lipid oxidation.
Oxidized fatty acids attached to phospholipids have been found to be more susceptible
to hydrolysis by PLA2 than fatty acids that were not oxidized [106]. Supporting
this linkage between oxidation and hydrolysis, Han and Liston [107] also found
increases in both activities when ferric iron was added to fish muscle.
E. Catalysts
Catalysts of lipid oxidation in muscle foods include both enzymic and nonenzymic
sources (Table 6), but by and large, the bulk of the research to date has focused on
the contribution of heme and nonheme iron to promotion of lipid oxidation.
1. Transition Metal Ions
Iron heme proteins, including myoglobin and hemoglobin, are abundant in muscle
tissue [108]. Relative concentrations depend on species and muscle type. Beef, lamb,
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