Abstract SCHIRACK, ANDRIANA VAIS. The Effect of Microwave ...

polyunsaturated acids that have a cis-cis 1, 4 pentadiene structure such as linoleic
and linolenic acids (Ory et al., 1992). Lipoxygenases activate oxygen to produce
hydroperoxides at the allylic carbon in polyunsaturated fatty acids, and conjugated
dienes can be subsequently made by rearrangement. Hydroperoxides subsequently
break down into alcohols, alkanes, ketones and aldehydes which can be the source
of off-flavors in the peanut. Oxidation by enzymes such as lipoxygenase is likely at
locations of cell membrane disruption, as reactants previously separated become
mixed and available for reaction (Ory et al., 1992).
When these enzymes are inactivated by high temperatures during roasting,
autoxidation becomes the principle source of lipid breakdown (Lee et al., 2002).
Although all enzymes are denatured during roasting, some enzymes such as
peroxidase, which contains iron, and polyphenoloxidase, which contains copper, can
become pro-oxidants after denaturation (Ory et al., 1992). Transition metals such as
iron and copper can promote lipid oxidation in peanuts by abstracting hydrogen from
unsaturated fatty acid to make a radical, or by indirectly generating reactive oxygen
species (Sanders et al., 1995).
Lipid oxidation can be identified using chromatographic analysis of the
samples. Regular lipid oxidation in raw or roasted peanuts is indicated by hexanal
and/or hexanol in high concentrations, and when present at levels greater than 2
ppm, these can be detected by taste panelists (Ory et al., 1992). The
monohydroperoxides that are formed from linoleate oxidation are precursors for
volatile decomposition products such as nonanal, octanal, decanal, and hexanal,
and the most predominant of these is hexanal (Min et al., 1989). Both the quality
49