The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 300

believed to be partly responsible for a more general deterioration in flavor, color,
texture, and the loss of some nutritional qualities in raw and processed foods 75,76
through free radical initiated reactions. Indeed a direct relationship between perox-
idase activity and off-flavor development in peas was claimed in 1958 by Wagen-
knecht and Lee 77 and later by Pinsent. 78 Active peroxidase is believed to spoil fruits
and vegetables at temperatures as low as -18°C and at low moisture levels the
development of off-flavors is often associated with the oxidation of indigenous lipids
and the phenolic constituents of foods. 79 It has been suggested that a significant
negative correlation between peroxidase activity and the flavor scores of high- and
low-yield orange juices indicates that it might be possible to use peroxidase activity
as an index of potential adverse flavor. 80 For string beans, the qualitative peroxidase
test was found to be the best index of adequate blanching. 81 Wagenknecht and Lee 77
found that off-flavors developed when horseradish peroxidase was added to pea
slurries and Zoueil and Esselen 82 also showed that the addition of horseradish
peroxidase to sterilized green bean puree caused an increase in the amount of
acetaldehyde in stored samples. Similarly, earlier work in 1936 by Arighi et al. 83
and later in1949 by Joslyn 84 established the accumulation of volatile aldehydes in
the raw and unblanched peas. In 1952 Joslyn and David 85 claimed that peas con-
taining more than 10 ppm of volatile aldehydes were objectionable.
Thermostability
High correlations between off-flavor development in frozen vegetables during stor-
age and the residual peroxidase activity 86-88 have shown peroxidase activity to be
useful as an index for the degree of blanching. Effective blanching of some vegeta-
bles is particularly difficult, not only because of the presence of thermostable
enzymes, but also because of either their large size, shape, or density. For instance,
for Brussels sprouts, due to their compact structure, inactivation of enzymes through-
out the sprout may be particularly difficult to achieve. 89 Similarly, whole
corn-on-the-cob is one of the most difficult vegetables to blanch effectively because
of its large size and the structural characteristics of its different parts. 90 Lee and
Hammes 91 determined significant correlations between residual peroxidase activity
in the outer cob and kernels blanched at 100°C and the off-flavor development
detected by a panel after 9 months storage at -18°C. The thermostability of perox-
idase activity has been investigated in other vegetables, e.g., turnips 92 and
cabbages 93-95 and in fruits, e.g., apples, 96,97 pears, 98 grapes, 99 and citrus products 100
and mango. 101 The flavor of mango is very delicate and is easily affected by thermal
processing and mango is marketed in the form of juice, pulp, or slices canned in
syrup. The thermal processes followed at present for these canned products are
mainly empirical. 102 Studies on the thermal inactivation of peroxidases have gener-
ally shown that plots of residual enzyme activity against heating time are non-linear
indicating deviation from first order kinetics. The most obvious explanation for non-
linearity is the presence in the whole foods and crude extracts of a mixture of
thermostable and heat-labile isoperoxidases. 103-109 However, while it seems very
likely that some of the deviation is probably accounted for by the presence of