Produce Degradation Pathways and Prevention

316 Produce Degradation: Reaction Pathways and their Prevention

blanching process leads to a swelling of the cell walls and individual cells begin to
separate. LTB could, however, improve the firmness of the produce. The effects of
pectin methyl esterase, which can be activated at low temperatures and inactivated
at higher temperatures, cause the increase in firmness. Low-temperature heating
activates demethylation of pectin, which is then cross-linked by calcium either
naturally present in the tissues or added^ (Quintero-Ramos et al., 2002). When the
blanching process is a pretreatment followed by other enzyme deactivating processes
such as pasteurization, total inactivation of enzymes is not required during blanching.
The enzymes present in fresh produce have different resistance levels to the heating
(Table 10.7). Peroxidaxe enzymes are often the most thermoresistant enzymes, hence
their use as a blanching efficiency indicator. Recent work has shown that a significant
but not well-defined proportion of active peroxidase can be left in some vegetables
without negatively affecting their shelf life (Selman, 2000). For various plant prod-
ucts it has been also concluded that the inactivation of lipoxygenase is most important
for the prevention of flavor changes during the subsequent storage of the products
(Williams et al., 1986).
The course of heat inactivation of enzymes depends on a number of conditions.
Osmotic pressure and the presence of substances reacting with protein, such as
sulfites and o-quinones, are among the factors to be considered. pH value is also
very important. Enzymes are most thermoresistant near their isoelectric point. For
example, PPO enzymes are generally most stable at about pH 6.0, and their ther-
moresistance decreases with changes of pH in both directions (Kyzlink, 1990). The
blanching process may cause losses of nutrients. These are mainly lost in exudates
leaking from the treated products. Heating also accelerates degradation changes of
labile compounds such as ascorbic acid. The total losses of nutrients depend on the

TABLE 10.7

Temperature Stability of Some Enzymes

in Plant Materials

Enzyme Plant

Z Value

(°C)

F Value

(min at 82°C)

Ascorbate oxidase Peach

Vegetables

59 2

Catalase Vegetables 28 6

Chlorophylase Spinach 22 2

Lipoxygenase Peas 16 <0.1

Pectin esterase Citrus juice 1.4 43

Peroxidase Peas 48 60

Phosphatase Orange juice 9 —

Polygalactorunase Citrus juice

Papaya

16

11

12

23

Polyphenol oxidase Fruits 12 1.1

Source:Adapted from Williams, D.C. et al., Food Technol., 40, 1986,

130, 1986.