584 Produce Degradation: Reaction Pathways and their Prevention
al. [169] also observed that the activity of cellulase from Clostridium thermocellum
was inhibited by the end products of cellulolysis, suggesting sugar repression in
these enzymes.
All produce contains carbohydrates, which are a good source of pulp, syrups,
gums, juices, gels, fiber, polysaccharides, oligomers, and monomeric sugars as well
as adhesives. Each of these substances, either directly or in a chemically modified
form, is a source of compounds that have potential use in food or feed and nonfood
industrial applications. The use of cell wall-degrading enzymes as a processing aid
has become normal practice by the fruit and vegetable processing industry worldwide
[170–172]. Cereal beta-amylases with high activities in situ are particularly well
known to the brewing industry for releasing easily fermentable sugars from cereal
grain starches [173]. Two fungal pathogens (Phomopsis cucurbitae and Rhizopus
stolonifer) are also known to produce endo- and exo-polygalacturonases with the
ability to macerate netted muskmelon tissue at different stages of fruit development
[174].
In response to various postharvest stresses, cell wall-associated polysaccharide
polymers also undergo modifications in structure and composition, which changes
their functional and material properties, including changes in their rheolological,
thermal, and hydration properties. For example, fluctuations in ambient temperature
could lead to pectin degradation, whereas changes in pH, ionic strength, etc. can
affect cell walls during storage and the ripening process [175–179]. It has been
shown that in untopped radishes and carrots the loss of postharvest quality, partic-
ularly changes in textural properties such as visible wilting and/or yellowing of
FIGURE 19.16Scanning electron micrographs of a blueberry surface: fresh surface (a), aged
surface indicating microbial growth (b), aged surface with cracks (c), and damaged surface
(d). Bars = 50 μm.