surfaces may release nutrients for microbial growth. This stress may also
allow growth of the otherwise quiescent endophytic flora.
The most frequently observed form of spoilage is a softening of the
tissue due to the pectinolytic activity of micro-organisms. Pectin, the
methyl ester of a-1,4-poly-D-galacturonic acid, and other pectic sub-
stances are major components of the middle lamella between the cells
making up plant tissue and once it is broken down the tissue loses its
integrity and individual plant cells are more easily invaded and killed.
Pectic substances may be quite complex and include unesterified pectic
acid as well as having side chains ofL-rhamnose,L-arabinose,D-galac-
tose,D-glucose andD-xylose. Several distinct enzymes are involved in the
degradation of pectin and their role is illustrated in Figure 5.10.
As described in the case of fruits, the prevention of spoilage during
storage and transport of vegetables must involve a range of measures.
The control of the relative humidity and the composition of the atmo-
sphere in which vegetables are stored is important but there is a limit to
the reduction of relative humidity because at values below 90–95%, loss
of water from vegetable tissues will lead to wilting. It is essential to avoid
the presence of free water on the surfaces of vegetables and temperature
Figure 5.10 Enzymic activities leading to the degradation of pectin
Chapter 5 155