Kasai et al. (1994) reported that the heating process caused
fruit and vegetables to soften and harden simultaneously; the
hardening occurs overwhelmingly at 50–70 C, above which the
rapid initial hardening is followed by a gradual softening. The
hardening and softening of vegetables are caused by denaturation
of the middle lamella. Bartolome and Hoff (1972) reported that
hardening was caused by the demethylation of cell wall pectin
by pectin methylesterase (PME), which was activated at temperatures
above 50 C, and the subsequent formation of a bridge
bond between the free carboxyl groups and Ca ions. The main
factor that causes vegetable softening during cooking is thought
to be the decomposition of pectin by b-elimination because of
the decrease in cell membrane function caused by heating
(Fuchigami, 1986). Fuchigami (1986) also reported that drying
caused decreased esterification and that the decomposition of
pectin by the b-elimination reaction was inhibited. Based on
these studies, sample transpiration during the drying process
can be considered to cause cell membrane damage. PME was
activated by the decreasing cell membrane function, and softening
was also inhibited by the decreased esterification during drying.
Thus, sample hardening was comparatively accelerated more
than softening.