286 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
expansins and polygalacturonases are among the
more abundantly expressed proteins in ripening to-
mato fruit, the modification of their expression has
not been sufficient to account for all of the cell-wall
changes associated with softening or for the overall
extent of fruit softening. The texture of fresh fruit
directly influences the rheological characteristics of
processed tomato products. Viscosity is influenced
by modifications of the cell-wall architecture but
also by changes that may affect polysaccharide mo-
bility and interactions of different components in
juices and pastes. Since cell-wall metabolism in
ripening fruit is a complex processes and many
enzymes have been identified that contribute to this
process, further insight to the effect of these enzymes
on fruit texture and processing attributes can be
gained by the simultaneous suppression or overex-
pression of combinations of enzymes. Researchers
are now preparing double transgenic lines with both
PG and-galactosidase gene expression suppressed
(John M. Labavitch, pers. comm., 2004). Moreover,
our knowledge of cell-wall enzymes increases, and
other genes are continuously identified that con-
tribute to pectin depolymerization and fruit softening
(Ramakrishna et al. 2003). Manipulation of the
expression of multiple ripening-associated genes in
transgenic tomato lines will further shed light on the
ripening physiology, postharvest shelf life, and pro-
cessing qualities of tomato fruit. Finally, the interest
exists and efforts are made to design optimal pro-
cesses for selective enzyme inactivation and thus for
production of products with desirable characteris-
tics, by introducing pressure as an additional (to time
and temperature) processing variable.
ACKNOWLEDGEMENTS
The authors would like to thank Asst. Professor P.
Christakopoulos, National Technical University of
Athens, School of Chemical Engineering, Biosys-
tems Technology Laboratory, and Dr. C. Mallidis,
Institute of Technology of Agricultural Products, Na-
tional Agricultural Research Foundation of Greece
for reviewing the manuscript and for their construc-
tive comments.
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