282 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
suppression of PG and Exp1 in the same transgenic
line showed an overall increase in viscosity compared
with the control, but only a 4% additional increase
compared with the single transgenic lines. Particle
size distribution in juices and pastes from the double
transgenic line are more polydisperse (Fig. 12.5), and
particles appear to be more rigid. In this line, an
increase in the size of carbonate-soluble pectin poly-
mers was observed. This size increase could suggest
differences in the degree of polymer cross-linking in
the particles, resulting in altered particle properties.
By modifying cell-wall metabolism during ripening,
the processing qualities of the resulting juices and
pastes were influenced. Hence, reducing the activity
of these cell-wall enzymes may be a route to the
selection of improved processing tomato varieties.
SUPPRESSEDPG ANDSUPPRESSEDPME
The activities of both PG and PME were suppressed
in the same line, and juices were evaluated and com-
pared to single transgenics and controls (Errington
et al. 1998). Small differences in Bostwick consis-
tency were observed in hot-break juices, with the
suppressed PG juices having the highest viscosity.
In cold-break juices from PG-suppressed fruit, a
time dependent increase in viscosity was observed.
Since a similar increase was not observed in the
double transgenic line, it was concluded that in or-
der for the increase in viscosity of cold-break juice
to occur, both absence of PG and continued action of
PME is required. Absence of PG activity will lead to
a larger size of pectin, whereas continuing deme-
thylesterification of pectin chains by PME will in-
crease calcium associations between pectin chains,
leading to gel formation and thus improved viscosity.
In conclusion, simultaneous transgenic suppression
of PG and PME expression did not result in an addi-
tive effect.
TOMATO PROCESSING
The majority of tomatoes are consumed in a pro-
cessed form such as juice, paste, pizza and pasta
sauce, and various diced or sliced products. Most of
the products are concentrated to different degrees
and stored in a concentrated form until ready to use.
Industrial concentrates are then diluted to reach the
desired final product consistency.
Textural properties of tomato fruit are important
contributors to the overall quality in both fresh mar-
ket and processing tomatoes (Barrett et al. 1998). In
some processed products, the most important quality
attribute is viscosity (Alviar and Reid 1990). It was
recognized early that the structure most closely
associated with viscosity is the cell wall (Whitten-
berger and Nutting 1957). Both the concentration
and type of cell-wall polymers in the serum fraction
and the pulp (particle fraction) are important con-
tributors to viscosity. In serum, the amount and size
of the soluble cell wall polymers influence serum
viscosity (Beresovsky et al. 1995), whereas in pulp,
the size distribution, the shape, and the degree of de-
formability of cell-wall fragments influence viscosity
(Den Ouden and Van Vliet 1997). Viscosity is influ-
enced partly by factors that dictate the chemical com-
position and physical structure of the juice such as
fruit variety, cultivation conditions, and the ripening
stage of the fruit at harvest. However, it is also influ-
enced by processing factors such as break tempera-
ture (Xu et al. 1986), finisher screen size (Den Ou-
den and Van Vliet 1997), mechanical shearing during
manufacture, and degree of concentration (Marsh et
al. 1978). These processes result in changes in the
microstructure of the fruit cell wall that are manifest-
ed as changes in viscosity (Xu et al. 1986). Since the
integrity of cell-wall polymers in the juice is impera-
tive, plant pectic enzymes usually have to be inacti-
vated during processing in order to diminish their
activity and prevent pectin degradation. Pectin de-
gradation could lead to increased softening in, for
example, pickled vegetable production or peach
canning, and loss of viscosity in processed tomato
products (Crelier et al. 2001). Although several
enzymes are reported to act on cell-wall polymers,
the main depolymerizing enzyme is PG. Therefore,
inactivation of PG activity during tomato processing
is essential for viscosity retention. However, there
are cases where residual activity of other pectic en-
zymes is desirable, as for example, with PME. Re-
tention of PME activity with concurrent and com-
plete inactivation of PG could lead to products with
higher viscosity (Errington et al. 1998, Crelier et al.
2001). Selective inactivation of PG is not possible
with conventional heat treatment since PME is
rather easily inactivated by heat at ambient pressure,
while PG requires much more severe heat treatment
for complete inactivation. PME is inactivated by
heating at 82.2°C for 15 seconds at ambient pres-
sure, while for PG inactivation a temperature of
104.4°C for 15 seconds is required in canned tomato
pulp (Luh and Daoud 1971). In order to achieve this