BLBS102-c12 BLBS102-Simpson March 21, 2012 13:11 Trim: 276mm X 219mm Printer Name: Yet to Come
12 Pectic Enzymes in Tomatoes 241carbonate-soluble pectin polymers 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.PG and PMEThe activities of both PG and PME were suppressed in the same
line, and juices were evaluated and compared to single trans-
genics and controls (Errington et al. 1998). Small differences in
Bostwick consistency 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 ob-
served in the double transgenic line, it was concluded that in
order 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 larger size of pectin, whereas
continuing demethylesterification of pectin chains by PME will
increase calcium associations between pectin chains, leading to
gel formation and thus improved viscosity. In conclusion, simul-
taneous transgenic suppression of PG and PME expression did
not result in an additive effect.TOMATO PROCESSING
The majority of tomatoes are consumed in a processed 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 market and processing toma-
toes (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 associ-
ated with viscosity is the cell wall (Whittenberger and Nutting
1957). Both the concentration and type of cell wall polymers in
the serum fraction and the pulp (particle fraction) are important
contributors to viscosity. In serum, the amount and size of the sol-
uble cell wall polymers influence serum viscosity (Beresovsky
et al. 1995), whereas in pulp, the size distribution, the shape,
and the degree of deformability of cell wall fragments influence
viscosity (Den Ouden and Van Vliet 1997). Viscosity is influ-
enced partly by factors that dictate the chemical composition and
physical structure of the juice such us fruit variety, cultivation
conditions, and the ripening stage of the fruit at harvest. How-
ever, it is also influenced by processing factors such as break
temperature (Xu et al. 1986), finisher screen size (Den Ouden
and Van Vliet 1997), mechanical shearing during manufacture,
and degree of concentration (Marsh et al. 1978). These processesresult in changes in the microstructure of fruit cell wall that are
manifested as changes in viscosity (Xu et al. 1986). Since the
integrity of cell wall polymers in the juice is imperative, plant
pectic enzymes usually have to be inactivated during processing
in order to diminish their activity and prevent pectin degrada-
tion. Pectin degradation 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 enzymes is desirable, as for example, with
PME. Retention of PME activity with concurrent and complete
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 com-
plete inactivation. PME is inactivated by heating at 82.2◦Cfor
15 seconds at ambient pressure, 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
selective inactivation, a combination of heating and high hydro-
static pressure can be used during processing or alternatively,
the expression of a particular enzyme can be suppressed using
genetic engineering in fruit.Thermal InactivationThermal processing (that is, exposure of the food to elevated
temperatures for relatively short times) has been used for al-
most 200 years to produce shelf-stable products by inactivating
microbial cells, spores, and enzymes in a precisely defined
and controlled procedure. Kinetic description of the destructive
effects of heat on both desirable and undesirable attributes is
essential for proper thermal process design.
At constant temperature and pressure conditions, PME ther-
mal inactivation follows first-order kinetics (Crelier et al. 2001,
Fachin et al. 2002, Stoforos et al. 2002).−dA
dt=kA, k=f(T,P,...)(1)whereAis the enzyme activity at timet,kthe reaction rate
constant, andPandT the pressure and temperature process
conditions.
Ignoring the pressure dependence, Equation 1 leads to
A=Aoe−kTt (2)
whereAois the initial enzyme activity and the reaction rate
constantkT, function of temperature, is adequately described by
Arrhenius kinetics through Equation 3.kT=kTrefexp[
−Ea
R(
1
T−1
Tref)]
(3)wherekTrefis the reaction rate constant at a constant reference
temperatureTref,Eais the activation energy, andRis the univer-
sal gas constant (8.314 J/(mol·K)).