Food Biochemistry and Food Processing

12 Pectic Enzymes in Tomatoes 279

The degree of methylesterification increased signifi-
cantly in antisense PME fruit, by 20–40% compared
with wild-type fruit. Additionally, the amount of
chelator-soluble polyuronides decreased in trans-
genic lines, a result that was expected since deme-
thylesterification is necessary for calcium-mediated
cross-linking of pectin. Furthermore, chelator-solu-
ble polyuronides from red ripe tomato pericarp
exhibited a larger amount of intermediate size poly-
mers in transgenic lines than in controls, indicating a
decrease in pectin depolymerization in the chelator-
soluble fraction. As described above, PME action
precedes that of PG. An increase of about 15% in sol-
uble solids was also observed in transgenic fruit peri-
carp cell walls (Tieman et al. 1992). Expression of the
antisense PME construct also influenced the ability of
the fruit tissue to bind divalent cations. Calcium accu-
mulation in the wall was not influenced; however,
two-thirds of the total calcium in transgenic fruit was

present as soluble calcium, compared with one-third

in wild-type fruit (Tieman and Handa 1994). Pectin

with a higher degree of methylesterification was

found in PME-suppressed fruit, thus presented fewer

sites for calcium binding in the cell wall.

GENETICENGINEERING OFPME ANDFOOD

PROCESSING

From a food-processing standpoint, the effect of

these modifications in PME activity in tomato cell

walls during ripening was further investigated. Juice

prepared from transgenic antisense PME fruit had

about a 35–50% higher amount of total uronic acids

than control fruit. Pectin extracted from juice pro-

cessed by a hot break, a microwave break, or a cold

break method was methylesterified at a higher de-

gree in PME-suppressed juices. In addition, pectin

from transgenic fruit was of larger molecular mass

Figure 12.4.Size exclusion chromatography of CDTA-soluble pectin fractions from juice (upper)and paste (lower)
prepared from control and suppressed PG fruit. Larger polysaccharide molecules elute close to the void volume of
the column (fraction 20),whereas small molecules elute at the total column volume (fraction 75). (Redrawn from
Kalamaki 2003.)