Enzymes in Quality and Processing of Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 38

of activity of two banana PG isozymes, PG 1 and PG 2,
with blanching at 60 ◦ and 70 ◦ C, but that of PG3 retained
50% to 60% activity. Though effective, thermal processes
are typically accompanied by degradation of color, flavor,
and nutrients and therefore must be optimized as much as
possible and/or linked with less severe methods.

Occurrence and function of PME in tropical and
subtropical fruits
PME in fruits normally exists as two or more isoforms, each
comprising a single polypeptide chain with molecular mass
ranging from 10 to 60 kDa (Markovic and Joernvall, 1992).
Two isoforms of PME have been identified in banana tis-
sue and their activity has been demonstrated to increase in
ripening banana (De Swardt and Maxie, 1967). In guava,
PME has been observed to contain two isoforms, where ac-
tivity increased with maturation (Carvalho et al., 2009). The
PME in guava was thermally stable, exhibiting an optimum
activity at 95 ◦ C and retained 96.8% of activity after 300 min
in 90 ◦ C. Several authors have also reported that orange juice
contains some thermally tolerant PME isozymes (Cameron
and Grohmann, 1996; Braddock, 1999). Table 3.5 shows the
differing conditions for optimal activity of PME in some
tropical and subtropical fruits.
According to Selvaraj and Kumar (1989), softening en-
zymes, such as PME, polygalacturonase, and cellulase, gen-
erally increased from harvest maturity to the color break
stage in mango fruits. However, Roe and Bruemmer (1981)
found that PME activity declined during the first stage of
ripening of mango fruits but then leveled off. It has been
observed that the activity of PME is higher in the break-
down pulp than in healthy pulp (Murthy, 1981) and that
peels have higher PME activity than the pulp of mango at
all stages of ripening (Ashraf et al., 1981). The postharvest
decrease in PME activity in the avocado fruit was shown to
decline from its maximum value at the time of picking to a
low level early in the climacteric stage (Awad and Young,
1979). PME has also been researched in papaya (Fayyaz
et al., 1995), pineapple (Perera et al., 2010), and passion
fruit (Shiomi et al., 1996).
The de-methyl-esterified homogalacturonan (HGA) with
negatively charged free carboxylic acid groups resulting
from the activity of PME (Ni et al., 2005) can either in-
teract with divalent ions, particularly Ca 2 + , thus forming
gels, or become a target for pectin-degrading enzymes, such
as polygalacturonases, affecting the texture and rigidity of
the cell wall (Pelloux et al., 2007). The cross-linking of
HGA with divalent ions to form gels is particularly im-
portant in processed fruits, as it improves the firmness and

PECTIN METHYLESTERASE (PME)

Nomenclature and reactions catalyzed
Pectin methylesterases (PME, EC 3.1.1.11), catalyze
the specific demethylesterification of homogalacturonan
(HGA) within plant cell walls. PME activity results in a
lower degree of esterification of the pectin as well as the
release of methanol and protons. In effect, this reaction
creates negatively charged carboxyl groups, which can im-
pact cell wall integrity and texture (Ni et al., 2005). It is
proposed that PME removes methyl esters in a blockwise
fashion (single-chain mechanism), giving rise to domains of
contiguous, deesterified galacturonic acid residues (Willats
et al., 2001).

Occurrence and function of PME in plants
Pectin Methylesterase is present in all higher plants and
is particularly abundant in citrus fruit (Johansson et al.,
2002). Plant PME is cell wall bound and typically occurs as
multigene isoenzymes (Bosch et al., 2005). The abundance
of PME and other pectin-degrading enzymes at different
growth stages in plants suggests that these enzymes play
important roles in major processes such as plant reproduc-
tion, growth, and defenses (Pelloux et al., 2007).
Research has demonstrated that PMEs are involved in
cell wall extension and stiffening (Al-Qsous et al., 2004),
cellular separation (Sobry et al., 2005), seed germination
(Ren and Kermode, 2000), root tip elongation (Pilling et al.,
2004), and internode stem growth (Saher et al., 2005). An-
other emerging role of PMEs is their involvement in the
regulation of fiber length by strengthening the cellular ad-
hesion between developing fiber and thus inhibiting their
intrusive apical elongation (Pelloux et al., 2007).

Table 3.5. Pectin methylestrase (PME) in tropical and subtropical fruits.

Fruit Substrates Properties or Role in Processing Reference

Optimum pH 8.5 and temperature 75 ◦ C and 85 ◦ C Guava Pectin Leite et al. (2006)

Optimum pH 8 and temperature 35 ◦ C Papaya Pectin Lim and Chung (1993)