A comparative study on wall degrading enzymes, pectin modifications and softening during ripening of selected tropical fruits

Cell wall modifying capacity and enzyme composition of rapid and slow-softening tropical fruits during ripening were investigated with respect to that of tomato, noted to contain significant endo-polygalacturonases (endo-PG; EC 3.2.1.15) activities. Besides PG, other enzymes selected to represent their pectin and/or hemicellulose modifying potentials were pectin methylesterase (EC 3.2.1.11), beta-galactosidase (EC 3.2.1.23), and (1-->4)-beta-alucanase (EC 3.2.1.4). Time for 50% firmness loss was about 1.5 days for Beaumont guava (Psidium guajava L.), 3 days for tomato (Lycopersicon esculentum Mill.), Mas banana (Musa acuminata, AA group) and Eksotika papaya (Carica papaya L.), 4.5 days for Harumanis mango (Mangifera indica L.), and 20 and 24 days for B 10 carambola (Averrhoa carambola L.) and Kampuchea guava, respectively. Capacity to markedly modify cell walls as reflected by capability to impact increased pectin solubility and depolymerization of chelator-soluble pectin, and which varied markedly with the fruit types, may not necessarily be correlated with softening rate, neither was it dependent on the presence of any significant endo-PG activities. Extensive pectin modifications occurred in ripening tomato, mango and papaya. Beaumont guava and Mas banana, though softened rapidly, experienced much limited pectin degradation, seemingly unable to match the modifications that occur in the slow-softening carambola and Kampuchea guava. All fruits tested, except tomato, contained significant exo-PG (EC 3.2.1.67) activities, thus explaining that endo-PG might not be the determining factor of differential softening. All other enzymes, viz. pectin methylesterase, (1-->4)-beta-glucanase, and in particular, p-galactosidase seemed relevant and might contribute significantly to the observed variations in softening rate amongst the fruits. (C) 2004 Elsevier Ireland Ltd. All rights reserved.