Cell Wall Degrading Enzymes and Degree of Methyl-Esterification at Different Stages of Maturity in 'Kaek Dura' and 'Red Maradol' Papaya

摘要

A comparative study on the cell wall degrading enzymes [i.e. Polygalacturonase (PG), Pectinmethylesterase (PME), and β-galactosidase (β-gal)] and degree of methyl esterification (DME) was conducted in papaya (Carica papaya) cultivars 'Kaek Dum' and 'Red Maradol' at different stages of ripeness. Fruits at color-break (<10% yellow peel color) were allowed to ripen under ordinary room conditions (25 + 2°C; 70-75% RH) and regularly monitored for maturity using degree of yellowing as indicator. Fruit at <10% yellow (initial), ripe (50-75% yellow), and over-ripe (80-100% yellow) were sampled for analysis of texture, cell wall degrading enzyme assay, and DME. Firmness progressively decreased with maturity in both cultivars. Pulp softening was observed to set in earlier in 'Kaek Dum' than in 'Red Maradol'. Softening was coupled with increased activities of the cell wall degrading enzymes, specifically PG and β-gal. These enzymes had markedly higher activities in 'Kaek Dum' which produced a softer pulp at the ripe and overripe stages compared to 'Red Maradol'. PME activity also registered a modest increase with ripening in both cultivars - indicating that the role of this enzyme in pulp softening could not be entirely discounted. DME in the water-soluble fraction decreased with ripening and did not differ with cultivar while DME in the EDTA and Na_2CO_3-soluble fractions were not detected. The results suggest that PG and β-gal were the major cell wall degrading enzymes that determine softening of papaya cultivars 'Kaek Dum' and 'Red Maradol'. The higher activity of these enzymes in ripe and over-ripe 'Kaek Dum' fruit corresponding to its higher rate of softening and softer texture at the ripe stage than that of 'Red Maradol' fruit.