The research purpose is to determine the pectin methyl esterase activity in papaya (Carica papaya L.) firmness, evaluated in three different ripening stages corresponding to edible fruits (4, 5, 6). Firmness was evaluated using a texture analyser TA-XT. Total soluble solids (TSS), total titratable acidity (ATT) and ratio (TSS/ATT) were simultaneously analysed. Pectin methyl esterase activity was assessed using a spectrophotometric method to quantify hydrogen ions which are released during the reaction. The results were fitted to a Michaelis Menten kinetic, in order to estimate the maximum value rate of activity (Vmax) and the Michaelis constant (Km). The results suggest that there is a significant difference (P<0.05) in firmness. At the same time TSS is increasing and TTA is decreasing during stages 4, 5 and 6 of ripening. It was further demonstrated that there is a linear relationship (R2=0.9622) between firmness and Km, suggesting that an increased in enzyme affinity for the substrate, a major loss of strength is observed, due to the loss of the structure of the plant tissue.