Nondestructive Firmness Measurement as a Maturity Index for 'Carabao' Mango (Mangifera indica L.) Fruit

'Carabao' mango was harvested green at different stages of maturity [100, 105, 110, 115, 120 and 125 d after flower induction (DAFI)] and subjected to nondestructive firmness test using a universal testing machine. A 7.9-mm diameter plunger with rounded tip (6.9-mm radius of curvature) was pressed onto whole fruit at 10 mm min -1 crosshead speed, to produce deformations of 0.8, 1.3, and 1.8 mm; additional fruits were tested until rupture for comparison. Total soluble solids (TSS), titratable acidity (TA) and pH were measured at the green stage and at the table-ripe stage (TRS) after ripening at 25 °C. Sensory analysis was performed at TRS to determine the best harvest maturity. Results showed no significant differences in TSS, TA and pH between maturities at the green stage; hence, physicochemical properties could not be used to test for maturity. Sensory analysis at TRS showed that fruit harvested at 115–120 DAFI gave the best eating quality in terms of aroma, sweetness, sourness, flavor and overall acceptability. Fruits harvested at 125 DAFI showed significantly lower sensory scores. Significant differences in rupture force (FR, kN) and fruit stiffness (B, kN mm -1 of deformation) were observed between maturities at the green stage; however, the nondestructive deformation test at 0.8 mm was more sensitive to fruit maturity. Fruits harvested at 115–120 DAFI had significantly lower values for B and load ratio at 0.8-mm to 0.5-mm deformation (F0.8/F0.5) compared with earlier harvest dates. Analysis of the normal distribution curve of a prediction set of 40 mature (MAT) fruits showed that 80% of the samples had B ≤ 1.360 kN mm -1 and F0.8/F0.5 ≤ 1.883. Based on the use of signal detection theory and these values as thresholds, 66.3% (by B) and 58.8% (by F0.8/F0.5) of immature or overmature (I/O) fruit could be detected. Analysis of a separate validation set of fruit using the same threshold values gave respective detection rates of I/O fruit of 68.8% using B and 60.0% using F0.8/F0.5.