NON-CONTACT FRUIT FIRMNESS MEASUREMENT BY THE LASER AIR-PUFF METHOD

The suitability of the laser air-puff method for measuring the firmness of fruits was investigated by constructing an instrument to deliver a puff of air on to a stationary fruit and record the resulting surface deformation with a laser displacement sensor. The instrument uses an 8.2-mm internal diameter nozzle to deliver a puff of 80 ms duration and 90 kPa peak pressure at an object-nozzle distance of 23.3 mm. The resulting fruit deformations were best modeled by the Boussinesq theory for die loading and this suggests that the measurements are insensitive to variation in the fruit’s radius of curvature. The laser air-puff method was proven suitable for accurate measurement of stiffness (R2 ~ 0.97, s = 0.45 MPa) on apples, kiwifruit and synthetic balls over a range of stiffness values from 0.5 to 8 MPa and radii of curvature from 11 to 30 mm. A good correlation (R2 ~ 0.88) was obtained between laser air-puff stiffness and penetrometer firmness measurements on kiwifruit over the firmness range 0 to 12 N. However, the mean residual error in regression (s = 1.0 N) is only moderate and the relationship shows a trend of increasing error with kiwifruit firmness. Consequently the laser air-puff method may be useful for sorting of kiwfruit where only very soft fruit (< 5 N) need to be identified and removed.