We were asked to devise a method to sort avocados so that selected fruit could be marketed as 'ripe and ready to eat'. We developed a method in which the fruit is tapped gently at several points around its circumference by small impactors containing piezoelectric elements. The characteristics of the impact allow the surface compliance to be calculated and hence the average firmness determined. The technique operates as the fruit is carried along a conveyor and can operate at up to 30 fruit per second per conveyor lane. The method is licensed to Sinclair International Ltd and is operating successfully on an industrial scale to sort avocados, mangoes and other fruit. You can watch a video here.