Sorting of fruits and vegetables by firmness and maturity is essential for marketing uniform high quality produce. In the US and in Europe, supermarkets increasingly demand high quality and uniform produce. Although machines for sorting fruits by size and external appearance are commercially available, a machine for sorting fruits by firmness and maturity does not exist.
This paper describes the development of a firmness sorting machine which was successfully tested in a commercial packinghouse. The machine comprises a unique conveying system which allows physical contact of the inspected items by a sensor finger 'on-the-go', thereby enabling sorting fruits into 'firm' -'soft' grades at 2)5 to 7)5 fruits/s per lane, without any damage to the fruit. The firmness sensors of the machine comprise a small electrodynamic shaker, for vibrationally exciting the bottom part of the inspected fruit. The root mean square (r.m.s.) level of the fruit-type-specific input signal X is measured in the shaker head. The r.m.s. level of the output signal X M is measured by a miniature accelerometer attached to the sensor finger contacting the top part of the fruit. A firmness index PFΒΉ (Peleg firmness test) is defined as:
Relatively firmer fruits pass a larger portion of the input vibration signal than softer fruits because the latter attenuate the input vibration energy more. Thus, larger PFΒΉ values indicate firmer fruits. Examples of firmness sorting of apples, nectarines and kiwifruit are presented.