Detection of internal defects in onion bulbs by means of single-point and scanning laser Doppler vibrometry

The number of onion consignments which are rejected or downgraded due to the incidence of internal defects is a continuing problem for wholesalers and growers. Defects may only be up to 4% incidence level, but result in the entire lot being lost for sale. Destructive quality control testing causes waste, so that there is a need for alternative non-invasive assessment. The aim of the current research was to demonstrate whether internal defects could be detected using Laser Doppler vibrometry (LDV). Several trials, with different types of sensors and levels of onion defect severity were conducted. Both scanning and single-point LDV were employed in order to develop a suitable measurement method to evaluate onion defects. It was necessary to measure resonant frequency at the neck or equator of the bulbs in order to segregate neck rot (Botrytis allii) or bacterial rot, respectively, but LDV could not differentiate sprouting and double hearted bulbs from sound onions. In conclusion, it was possible to non-destructively identify onion bulbs with only a 5% area affected with neck rot (visible after cutting). It would be necessary to calibrate for different onion cultivars and origins, if the technique is to be implemented on a commercial sorting line.