Imazalil on-line control in post-harvest treatments of citrus fruit

Penicillium digitatum and Penicillium italicum represent the main causes of post-harvest loss in citrus fruit for medium and long term storage. They can be controlled using fungicide as Imazalil, (IMZ) and fruits are generally treated on-line in the packing line. One of the more effective treatments is the fruit immersion in a tub located on the packing line after the fruit dumping. However, at the present the fungicide concentration during the whole day, related to the amount of fruit packed in the line, is not controlled. The aim of this work has been to set up a technique to monitor the Imazalil concentration in the washing tubs used for the dip treatment of citrus fruits. With this aim, several parameters of the water/Imazalil solution have been tested, as pH, conductivity and redox potential, but they have shown no correlation with the Imazalil concentration. Then the spectrophotometry has been used to assay the Imazalil concentration: sample solutions (0.5 to 250 ppm) have been exposed to ultraviolet (UV) and near infrared (NIR) wavelengths. The tests performed have shown that NIR gives good correlation between Imazalil and the prediction model: the standard error of correlation (SEC) is 0.6 and the standard error of validation (SEV) is 3.7, such errors are acceptable both for the quick analysis method and for the minimum measurable concentration. Then UV has been successfully employed for the quick determination of the Imazalil concentration (SEC= 0.6, SEV= 1.5), then it was built a feedback control system that controls the variation of the IMZ concentration in the tub at a rate of 3.2 ppm per minute: this is accurate enough for most application and particularly for the purpose of constantly monitoring the IMZ concentration in dipping tubs for citrus fruits.