Full-scale experiments in forced-air precoolers for citrus fruit: Impact of packaging design and fruit size on cooling rate and heterogeneity

Forced-air cooling (FAC) is a widely applied postharvest technology to rapidly remove the field heat of packed fresh fruit. The cooling uniformity of the fruit in different pallets and cartons during FAC is critical but often remains unknown in commercial operations. This study investigated the cooling rate and heterogeneity of packed citrus fruit in a full-scale, forced-air precooler with 40 pallets of fruit. The influence of package design (package type and wrapping) and fruit size on the precooling performance was quantified in several experiments with three types of citrus fruits ('Navel' orange fruit, 'Nova' mandarin fruit and 'Eureka' lemon fruit). Results showed that the cooling heterogeneity mainly occurred along the flow direction. Cooling was uniform between horizontal regions at the same side of the pallets and between the different heights in the precooler. High resolution measurements with 25-30 sensors in a single pallet gave an even better insight in this heterogeneity. Fruit wrapping induced a much slower cooling rate and larger cooling heterogeneity, especially in the cartons at the outflow side of the pallet. The 'Nova' mandarin fruit in Opentop cartons cooled 24% (at the inflow side of the pallet) and 42% (at the outflow side of the pallet) faster than the 'Eureka' lemon fruit with similar fruit size in Supervent cartons, showing the impact of packaging design. These experiments quantified the cooling heterogeneity of the commercial precoolers.