Effect of irradiation as a phytosanitary treatment on the post-harvest quality of apples

The project comprised of four main objectives, namely: (i) to establish the effect of cobalt-60 on storage quality of apples, (ii) to determine the minimum and maximum absorbed irradiation dose in a pallet of apples, (iii) to test methods to reduce the irradiation dose ratio, (iv) to ascertain whether SmartFresh^SM application prior to packing can maintain flesh firmness of irradiated apples

Three populations each of Royal Gala, Granny Smith, and Pink Lady apples stored either under regular atmosphere, or controlled atmosphere conditions prior to packing, were irradiated within 10 days after packing with cobalt-60 at different dosages, ranging from 600 to 2500 Gy. Apples were evaluated after storage for 8 weeks at -0.5°C, followed by 7 days at 20°C.

In one population of Royal Gala and one population of Granny Smith apples, irradiation between 600 to 900 Gy adversely influenced storage quality, compared to non-irradiated apples, while the three Pink Lady apple populations were not negatively influenced by the different irradiation dosages.

A minimum:maximum absorbed irradiation dose ratio of 1:2.5 was obtained when a pallet of Royal Gala apples (18.2 kg cartons stacked five layers high) was irradiated. The ratio was reduced to 1:2 when the middle cartons of the stack were removed, or when a 3-layered stack of apples was elevated, by placing three empty pallet bases below the stacked pallet. If a larger cobalt-60 source could be used, the minimum-maximum absorbed irradiation dose ratio in a stacked apple pallet may be further reduced. This aspect of the project will need to be further investigated if required.

The effect of SmartFresh^SM application prior to irradiation of Pink Lady apples at 900 Gy was evaluated during the 2008 season. The Pink Lady apple populations treated with SmartFresh^SM before irradiation appeared to be firmer than fruit not treated with SmartFresh^SM, after storage for 7 weeks at – 0.5°C + 7 days at 20°C. Since SmartFresh^SM is registered for use on most apple cultivars, it suggested that only apples treated with SmartFresh^SM be irradiated, if irradiation technology is used on a commercial scale as a phytosanitary treatment.