Minimal impact of phytosanitary irradiation on plum quality after long-term cool storage

Research shows minimal differences in package atmospheres, fruit maturity, flesh firmness and sweetness between control and irradiated plums

• Introduction
• Materials and methods
• Results and discussion
• Conclusion

Introduction

Global trade enables the movement of seasonally fresh produce between countries. However, there are quarantine risks due to the potential spread of pests and diseases.

Phytosanitary irradiation is an efficient, quick and chemical free alternative to existing export treatments such as methyl bromide fumigation, vapour heat treatment and cold disinfestation.

Phytosanitary irradiation is a highly efficient process that sterilises insects and prevents them from breeding in new geographic locations. The commonly used radiation dose for export is 400 Gray (Gy) to treat all insect pests of quarantine concern and the process usually takes less than one hour.

Australian peaches and nectarines were granted market access to Vietnam in early 2022; however, plums were not included in the application. In a recently completed Hort Innovation project (AM19002: Building capacity in irradiation), researchers at Agriculture Victoria conducted a long-term storage trial to determine the effect of irradiation treatment on plum quality.

Materials and methods

Twelve cartons of a mid-season plum packed in Polywrap™ modified atmosphere packaging (MAP) was sourced from a commercial grower in Swan Hill in February 2023 and six cartons treated at a dose rate of 400 Gy by Steritech prior to cool storage at 2 °C at the Agribio Centre, Victoria. Among one or two cartons per treatment, 20 to 40 fruits were assessed after 7, 35, 42 and 49 days of storage followed by 0 or 3 days ripening at 18 °C.

Results and discussion

A similar reduction in oxygen (O₂) and increase in carbon dioxide (CO₂) concentrations within MAP was observed among both control and irradiated fruit over seven weeks storage at 2 °C (Fig. 1), as measured with a portable CheckPoint gas analyser (Dansensor, Denmark). Oxygen concentration initially decreased from 21 % to between 9 and 13 % in control fruit, and down to between 9 and 15 % among irradiated fruit. Carbon dioxideconcentrations increased from 0 % up to 4 to 5 % in both treatments which suggests all fruit were responding similarly to atmosphere modification, and that irradiated fruit were not physiologically stressed due to treatment, which would have been indicated by lower O₂ and higher CO₂ concentrations than observed.

Little difference in I_AD value — an indicator of fruit physiological maturity — was observed between treatments at each removal and subsequent ripening period as measured with a Difference in Absorbance (DA) meter (Turoni, Italy) (Fig. 2). Lower values (i.e., more mature fruit) were found more so in fruit that had been stored for at least five weeks and then ripened; however, similar trends were observed in both treatments.

Non-destructive fruit flesh firmness as measured with an Agrosta®100 USB durometer (Agrosta, Serqueux, France) was similar among both treatments at each removal. However, fruit softened more rapidly at all ripening periods after 5 weeks of storage indicating that this fruit would require immediate sale during marketing or marketed at a significantly lower temperature than 18 °C to reduce the rate of fruit softening (Fig. 3). Plums from both treatments showed similar destructive flesh firmness as measured with a fruit texture analyser (Guss™ Fruit Texture Analyser GS-14, South Africa) after long-term storage and ripening, although irradiated fruit were consistently softer by approximately 0.5 kg/cm² than control fruit (data not shown). All non-destructive values corresponded to destructive values in the range of 1.0 to 2.1 kg/cm². Minimal differences in non-destructive fruit firmness were found between the two treatments after seven days of storage and marketing, which potentially represents a similar time duration for air freight of stone fruit from Victorian farms to retail shelves in Vietnam.

As with other fruit quality parameters, there was little change in soluble solids concentration (SSC) after each storage and ripening period as measured using a digital refractometer (Atago PAL-1, Atago, Tokyo, Japan) which suggests that irradiation had minimal effect on sweetness in both control and irradiated fruit (Fig. 4). Only non-destructive sampling of fruit was conducted directly out of cool storage (i.e., day 0), so no destructive SSC measurements were recorded. Interestingly, the taste of all plums was less sour after long-term storage which suggests that flesh acid concentrations must have decreased substantially as SSC levels in both treatments remained relatively constant at approximately 14 °Brix during storage.

Conclusion

This trial demonstrated that the impact of phytosanitary treatment on plum quality was minimal during long-term cool storage. Similar results were observed between control and treated fruit in terms of plum package atmospheres, maturity, flesh firmness and sweetness. Datasets from irradiation and cool storage experiments on fresh produce grown in Victoria will be used to open new export markets and improve existing ones, such as Vietnam.

Acknowledgement

The ‘Building capacity in irradiation – pathways to export’ project is funded by the Hort Frontiers Asian Markets Fund (Project AM19002), part of the Hort Frontiers strategic partnership initiative developed by Hort Innovation, with co-investment from the Department of Jobs, Precincts and Regions (Victoria), Steritech, NSW Department of Primary Industries, SA Research and Development Institute, NZ Plant and Food Research, Aerial (France) and the Australian Government.

For more information, contact Glenn Hale on glenn.hale@agriculture.vic.gov.au

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