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) were sourced from a commercial grower in Swan Hill in February, and six were treated at a dose rate of 400 Gy by Steritech, before cool storage at 2°C at the Agribio Centre, Victoria.

Among one or two cartons per treatment, 20 to 40 fruits were assessed after seven, 35, 42 and 49 days of storage followed by zero or three 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 per cent to between 9 and 13 per cent in control fruit, and down to between 9 and 15 per cent among irradiated fruit. Carbon dioxide concentrations increased from 0 per cent up to 4 to 5 per cent 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.

Figure 1. Change in oxygen (O2) and carbon dioxide (CO2) concentrations among control (untreated)
and irradiated (treated) plums during seven weeks storage at 2 °C.  

Little difference in IAD 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 stored for at least five weeks and then ripened; however, similar trends were observed in both treatments.

Figure 2. Change in fruit maturity (IAD) among control (untreated) and irradiated (treated) plums during seven weeks storage
at 2°C and ripening for three days at 18°C. Error bars are the standard deviation of each mean.

Non-destructive fruit flesh firmness as measured with an Agrosta100 USB durometer (Agrosta, Serqueux, France) was similar among both treatments at each removal. However, fruit softened more rapidly at all ripening periods after five weeks of storage, indicating it 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.

Figure 3. Change in non-destructive fruit firmness among control (untreated) and irradiated (treated)
plums during seven weeks storage at 2°C and ripening for three days at 18°C. Error bars are the standard deviation of each mean.  

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.

Figure 4. Change in plum soluble solids concentration (SSC) among control (untreated) and irradiated (treated)
plums during seven weeks storage at 2°C and ripening for three days at 18°C. Error bars are the standard deviation of each mean.  

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