John Lopresti, Janine Jaeger, Christine Frisina and Glenn Hale*

With air freight costs expected to remain high for the foreseeable future, a large proportion of Australian stone fruit is likely to be exported via sea freight in upcoming seasons.

Current stone fruit sea freight export chains are generally too long relative to the potential storage life of most exported cultivars.

Orchard manager at Cutri Fruits in Swan Hill, Mick Tempini said, “unreliable and uncertain export supply chains increase commercial risk when exporting, and potentially impact on access to new export markets.

“New technologies and postharvest protocols that can extend the storage potential of our export cultivars are vital to mitigate the negative effect on quality of sea freight.”

Sea freight durations of over four weeks are very common based on temperature monitoring of over 70 stone fruit sea freight consignments using real-time loggers over the past five years.

Excessive sea freight durations, and/or delays during trans-shipping, significantly increase the risk of sub-standard fruit quality in importing markets due to storage disorders such as flesh browning, poor flesh texture and lack of juiciness after ripening.

Storage disorder symptoms in affected fruit include incomplete or inconsistent ripening and ‘mealiness’. Such symptoms are usually not apparent until fruit arrives at retail or fully ripens after purchase. Current commercial postharvest practices are unlikely to mitigate the effects on fruit quality of long sea freight durations, particularly in highly susceptible cultivars.

Recent work by Agriculture Victoria Research (AVR) indicates that the combination of the following practices can potentially reduce the risk of commercially important storage disorders in fruit after sea freight export:

- Selection of appropriate export cultivars

- Harvest at optimum fruit maturity

- Stepwise cooling

- Use of modified atmosphere (MA) liners.

Below we briefly discuss the merits of each component of a novel postharvest protocol for stone fruit, that if proven to reduce risk of storage disorders, can be implemented commercially with relatively minor adjustments to current postharvest practices.

Figure 1. Effect of simulated sea freight duration and marketing on incidence of
severe flesh browning in ‘Majestic Pearl’ and ‘September Bright’ nectarine. 

Export cultivar storage potential

A recent assessment of peach and nectarine cultivars currently grown, or being considered for export markets showed that a majority of these are unlikely to perform optimally in terms of quality and ripening when sea freight durations exceed three to four weeks.

Information on the effect of cultivar on storage potential, and recommended maximum sea freight durations for export cultivars, is available online at ‘Stonefruit: Cultivar performance for export’.

An example of the effect of excessive sea freight durations on the incidence of storage disorders, as indicated by flesh browning severity, is provided in Fig. 1, where ‘Majestic Pearl’ and ‘September Bright’ nectarines underwent simulated sea freight for up to 42 days at 2°C.

Low incidence of commercially significant flesh browning (i.e., < 10%) was found after 35 days of simulated sea freight but most fruit in both cultivars was severely affected after 42 days.

Although growers and exporters have little control over shipping times, regular real-time temperature monitoring of export consignments and sea freight durations, can inform discussions with shipping companies, and selection of appropriate cultivars whose storage potential more closely aligns with expected sea freight durations.

Optimum harvest maturity

It is relatively well established that storage potential and eating quality after ripening of most stone fruit cultivars is reduced when they are harvested ‘immature’. In terms of sea freight export, the aim should be to harvest fruit at the ‘onset’ of maturity where fruit is just beginning its ripening phase (i.e., when fruit begins to produce low levels of ethylene).

At this harvest maturity, the storage potential of fruit is likely to be maximised whilst not compromising fruit quality for the consumer. Fruit size, skin colour and flesh firmness are a measure of fruit quality; however, a tool like the DA meter provides a better estimate of fruit physiological maturity, particularly when DA measurements are correlated to the rate of ethylene production.

Tagging of up to 50 fruit in the weeks prior to expected harvest and regular DA measurements on the tagged fruit can provide valuable information to optimise harvest maturity for export, as well as assist with harvest scheduling and labour requirements.

Information on using the DA meter to optimise harvest maturity and a database of recommended DA harvest maturity values for 20 stone fruit cultivars is available online (search for ‘Stone fruit maturity and fruit quality’ in profitable-stonefruit-research).

Figure 2. Potential stone fruit postharvest protocol incorporating stepwise cooling directly after harvest. 

Stepwise cooling directly after harvest

Recent studies by AVR have shown that stepwise cooling of stone fruit directly after harvest – also known as ‘preconditioning’ – can significantly reduce the incidence of storage disorders in fruit after sea freight export or extended cool storage compared to fruit that are ‘fast’ cooled down to 2°C (See article ‘Preconditioning key to top stone fruit quality’, Australian Tree Crop, Dec/Jan 2021). A commercially workable postharvest protocol has been developed incorporating stepwise cooling (Fig. 2).

Commercial testing of stepwise cooling will be conducted during the 2022–23 stone fruit season utilising two protocols, cooling to 12°C for 48 hours, and cooling to 18–20°C for 24 to 36 hours, with the former conducted in a preconditioning cool room, and the latter fruit stored at ambient temperature in the pack house.

The quality of fruit undergoing stepwise cooling will then be compared to ‘fast’ cooled fruit after storage at 2°C for five weeks and ripening at 18°C for up to seven days. Further information on stepwise cooling research and potential implementation of the protocol is provided on the Horticulture Industry Network website.

MA liners to extend fruit storage life

Modified atmosphere (MA) liners or ‘bags’ are now commonly used by growers and exporters for sea freight consignments mainly to reduce fruit weight loss, but also because many importers prefer consignments to be packed in liners.

Discussion with major stone fruit exporters suggest that although MA liners are understood to extend fruit storage life, little information is available for specific cultivars regarding their benefits when used correctly to reduce oxygen levels and increase carbon dioxide concentrations during low temperature storage.

Recent studies by AVR found that commercially utilised MA bags resulting in similar atmosphere modification, significantly reduced the incidence of flesh browning in nectarine after simulated sea freight and ripening (Fig. 3).

It is likely that MA liners extend storage potential of most stone fruit cultivars by reducing the risk of storage disorders, although these benefits may be outweighed by poorer eating quality after ripening if fruit is stored well beyond its inherent storage potential, regardless of the MA liner used.

Figure 3. Effect of high humidity perforated bag compared to two modified atmosphere liners
(MA liner #1, MA liner #2) on incidence of severe flesh browning in ‘Majestic Pearl’
nectarine after simulated sea freight export of 35 days at 2°C and ripening at 18°C for four days.
Mean O2 and CO2 concentration within both MA liners was 5 to 6 %, and 14 to 17 %, respectively. 

Novel stone fruit postharvest protocol

Postharvest management of stone fruit for sea freight export can be significantly improved by reducing the risk of storage disorder symptoms after fruit ripening by consumers.

A postharvest protocol combining appropriate cultivar selection, optimal harvest maturity, monitoring of temperature and shipping durations, stepwise cooling and use of MA liners is commercially feasible but requires further testing, both experimentally and commercially.

During the 2022–23 stone fruit season, AVR will be conducting two large experiments to test the proposed novel stone fruit protocol. These experiments will confirm if the protocol reduces the risk of storage disorders compared to current practices, as well as pinpoint which practices have the most impact on maximising fruit quality after export.

Acknowledgement:

The Food to Market Program (Stone fruit) is funded by the Victorian State Government.

*Written by John Lopresti, Janine Jaeger, Christine Frisina and Glenn Hale – Horticultural Crop Physiology, Agriculture Victoria Research. Contact John Lopresti 0419 997 740 or john.lopresti@agriculture.vic.gov.au