By John Lopresti and Glenn Hale - horticultural consultants Fruitful Insights

Australian stone fruit is exported by both air and sea freight typically in plastic film liners ranging from perforated high humidity (HH) bags to modified atmosphere packaging (MAP).

Film liners are used to minimise fruit weight loss, extend storage and shelf life, reduce rot development, and are the preferred packaging option in some export markets.

For air freight export of relatively short duration, stone fruit is usually packed in HH bags, which enable effective fumigation treatment for market access, and minimise condensation caused by fluctuating storage and handling temperatures which usually occur along air freight supply chains.

These perforated liners do not extend fruit storage life but can significantly reduce fruit shrivel while minimising fruit softening during export.

Airtight MAP liners are mostly utilised to reduce fruit respiration rates and extend fruit storage life during sea freight export, where shipping durations can range from three to six weeks, which often exceeds the inherent storage potential of many export cultivars.

MAP liners can extend low temperature storage life of peach and nectarine cultivars by one to four weeks compared to cool storage in air (without MAP), slightly increase shelf-life during retailing, and reduce the risk of poor fruit quality due to shrivel (moisture loss), softening, and internal flesh disorders such as browning, bleeding, and lack of juiciness.

These symptoms are usually only apparent when fruit is removed from MAP at the retailers or when fully ripened after purchase.

MAP should only be used at storage and handling temperatures below 4°C and where little variation in storage temperature is expected (i.e., during sea freight), while MAP cannot extend storage life of fruit indefinitely as, depending on the cultivar, more than six to nine weeks of cool storage in MAP will likely result in incomplete fruit ripening and poor eating quality.

The extent to which MAP increases fruit storage life compared to fruit stored in air can vary depending on factors such as cultivar, liner type and manufacturer, fruit harvest maturity and variability, latent rot risk, market access treatment and temperature management (see table).

Some current export cultivars (e.g., white peaches) have a relatively short storage life hence may be better suited to air freight.

End of storage life for sea freight cultivars is usually due to flesh browning, rot development or excessive fruit softening.

Previous research has shown MAP can significantly reduce the incidence of flesh browning in white and yellow nectarines after simulated sea freight and ripening.

MAP has also been shown to increase the storage life of early-season white peaches from three to five weeks whilst mid-to-late season yellow nectarine cultivars can usually be stored in MAP for at least seven weeks without any negative impact on fruit ripening and eating quality.

Although growers and exporters have little control over shipping times, regular monitoring of export consignments can inform selection of appropriate cultivars whose storage potential more closely matches expected sea freight durations when MAP is utilised.

Once fruit are packed and sealed in MAP, oxygen (O 2 ) concentrations fall and carbon dioxide (CO 2 ) concentrations rise to an equilibrium level generally within three days of packing, with the final ratio of O 2 to CO 2 dependent on storage temperature, fruit maturity and liner type.

Oxygen concentrations can vary from 8 to 16 % between MAP types. When selecting an appropriate MAP growers and exporters need to consider the differences between liner types in terms of storage life extension and their compatibility with cultivars exported, and not purely on liner price, which is often the case.

Ideally MAP for commercial shipments should be selected after simulated sea freight export trials conducted on-farm for several key export cultivars.

It is well established that among stone fruit cultivars storage potential and eating quality after ripening is reduced when fruit are harvested ‘immature’ (pre-climacteric or no ethylene production).

In terms of sea freight export, the aim should be to harvest fruit at the ‘onset’ of climacteric maturity when fruit begins to produce low levels of ethylene.

At this harvest maturity the storage potential of fruit is likely to be maximised when using MAP whilst not compromising fruit quality for the consumer.

Extended cool storage or shipping of immature fruit in MAP is not recommended as these fruits are unlikely to fully ripen after retail, whilst better eating quality can potentially be achieved by harvesting slightly more mature fruit that can be safely exported in MAP without compromising fruit firmness or storage life.

MAP is also likely to reduce the risk of rot development during long term cool storage or sea freight export provided an efficient cool chain, and where effective fungal disease control has been implemented in the orchard. MAP is unlikely to reduce postharvest rot risk in seasons with very wet weather during flowering or at pre-harvest.

MAP is an important technology for the postharvest management of stone fruit during long-term cool storage or sea freight export, especially when used correctly with carefully selected cultivars that are harvested at the optimal maturity.

It can significantly extend fruit storage life by up to four weeks and increase subsequent shelf-life by up to three days at retail and reduce the risk of storage disorder symptoms after fruit ripening by consumers.

In the medium-term it is recommended the various commercial MAP bags available are compared in terms of extending storage life and in reducing storage disorders under simulated and commercial sea freight conditions for a range of export stone fruit types and cultivars.

For more information contact John Lopresti ( john.lopresti@fruitfulinsight.com.au ) or Glenn Hale ( glenn.hale@fruitfulinsights.com.au ) at Fruitful Insights ( www.fruitfulinsights.com.au ).