The 2014/15 growing season has been cool across a range of apple production districts, which could lead to an increase in the incidence of internal browning in long term stored ‘Cripps Pink’ fruit. To avoid this, growers and cool store operators should plan on marketing their fruit within six months of harvest or stepwise cool their fruit down to a storage temperature of 2.5°C.
Dr Gordon Brown
As the harvest season draws to a close we approach the time for picking our precious Cripps Pink apples. With the exception of the hottest growing regions, Cripps Pink apples are prone to flesh browning when stored for six or more months.
The Horticulture Australia Ltd (HAL) project AP04008, which was conducted by researchers in Australia and New Zealand and led by Jenny Jobling (Applied Horticultural Research Pty Ltd), investigated flesh browning, and identified two separate types of flesh browning induced by the climate:
Diffuse Flesh Browning – a chilling injury that occurs in seasons where less than 1,100 Growing Degree Days (GDD) above 10°C are accumulated.
- Radial Flesh Browning – a senescent breakdown that occurs in seasons where 1,100 to 1,700 GDD above 10°C are accumulated and is exacerbated when stored at 0°C or above 1 per cent CO2.
In seasons accumulating more than 1,700 GDD there is little risk of developing flesh browning.
Project AP08004, completed in 2014, identified that detailed climatic records could not predict the incidence of flesh browning at a regional or orchard level, however it did confirm that flesh browning is more severe in cold seasons.
From the climatic records collected by the Australian Pome Fruit Improvement Program® Ltd (APFIP) for the three sites with GDD data in the current and 2013/14 seasons it is clear that the current season has been substantially cooler than last season. If the current cool season trend continues then by the Cripps Pink harvest period there will be between 800 and 1.100 GDD for the current season at these sites.
This means that for both the Huon and Orange districts the probability of encountering diffuse flesh browning has increased from 0 to over 50 per cent while Manjimup growers may see less radial flesh browning and increased diffuse flesh browning. While climatic records are not available for all growing regions this data suggests that most regions will encounter diffuse flesh browning (storage chilling injury) with the current crop. It is also worthy to note that radial diffuse browning is more severe when fruit are stored under cold conditions (0°C) or under CO2 levels above one per cent.
Grower actions to reduce the incidence of flesh browning
- Market fruit within six months of harvest.
- Store fruit at a higher temperature.
- Stepwise cool fruit after harvest.
High temperature storage
In 2002, Angelo Zanella, in northern Italy sent information on his trial results on the storage of Cripps Pink apples. Cripps Pink apples were stored under Controlled Atmosphere (CA) at 1.5 per cent O2 and 1.3 per cent CO2 for five months at either 0 or 2.5°C. The higher storage temperature reduced internal browning from 46 per cent down to 16 per cent. This research was repeated for Australian fruit in 2007, as part of HAL project AP04008, for fruit from the Huon Valley and Batlow and simultaneously in Tasmania in 2007 and 2008 as a privately funded project.
This research confirmed the earlier studies in Italy and demonstrated the effect of storage temperature on Australian grown fruit. The impact of storage temperature on the appearance of internal browning clearly show that storage at 0°C resulted in almost half the fruit expressing internal browning while storage at 3°C reduced this to commercially acceptable levels.
While the appearance of internal browning can be clearly observed, what about other fruit quality parameters such as fruit firmness? Storing fruit at 0°C resulted in softer fruit after storage than fruit stored at higher temperatures indicating that chilling injury also leads to increased rates of fruit softening as has been observed in other crops.
The data indicates that long term storage temperature of Cripps Pink should be at temperatures above 0°C and closer to 3°C. Care needs to be taken however as fruit greasiness and fruit storage rots may become a problem. Both of these can be controlled to a certain degree by storage at 1 per cent O2.
Another advantage of storing fruit at a higher temperature is potential savings in electricity costs for fruit storage. Research in Europe and two reports for Gala apples were presented at the International Horticulture Congress in Portugal in 2010. These trials both studied the possibility of using SmartFresh® in conjunction with higher storage temperatures to reduce storage costs and greenhouse gas emissions. In both cases there was a financial saving of close to 30 per cent of storage electricity costs.
Storing fruit at a higher temperature increases the rate of fruit respiration and hence the rate of CO2 accumulation in the cold room. This means that there will be increased demand on the cold store operator to remove CO2 from the rooms. In this trial work, SmartFresh® plus higher storage temperature resulted in superior out turn of fruit compared to conventional CA at lower storage temperatures. This challenges current commercial practices and demonstrates the usefulness of this treatment at reducing storage costs, reducing CO2 emissions and increasing fruit quality for the consumer.
While stepwise cooling is reported (by Little and Holmes in their book on apple and pear storage) to reduce the incidence of chilling injury, the effectiveness of this treatment is not provided. Further, a search of the scientific literature has identified that while it is reported that stepwise cooling reduces the incidence of chilling injury in banana, cucumber, eggplant and Yali pears there is no published scientific trials reported on the impact of stepwise cooling on the appearance of chilling injury in apple.
Back in 2002, Angelo Zanella in Italy sent through his storage trial results which showed that stepwise cooling reduced the incidence of internal browning from 46 per cent down to 18 per cent of fruit, similar to storing fruit at 2.5°C for mid season harvest and after five months of storage and a period of shelf life. After this study he recommended step wise cooling, one week at 5°C while the room was filling, one week at 4°C, one week at 3°C and then hold fruit at 2.5°C. No internal browning was encountered in the following season when this cooling strategy was used.
APAL would like to acknowledge the original work of the researchers involved in the AP04008 project:
- Jenny Jobling Applied Horticultural Research Pty Ltd (Project Leader)
- Hannah James University of Sydney
- David Tanner Food Science Australia
- Ian Wilkinson IHD Knoxfield
- Gordon Brown Scientific Horticulture (Tas)
- Tony Portman Agriculture WA
- Stephen Morris Sydney Postharvest Laboraotory
- Stuart Tustin HortResearch NZ
- Beth Mitcham University of California, Davis USA
- Angelo Zanella Laimburg Research Station, Italy