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Internal browning – what causes it and how to reduce the risks

Research & Extension

The 2021 season saw higher reports of internal browning in Pink Lady apples across Australian apple growing regions. This webinar, presented by Dr Hannah James, reviewed the different types and causes of internal browning disorders and how risks associated with internal browning can be identified and managed, both in the orchard and postharvest.

What is internal browning?
Internal browning is caused by the oxidation of phenolic compounds that occurs when cells in apples are damaged. This is similar to the browning reaction that happens when you cut an apple and expose the surface to air.

There are different types and causes of internal browning disorders
Three main types of internal browning have been identified in Pink Lady apples – diffuse (chilling injury), radial (maturity or senescence related) and CO2 injury (storage atmosphere). Diagnosing the type of internal browning is important so that the appropriate management strategies can be employed.

What causes internal browning?
Both pre-harvest and post-harvest factors can contribute to the risk of internal browning developing. Hannah explained that pre-harvest factors (eg. fruit maturity, climate, nutrition, crop load) establish the risk for the given season and the way fruit is handled (eg. storage conditions) post-harvest can be used to manage the risk.

How can the risk be reduced?
Managing internal browning disorders requires an integrated approach – both pre- and post-harvest. Seasonal climate can help to determine if the risk of internal browning is likely to be low, moderate or high. In her presentation, Hannah describes how looking at the accumulation of growing degree days can be used to help determine the risk. This information can then be used to help make decisions about pre-and post-harvest management strategies.

Internal browning is more likely to occur in fruit that is harvested at less than optimum maturity or stored at temperatures and CO2 levels that are also not optimal.

Pre-harvest – Make sure that fruit is harvested at optimum maturity by testing maturity ahead of the predicted harvest. Aim for consistent maturity in bins.

Post-harvest – Manage temperature, CO2 and the fruit inventory to optimise storage conditions and duration.

Calculating growing degree days

You can find an example of a calculator for growing degree days here:  http://grf-smartfarm.daf.qld.gov.au:3838/apps/chillcalculator/ or tinyurl.com/applegdd

This site accesses daily weather station data for 600 locations across Australia. The data can be used to calculate the accumulation of winter chill and growing degree days or hours

Best practice for harvest and storage
Recommendations for harvest maturity for optimal storage in Australia:

  • Starch Pattern Index of 2.0-3.5 (using a 1-6 SPI scale; eg. Little C (2011). Apple and Pear Maturity Assessment)
  • Minimum firmness of >8 kg
  • Brix of >14%

Storage atmosphere
SmartFresh to be applied as close to harvest as possible and must not exceed 5 days from harvest to provide control of superficial scald.

Pink Lady are a CO2 sensitive variety. CO2 should be maintained at <1% during the first 6-8 weeks of storage. It is critical that CO2 levels are monitored during the room loading and cooling periods.

You can contact AgroFresh for more information on managing CO2 levels for sensitive varieties.

If the fruit are going into CA storage, CO2 levels should be maintained at or below 1%.

Storage temperature
Pink Lady are a chilling sensitive variety. A stepwise cooling regime should be followed. A standard stepwise cooling regime would be to set the room to 4°C during fruit loading, then cool by 1°C per week until the destination temperature is achieved.

Managing the risk

  • Harvest maturity
    • Harvesting fruit at optimal maturity is critical to manage the risk of developing internal browning
    • Start checking maturity in the field well ahead of predicted harvest
    • Plant growth regulators can help to hold maturity around weather and labour issues
    • Harvest fruit in a manner to get consistent maturity in the bins (multiple picks)
  • Postharvest management
    • Temperature management: do not rapid cool fruit, follow a stepwise cooling regime
    • CO2 management: monitor CO2 levels, especially over the first 8 weeks of storage, try to keep CO2 levels below 1% as much as possible
    • Inventory management: where pre and postharvest risks cannot be well managed, identify this inventory and be aware that it is higher risk for developing internal browning during storage; plan accordingly.

 

Further information:

Managing maturity pre-harvest

ANABP 01A apple maturity testing: A step-by-step guide

Improving Your Apples with ReTain®

Harvest timing key to consistent quality

Content is for education and information. Please check with your crop advisor or AgroFresh for more information and follow any product labels.

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