Factors influencing flesh browning in ‘Cripps Pink’ apples

PRINCIPAL INVESTIGATORS: Dr Ian Crouch and Dr Elke Crouch
Dr I Crouch: +27 21 887 1134 / ian@experico.co.za
Dr E. Crouch: +27 21 808 4763 / elke@sun.ac.za
DURATION: Two years
LEAD INSTITUTIONS: ExperiCo (Fruit Technology Solutions) and Stellenbosch University (Department of Horticultural Science)
Beneficiary: The pome fruit sector

The occurrence of flesh browning in ‘Cripps Pink’ apples caused major economic losses to the South African fresh fruit export industry in recent years. Research by Dr Jenny Jobling in Australia provides interesting theories on the causes, but not much is known about this post-harvest condition in South Africa. The effect of South African harvest and cold storage factors on the development of flesh browning in ‘Cripps Pink’ apples was studied in a project conducted by ExperiCo (Fruit Technology Solutions) and the Department of Horticultural Science, Stellenbosch University. The Post-Harvest Innovation Programme co-financed the project, together with Fruitgro Science, on behalf of the South African Apple and Pear Producers’ Association (SAAPPA).

Factors influencing the development of flesh browning

This project was made possible by the equal input and contribution of everyone involved – the ExperiCo team: Dr Ian Crouch and Heleen Bergman, and the Department of Horticultural Science team: Dr Elke Crouch, Mariana Jooste and MSc student, Joanna Majoni.

Initial findings confirmed some of the Australian results presented by Dr Jenny Jobling. The South African study focused on the factors believed to have the biggest influence on the development of flesh browning in ‘Cripps Pink’ apples. These included storage temperatures and duration, cooling rate (rapid cooling versus stepwise cooling), harvest maturity, cell membrane studies and biochemical antioxidant analysis.

Cold storage and biochemical analysis results over two seasons

In 2009, flesh browning was mainly found in asymmetric fruit. This type of browning is known as bulge browning and can be eliminated during packing.

In 2010, diffuse flesh browning was the main type of browning, associated with a loss in cell membrane integrity. The polyunsaturated fatty acids were lower in brown fruit. The higher saturated:unsaturated fatty acid ratio indicated leakiness of the membrane that led to flesh browning.

  • Higher storage temperatures retarded the development of flesh browning – it took much longer to develop when fruit was kept at 2.0°C (instead of the usual -0.5°C) in controlled atmosphere (CA) storage. However, it also increased the risk of greasiness and peduncular scald development.
  • Fruit stored at -0.5°C indicated diffuse flesh browning, with a much higher concentration of total phenolics and a low ascorbate (antioxidant) content.
  • Cooling rate trial results were inconclusive on the development of flesh browning. However, stepwise cooled fruit were generally firmer with a more intense overall skin colour development than rapidly cooled fruit. Total phenols were also higher for stepwise cooled fruit.
  • When fruit was rapidly cooled to -0.5°C, the oxidised glutathione and ascorbate levels increased, which is an oxidative stress response.
  • Harvest maturity has a direct influence on flesh browning – much more flesh browning occurred in fruit harvested at advanced maturity than fruit harvested at optimum maturity, regardless of the growing area.
  • Cell membrane and antioxidant test results confirmed the influence of harvest maturity. Late harvested fruit showed higher stress levels and more advanced senescence.
  • The total ascorbate (antioxidant that prevents browning in other fruit) of late harvested fruit decreased between one and five months of CA storage, whilst it took five to seven months to decrease in fruit harvested at optimum maturity.
  • On the other hand, reduced glutathione increased more in fruit harvested at optimum maturity than in fruit harvested later.

Outcome of study and benefits to industry

  • The study proves that harvest maturity, storage temperature and duration are critical factors influencing the potential development of flesh browning in ‘Cripps Pink’ apples.
  • Thus, producers can optimise export yields to increase profitability.
  • Due to the influence of harvest maturity, producers should not wait too long for colour development before harvesting.
  • Fruit stored for five months or longer developed flesh browning, which clearly shows the influence of storage duration.
  • Biochemical indicators (total, oxidised and reduced ascorbate) must be further studied for use as early indicators of stress and flesh browning.
  • Fruitgro Science (on behalf of SAAPPA) will finance further research to define the influence of pre-harvest factors on the development of flesh browning in ‘Cripps Pink’ apples.