What have we learnt over the past ten years? By Anna Mouton.
Internal browning is a post-harvest disorder in which the flesh of apples turns brown while remaining firm. The two main types are diffuse browning and radial browning. These can occur in combination — this condition is creatively called combination browning.
Internal browning is among the most important causes of losses during the post-harvest period, according to Dr Elke Crouch, a post-harvest physiologist with the Department of Horticultural Sciences at Stellenbosch University. But, she says, it’s very important to distinguish which type of browning affects your fruit.
Diffuse browning: all about maturity
Diffuse browning is a problem in fruit that was harvested at post-optimal maturity —characterised by starch breakdown of 40%–50% or greater. Preharvest factors that speed up ripening increase the risk of diffuse browning because fruit are more likely to be harvested when beyond their optimal maturity. “If you see your orchard is moving fast, the chances of that orchard storing well are not good,” cautions Crouch.
Preharvest factors that are associated with a greater risk of diffuse browning include younger trees and orchards on sandy soils. Crop load also plays a role in that larger fruit ripen faster and store less well.
Crouch explains the tension between optimal maturity and colour development. Better colour translates into higher value — but better storability enables growers to sell at a more favourable price. “People want to store long-term, and they want red fruit. Not all fruit will be able to make that grade.”
Diffuse browning can develop as early as three months into the storage period. Fruit that is left longer on a tree so that it can colour — typically those on the inside of the canopy — has a greater risk of browning and Crouch doesn’t recommend storing these fruit for more than three months.
Radial browning: a seasonal problem
Radial browning seems most common when fruit are exposed to cool weather during the cell enlargement phase. This may lead to denser fruit which could slow the escape of carbon dioxide — high tissue carbon dioxide levels are damaging to cells. Maturity may also impact gas exchange but is not a determinant of radial browning to the same extent as it is for diffuse browning.
Tree age and soil type do not affect the occurrence of radial browning.
Radial browning was thought to develop early in storage and packhouses use hyperspectral sorting to eliminate brown fruit prior to shipping. “But you risk that fruit goes brown on the water,” says Crouch. Her team’s data showed that radial browning can become worse during shipping and shelf life. This is especially true for high-risk seasons and after prolonged storage.
Best practices for storage
The stage at which you harvest is critical and fruit that has passed optimal maturity is likely to develop internal browning. “The bulk of your fruit will have to be sold before six months,” states Crouch. “If you want to risk storing fruit, which fruit are you going to choose for that?” She advises growers to measure starch breakdown to inform their decision.
Step-down cooling has been effective in reducing the occurrence of diffuse browning. New research confirms this and shows that step-down cooling is also essential for the control of radial browning. Crouch thinks that newer cultivars may be more sensitive to chilling injuries. “All the new protocols have very long step-down cooling,” she observes, “not like the instant we used to have.”
Internal browning in Cripps Pink is well-understood, according to Crouch. “We know that if you harvest them over-mature and store them at really low temperatures you will get diffuse browning,” she summarises. “We know that in some seasons we get radial browning and in some seasons we don’t and generally the cooler seasons your risk might be higher.”
Browning in other cultivars is not always as simple. “In other cultivars, like Fuji, we’re still trying to figure all the browning patterns out,” says Crouch. “One needs research to make a storage recipe for each specific fruit and each type of browning.”
Image: Combination browning affects both the cortex and the vascular tissue.
All images supplied by Ian Crouch | ExperiCo.