Internal browning in Fuji

The chemistry behind browning

The process of internal browning in Fuji and other cultivars is basically the same as that which causes apple slices to turn brown.

Apples contain various phenolic compounds that play an important role in normal metabolism. Phenolic compounds can be changed into quinones by enzymes called polyphenol oxidases. Quinones react with amino acids to form pigments – melanins – that show up as brown discoloration.

Phenolic compounds and polyphenol oxidases are usually kept in separate membrane-bound containers inside cells. But weakened or damaged membranes can allow these compounds to escape and interact, resulting in browning. "It's a physiological response to damage," explains Chigwaya.

Cultivars that have naturally high levels of phenolic compounds, as well as high levels of activity of polyphenol oxidase, are more prone to browning. The link between high phenolic content and browning potential in Fuji has been noted in more than one study.

For example, in a French research project, the browning response after bruising was assessed in 11 cultivars. Fuji, together with Red Delicious and McIntosh, showed the greatest degree of browning, as well as the highest concentration of oxidised phenolic compounds. Red Delicious is of course a parent of Fuji.

Understanding that leaky cell membranes lead to browning is only part of the story – what causes cell-membrane damage in the first place?

Energy, oxygen and carbon dioxide

Apples in storage are alive. They use oxygen to generate energy for maintaining cellular functions, producing CO₂ in the process. If oxygen levels in the fruit are too low, and CO₂ levels too high, the apples change their energy metabolism from aerobic to anaerobic respiration. Anaerobic respiration, also known as fermentation, is less efficient, so the fruit lacks energy to keep its membranes in good shape – phenolics and enzymes escape, and browning results.

For his doctoral degree, Chigwaya looked at why some Fuji apples seem to be more sensitive than others to the high CO₂ levels in controlled-atmosphere storage. He scanned fruit using X-ray computed tomography to see what was happening inside them.

Chigwaya found that low porosity and high density affected browning in Fuji. "Porosity refers to the intercellular spaces inside the apple. Gas transport takes place in these spaces – oxygen to the cells and carbon dioxide away from the cells. So fruit with high porosity can efficiently transport gases. If efficiency is low, the fruit is more prone to browning because carbon dioxide accumulates inside the fruit, pushing the fruit to anaerobic respiration."

Poor gas diffusion is also why water core can increase the risk of internal browning, explains Chigwaya. "In fruit with water core, the intercellular spaces are completely flooded. So they cannot transport oxygen and carbon dioxide, which makes the fruit very sensitive to high carbon dioxide conditions."

The structure and connectivity of the intercellular pores is another factor that may affect the sensitivity of fruit to internal browning. For example, research has shown that firmer cultivars such as Scifresh – marketed as Jazz – tend to have smaller intercellular spaces compared to softer cultivars such as Golden Delicious. Fuji has similar porosity to Golden Delicious, but Fuji has smaller intracellular spaces that are less interconnected, and therefore less able to transport gases, than Golden Delicious.

Preharvest risk factors for Fuji browning

Although correct postharvest handling is critical for prevention of CO₂ damage in Fuji, the susceptibility of fruit is largely determined by pre-harvest factors. "You should look at orchard history," says Chigwaya. "There are some orchards that are just very prone to internal browning."

As for many postharvest problems, fruit size and harvest maturity play a role. "Harvest maturity can sometimes be an issue if you wait for red colour to develop," comments Daniël Viljoen, senior researcher at ExperiCo Agri-Research Solutions. "You can develop very high starch breakdown levels, and that can be a risk."

In addition, scientists have shown that the intercellular spaces in Fuji can become blocked by clumps of cells as the fruit matures, obstructing gas movement.

Large fruit can be more susceptible to browning for two reasons, according to Viljoen. One is that large fruit have a smaller ratio of surface area to volume, and longer gas diffusion distances, than small fruit. The other is that minerals such as calcium are often diluted in larger fruit, leading to lower tissue concentrations, which also cause these fruit to ripen faster and store less well.

"If you have less calcium in your fruit, then the cells are weaker and can easily be damaged, leading to internal browning," agrees Chigwaya.

Whereas climatic conditions increase the risk of especially radial browning in Cripps Pink, the impact on Fuji is uncertain. "We haven't done studies but there is literature where the researchers found that a cooler spell during the cell division phase – which is just after full bloom – led to denser fruit," says Viljoen. "And denser fruit leads to more browning." It has also been suggested that previous episodes of Fuji browning may have been linked to rain before harvest.

Optimal postharvest handling

Predisposed Fuji may start developing browning from the moment they are stored in controlled-atmosphere conditions. But, says Viljoen, the problem usually becomes noticeable afterwards, when the fruit is exposed to the regular atmosphere. "That's when the fruit wakes up and respiration starts. In my experience, that's when we see defects."

After harvest, apples should be transported to the cold store as soon as possible. Fruit should never be left outside overnight to lose field heat – this can promote ripening and lead to senescence browning.

Once at the cold store, the fruit must be cooled slowly. Keep fruit at 5°C for the first week and then cool gradually to 1°C.
The current best-practice guidelines for Fuji recommend starting controlled-atmosphere storage after 14 – 28 days. Levels of CO₂ should ideally be below 0.5% – lower is even better.

"In the past, people used to store Fuji at a very high CO₂ concentration," recalls Viljoen. "After the browning year – 2015 – they started to pull down the CO₂ levels to as low as possible." This has gone a long way to preventing similar losses.

Cooling slowly and acclimatising fruit to controlled-atmosphere storage conditions give them time to adapt. Although the first few weeks are the most critical, Fuji must still be protected against high CO₂ levels right up until it reaches the market. This means using only perforated bags for packaging, and guarding against the build-up of CO₂ in shipping containers.

Research on Fuji browning continues. "Currently, we're doing a density trial to see to what extent density influences internal browning," says Chigwaya. This Hortgro-funded project aims to test whether density could be used as a predictor of browning. If successful, it will provide packhouses with another tool to ensure that only the best quality fruit is sent to market.

Featured Image: Cavities form in Fuji when lesions due to internal browning dry out during storage