When pink apples go brown

The many browning disorders of Cripps Pink clones. By Anna Mouton

“What’s amazing about browning in the Cripps Pink clones is that there are so many kinds,” says Dr Elke Crouch, post-harvest physiology and technology researcher at Stellenbosch University. “And every single type of browning is different: both in the factors that cause them and how we rectify them.”

Browning of the flesh in Cripps Pink clones fall into five categories: diffuse, radial, combination and bulge browning, and carbon dioxide (CO₂) damage. Diffuse browning is the most common kind in South Africa. “The rest of the world has a problem with radial browning,” states Crouch, “and that’s difficult to rectify because we don’t really know why it occurs.”

In diffuse browning the cortex is affected whereas in radial browning it’s the vascular tissue. Combination browning shows discoloration of both areas. Browning is not present at harvest — it develops after several months of storage.

Getting rid of diffuse browning

Crouch and her colleagues have been researching browning for several years. “We started with post-harvest — I think the post-harvest work is really important because that’s where you can make a big difference.

“You have to harvest at less than 40% starch breakdown. You cannot store long-term if you don’t have that. If you harvest over 40% starch breakdown, your chances of getting diffuse browning are really good.”

The second critical factor is storage temperature. “It seems like slightly higher storage temperatures actually prevent diffuse browning,” explains Crouch. She recommends storage at 2˚C. Application of 1-MCP (1-methylcyclo propene or SmartFresh) will reduce the risk of greasy fruit.

Studies of preharvest variables indicate that factors which speed up ripening — such as sandy soils and younger trees — also increase the risk of diffuse browning. “We found that anything that enhances maturity led to more diffuse browning. Or combination browning because you need diffuse browning to get combination browning,” says Crouch. “Radial browning wasn’t related to any of those factors.”

According to Crouch, the danger of diffuse browning is greater when the season has favoured early ripening. Producers need to be aware of this risk and adjust harvest schedules and storage decisions accordingly.

Other shades of browning

Although researchers overseas found harvest maturity to affect radial browning, this was not seen in South African studies. Instead, radial browning seems to be associated with seasonal factors, especially cooler seasons. “I think a lot of people are going to get radial browning this season,” warns Crouch. “I already see the claims coming through.

“Radial browning, like diffuse browning, only manifests with longer-term storage —  three to four months under controlled atmosphere. And you definitely want to take fruit out after five months if there is a risk of either of these.”

Crouch advises non-destructive sorting when radial browning is found on opening a room. In their study, radial browning didn’t increase much during shelf-life, so removal of affected fruit prior to export should go a long way toward avoiding claims.

In other countries, radial browning occurs in areas that accumulate more than 1 100 GDD (growing degree days) between full bloom and harvest. In these regions, it manifests during seasons with GDD in the range 1 100 to 1 700. Lower spring temperatures seem to increase later risk of radial browning, perhaps by affecting early cell division and expansion processes.

In contrast, bulge browning occurs when fruit is misshapen and browns on the deformed side. “The theory is that pollination was faulty,” says Crouch, “that something went wrong at fruit development.” The bulging side of the fruit is weaker and more susceptible to browning. This problem has not attracted much research interest — the prevalence seems low and badly shaped fruit are easily identified and removed during sorting.

CO₂ injury is characterised by small cavities in the cortex. It develops due to incorrect storage practices. “If you don’t cool the fruit down properly before you put it under controlled atmosphere,” explains Crouch, “then the fruit respires causing carbon dioxide build-up and low oxygen levels. Pink Lady especially doesn’t like that and develops carbon dioxide injury.” High respiration rates and rising carbon dioxide levels in a cold store indicate a problem and need investigation.

Digging deeper into browning

“There’s still a lot of research to be done,” stresses Crouch. An ongoing project is looking at long-term —  up to nine months — storage. “Nine months and a six-week shipping period and then seven days shelf-life: it’s risky and this season was very successful.

“Our last evaluation was toward the end of February this year, of fruit harvested last year in April — almost a year. We had very good results. But we need to repeat it,” cautions Crouch. “We need to confirm that it wasn’t just a good season.”

Orchard effects on browning also require further study. “The orchard differences are huge so there are definitely orchard factors involved in all of these types of browning,” says Crouch. “So it’s easy to say: ‘Harvest at the right time, store correctly’ but then one goes back to the orchard factors, and you can’t really put your finger on the cause.”

Crouch is optimistic that diffuse browning in Cripps Pink clones is controllable. “I think the problem is not as bad as it was because more people are doing what they’re supposed to be doing. But I think we also had a tough season — fruit are generally riper, so maybe things might be popping out. And we had a cool spring, so we are likely to see more radial.”

Image: Combination browning.

All images supplied by Ian Crouch | ExperiCo.