What have we learnt since 2015? By Anna Mouton.
In 2015, Fuji apples shipped to Asia developed severe internal browning — up to 80% in some batches. Ever since, researchers in the fruit industry set out to find why this happened, and how to prevent future occurrences.
“In physiological disorders you get an alignment of stresses,” says Dr Ian Crouch, director of research at ExperiCo. “All the factors just line up — like dominoes.” When certain growing conditions and post-harvest practices combine, one falling domino can trigger a disaster.
Getting to know internal browning
The clue is in the name: internal browning refers to brown discoloration of fruit flesh. The peel is undamaged. Internal browning is not the only disorder to cause browning, but the brown areas remain firm. In other physiological disorders, the flesh becomes soft. Cripps Pink clones and Fuji apples have been most affected by internal browning in South Africa.
Fuji has been grown locally since the mid-1990s. “And right back then, when Unifruco Product Development was looking at the cultivar, they found that, even though it tasted wonderful, it had browning issues,” recalls Crouch. Even so, Fuji was a success and orchards have increased fifteenfold between 1995 and 2017. No surprise then that the losses of 2015 prompted a concerted research effort.
Crouch describes a multipronged attack. The first step was to share pictures and a description of the disorder to an international network of post-harvest scientists. Feedback soon pointed to carbon dioxide as a potential cause of internal browning. “One of the people who really made an impact on us was Professor Emeritus Michael Reid, from the University of California, Davis,” says Crouch. The industry subsequently brought Reid to South Africa as a technical expert.
Crouch also conducted a literature review. “We firmly believe that the cheapest research is somebody else’s,” states Crouch. “I read all the articles I could find on internal browning.” Available research confirmed the role of carbon dioxide (CO2) injury. Hortgro published a summary of the literature review, including guidelines for control, in a Fresh Notes.
CO2: not just a greenhouse gas
So how does carbon dioxide cause browning? “Any ripening fruit produces CO2,” explains Crouch, “and if that CO2 can’t leave the fruit, it leads to leaking of phenolics in the cells and that gives browning. Factors that prevent normal diffusion of CO2 out of the fruit promote browning.” Ethylene contributes to browning by increasing respiration and therefore CO2 production. Ethylene may also indirectly affect the leakiness of cell membranes to phenolics.
“Normally when we talk carbon dioxide damage, we’re thinking of build-up in controlled-atmosphere storage or in bags,” says Crouch. “What we didn’t realise is that, even in the orchard, there are all kinds of factors that also prevent diffusion of CO2 — like the density, peel, and size-volume ratio of the fruit.
“We put together projects linked to some of the hypotheses around CO2. But we can’t reproduce all of the dominoes,” adds Crouch. “You can’t make a fruit do something that it’s not programmed to do. That’s always a problem in research.”
Experiments showed that exposure to extreme CO2 levels immediately after harvest can produce internal browning. Apples from different orchards vary in their response with some more susceptible than others. “The idea was to rank orchards according to this simple test,” clarifies Crouch, “to see if we could link the predicted risk to actual development of internal browning in storage.
“But we only got low levels of browning during storage, so it was difficult to show a correlation.” Preharvest risk factors identified so far include suboptimal harvest maturity, harvest after 180 days post bloom and previous occurrence of internal browning in an orchard.
Keep those browning blues away
At harvest, fruit should be moved to the cold store as soon as possible—but not cooled too quickly! Fruit harvested on a warm day can undergo severe stress during rapid cooling. “A darker fruit like a Pink Lady will be hotter than ambient,” Crouch points out. Reduce stress by using step-down cooling. Start storage at 2.5˚C and low humidity, followed by gradual cooling over seven days.
“Even if you’re going to put fruit in controlled atmosphere, don’t do it immediately,” advises Crouch. “Give it a couple of weeks to acclimatise. Cooling is one stress and controlled atmosphere is another. So it’s like a domino effect with all these stresses on top of preharvest stresses — we don’t even know what all of those are.”
Long-term storage under regular atmosphere increases the prevalence of internal browning. Browning may occur as early as three weeks into storage, so fruit stored under regular atmosphere should be packed no later than three weeks post harvest.
Keeping CO2 levels to a minimum — ideally less than 0,5% — is essential for controlled-atmosphere storage. This is most critical during the first four to eight weeks. Note that late-harvested fruit is not suitable for controlled-atmosphere storage.
CO2 build-up in packaging and during transport must be avoided. Increase the re-cooling period from two to five days and use delivery air not colder than –1˚C.
The guidelines for control have recently been updated and include best practices for both internal browning and lenticel breakdown in Fuji apples.
Cracking under pressure
“The problem with research is that it’s very difficult to predict when you’re going to have the same problem,” observes Crouch. Internal browning has receded since 2015, hampering efforts to understand the disorder.
New research suggests that markets affect internal browning. Crouch wonders whether better cold-chain management on the receiving end is one reason why browning has decreased. “Maybe they’re now trying to look after the fruit better when it arrives.”
But producers have no reason to relax. Crouch says that cracking is the new problem with Fuji apples — to the extent that some producers are removing trees. “We may be able to solve the browning. The cracking may be something that we can’t prevent.”
Image: Typical signs of Fuji internal browning.
Supplied by Kenias Chigwaya | ExperiCo