A recent Hortgro-funded project assessed risk indicators and storage protocols for improved superficial scald management in Granny Smith apples. By Anna Mouton.
Superficial scald is a postharvest disorder of apples and pears characterised by areas of brown or black skin discoloration. The underlying flesh is normal, but the fruit’s appearance disqualifies them from fresh markets.
“If more than 1% of the fruit is affected — even with small spots — the entire batch is downgraded,” says Monja Gerber. Stringent standards are necessary because superficial scald intensifies during shipping and shelf life.
Granny Smith is the most scald-susceptible apple grown in South Africa. Gerber investigated superficial scald physiology in this cultivar for her PhD. Her research supervisor was Dr Elke Crouch, Postharvest Physiology Research Chair in Deciduous Fruit in the Department of Horticultural Sciences at Stellenbosch University. The ARC Infruitec-Nietvoorbij, ExperiCo and the University of the Western Cape collaborated on the project.
“The project’s main aim was to see which postharvest treatments work,” explains Gerber, “but also to look for possible drivers that could indicate an increased risk for superficial development before fruit is shipped.”
The evils of ethylene
Superficial scald occurs when reactive oxygen molecules damage fruit cells. The destructive molecules include oxidation products of a-farnesene and other compounds. Gerber measured the levels of different molecules in the biochemical pathways involved in superficial scald in Granny Smith apples stored according to 12 protocols — see Table 1.
The apples were harvested at pre-optimal maturity — ±20% starch breakdown — to increase their scald susceptibility. Gerber performed biochemical measurements, maturity indexing, and superficial scald assessments after 4, 8, 16, 24 and 33 weeks of storage at 1 °C, followed by six weeks of simulated shipping at -0.5 °C and 10 days of 20 °C shelf life in regular atmosphere (RA).
She found that internal ethylene had the strongest correlation with superficial scald development.
Gerber cautions that any ethylene in a cold room is a red flag. “I would say that you need to do another 1-MCP [1-methylcyclopropene] treatment if you want to store the fruit longer. Alternatively, you might decide to send it to a closer market.”
Table 1
Storage treatments used in trials. Storage was followed by six weeks of simulated shipping and 10 days of 20 °C shelf life at RA.
Dominant storage conditions1,2
Treatment at harvest
Treatment
after removal from storage3,5
RA
±20% O2. 95% RH.
–
–
DPA3
–
1-MCP4
–
CA
1.5% O2. 1.0% CO2.
–
–
1-MCP4
–
1-MCP4
1-MCP4
1-MCP4
7 days of DCA-CF ±0.4% O2. 0.7% CO2.
1-MCP4
1-MCP4. 7 days of DCA-CF ±0.4% O2. 0.7% CO2.
DCA-CF
±0.4% O2. 0.7% CO2.
–
1-MCP4
RLOS + ULO
Cycles of 0.5% O2 for 10 days
followed by 0.9% O2 + 0.8% CO2 for 21 days + 0.5% O2 for 7 days.
5Treatments were applied after removal from 24 weeks and 33 weeks of CA storage.
1-MCP for the win
Gerber’s trials ran for three seasons. When considering storage without 1-MCP application, dynamic controlled atmosphere-chlorophyll fluorescence (DCA-CF) storage inhibited superficial scald for longer than controlled atmosphere (CA) storage in two out of three seasons. Its failure in the other season was ascribed to a malfunction of the nitrogen generator that led to oxygen levels fluctuating above 1%.
“The fruit quality is very good in DCA-CF, but you need a system that functions 100%,” comments Gerber. “During the season where we had problems with the DCA, we observed scald earlier than in the other two seasons, although still only after shelf life.”
Lower oxygen levels, such as those maintained in DCA-CF storage, are associated with slightly elevated ethanol levels in fruit. Ethanol seems to protect the fruit against oxidative stress during storage but is associated with more significant oxidative stress after shelf life. This could be due to accelerated ripening and senescence.
According to Gerber, her main finding was that superficial scald after long-term storage was entirely prevented by 1-MCP application before CA storage, followed by a second 1-MCP application on removal from storage but before shipping.
“That second 1-MCP treatment worked every time,” says Gerber. “It’s a safety net in case undetected metabolic changes cause fruit to start making ethylene again.”
However, she cautions that a second 1-MCP treatment is not a panacea for ethylene exposure during storage. “Low levels of ethylene can induce superficial scald that treatment with 1-MCP cannot reverse. But the double treatment is definitely advantageous if applied before internal ethylene is detected for Granny Smith, especially if you’re shipping far.”
Start in the orchard
Despite being a postharvest researcher, Gerber emphasises the impact of preharvest factors on postharvest disorders. “At the end of the day, it’s about management,” she reflects. “My samples showed that ethylene levels were much higher in seasons with more mixed maturities. There was much more superficial scald.”
Although 1-MCP can paint over some of the differences of mixed maturities, Gerber recommends trying to keep potentially ethylene-generating fruit out of long-term storage. “It only takes one apple that’s more mature than the others to trigger all of them,” she warns.
Going forward, Gerber hopes to continue her research into predictors of superficial scald. Her work so far only examined fruit from one farm, but she wants to source more varied fruit.
“It doesn’t help to have a wonderful model, but it only applies to a specific orchard,” she says. “You want to include more variation so the model applies to everyone.”
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