Successful storage to prevent superficial scald

Technology can safely maintain fruit at minimal oxygen levels during long-term storage to effectively inhibit superficial scald in Granny Smith. By Anna Mouton.

The idea that oxygen removal can prevent superficial scald in apples isn’t new — it was first explored by Powell and Fulton in 1903. But they also recognised that lack of oxygen would eventually suffocate the fruit.

Research has since confirmed that fruit store longer if their metabolism is slowed by oxygen reduction. When oxygen drops too low, the fruit survives by switching to fermentation, which generates quality-degrading by-products.

“People used to think that the ideal oxygen level is where respiration is at a minimum without any ethanol formation,” says Braam Mouton, technical postharvest manager at Dutoit Agri. “But going from aerobic to anaerobic respiration isn’t like a light switch — it’s more like a slider.”

Mouton has been studying different systems for controlled atmosphere (CA) storage for his master’s degree as part of a Hortgro-funded project. His supervisor is Dr Elke Crouch, Postharvest Physiology Research Chair in Deciduous Fruit in the Department of Horticultural Sciences at Stellenbosch University.

Oxygen makes up about 21% of the atmosphere, and conventional CA systems reduce oxygen levels to around 1.5%. Mouton looked at the effect of further lowering oxygen using dynamic controlled atmosphere-chlorophyll fluorescence (DCA-CF), dynamic controlled atmosphere-respiratory quotient (DCA-RQ) and extreme low oxygen (XLO). He also investigated the impact of ethylene scrubbing.

“All these systems regulate oxygen,” he says. “I looked at the oxygen data to see what happens to the fruit’s physiology.”

DCA versus XLO

Granny Smith apples were harvested at pre-optimal maturity in two seasons. No 1-methylcyclopropene (1-MCP) was applied. The susceptibility of the apples to superficial scald was confirmed by holding a sample in regular atmosphere for 16 weeks plus shelf life — almost all these fruit developed scald.

The low-oxygen treatments were stored for 42–43 weeks at -0.5 °C. Superficial scald was assessed immediately post-storage and after a six-week simulated shipping and 10 days of shelf life at 20 °C.

Oxygen levels in the DCA-CF systems were generally at the lower oxygen limit of 0.4% ± 0.2% safety margin. Mouton notes that managing the small-scale experimental DCA-RQ system was challenging because regulation depends on measuring the ratio of CO₂ production to oxygen consumption. The greater surface-to-volume ratio of a small container magnifies the impact of leaks on these values.

The XLO treatment was included to represent a static system. “In reality, oxygen levels in the extreme low-oxygen treatment were not so low,” comments Mouton. During his trials, fruit were exposed to the highest cumulative oxygen in this system.

Oxygen levels in the XLO system were nearly triple those in the DCA-CF system in the first season and nearly double in the second season.

None of the DCA treatments showed superficial scald. High oxygen levels of more than 1.5% in the XLO system in the first season led to uncontrollable ethylene production and unacceptable scald development. Oxygen levels were reduced in the second season, but 0.8% of one XLO replicate nonetheless had scald after the six-week simulated shipping.

The ethylene connection

Superficial scald is an oxidative process resulting from cumulative oxygen exposure. Mouton analysed cumulative oxygen for the different treatments and found that oxygen levels positively correlated to scald occurrence and severity.

“With Granny Smith, which gets superficial scald, you need to drop the oxygen levels as fast as you can and keep them as low as you can,” he says. “The reverse is true for cultivars sensitive to CO₂ damage — those need step-down protocols.”

Achieving low temperatures and oxygen levels promptly stops the fruit from generating ethylene. Ethylene is associated with superficial scald and accelerated ripening. Its effects are difficult to control because ethylene promotes the production of more ethylene.

Two of Mouton’s treatments included ethylene scrubbers. “In the DCA-CF and DCA-RQ systems, the oxygen levels were so low that there was never enough ethylene to need scrubbing,” he recalls. “And scrubbing ethylene out of a system where oxygen levels are twice as high doesn’t work.”

One problem with scrubbers is that they can’t remove ethylene from inside the fruit. “You’re depending on passive gas diffusion to get that ethylene out,” says Mouton. “So, in my opinion, prevention is better than cure, which comes down to the oxygen levels used during storage.”

What happens in practice?

When correctly applied, DCA storage can improve storage life and reduce postharvest defects by maintaining optimal oxygen levels. It can be implemented in existing CA stores, provided these are airtight and allow effective oxygen removal and CO₂ scrubbing.

Mouton notes that oxygen levels are pulled down faster in DCA-CF than in DCA-RQ, translating into better superficial scald control with DCA-CF. “One should be led by cultivar characteristics when choosing your storage techniques,” he advises.

However, he has observed that many pack houses use DCA-CF to set oxygen levels at the start of storage and then run it as a static system.

He worries that pack houses might measure chlorophyll fluorescence on too few samples. “Representative samples are pivotal, but they remain a challenge,” he says.

The performance of DCA — and other — stores can be improved by loading them with uniform fruit. “If you compare an enormous fruit to a tiny one, then gas diffusion will be much easier in the small fruit,” Mouton explains. “We remove oversized and undersized fruit in our trials to reduce our error margin, but if you have a commit-to-pack pack house, you store all those sizes together and pack them in their respective counts.”

Variability can be especially troublesome in Granny Smith because the trees are often planted as cross-pollinators. This can lead to mixing fruit from several orchards in the same cold room.

Although Mouton omitted 1-MCP in his trials, he thinks using it will buy fruit time while negotiating the long and often unreliable cold chain to overseas markets. “If your DCA storage was successful and you lost no time between opening the room and shipping, it works. But do you want to take that risk?”