Low-oxygen strategies against scald

Initial and repeated low-oxygen stress can extend the scald-free storage life of Packham’s Triumph pears. By Anna Mouton.

Packham’s Triumph remains South Africa’s biggest pear cultivar by area planted and volume exported — nearly 40% of pear cartons we shipped in 2022 contained Packham’s. Good postharvest quality is one factor helping this veteran hold its own against more modern cultivars.

But Packham’s pears have one postharvest shortcoming: they are susceptible to the disfiguring brown and black skin discoloration known as superficial scald. Superficial scald is initiated early in storage, but signs usually only show up within a week of removal to ambient temperatures.

Superficial scald is readily controlled by drenching fruit with the antioxidant diphenylamine (DPA), but this synthetic chemical is no longer acceptable to many of our markets. Treatment with 1-methylcyclopropene (1-MCP) immediately after harvest is not advised for Packham’s as it prevents further ripening.

“When I started here in 2015, our first work was looking at DCA-CF [dynamic controlled atmosphere-chlorophyll fluorescence] for Packham’s,” recalls Dr Anél Botes, a postharvest researcher at the ARC Infruitec-Nietvoorbij. “Kobus van der Merwe did the initial research .”

Van der Merwe, of the ARC Infruitec-Nietvoorbij, and MSc student Melrose Ramokonyane, of the Department of Horticultural Science at Stellenbosch University, found that DCA-CF prevented superficial scald for up to seven months’ storage — the longest period he assessed.

Storage in DCA aims to provide fruit with the minimum oxygen required to avoid fermentation. Fermentation allows fruit to generate energy for survival without oxygen but produces ethanol as a by-product. Too much internal ethanol spoils the fruit’s flavour.

Different DCA systems use different technologies to establish the fruit’s oxygen requirement. The ARC researchers used DCA-CF, which measures the chlorophyll fluorescence of the fruit peel. A spike in chlorophyll fluorescence indicates the low-oxygen limit. Oxygen is adjusted to 0.2% above this level, while CO₂ is kept at less than 1%.

Although DCA maintains the quality of Packham’s during storage, the pears ripen rapidly and turn yellow once removed. This can be countered by applying 1-MCP after storage and immediately before packing and shipping. The fruit still ripens, but retains its green colour and shelf life.

Initial low-oxygen stress (ILOS)

“We know that DCA works,” says Botes, “but not all pack houses have DCA facilities. Low-oxygen stress seemed a good alternative because it can be applied in a normal CA [controlled atmosphere] room — the room must obviously not leak, or you’ll never reach the low oxygen levels required.”

Results of trials conducted in 2012–2013 by Ramokonyane and Van der Merwe compared superficial scald development in regular atmosphere (RA) plus ILOS, CA with and without ILOS, and DCA-CF to a control in RA.

The ILOS consisted of holding fruit at 0.5% oxygen for 10 days before transferral into RA or CA rooms. Fruit quality and superficial scald were assessed after two, three, five, and seven months’ storage, followed by a six-week shipping and a three or seven-day shelf-life period.

The Packham’s pears stored in RA developed scald after only three months — none of the other treatments scalded this soon. Conventional CA without ILOS limited scald to 0.5% after five months’ storage.

ILOS extended the scald-free storage period for fruit in RA to five months and CA to seven months. However, DCA-CF came out on top with zero scald after seven months — oxygen levels in DCA-CF are low from start to finish.

Superficial scald is an oxidative process linked to ethylene levels. Low oxygen levels inhibit ethylene production, while exposure to ethylene stimulates the generation of more ethylene in what soon becomes a runaway process.

Early application of control measures is critical to scald prevention. Botes recommends treating with ILOS within five days of harvest. “The stress at the start is the important stress,” she notes.

“With most cultivars, you have to decide your goal,” says Botes. “If the fruit quality isn’t so good that you want to store it long-term, then ILOS with RA is an option that can give you five months of scald-free fruit. Or you can use ILOS with ultra-low oxygen (ULO) and easily store Packham’s for ten months without any superficial scald.”

Repeated low-oxygen stress (RLOS)

Whereas ILOS is based on a single low-oxygen stress at the start of storage, RLOS consists of repeated low-oxygen stress events. In 2017–2019, Botes investigated RLOS for Packham’s Triumph as part of a Hortgro-funded project.

Pears were harvested at optimum maturity and cooled to -0.5 °C within a day. They were then stressed by exposure to 0.5%–0.6% oxygen and 0.6%–0.8% CO₂ for 10 days. Different treatment groups received one to nine repetitions of one week of similar low-oxygen stress at monthly intervals.

The RLOS trials were conducted in CA and ULO storage with the control in RA. Oxygen levels were 1.5% and CO₂ levels 2.5% in CA storage and oxygen levels were 0.9% and CO₂ levels 0.8% in ULO storage when fruit were not undergoing low-oxygen stress.

Botes evaluated fruit quality, ethanol levels, and superficial scald development after two, four, six, eight, and 10 months of storage, followed by six weeks of shipping and handling and a seven-day shelf-life period.

Nearly 60% of the pears held in RA developed superficial scald after only four months in storage, and all of them were scalded after eight months. The RLOS-ULO treatments all completely inhibited superficial scald, while the RLOS-CA treatments prevented scald after eight but not after 10 months of storage.

Ethanol levels increased but weren’t as problematic as feared. “The guidelines for Packham’s stipulate levels of 100–150 ppm,” says Botes. “In fruit that we stressed every month for 10 months, the ethanol concentrations sometimes reached 700 ppm. But the fruit lost that ethanol during the six-week shipping period.”

She cautions that fruit may not be able to metabolise ethanol should levels become too high. “We were never able to pinpoint the problem value,” she admits.

Although DCA remains the best option for long-term storage of Packham’s Triumph, this research shows that low-oxygen stress is a viable alternative for pack houses that lack DCA facilities.