The following applies to all scald-susceptible fruit.

Preharvest risk factors

Hot and dry weather in the weeks before harvest. Increased risk could be the result of drought stress. Both increase ethylene and α-farnesene — important precursors of superficial-scald promoting compounds.

Larger fruit usually have lower calcium and high potassium and nitrogen levels.

Low crop load and vigorous trees. In addition to leading to larger fruit, shaded fruit are also more prone to superficial scald as they have lower levels of vitamin E and higher levels of α-farnesene.

Harvesting fruit too early increases susceptibility by lowering saturated fatty acid levels and antioxidant to α-farnesene ratios.

Mixed maturities within orchards. Pre-optimally harvested fruit are more prone to superficial scald while more mature fruit can release ethylene which initiates superficial scald in all the fruit. An even maturity helps in managing the disorder during storage.

Handling after harvest and during storage

Do not leave bins of fruit in the sun. Accumulate them in the shade before transporting them as soon as possible — the same day as they were harvested — to the pack house.

Load the fruit into the cold room as soon as possible — within six hours — after delivery to the pack house. Do not leave fruit outside the pack house in the shade or overnight to cool.

Cold-room temperatures must not fluctuate above 3°C–4°C while warm fruit is added. Excessive temperature fluctuation can impair the cooling of fruit and stimulate ethylene, which is an important initiation factor for superficial scald during the first week after harvest. Curtains that separate warm and cold fruit inside the cold room can help to reduce temperature fluctuation.

The cooling capacity of the cold room must be sufficient for the volume of fruit — monitor this. Loading two to three cold rooms simultaneously may achieve more even cooling depending on cold-room cooling capacity. Do not exceed the cooling capacity of the rooms.

Pulp temperature must reach 4°C after 48 hours.

Monitor CO₂ levels when warm fruit is closed in a cold room. Aim to keep CO₂ levels below 1%. Use additional lime or new-generation scrubbers to reduce CO₂ levels to 0.5%.

Avoid anything that promotes ethylene production or build-up. Although superficial scald is usually only seen after storage, it is initiated by ethylene during the first days of accumulation.

Cultivars need to be stored separately. Similarly, mixed maturities of the same cultivar in one cold store will result in ethylene from one batch affecting the rest of the fruit, thereby initiating superficial scald.

Granny Smith

The following guidelines apply to Granny Smith apples.

For red apples, superficial scald can be prevented by following the best postharvest practices for Cripps Pink.

What is the correct harvest maturity for long-term storage?

• Release: 20% starch breakdown.
• Optimum: 25% starch breakdown
• Post-optimum: more than 40% starch breakdown.
• The minimum starch breakdown according to DALRRD is 15% but starch breakdown below 20% increases susceptibility to superficial scald and core flush in high-risk seasons.

Manage ethylene to slow ripening and reduce greasiness and superficial scald by applying ethylene inhibitors (1-MCP) within 3–7 days of harvest. This will support storage in regular atmosphere for up to three months and in controlled atmosphere (CA) for longer than three months.

Should the cold room not be filled within three days, apply 1-MCP and repeat the treatment once the room is full. Vent the rooms well before loading and treating with 1-MCP. Check ethylene before and after treatment — levels should preferably be below 1 ppm. Leave the door open for a few hours to allow CO₂ to leave the room after treatment.

Fruit can be stored in CA (1.5% O₂ and 1% CO₂) or dynamic controlled atmosphere-chlorophyll fluorescence (DCA-CF) after 1-MCP treatment. Storage in CA is possible for up to 11 months but is not recommended when scald risk is high and should not be done without 1-MCP. In general, lower oxygen storage options are preferred (<1%) as these provide better ethylene control especially in the absence of 1-MCP use.

Monitor ethylene in cold stores — increased levels increase superficial scald risk. Ethylene should not exceed 3 ppm. A second 1-MCP treatment can be applied if ethylene rises above this level. Research has also shown that a second 1-MCP treatment after 24 or 34 weeks of CA storage inhibited superficial scald completely compared with an untreated control.

Granny Smith apples should be stored alone. Do not load them into a room previously loaded with other cultivars (especially high ethylene producers like Royal Gala) without venting properly.

Guidelines for O₂ and CO₂ levels in CA storage

O₂ levels of 3% and CO₂ levels of less than 1% are achieved by displacement with N₂ during the first 48 hours after the room is sealed. Thereafter, levels are maintained as set out in Table 1.

Recommendations for O₂ levels are based on recent research from Stellenbosch University that showed that the risk of superficial scald increases with cumulative O₂ exposure and time above 1% O₂.

Table 1

Target levels of O₂ and CO₂ in CA storage of Granny Smith. Note that long-term storage without use of 1-MCP is not recommended.

	%O2	%CO2
Minimum	0.6%	0.5%
Optimum	Below 1.0%	1.0%
Maximum	1.0%	1.5%

Guidelines for DCA-CF storage

The guidelines in Table 2 are based on research conducted at the ARC and updated with research findings for Granny Smith from Stellenbosch University.

It is good practice to monitor ethanol levels in fruit in DCA-CF storage.

Table 2

Guidelines for DCA-CF storage for Granny Smith apples and Packham’s Triumph pears. Fruit temperature at all times is 1.0 °C for Granny Smith and -0.5 °C for Packham’s Triumph. Higher fruit pulp temperatures during DCA-CF could lead to internal browning especially in Packham’s Triumph pears. LOL = lower oxygen limit.

	Period	%O2	%CO2
Load room and cool to core temperature within 48 hours of harvest.	5 days	–	Below 1.5%.
Seal room and reduce O2 to 3%	2 days	3.0%	1.5%
Reduce O2 to 1.5%.	2 days	1.5%	1.0%
Reduce to LOL.	2 days	Granny Smith Usually ≈0.4% Packham’s Triumph Usually 0.2%–0.3%	Below 1.0%
CA. O2 at 0.2% above LOL.	Remainder	Granny Smith LOL + 0.2% Usually ≈0.6% or below. Keep below 1% as cumulative O2 above 1% increases risk of superficial scald. Packham’s Triumph LOL + 0.2%–0.3%	Below 1.0%

Packham’s Triumph

These guidelines apply to Packham’s Triumph pears. The recommendations can also be applied to other scald-susceptible pears.

What is the correct harvest maturity for long-term storage?

• Release: ≈5 kg firmness.
• Optimum: 7.0–6.6 kg firmness.
• Maximum according to DALRRD: 8.0 kg.

Packham’s Triumph placed in DCA-CF within seven days of harvest can be stored for up to nine months.

Note that application of 1-MCP to Packham’s Triumph directly after harvest will prevent ripening and is not recommended. Application of 1-MCP to fruit stored for more than six months on removal from storage and before shipping will slow ripening and control superficial scald and yellowing.

An initial low-oxygen stress of 0.5%–0.6% O₂ for 10 days at the beginning of storage can control superficial scald for up to eight months in conventional CA (1.5% O₂ and 2.5% CO₂) storage.

Repeated low-oxygen stress can extend storage for up to 10 months but storing this long is not recommended when the risk of superficial scald is high.

Guidelines for O₂ and CO₂ levels in CA storage

O₂ levels of 3% and CO₂ levels of less than 1% are achieved by displacement with N₂ during the first 48 hours after the room is sealed. Thereafter, levels are maintained as set out in Table 3 based on research conducted at the ARC.

Table 3

Target levels of O₂ and CO₂ in CA storage of Packham’s Triumph.

	%O2	%CO2
Minimum	1.0%	1.0%
Optimum	1.5%	1.5%–2.5%
Maximum	2.0%	3.0%

Guidelines for DCA-CF storage

The guidelines for Packham’s Triumph based on research conducted at the ARC are summarised in Table 2. Note the different lower oxygen limits for Granny Smith and Packham’s Triumph.

It is good practice to monitor ethanol levels in fruit in DCA-CF storage.

Contributors

Dr Anél Botes, Postharvest Technology: Fresh Produce, ARC Infruitec-Nietvoorbij

Dr Elke Crouch, Postharvest Physiology Research Chair in Deciduous Fruit, Department of Horticultural Sciences, Stellenbosch University.

Dr Ian Crouch, Managing Director, ExperiCo Agri-Research Solutions

Henk Griessel, Quality Assurance Manager, Tru-Cape

Angelique Marais, Research and Innovation Manager: Postharvest, Fruitways Packing Services

Braam Mouton, Technical Manager: Postharvest, Dutoit Agri

Adriaan Theron, Postharvest Technical Manager, Ceres Fruit Growers

Daniël Viljoen, Research Manager, ExperiCo Agri-Research Solutions