skip to Main Content
202403 Fresh Quarterly Issue 24 01 Superficial Scald Web
Issue 24March 2024

Superficial scald 101

Why does it occur, and how can it be prevented? By Anna Mouton.

Superficial scald is a postharvest storage disorder of apples and pears characterised by brown or black areas on their skins. Their flesh is usually normal, but the fruit’s appearance renders them unmarketable except for processing.

The disorder was first reported in 1903 in a book of guidelines for cold storage of apples written by pomologists with the United States Department of Agriculture — large-scale cold storage of apples had been booming since around 1890, but fruit quality was not always maintained.

Superficial scald, also called storage scald, develops during cold storage but is mostly only seen after shipping or shelf life. Buyers will reject consignments with even minimal scald because they’ve learned that the disorder will intensify over time.

Granny Smith apples and Packham’s Triumph pears are the main cultivars that suffer from superficial scald in South Africa. Losses also occur in Cripps Pink apples.

How does superficial scald develop?

Researchers agree that superficial scald is caused by oxidative stress, but they have been arguing about the details since at least the 1960s.

Oxidative stress occurs when highly reactive molecules called oxidants damage cells. Cellular processes continuously generate oxidants, which are neutralised by antioxidants and antioxidant enzymes in healthy cells. Uncontrolled oxidative stress is a vicious cycle in which products of oxidation are themselves oxidants.

All apples and pears experience increased oxidative stress after harvest and during storage, but not all are susceptible to superficial scald. Studies suggest that sensitive cultivars such as Granny Smith have lower levels of antioxidants and antioxidant enzymes.

The brown or black areas on the fruit skin form when damaged cells die and release enzymes that interact with phenolics to produce brown pigments.

What are the risk factors?

Superficial scald mainly affects Granny Smith apples and Packham’s Triumph pears but can also occur on Cripps Pink and Red Delicious apples.

Apples are more susceptible to scald when harvested at pre-optimum maturity as fruit harvested too early have lower antioxidant levels. Harvest maturity appears less strongly correlated with superficial scald development in Packham’s Triumph.

Hotter and drier weather in the weeks before harvest increases the risk of superficial scald. Research links water stress to oxidative stress and the accumulation of scald-promoting substances in fruit.

Some researchers have reported that cool nights before harvest reduce the occurrence of scald. Red skin seldom develops superficial scald, and the effect of cool nights may be due to enhanced red colour and antioxidants and the formation of unsaturated fatty acids that support cell-membrane resilience.

Large fruit are at greater risk of superficial scald than smaller fruit. Low calcium and high potassium and nitrogen levels in fruit are associated with greater scald sensitivity. It follows that the risk of superficial scald is likely to rise when the crop load is too low.

Even though scald usually only manifests after storage, the underlying cellular damage occurs early in the postharvest period. Its development is strongly correlated to ethylene levels in fruit, so anything that promotes ethylene production is a risk factor.

How can superficial scald be prevented?

Temperature

Step-down cooling helps prevent superficial scald in Cripps Pink but causes premature ripening in Granny Smith and Packham’s Triumph. These latter two cultivars should be cooled as quickly and safely as possible.

Superficial scald is a chilling injury, so its prevalence and severity tend to worsen the colder and longer the fruit are stored. Although scald may manifest earlier at higher storage temperatures, the fruit are usually less badly affected than they eventually become at lower temperatures.

Local research showed scald development after shelf life in Granny Smith stored in regular atmosphere for ten weeks was greater at storage temperatures of -0.5 °C than 0.5 °C. Fruit stored at 5 °C developed no scald.

Chemicals

Treatment with 1-methylcyclopropene (1-MCP) within seven days after harvest provides excellent control of superficial scald in apples. Recent Hortgro-funded research showed that Granny Smith benefits from a second treatment — refer to this article in this issue for more details.

Packham’s Triumph can be treated with 1-MCP after removal from cold storage and before shipping. Treated pears will still ripen, but softening and superficial scald will be reduced.

Preharvest application of 1-MCP within seven days before harvest also provides good scald control in Granny Smith, but coverage and efficacy are better with postharvest applications.

Experiments have demonstrated that ethanol and some other alcohols can reduce superficial scald, probably by acting as antioxidants, but there are no application technologies for commercial pack houses.

However, natural ethanol production by fruit stored under very low oxygen is thought to support scald control and is likely safer and more effective than ethanol vapour treatments.

As superficial scald results from oxidative stress, antioxidants such as diphenylamine and ethoxyquin provide excellent control, but these products are no longer acceptable in many international markets. Applying antioxidants such as ascorbic acid is ineffective at controlling superficial scald.

Atmospheres

Controlled atmosphere storage will generally delay but not prevent superficial scald development. Flow-through controlled atmosphere storage is superior because it removes volatile compounds that drive scald development.

Low oxygen levels reduce scald so long as safe ethanol and CO2 levels are maintained. Dynamic controlled atmosphere systems based on respiratory quotient or chlorophyll fluorescence allow cold-room operators to achieve low-oxygen storage without excessively stressing the fruit.

A recent Hortgro-funded project investigated applying different controlled atmosphere systems to control superficial scald in Granny Smith. Read this article in this issue for more details.

An initial low-oxygen stress followed by conventional controlle atmosphere storage has been shown to prevent superficial scald in Packham’s Triumph — read more about this research elsewhere in this issue.

Several researchers have examined the use of ethylene scrubbers in cold rooms, but the results have been inconsistent. Preventing ethylene production in the first place is likely to be more effective than scrubbing for reducing postharvest disorders.

Coatings

Before the advent of diphenylamine, scald was controlled by wrapping fruit individually in paper containing mineral oil. Scald could also be reduced by placing oil-impregnated paper strips in bulk bins. Oil wrappers were used by Australian and New Zealand exporters as late as the 1980s.

The oily papers probably worked as primitive scrubbers to remove volatiles that drive oxidative stress.

Direct application of mineral oils to fruit helped maintain fruit colour and firmness but was reported to reduce flavour and quality.

Other coatings have not proven effective in reducing superficial scald.

A summary of the best-practice guidelines for controlling superficial scald is provided in this issue.

Back To Top