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This article discusses the possible causes of skin browning which was a major fruit quality problem again during the 1991/92 season. Author Scott Ledger believes that ooze sap is a primary cause.

Detailed assessments of grower lines on the markets found skin browning to be a major problem again last season.

The frustrating part about skin browning is that it is not obvious at the time of packing but develops during marketing.

Possible causes are sap, moisture, heat, detergent, abrasion and chilling.

SAP
We know that the initial spurt sap causes burning of the skin but what about the ooze sap? In the past we have said that ooze sap does not cause any problems. I have changed my opinion and now believe that ooze sap is a primary cause of skin browning.

During visits to packing sheds I observed that where fruit had been de-stemmed and held overnight in crates, skin browning started to develop at the contact points between fruit and between fruit and the crate.

Ooze sap either runs to the contact points directly or dissolves in water and collects at the contact points. The end result is a concentration of ooze sap at the contact point and subsequent skin browning.

Fruit at the markets with skin browning often appears to have sap smeared over the skin.

This sap is probably collecting on brushes, rollers, conveyors, grader cups and packing brings.

Regular, thorough cleaning of equipment is essential to prevent a build-up of sap.

Sap can accumulate in water baths used for de-stemming. If you are de-stemming under water or placing de-stemmed fruit into water, add lime at a rate of 2 per cent (10 grams per litre). The lime helps to dissolve and de-activate the sap. Change the water baths regularly.

The hot dip is another potential source of sap build-up.

Do not keep the dip for longer than three days and make sure any sap sticking to sides and bottom of the tank is removed when cleaning.

You cannot be too careful with sap!

MOISTURE
The floods during the 1990/91 season showed us that moisture continually on fruit can cause skin browning. Packing of wet fruit should be avoided. Pulling fruit in and out of coolrooms is not a good practice as it results in a build-up of moisture in the plastic cups from condensation.

HEAT
Mild heat injury can show as light brown patches on the skin. Recent research and grower experience have found that mangoes are susceptible to heat injury when harvested during wet weather, dipped immediately after harvest, cooled before dipping and harvested from the inside of the tree.

DETERGENT
High concentrations of detergents used as a pre-treatment before de-stemming may cause browning of the skin. We recommend fruit wetting agents such as Agral® rather than household detergents because of the dangers of side effects. The rate of use is 1ml per litre of water.

ABRASION
Abrasion injury can be caused by rubbing of fruit in field crates and on grading and packing equipment.

Loads of field crates should be covered if transported long distances over dusty roads.

Clean grading and packing equipment at least once a day, especially brushes.

Replace worn or stiff brushes. Fruits are more susceptible to brushing injury following the hot dip. Avoid over-filling equipment with fruit.

CHILLING
Chilling injury can show as a dull grey or browning of the skin. Damage occurs when fruit are held below 10°C for periods longer than three days.

DATE: January 1993

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