Avoid inking, skin burn and ‘gravel rash’ stains

Stone Fruit Dec. 5, 2021

Peach, nectarine and apricot skin discoloration disorders are described as inking, streaking or spotting – these disorders can often look like gravel rash, or a friction burn on the skin of the fruit.

This inking is usually associated with red areas of the fruit, with browning of the skin developing in patches on the yellow and orange areas of the fruit.

Director of Insense, Russell Fox says highly coloured fruit are more susceptible to this discoloration, while sensitive skinned fruit like apricots, white peaches and nectarines are very vulnerable.

He said apricot skin is very tender and easily susceptible to discoloration resulting from what is often referred to as ‘roller damage.’ Roller damage is where the fruit would be roughed up along the packing the lines on the rollers or belts. Any type of abrasion, from picking the fruit, to unloading the fruit from the bins, to packing the fruit – even the apricots rubbing up against each other may cause this type of discoloration.

“Frequently inking does not show up on the fruit until after packing and fruit is in the carton, causing dismay as to why, where and how this disorder develops,” he said. Symptoms can become evident within 48 hours after harvest and packing.

Inking is the result of a combination of physical, chemical, and physiological factors. Abrasion of the skin significantly contributes to this discoloration, especially abrasion during harvest and packing.

The red colour of stonefruit is due to the antioxidant pigment anthocyanin. There are other plant phenolics and other chemical antioxidants that also supplement colour in the cells of the skin.

When cells are ruptured by physical injury, particularly by abrasion, these anthocyanins and phenolics react with heavy metals, causing the pigments to turn various shades of red, purple, brown, and in severe cases, completely black.

The heavy metals iron (especially from dust), copper and aluminium are the most damaging contaminants, with zinc and manganese implicated as underlying factors.

Where do the heavy metals come from?

These heavy metals are from dust, insecticides and fungicides, and foliar nutrients sprayed on to the fruit,” Mr Fox said. They can also be contaminants in water used in the packing line from a water dump or spray wash.

What are heavy metals?

Heavy metals are generally defined as metals with relatively high densities, atomic weights, or atomic numbers, and include copper (Cu), iron (Fe), lead (Pb), Molybdenum (Mo), Zinc (ZN) nickel (Ni), stannum (Sn), and aluminium (Al).

Mr Fox said many commonly used miticides, fungicides and insecticides have high contents of aluminium (Al).

“The susceptible period for inking to develop on fruit is from three weeks before harvest and throughout harvest and packing,” he said. “The packing and grading processes can contribute significantly to skin discoloration.”

Factors to consider and eliminate to reduce inking

At a pH less than 5.2, iron exists in a soluble, discoloration-prone ferrous state. Rainwater has a typical pH of 5.3–5.5 from the carbon dioxide in the air dissolved in the water droplets. With a physiological fruit pH (~3.5), the dust on fruit provides the iron to react with rainwater.

This is especially significant when fruit is wet for a long period often from a rain shower in the evening. Rainfall or overirrigating near harvest can increase the water pressure (turgor) in cells, this makes the skin cells more vulnerable to abrasion damage during harvest. Mr Fox said this combination of rain and dust, and other heavy metals accumulated in the fuzz on peaches especially, is often the primary cause of severe inking in susceptible peaches and brown staining in nectarines.

“Packing line management is important. Flume water with excessive chlorination and pH under 6.5 make fruit more susceptible to inking. Dirty water increases the risk of fruit discoloration (due to higher heavy metal contamination).”

High iron content in water is a common cause of skin discoloration. Dust accumulation in dump tanks can result in elevated levels of iron in the water, which predisposes fruit to inking and discoloration. Keeping pH levels above 6.5 helps minimize risk of iron-related inking.

Strategies to eliminate or reduce inking in stonefruit

Mr Fox said Parka is a specialist cuticle supplement that provides a protective barrier between the fruit skin and these external contaminants to eliminate or reduce inking peaches and nectarines and protect apricot skin during the packing process.

Before harvest

- Three to four weeks before harvest apply Parka to supplement the fruit cuticle

- One to two days before harvest apply Parka to apricots and susceptible peach and nectarines to help protect their delicate skin during the harvest and packing process.

Harvest

- Reduce fruit abrasion damage by treating fruit gently at harvest and transporting

- Reduce fruit contamination by keeping picking containers dirt-free and clean; avoid dust contamination on fruits

- Check your water quality for contamination from heavy metals and pH

- Do not spray foliar nutrients or preharvest fungicides containing Fe, Cu, or Al within 21 days of predicted harvest to avoid having these metals on the fruit surface

- Check the pH of your irrigation and water used for spraying. Water with pH <6.5 can aggravate inking due to increased iron and heavy metal availability

- With orchards that have a history of inking, leave harvested fruit in cooler for 48 hours prior to packing for inking to show out.

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