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Reduce Bruising and Save Money

2 August 2011

Growers could save themselves thousands of pounds a year by reducing the amount of tuber bruising that occurs during and after harvest.

Overall, bruising costs the potato industry about £26m a year in lost quality and marketable yield, the equivalent of £200 for each ha grown in Great Britain, says Eric Anderson of Scottish Agronomy. 

To manage bruising successfully, a basic understanding of the complex science affecting susceptibility is needed, he explains.

“Bruising occurs when a mechanical impact causes internal disruption of the tuber at the cellular level. The disrupted membranes allow an amino acid, tyrosine and an enzyme, polyphenol oxidase to mix, resulting in blackspot pigments.”

The strength and permeability of cell membranes are therefore crucial to prevent these two substances coming into contact, Mr Anderson notes.

Elasticity of the cell walls is also key as it affects their ability to resist damage. This elasticity is governed by turgor, the outward force exerted on a cell wall by the water contained within it.

Small changes in elasticity of tissue have a large influence on bruising.  “Slight reductions in hydration, the equivalent of 2% tuber weight loss, can reduce turgor enough to double the bruising caused from a similar drop height,” he explains.

So what other factors affect bruising sensitivity in the field, and how can they be managed? Variety plays a role, mainly due to tuber size and shape. Big tubers and those with small radius of curvature tend to bruise more easily.

“Many people think high dry matter levels cause bruising, but it actually accounts for less than 30% of the difference between varieties. Marfona has a dry matter content of 17%, but it can suffer badly from bruising, including this year. Tuber age is important. Tyrosine levels rise with physiological age, and cell membranes become more permeable, making tyrosine leakage more likely” says Mr Anderson.

There is a fine balance in the water relationship of potatoes and when tubers are most resistant to damage. Soil moisture levels in the run-up to desiccation are key – too little (or too much, waterlogging causing root die-back), and turgor will be adversely affected, raising the risk of bruising.
Sufficient potash is needed for water regulation within the tuber. Amongst other things, K helps maintain cell turgor, regulates opening of leaf stomata and promotes water uptake.
Warm tubers are less likely to bruise, so lifting earlier can reduce the incidence dramatically. “Soils in mid-September can be 3˚C warmer than in mid-October, which, together with the fact that tubers are a month younger, can reduce bruising by 10%,” says Mr Anderson.

Soil type also affects bruising levels. Clay and silt soils providing the best protection at harvest, cushioning tubers on the primary web, providing they are free of clods.

But, perhaps most importantly, gentle handling and minimising drop heights are vital to keep bruising to a minimum. Most damage on the harvester is caused by haulm separation systems and operators not tweaking the settings enough.

“A one per cent increase in damaged tubers on a harvester lifting 60t/hour could reduce saleable tubers by 6t/day,” he explains. “Even at £100/t that could cost £600 per harvester per day. Potatoes can often be worth twice as much money, so losses can quickly mount up.”

That’s a pretty strong incentive for correct harvester set up, but growers should not focus on this operation alone, Mr Anderson adds.
 
“Bruising is cumulative – care needs to be taken at all stages of the process, including how the trailer is driven and tipped, and grading into and out of store.”

Regular samples taken at intake for hot boxing will help reduce bruising problems, he advises. “Taking five or six samples each of 25-30 tubers from each harvester per day will help identify what’s happening, and steps can be taken to correct any problem before it gets out of hand.”

For more information on bruise reduction visit www.potato.org.uk/bruising.

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