Improve potato quality by minimising mechanical damage

Page last updated: Wednesday, 20 April 2016 - 8:41am

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Reducing damage

Bruising and damage prevention in ware potatoes begins with careful planning before planting.

Paddock selection

Potato crops should be grown in soil that will produce fewer bruised tubers. Avoid rocky and cloddy soils which cause a lot of damage during harvesting.

Soil moisture

Irrigation prior to harvesting is very important in increasing the soil moisture content in clay soils. This reduces the number of clods and provides a layer of wet soil between the tubers and machine components, reducing impacts and consequent damage. On sandy soils, manage moisture so soil will not separate from the conveyor thereby increasing tuber damage. If soil is too wet, delay harvesting. Soil moisture for harvest should be between 60 and 80% of field capacity.

Skin maturity

Allow two to four weeks after top removal for skins to harden. Maintain soil moisture so tubers do not dehydrate underground while the skin is hardening.


Tuber temperatures between 12 and 18ºC are considered ideal for harvesting as this is when less bruising occurs and microbial growth is not too fast. Tubers warmer than 18–20°C and under drought stress are susceptible to blackspot bruising.

Harvesting when tuber pulp temperature exceeds 18°C or soil temperature is greater than 25°C increases the risk of microbial rots, especially in damaged tubers. Tuber temperatures should also be kept within the optimum range during washpacking.

Harvester and washpacker set-up

Potato harvesters have many adjustments to allow gentle handling. Set-up can be assessed using an electronic potato (instrumented sphere).

Assessing set-up

Assessment of impacts during harvesting can be measured using an electronic potato such as a Smart Spud®. The Smart Spud (produced by Sensor Wireless Inc) contains an accelerometer that measures impacts as G-forces as it passes through handling equipment (see Figures 3 and 4).

G is a measure of gravitational units where 1G = 9.81m/s2. The higher the level of G-force, the more severe the impact. By burying the Smart Spud in the ground it can be lifted with other tubers and pass through the harvester. This allows you to identify specific locations where damage is occurring. Similarly, the Smart Spud can be placed in a potato line to determine impact points through the washpacking process.

Harvesting potatoes showing the tubers on the elevator
Figure 3 Harvesting potatoes
Harvested potatoes showing the Smart Spud which is used to measure impacts during the handling processes
Figure 4 Smart Spud measures impacts during the handling processes

Results from calibration of WA potato varieties against G-force readings from the Smart Spud show differences in bruising occur with drop height, variety and temperature (Table 1).

At 10°C, 20% of Nadine tubers were damaged from a drop of 40cm. Ruby Lou was more tolerant at this temperature with no bruising.  Bruising in Ruby Lou only started at 60cm with 30% of tubers bruised.

Table 1 Percentage of Nadine, Ruby Lou and Royal Blue tubers (120–350g) damaged when dropped from 10 to 100cm at 10 and 15oC. Mean G-force was recorded with a Smart Spud and averaged for each height

Drop height


Mean G-force


Ruby Lou

% tubers damaged at 10oC


% tubers damaged at 10oC

Ruby Lou

% tubers damaged at 15oC


% tubers damaged at 15oC

Royal Blue

% tubers damaged at 15oC

10 52 0 0 0 0 0
20 102 0 0 0 0 5
30 164 0 0 0 0 10
40 194 0 20 0 0 20
50 218 0 80 0 0 35
60 223 30 60 70 90 40
70 226 40 60 20 70 80
80 232 30 100 20 90 80
90 279 50 90 30 90 80
100 244 40 100 30 100 80

At 15°C, a 20cm higher crop was required for Nadine to show bruises but here 90% were affected. Bruising in Ruby Lou again did not occur below at 60cm drop height but at the higher temperature the bruise percentage at 60cm was more than double that of the lower temperature.

Royal Blue tubers at 15°C were susceptible to bruising at lower drop heights than Ruby Lou and Nadine, with 5% of tubers damaged at just 20cm.

Reducing drops and adding cushioning

Modifying equipment and decreasing harvesting and washpacking speeds can minimise damage. Reducing drop heights from conveyors will improve potato quality.

Considerable bruising can occur where drops are greater than 40cm, particularly when the drop area is on belting supported by a hard metal roller or plate.

Plan view of a potato harvester showing main impact sites where 1 = first short main web onto diviner web; 2 = first separator; 3 = second separator; 4 = bunker filling elevator drop; and 5 = bin filling from bunker drop
Figure 5 Plan view of a potato harvester showing main impact sites where 1 = first short main web onto diviner web; 2 = first separator; 3 = second separator; 4 = bunker filling elevator drop; and 5 = bin filling from bunker drop

The impact surface affects the amount of damage caused to a potato. Cushioning materials considerably reduce impact. A 6mm thick rubber layer on steel increases energy absorbance considerably, relative to uncovered steel. This means tubers are less likely to get damaged if mechanical components are covered in rubber.

Case studies in WA have found that an instrumented sphere will record higher impacts with greater drop heights and no cushioning. For example, from the elevator into the bunker, extremely high impacts (above 300G) have been recorded.

Contact information

Peter Dawson
+61 (0)8 9892 8461