Safely behind doors

22 September 2001

Safely behind doors

During and immediately after loading stores is the most crucial time for close monitoring of potatoes in store. Lucy de la Pasture reports

POTATO harvesting is always a testing time: men and machinery at full stretch and always the threat of a break in the weather. But take care not to overlook the needs of the potatoes already in the relative safety of the store – it can all go wrong in the three weeks after harvest, warns Adrian Cunnington, British Potato Council storage expert at Sutton Bridge.

Monitoring should start from the moment the first load arrives in the yard. Taking samples of the crop before loading into store can avoid nasty surprises when the crop is marketed. Washing will reveal any pest damage, harvest damage or disease problems that may make long-term storage inadvisable.

"Install temperature probes in the crop at loading, because problems can occur with ventilation right from the start," explains Mr Cunnington. "Waiting until the store is fully loaded before regular checks are made is a recipe for disaster.

"Should a temperature gradient be allowed to develop, condensation will probably occur and the resulting free moisture will enable skin blemish diseases and soft rots to thrive."

The newly lifted crop will introduce moisture into the store either on the tuber surface or from soil particles adhering to the crop. This free moisture will humidify air moving through the crop, making condensation on other parts of the stored crop likely.

"Make sure that air is moving around the crop within a couple of hours of entering the store to reduce the humidity and allow the free moisture to evaporate off the potatoes," advises Mr Cunnington.

Warm air can hold more moisture than cold air, so crops will dry much faster earlier in the season than later when the ambient temperature is much cooler. Similarly, healing of wounds occurs at a faster rate when tubers are at a higher temperature.

Wounds allow the entry of fungae and bacteria present on the surface of tubers, to develop causing disease. The first stage of wound healing is the production of a substance called suberin at the surface of the wound, which provides an initial barrier against infection. The skin or periderm is ultimately formed, providing a permanent less permeable barrier.

Once the potatoes are dry it is essential not to allow them to become wet again as all the benefits of dry-curing can be undone. Re-wetting of potatoes can initiate diseases such as silver scurf. Keeping the store temperature within  4íC of the ambient air will minimise the risk of condensation problems occurring. Should the external air not fit the  4íC criteria then air should be re-circulated around the store to maintain ventilation and avoid temperature gradients from becoming established.

Simple things, such as having the store door open for long periods during store loading, can have dire results. External air entering via the door can condense on potatoes if the potatoes are cooler than the incoming air. These potatoes then become wet and are susceptible to disease and sprouting.

Where potatoes are stored in boxes, attention to detail when stacking will pay dividends. Boxes should be stacked so that air can move around the store but cannot bypass the boxes.

"Air will always take the path of least resistance, so make sure there are no easy routes back to the fan," advises Mr Cunnington. "If one part of the store is being filled and one side is left empty the air will hardly pass through the potatoes at all. Placing a wall of empty boxes across the store will prevent the air from taking the easy route."

Pallet slots should face in the direction that the stores fans are moving the air and all apertures should be open so that air movement occurs through the whole of the stack. And dont be tempted to stack boxes into the eaves to maximise the storage capacity of the store. The top of the boxes should always be left level.

"A stepped top of the stack will cause warm and cool air to mix which will result in condensation. The potatoes will then become dank and drip with moisture allowing disease to flourish and sprouting may occur," comments Mr Cunnington.

Making sure that all wounds are suberised before pull-down to the desired storage temperature begins will minimise crop weight loss. Moisture loss from wounds is in the region of 100 times more than from undamaged skin.

An earlier start for pull-down may be appropriate where tubers are destined for pre-pack markets where blemish diseases such as silver scurf and black dot are present because these are discouraged from developing by low temperatures.

"When using refrigeration systems to dry the crop, dont go overboard and take too much heat out of the crop in one go," advises Mr Cunnington. "More than 0.5íC a day may allow temperature gradients to develop and condensation events to occur."

Ware crops will normally take 2-3 weeks to reach holding temperature, whereas processing crops need a slower rate of pull down and may take 4-6 weeks before holding temperatures are reached.


To produce the BPC Store Managers Guide providing a new reference for growers and store managers to help makes savings and maintain quality in store

Potential benefit

£9m a year –

by preventing storage losses (rotting and sprouting) and failure to meet packing and processing specifications

Key features

&#8226 68 pages split into 27 easy-to-read sections with clear cross-referencing and action points

&#8226 New R&D combined with established practice ensures advice is up to date

&#8226 Self-assessment feature enables readers to see how their store measures up

&#8226 Basic section outlines underlying principles

&#8226 Market-specific features to focus management on the requirements of each sector

&#8226 Energy management chapter identifies potential savings

&#8226 Dew-point tables to help assess condensation risk


&#8226 Guide is free to BPC levy-payers – 01865 782222, £80 to non levy-payers

&#8226 Store managers courses: Forfar 15/16 Jan 2002 (SAC); Spalding 27/28 Feb 2002 (BPC)

&#8226 BPC Potato Storage Event: Sutton Bridge, Lincs. 1 May 2002

Details: Adrian Cunnington, BPC, 01406 351998; Bob Pringle, SAC, 01224 711093

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