Potato Review Group

Contents

Principles

Suggested storage conditions

Overview of storage losses

Heat balance in potato stores

Water and weight loss from stored potatoes

Condensation

Relationship between dew point temperature and condensation

Further information:

Storage notes

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Principles

N.B. This page covers principles of storage, only. For information on practical store management consult Sutton Bridge Crop Storage Research.

  1. “Cure” tubers after harvest.
  2. Cool store to achieve the target temperature for the tubers.
  3. Use a storage temperature suitable (see below) for the purpose of the tubers (seed, processing, etc.).
  4. Aim to maintain a constant temperature throughout the store and the season.
  5. Monitor the environment regularly in different areas of the store.
  6. Depending upon ambient environment, duration of storage and end use of the tubers, positive ventilation, refrigeration and / or humidification may be required.
  7. Inspect tubers regularly for disease, sprouting and quality for processing, as appropriate.

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Suggested storage conditions

Seed tubers

  • 2 – 5 ºC
  • The temperature range throughout any batch of seed should not exceed 2 ºC.
  • Uniformity of seed ageing is important.

Pre-pack crops

  • 4 – 6 ºC or
  • 5 – 10 ºC if sprout suppressant used to control sprout growth during later storage.
  • Must be protected from light.
  • Water loss should not exceed 5 %, if possible.

Processing crops

  • 5 – 7 ºC for chips
  • 7 – 10 ºC for crisps
  • Reducing sugars should be maintained at 0.1 %, preferably, and should be uniform. A range of + 1 ºC is suggested.
  • Some low temperature sweetening may occur but may be reduced by reconditioning at up to 20 ºC.
  • A temperature of 10 ºC would avoid low temperature sweetening but during long term storage would result in irreversible senescent sweetening.
  • Sprout suppressant required.
  • Water loss should not exceed 5 %, if possible.

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Overview of storage losses

  1. Storage losses cannot be avoided even by optimal storage.
  2. Good storage can merely limit storage losses in good product over relatively long periods of storage.
  3. Bad storage results in high storage losses, even in an originally good product.
  4. Wound healing is essential immediately after harvest and may also be necessary after grading.
  5. Storage can affect the propensity for bruising and damage.
  6. Storage losses (due to losses in weight and / or quality) can be caused by the following processes:
    • evaporation of water from tubers
    • respiration
    • sprouting
    • spread of disease
    • changes in the chemical composition and physical properties of the tuber
    • damage by extreme temperatures

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Heat balance in potato stores

  1. Respiration of a tuber results in a slightly higher temperature than that of the surrounding air.
  2. At “normal” storage temperatures a 200 g tuber could raise the temperature of 1 litre of air by 8 ºC in 1 hour in a closed environment.
  3. In a stack, 1 tonne of fairly clean potato tubers will have only about 0.5 m3 air in the spaces between them.
  4. If the air could not escape, the temperature could rise by 65 – 130 ºC in 1 hour.
  5. The more the air temperature increases, the more rapidly the warm air rises out of the heap, resulting in convective air movement.
  6. Fresh air is required to provide oxygen and to remove carbon dioxide: the temperature of the outside air will therefore affect store temperature.
  7. Ventilating stores to cool them also removes water from the potatoes at a rate proportional to the water vapour pressure deficit (WVPD).
  8. The heat balance in a store thus involves the following:
    • the temperature of the potatoes entering store is usually higher than the storage temperature required;
    • in a store filled with potatoes a certain amount of heat is continuously produced;
    • more heat than is being produced has to be removed during the cooling period;
    • once storage temperature has been reached, heat evacuation must equal heat production;
    • heat flow can occur through walls, floor and ceiling.

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Water and weight loss from stored potatoes

  1. The rate at which water will evaporate from a tuber is proportional to the difference between the water vapour pressure deficit (WVPD) of air at the tuber surface and the WVPD of the surrounding air.
  2. The maximum water vapour content of air varies with temperature. The WVPD is the difference between the potential and actual water vapour content at a given temperature.
  3. Weight loss due to evaporation from a typical mature stored tuber may be 0.17 % week-1 mbar-1 WVPD.
  4. As air moving through a potato stack is warmed, the WVPD increases and potential water loss from tubers increases.
  5. Refrigeration may increase the WVPD of incoming air.
  6. Humidification may be required to reduce the WVPD of incoming air.
  7. As an example, suggested WVPDs for eastern England are:
    • 0.8 mbar during winter,
    • 2.7 mbar (maximum) in May and June,
    • 1.4 mbar (mean) for an October – May storage season.

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Condensation

  1. As air moves through the stack it is warmed and can thus take up more water.
  2. When the air reaches the top of the stack, it is cooled by mixture with the incoming air and by potatoes in equilibrium with this mixture.
  3. As the air is cooled to ambient temperature any excess water will condense out.
  4. In a stack without a covering (fleece, straw etc), this condensation will occur on the top of the potatoes.
  5. If there is a sufficiently thick covering, the potatoes will be warmer and the air will have moved above them before being cooled to dewpoint, condensation occurring in the covering.
  6. Water may condense on the inner surface of the store structure (particularly if poorly insulated) and drip on to the potatoes.
  7. The risk of condensation within a stack or within boxes increases if tubers begin to sprout.
  8. Condensation increases the risk of disease development during storage.

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Relationship between dew point temperature and condensation

  1. As air is cooled, the quantity of moisture that it can hold declines and its relative humidity therefore rises until it reaches 100%.
  2. The dew point temperature of air is the point at which the relative humidity becomes 100%. If air is cooled below this point it will release water.
  3. The dew point is therefore determined by the initial amount of water held by the air (absolute humidity).
  4. The more water the air holds the higher its dew point temperature will be.
  5. If the dew point temperature of the air is above the temperature of the crop, then air cooled by coming into contact with the crop will be unable to hold all its water and moisture will be released.
  6. The “vapour pressure deficit” (atmospheric dryness) measures how much more water the air can hold and so also how much it can be cooled before releasing its water.
  7. Dew point temperature, T(d), is calculated as;
    • T(d) = T – (‘v.p.d’/D ),
    • where T is air temp; D is taken from tables;
    • v.p.d = es(T)*(1-RH); es(T) is taken from tables.

If T(d) is higher than the crop temperature, ventilating the store with ambient air will result in condensation in the store. (In the Netherlands, T(d) is given for all potato growing areas.)

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Further information

Storage notes

Storage problems 2001 (Potential problems after poor harvest conditions)

Storage principles 2001 (Effects of store conditions on atmosphere and crop response)

Storage 1999 (Influence of storage conditions on tuber physiology)

References:

  • Burton, W.G., van Es, A. and Hartmans, K.J. (1992). The physics and physiology of storage. pp 608-727 In The Potato Crop. Ed. P. Harris, Chapman & Hall, London.
  • PMB (1996). Potato Storage. Potato Marketing Board / Sutton Bridge Experimental Unit. 12pp.
  • SAC. Condensation on Potatoes in Box Stores. SAC Information, 2 pp.

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