Storage

Condensation in Potato Stores

What Is Condensation:

Condensation only occurs when moist air that is warmer than the tubers, comes into contact with cooler potatoes. The mechanism is the same as happens in a crowded room on a cold day, when moisture in the air condenses on the cooler windows. The air at the window is cooled until the air reaches its dew-point, whereupon moisture in the air condenses on the glass.

The heat given off by newly harvested potatoes causes air surrounding the tubers to warm up. This heat dries the surrounding air, just as a heater bank in a grain drier reduces the relative humidity of air for drying grain. This dry air then causes moisture from the flesh of the tuber to migrate through the tuber skin. The warm air surrounding the tubers therefore gains moisture from the flesh of the tubers as it rises up the box or pile. The problem of condensation arises when this warm moisture laden air reaches the colder tubers on the surface of the box. The amount of condensation will be greatest with early harvested, badly damaged crops and least with late harvested undamaged crops. Skin thickness and structure may influence the degree of condensation.

Condensation on potatoes in box stores.

Condensation on the crop in store results from warm humid air meeting colder tubers whose skin temperature is below the dew-point temperature of the air. Condensation may occur on the crop,

• during wound healing, when the potatoes are giving off a lot of heat
• when warm humid spell of weather follows a cooler spell
• if temperature differences occur within boxes or between boxes
• when warm humid outside air flows into a store through open or leaks in the store fabric.

1. When potatoes are active and giving off considerable heat.

During wound healing, the heat produced by the potatoes in the box causes its centre to heat up more than the surface tubers. This causes warm moist air from the centre of the box to rise and condense on the cooler tubers on the surface of the box. Positive or forced air ventilation can minimise or completely eliminate this form of condensation by

• cooling the centre of the box
• equalising the temperature throughout the box.

Regular blowing is required for two to three weeks after harvest to prevent this form of condensation, two days is not enough. Such condensation increases if early, very active crops are harvested, or if the natural convective airflow through the box is restricted by tubers being small or dirty. Firm evidence shows there can be an increase in the development of surface blemish diseases if positive ventilation is not used during this period. In addition a number of cases of watery wound and soft rotting could be avoided if positive ventilation is not used.

2) Crop temperature is below the dew-point temperature of the ventilating air.

This is very common during wound healing. The crop may be cooled by cold winds from the east or north only to be ventilated soon after by warmer humid winds from the south-west when the weather changes. Positive ventilation, with its greater air volume flow through boxes compared to natural convective ventilation, can actually make condensation worse if it is not properly controlled. Nowadays, however, such control can be purchased for a relatively low sum. Positive ventilation will however allow moisture to be quickly dried if it does occur.

3. Uneven temperatures within the crop in store.

The air in potato stores is usually between 85-95% relative humidity, which is very humid. If some potatoes in store are a couple of degrees cooler than the rest of the crop, the warm humid air from one part of the crop can cause condensation to form on another part. Differences in crop temperature often result from ventilating the boxes with air, which is much cooler than the crop. The potatoes nearest the ventilation inlets are the ones that often suffer this form of condensation. Uneven air circulation in store can also cause temperature differences in the crop. Localised areas of "dead" air may be extremely humid, so only small differences in crop temperature can cause condensation to form on the cooler tubers. Lean-to stores are renowned for having problems of moist tubers.

4) Leaky stores and open doors.

Probably the most common reason for condensation on the crop is from the air entering the store through doors that are permanently open during the day and through holes in the fabric. Mild SW winds sweep over Ireland, enter the store and condense on the cold crop within closed doors.

During Harvest:

Growers have to be careful to make sure no condensation forms on potatoes already in store, when loading new crop. If the store crop is at 10C, and new crop comes in at 16C, there is huge potential for condensation to form on the cooler crop. Try and minimise the temperature between the two. A gap of 4C is reasonable. Once all the crop is in store, bring the temperature down quickly to the holding temperature.

Finally, monitor the crop daily in store to check that any rots are drying out, which should happen in between first week and when the temperature is stabilised.

Filling the Store

1. 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.
2. Monitoring should start from the moment the first load arrives in the yard. Install temperature probes in the crop at loading, because problems can occur with ventilation right from the start.
3. Waiting until the store is fully loaded before regular checks are made is a recipe for disaster.
4. 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.
5. Wounds allow the entry of fungi, 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. This provides an initial barrier against infection. As the days go by this suberin begins to form into a skin. This skin is also called the periderm. When the skin, or peiderm, is fully formed it provides a permanent barrier against infection. The time taken to form this periderm depends on the temperature of the tubers (see chart)

Impact of temperature on wound healing

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

Pallet slots should face in the direction that the store's fans are moving the air so that air movement occurs through the whole of the stack. 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.

Condensation:

1. Make sure the 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.
2. 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.
3. 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.
4. 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 scruf.
5. 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.

Fridge Drying:

1. When using refrigeration systems to dry the crop, don't go overboard and take too much heat out of the crop in one go. More than 0.5ºC a day may allow temperature gradients to develop and condensation will occur.
2. Keeping the store temperature within +/-4ºC of the ambient air will minimise the risk of condensation problems occurring. Should the external air not fit the +/-4ºC criteria then air should be re-circulated around the store to maintain ventilation and avoid temperature gradients from becoming established.
3. Ware crops will normally take 2 - 3 weeks to reach holding temperature