Different drying techniques

What effects do the various drying techniques have?

Conclusion: from the article: Drying onions: Common truths and misconceptions
Moist onions lose a considerable amount of quality within just one week. It is advisable to fully dry them out within this space of time.

The underlying fact is that onions need to be dried as quickly as possible. This is generally achieved by ventilating the warm onions with dry air (with a relative humidity of below 50%).

After drying onions via condense drying for the last few years, we have learnt that a relative humidity (RH) of 50% produces the best results, as the risk of over-drying the onions is minimal. Previous research showed that onions can also be dried rapidly using heaters, but in practice this method is not used due to the high costs involved. Hence, the only remaining alternative is condense drying drying.

In humid climates, the air must be heated to between 4°C and 8°C above room temperature, or higher if the onions need to be dried quickly, to lower the RH to 50%. Powerful gas or oil heaters are required to reach these temperatures, and this comes at a price. Using less powerful heaters over the course of a few days, inside fully enclosed or partially enclosed store houses is not at all advisable, due to the risk of condensation forming on the onions themselves.

The main difference between using heated air and condense drying to dry onions is that the condense drying method starts to dry the onions out straight away.

relative humidities (RH)

Red line: RH of external air (varying from 40-95%)
Blue line: RH of internal air (remaining between 50 and 65%)

temperatures

Bottom red line: External temperature, 32-35°C
Upper fuchsia line: Internal temperature, 32-35°C
Other 3 lines: Temperature of stored produce, 30-32°C (this does not tend to exceed 32°C)

Generally speaking, it is impossible to ensure dry, high quality onions "for free", no matter which method is used, when a store house is ventilated with untreated external air.

So, what can be done?

Below we summarise the pros and cons of the various existing methods and techniques.

1. Curing without heaters, using only external air

This method only works when the external air is dry. If the air is humid, the day-night temperature variation increases the level of humidity in the storehouse, rather than decreasing it.

Heaters

2. Curing with small heaters

The RH of the air does little to stop diseases spreading within each bulb, but it does affect how it spreads throughout the store. If the air inside the storehouse is kept below 65% humidity, then any infected bulbs will remain as such, but the disease will not spread to the other bulbs. If the heaters are not very powerful, the relative humidity could remain at 70-75%, which is too high.

It is often difficult to heat the produce to 30-32°C, which is the temperature required to reduce drying time by several days, and consequently reduce neck rot by approximately 5%. Another advantage of heating the onions is that the infected bulbs will rot faster and become easier to pick out.

Hence, in many cases, this method does not suffice. In order to reach the necessary RH and temperature, no expense can be spared on either heater strength or fuel.

The two alternatives

There are two ways to reduce the RH of the air: heat it so that the air can absorb more moisture per kilo (using heaters), or reduce the level of air moisture (via condensation).

Using heaters produces water via the very combustion process itself. For example, a kilo of burnt propane gas produces 1.6 kg of water vapour (and CO2); hence any vapour released into the house must be eliminated, and this is a rather inefficient use of energy.

The alternative involves eliminating vapour from the air, but keeping the energy required to do so inside the storehouse. How is this done?

Evaporators, the cooling part of the cooling system. The compressors are outside.

Eliminating vapour via controlled condensation

The initial phase of the condensation drying method is simple.

When humid air comes into contact with a colder surface it rapidly cools down, and reduces its capacity to retain water vapour just as quickly. Part of this vapour then condenses in droplets of water on the cold surface. This is what causes dew to form on flowers, windows to steam up in cold weather, droplets to form on cans of beer when taken from the fridge in the summer, and air conditioning units to drip.

For condense drying, the air in the storehouse (warm and humid) is passed through the radiators or condensers (cold) of a refrigeration unit. The vapour condenses on these radiators, and the resulting water is removed from the storehouse.

However, although this process dries the air, it also makes it cooler than when it entered the condenser, meaning it requires reheating before being funnelled back into the storehouse, to lower its RH and dry the onions quickly. After ten days, when the onions are properly dried, it is advisable to keep the air cool and dry.

Condensation releases a lot of heat, known as latent heat; this heat is mainly absorbed by the cold surface. The air also absorbs part of this heat, but this does not make up for the heat lost when the air came into contact with the cold surface.

Condense Drying was first trialled in the 1970s, but the level of technology at the time meant that too much energy was lost reheating the cooled air, to keep the drying air at a stable temperature instead of cooling down. It was only ten years ago that this technique was revisited. Modern systems redirect the latent heat produced through condensation into the heating unit, which is actually the condenser coolant. This excess heat then reheats the air, before being pumped back into the storehouse. This technique works, although it does bring practical inconveniences:

  -- The storehouse has to be filled with radiators, pipework and machinery. As well as cooling evaporators, heating units are also required, which in turn means additional engines and flow and return pipes. This all inhibits air circulation, meaning the machinery must be closely monitored to ensure it is well adjusted. In practice, a different setup is required for each storehouse.

  -- If the storehouse comprises several spaces, these systems require extra back-up capacity and a complex network of pipes and valves. In reality, this simply means that not all spaces can be drying at 100% capacity at any given time.

  -- In larger store houses, a secondary circuit is built for evaporation and heating, combining water and ethylene glycol, however this is not recommended for a space used for storing food items. Over time, these circuits will inevitably require inspection and maintenance works.

Conclusion: These "traditional" systems can work well, but they require a very specific combination of various techniques.