Drying and storing onions in the tropics

Drying onions in the hot, humid tropics.

As an example, let's take a look at Indore in India.

Climate graph for Indore:

In Indore, onions are harvested between the end of the dry season and the beginning of the monsoon season. Onions are sold at very low prices during the harvest, not only because there is such a high supply of onions in the area, but also because farmers lack adequate storage facilities and are forced to sell their produce quickly before the monsoons start and cause it to rot.

On average, as much as 50% of the onion harvest in Indore is lost, and sometimes this percentage is even higher. For further information, see: Financial losses incurred by inadequate storage. Have a look: Financial losses incurred by inadequate storage..

Whilst many cold stores for potatoes can be found in the area, none can be found for onions.

This comes as no surprise to us.

Potatoes have to be kept cold at all times. To find out why, visit: Potato Storage. How to store potatoes long term?.

Why?
Onions do not always have to be stored at low temperatures; they can be stored at 2° Celsius as they are in the north of Europe, but they can also be stored at 25 to 30° Celsius..

Why is there a general misconception that onions must always be stored at 2° Celsius?

Winters are very cold in the north of Europe, with temperatures generally ranging from -5° to 5° Celsius. Hence, storing onions at 2° Celsius in these conditions is very energy efficient.

However, even in northern Europe, onions have to be heated to just above the local dewpoint temperature before they are sold so as to avoid condensation and them becoming too wet during transit.

However, in the tropics, it would be madness to store produce at 2° Celsius, when it can be stored at between 25° and 30° Celsius.

There are two main reasons for this:

1. Imagine onions that have been chilled to 2° C suddenly coming into contact with hot and very humid air. Just think about what happens to a cold glass of lemonade when you take it out of the refrigerator, the same would happen to the onions; they would end up too wet and the subsequent rotting process could lead to you losing your entire supply.
2. Why waste all that energy chilling hundreds of tonnes of delicate onions, keeping them cold for months on end while the outdoor temperature is 30° or more, only to waste even more energy warming them to above the high dewpoint? This is not only a tremendous waste of energy, but also a waste of time. It takes about two weeks to heat a couple of hundred tonnes of onions to 25° and takes about another 2 weeks to cool them down to 2° Celsius. The reheating process is particularly risky, as although the onions might feel warm at first, the cores remain cold for a long time.

Consider the following table:

The secret to storing onions successfully is drying them as quickly as possible.

• Once the first three outer layers of an onion and its stem have been dried, the onion is then completely sealed from the outside environment and hardly loses any more weight (water).
• As the onion is sealed, the inside is completely protected against rot and fungi.

We would recommend:

1. Drying and sealing the onions immediately after harvest for a maximum of three to four days. Which is possible now, with our condense drying technology.
2. In the tropics, keeping them at a constant temperature of 30° Celsius, in total darkness with as little ventilation as possible and a relative humidity of about 65%.

Why a constant temperature?
An onion, even in storage, is a living thing. Every time the temperature changes it will start sprouting.

Why total darkness?
An onion, even in storage, is a living thing. Every time it is exposed to light it will start sprouting.

Why as little ventilation as possible?
Only the CO2 between the onions should be removed, otherwise they continue to breathe and will sprout again. And every time we ventilate using air with a humidity of 65% they will lose some water, even though they are sealed.

Why a relative humidity of 65%?
This dryness is to keep bacteria and fungi at bay. Bacteria and fungi are not killed by dryness, but they remain more or less dormant.

For more details see:

1. How to and store onions

Onion Storage. How to store onions long term?

2. Drying onions: common truths and misconceptions:

Drying onions: common truths and misconceptions: and the sub-sections within this section

In sum, this is a technical challenge:

To successfully store freshly harvested, humid onions for a long period of time at around 30° Celsius in an environment where the temperature is also 30° Celsius and the relative humidity is around 80%, the onions have to be dried very quickly at a constant temperature, and then kept at 30° Celsius with an ambient relative humidity of 65% and an outside relative humidity of at least 95%.

VaccTek. is the answer. For more information see the full section and sub-sections here: Drying onions: common truths and misconceptions

Instead of using warm or cold air (heating or cooling the onions) we create an environment with a temperature of 30°C, and a relative humidity which is about 55% during the drying period, and rises to 65% during storage.

This way, no energy is wasted on heating or cooling the onions.

How do we do this?

VaccTek sucks in the very humid, warm air from outside and cools it down to far below the local dewpoint. As a result, the water is extracted from this air via condensation.

Next, 80 to 95% of the heat generated by the cooling equipment is transferred back into the cooled, dried air. As a result, the air entering the facility from the VaccTek unit has a temperature of 30°C and a relative humidity of 55 to 66%.

Consequently, we only use energy to evaporate the moisture from the onions, but never change their temperature. This is ideal for onions.

We therefore recommend the following systems to store onions in the hot, humid tropics:

1. Onion and potato box storage, forced ventilation through pressure wall, see: Onion and potato box storage, forced ventilation through pressure wall
2. VaccTek installed in each storage unit, see: VaccTek, the condense dryer for your crop

A comment for engineers with some basic understanding of thermodynamics:

If you remember your thermodynamics, you will understand that we reuse 80 to 95% of the energy that is enclosed in the cooling system's enthalpy diagram. And if you really remember your thermodynamics, you will realise that we hardly raise the entropy of the universe with this system.

VaccTek dries by regulating the relative humidity of the air, while keeping the air temperature constant.