The processes required to store onions properly.
Once they are stored, onions undergo the following processes:
– The drying process
– The processes of raising or lowering their temperature
– Conservation or storage proper
These processes are controlled by the Multiserver from AgroVent systems BV.
1. The onion drying process
The aims of the onion drying process are:
– To remove all of the surface water or moisture from the onion
– To dry three or four layers of the onion skin, therefore sealing the onion
– To dry the stem and therefore also seal this part of the onion (remember that once the stem is cut, it is left open and must be kept as dry as possible.)
It is best to dry onions at between 25 and 30° Celsius..
The Multiserver programme for drying onions will ensure that the turbines work the maximum number of hours to extract moisture from the onions as quickly as possible, whilst maintaining the temperature between 25 and 30°C, and the relative humidity between 60% and 65%. In doing so, the aim is not to dehydrate the insides of the bulbs, but rather the 3 or 4 outer layers which separate the bulb from the outside.
This process is completely controlled by the Multiserver, which constantly searches for the mixture of air with the highest drying capacity, by managing the doors, turbines, heaters and condensers.
The processes of raising or lowering the temperature of the onions
Any changes to the temperature of the onions must be gradual and controlled, for two reasons:
– To prevent the cells in the bulb from being destroyed.
– To ensure that the centre of the bulb is at the same temperature as the outer layers.
This is why the Multiserver has two special programmes to control the heating or cooling of the produce. Onion storage specialists advise cooling or heating the onions by 2 degrees per day at the very most, and recommend only 0.5 degrees per day.
Such a slow change in the temperature of the onions has important consequences when planning to move the onions from the storage facility.
Let’s assume that the onions are stored at 2° Celsius.
The destination of the onions is 25° with 85% relative humidity, making the dew point 22.3°. This means that the onions must be heated by at least 20.3°, to 22.3°. At the maximum allowed increase of 2° per day, this process will take at least 10 days.
These days must be kept in mind when planning the onion delivery.
Especially in hot and humid climates, if the onions are not heated above the dew point, many of them will rot in the humidity.
3. The onion storage process
A well-dried onion has a Crispy skin and a juicy middle.
Before they can be stored, onions must tick the following boxes:
– The onion must be the right variety for storage.
– It must be harvested when it is completely physically ripe.
– The bulb must be firm and compact.
– The skin must be strong.
– The bulb must have a low water content.
– It must be grown to Good Agricultural Practices.
– It must not be fertilised with nitrogen during the last 6 weeks of its growth.
– It must be applied sufficient amounts of aglime.
Onions can be stored at two ranges of temperatures.
– Between 0 and 4° Celsius
– Between 25 and 31° Celsius