How to Dry Onions

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When it comes to producing dehydrated foods, the drying process can be long and the utmost care must be taken to ensure the highest quality of the product. Onion drying is a process that takes time and involves several different drying stages.

Proper drying conditions need to be maintained and they vary according to each drying stage. The first stage of onion drying is the most critical and involves the sliced onions to be subjected to high volume air flow conditions and moderate high temperatures of 180º - 220º to remove most of the moisture content in a short time. If drying conditions are not properly maintained the dried onions can discolor and become pink. A good drying result is pure white in color.

The second stage of onion drying is to rid the difficult to remove moisture content and is done under reduced temperature conditions for slow withdrawal.  The onions are then moved onto the third stage to remove the even more difficult to remove and deeply diffused moisture content. The final drying stage is where the final moisture reduction is carried and requires dehumidified air.

The onion drying at this final stage is only possible with a desiccant system to produce the low relative humidity levels needed for the right drying conditions to bring the moisture out from the deepest layer of the onions. It is when the onions reach the point to about 4 percent moisture that dehydrated onions can maintain their long shelf life.

Dehumidified air that is below the relative humidity of the ambient air is supplied by the desiccant system. There are two types of desiccant dehumidifiers – dry and liquid systems, of which the former is most commonly and ideally used for food dehydration.

The system uses a rotating wheel and as the wheel turns, the air movers in the machine pass the air through a regenerating zone where the air is dried and the moisture is exhausted to the outdoors. During the regeneration, the desiccant media is heated by some type of heater within the machine. Generally these systems use a desiccant material, such as silica gel, to absorb the moisture. The dry air that leaves the desiccant dehumidifier unit and returns to the space you are controlling the humidity in, is at a temperature of around 120º.

As far as onion drying is concerned, to achieve high quality end products with minimal colour degradation and maximum shelf life, favourable drying, thermal and dehumidifying measures for each drying stage has to be perfectly maintained. When properly dehydrated, onions are white in colour and solid with a more pungent flavor.

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This article was published on Wednesday 05 September, 2012.

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