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  • Karel Schmiedberger

Freeze-dried onion


Freeze-dried onion

Self-grown and cared for onions will last, under the right cellar conditions, for many weeks without harm. However, drying by freeze-drying will not only result in a much longer possible storage time, but also in weight loss.


Note: This test was performed on an earlier model of the AMARU lyophilizer. We have made several improvements since then and today the capacity would be higher and the process faster.

Preparation

I have purchased a classic white onion, supposedly weighing about 7 kg. After peeling and slicing, we managed to load almost exactly 5 kg into the freeze-drier, with about seven onions left over. We had to throw away some of the pieces because the traders are always completely careless about proper handling and so losses are incurred.

The original intention was to cut a few trays into rounds and a few into blocks. However, during processing, we found that neither of us could cut onions properly, but also that the onions we bought were not quite as hard as they could be and cutting them into nice rounds was almost impossible. So we ended up with plus or minus 1000 grams of cubes on each tray.

As with the horseradish, in addition to a cutting board and knife, I would recommend including diving goggles in your basic onion slicing equipment. By the way, did you know that the irritant gas Thiopropanal S-oxide causes teary eyes?

Freezing

I set 14 hours as the freezing time, which ended up being ideal because even the middle shelf of the Lyon got below -25°C. The commonly offered AMARU has a slightly more powerful cooling system and in the end maybe another hour could have been saved.

Drying

According to the tables, onions have about 90% water. However, according to the sensors we use in Lyon to monitor what is happening inside the machine, the evaporation of water is minimised (marking the end of the process) sometime slightly after the 25th hour of drying. Just to be on the safe side, we left the onions in the machine for a while longer and ended the process after 28.5 hours.

The rapid drying is also due because the layers of onions allow the water to evaporate easily.

Result

Thanks to the above, the onions were perfectly dry. Of course, after drying, the individual skins do not stick together as much and so any slices would probably tend to fall apart. Dicing proved to be ideal both for drying and for subsequent processing.

About 115 g of each kilogram remained after drying, which corresponds to about 11% of the original weight before drying.

Rehydration

Bringing dried onions back to a more or less original state is not difficult. It's just worth bearing in mind that the water will impart most of the flavour during rehydration. In practice, it would be ideal to use the freeze-dried onions directly in a salad, for example, which will be significantly more flavourful.

Theoretically, there is nothing to prevent the use of the onion for scrambled eggs, but first the onion needs to soak up the water - this takes away some of its "strength" (it remains in the water).

Uses

In addition to the above, freeze-dried onions can be conveniently blended into a powder and used to flavour de facto anything that does not require larger pieces of onion.


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