We have been tempted to freeze-dry potatoes for some time. After all, it is a widely used food in our country and, like eggs, potatoes have a wide range of uses not only in Czech cuisine.
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 will admit right at the beginning that I wanted to test how potatoes can be dried and what the result would be. Scraping potatoes is not one of my favourite activities, so I bought 6.6 kg of sweet potatoes, pre-cooked, but mainly peeled, from a local shop. For the purposes of the test, absolutely sufficient.
However, these potatoes also need to be cut into trays. The basic rule is to cut them as thinly as possible. However, I deliberately left some really large pieces on the trays, which I hoped would keep their shape even after drying.
I would also like to mention for the record of the experiment that my greedy colleague ate a few potatoes and therefore it was necessary to reduce the input weight from 6.6 to 6.5 kg. However, this phenomenon can be eliminated by taking raw potatoes.
Freezing
The potatoes were not refrigerated, but transport in the car did bring them down to about 18°C. AMARU then brought the temperature down to below -30°C within 9 hours, but I added another hour to make sure that even really thick pieces were frozen.
Drying
I set the freeze-drying time itself at 28 hours. This went out at 6am, so by the time I got to the machine, another 3 hours had passed in a state of process completion - AMARU just keeps the product slightly heated at this stage, so drying is still happening, but only really lightly.
If I had cut all the potatoes into normal chunks, this would probably have been plenty, but on careful inspection of the largest chunks, I could still see the cold center of the potato, where there was still moisture. So I started post-drying for about 5 hours, which got rid of the last signs of water even in the thickest pieces of potatoes. Even those 5 hours may have been too much, but you can't "over-dry" food in a lyophilizer.
If I dry the same amount of potatoes next time, I would set the drying phase to maybe an even 30 hours and try to reduce the time by 30 - 60 minutes each time I load it.
Result
The potatoes were perfectly dry. As you can see in the video below, there was the requisite crack when broken and even the largest pieces were not cold to the touch. The weight loss is brutal, of course. Unfortunately, we still don't have a scale with the dimensions for the product tray, so we can only estimate that less than a kilogram of the 6.5 kg remained after drying.
Rehydration
I'm going to look at this a little more carefully today, because there's no point in eating potatoes dry. In fact, I wouldn't even recommend it. Thanks to the fact that I bought them already pre-cooked it was possible to taste a bit. I can still taste the awful dry-starchy taste now.
However, the question was whether the freeze-dried potatoes can be made into a full-fledged, cooked side dish again or whether they are "only" suitable for mashing. The opportunity presented itself at home when my wife fried the schnitzels. I tried putting the potatoes in salted water and boiling for a while. The raw ones would need to cook longer, but I was more concerned with re-saturating the water and reheating. The result is not too bad.
Freeze-dried potatoes need to be handled with care. After all, they are quite fragile and running a stream of water over them is not the way to go. With a little care, I managed to prepare a normal side dish for the schnitzel, which was only missing butter. A few of the potatoes did fall apart with the boiling, but I'd say that's no more of a problem than with cooking regular potatoes. You just have to remember that there needs to be more water. After a while in the pot, the top bits of my potatoes were sticking out above the surface because they soaked up the water quickly.
Uses
There are no limits to imagination. If someone wanted to sell freeze-dried potatoes - cut up pieces to be heated, they would have to provide very good packaging so that the customer would not bring home only potato fragments.
Better use, of course, will be found for blended or just crushed potato powder. In good packaging, one can keep it at home for years and make mash from it at any time - mashed potato powder, but only from potatoes, with no added chemicals. Beyond that, it is more up to the person standing at the stove. There is probably nothing to stop one using freeze-dried potatoes to make potato dumplings or pancakes. A combination, for example with freeze-dried milk, could also be considered for the final product.
We can forget about echt potato soup, because the point of it is coarsely grated potatoes, which the freeze-dried version does not allow, but what prevents the drying of already grated potatoes...?