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  • Karel Schmiedberger

Freeze drying food - leave whole or cut?

Originally my intention was to bring an article about lyophilization of mushrooms, preferably true mushrooms of course. Although the mushroom season has not gone badly for most, I am not what one would call a mushroom picker myself. I simply have not been able to collect the necessary amount of mushrooms to meaningfully fill an AMARU.

Still, for the curious, I can state that I managed to load less than 3 kg of different types of mushrooms into the lyophilizer. With such a small quantity, it probably did not make sense to measure the freezing time or the drying time itself. Nevertheless, the result was excellent, almost surprising considering how little dry matter the mushrooms contain and the result is really very light.

Still, loading mushrooms into the freeze dryer gave me the idea for this article. Quite a few people ask us about the possibility of freeze-drying whole foods. Obviously, such an apple or banana freeze-dried whole would certainly be a unique product, but with any food processing for sale, it is all about the economics of the operation. This, in turn, is countered by slicing into fine pieces. It is therefore a good rule to remember:

When you cut food into smaller pieces, you get a larger surface area for each such piece over which water can more easily sublimate from the food.

According to this rule, it is clear that an apple cut into slices will dry much faster than a whole apple, but the apple pulp, spread evenly over the tray, will dry the fastest.

For most fruits and vegetables, it is also important to keep the peel in mind. This is a very effective barrier to prevent the sublimation of water from the product. Often this can be aided by piercing or cutting through the peel. A good example is strawberries, which are often asked about the possibility of freeze-drying whole foods. When the strawberry stem is pulled out, it not only breaks the outer skin, but in the case of strawberries it also creates a hole, sometimes leading to the interior of the fruit, which aids drying. However, the above rule still applies, and therefore halved strawberries will always dry faster than whole ones, and strawberries cut into quarters or slices will dry the fastest.

We decided to demonstrate the difference with freeze-dried mushrooms in the video below:

A chapter in itself is the small berry fruits, such as blueberries. These are self-evidently not to be halved or even quartered. Yet again, the repeated rule still applies here. Therefore, some large-scale producers of freeze-dried blueberries have their own system of slicing or piercing small berries in order to speed up freeze-drying and reduce the cost per process. The whole article could thus be summarised in one sentence:

Yes, it is possible to freeze-dry fruit whole, but the same fruit will always dry faster (i.e., at a lower cost) when cut into the smallest possible pieces, and it is up to everyone to find the ratio between the size of the individual pieces and the economics of drying.



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