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  • Karel Schmiedberger

Optimising the process of freeze drying food

Thermometer with K-type thermocouple for optimizing the freezing of food in the AMARU freeze-dryer

It is common during installation and operator training to discuss with the user how to shorten the freeze drying process. Of course, a shorter process means lower costs and therefore higher yields. There are several ways, but I deliberately omit those that shorten the freeze-drying process at the expense of the quality of the result.

In a typical industrial freeze dryer, the process is divided into several stages and the user must be familiar with freeze drying to be able to develop a workable recipe for freezing the product, primary drying, and eventually secondary drying.

AMARU is an automatic lyophilizer where the process is much simpler, but there is still room for optimization by the user. However, this will not require years of experience in freeze drying.

Drying optimisation

In the case of AMARU, this is definitely the easy part. The important thing is to approach the optimization at least a little scientifically and know what and how much to load into the freeze dryer. In other words, I need to weight the loading so that I can repeat the experiment under the same conditions.

I simply need to then run the process with a longer drying time. Let's say that for 8 kg of cheese I prefer to set the drying time to 35 hours. The product should be well dried and now I need to prepare another, identical batch, where I take two hours off the time and set only 33 hours of drying. I repeat this process (eventually taking away perhaps only 1 hour) until I reach a result that no longer seems as well dried as I would like. That said, just go back to the last setting that gave a good result and we have the drying process figured out.

So optimizing the process on the AMARU is a matter of 3 - 5 processes and not months as is common with more complex freeze dryers.

Freezing optimisation

In the case of freezing, we do not need to use trial and error, but good planning will be required. It doesn't matter if we are loading food at room temperature or if the freezer saves us time. In either case, we will need to measure the temperature either with a wireless thermometer or with a thermocouple, which is more reliable in this case.

For the sake of example, let's assume that we are again loading 8 kg of cheese, which is at around 8°C. For proper freezing in AMARU we set a freezing time of 15 hours. Before the set time expires, we need to interrupt the process, remove the middle shelf and measure the temperature of the product.

  • If the thermometer shows temperatures of -25°C or below, the product is well frozen and the process can continue. Simply start a new process, set only 2 hours of freezing (the minimum allowed by the control unit) and then reset it to zero, letting AMARU go straight into the drying phase.

  • However, if the measured temperature is -30 or even -35°C or less, it is possible to take a few hours off the freezing time for the next process.

  • On the other hand, if the product is only -20°C or more, you definitely need to add at least 2 - 3 extra hours of freezing.

As I have already mentioned, the measurement can be done with a non-contact thermometer. However, it is important to remember that we are only measuring the surface temperature in case the product is cut into larger pieces, it may still be insufficiently frozen inside.

This potential error is eliminated by a thermocouple which is inserted into the product before freezing and only connected to the thermometer when the temperature needs to be read. Moreover, it is not an expensive device. With a price tag of around a few dozen Euros, it is a suitable piece of equipment for anyone who is serious about the business of freeze-drying food. :)

Teploměr s termočlánkem typu K
The thermocouple can be plugged into the product before freezing and when connected to a thermometer, it will give the exact temperature of the product.



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