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  • Karel Schmiedberger

How food freezes before freeze-drying

Kostky ledu

Proper freezing of food before freeze-drying is the first step to success. If the temperature is too high, boiling will occur instead of drying, which will affect the appearance of the food. The AMARU is capable of freezing the load itself, but most users are more likely to reach for a storage freezer to save machine time on the lyophilizer. Either way, it is good to understand how food actually reaches the required temperature of around -30°C.

If we store food at room temperature, the freezing process starts in the same way as if we put it in the freezer, i.e. the temperature slowly drops to around -2°C. Somewhat surprisingly, this value can be reached more slowly when the load is kept in the fridge. This is the so-called Mpemba phenomenon, which has not yet been precisely explained (see Wikipedia).

Although we all consider the "freezing point" to be 0°C, in practice, more complex loads usually freeze completely at around -2°C. Even if the storage freezer or AMARU is constantly cooling, here the temperature drop stops even for a few hours. This continues as long as the water in the food is in a liquid state.

Graf zamrazení potravin v lyofilizátoru

When the product is frozen, the temperature continues to drop to lower values, down to the required -30°C. The process is best illustrated by the graph of the whole milk temperature record above.

It is thus clear that keeping food at temperatures below about -5°C before loading into the freeze dryer will significantly reduce the time required for freezing in the freeze dryer. However, it should also be noted that even freezing the product to -30°C will not allow drying to start immediately after loading into the AMARU. Even a short transfer of the food from the freezer to the freeze dryer will cause considerable heating and it is necessary to let AMARU stabilize the product at around -30°C before drying.

However, we will show another graph that is a little different. It is a record of the freezing of condensed milk.

Graf zamrazení kondenzovaného mléka

As you can notice, the "freezing" moment is completely missing in the graph. It is not there simply because it did not occur and although the condensed milk reached a temperature of -35°C, it was still a viscous liquid and therefore incapable of being freeze-dried.

Honey, for example, or anything containing large amounts of sugar (or salt) behaves the same way. When the food is not frozen, freeze-drying does not exactly occur and the result is just a dirty freeze-drier chamber. Incidentally, the video of us trying to make freeze-dried bee honey says it all.



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