How to tell if a freeze-dried product is really dry?
With freeze-drying, it is not enough to watch only the set drying time. The pressure inside the chamber is also a very important indicator, because it is related to how much water vapour is still leaving the product. It can often tell you whether it already makes sense to stop the process or whether it is better to let the freeze dryer keep running. With AMARU, there is one simple rule:
For the vast majority of products, there is little point in taking them out if the pressure is 200 mTorr or higher.
Higher pressure usually means that water vapour is still leaving the product and keeping the pressure in the chamber elevated. As the product gradually dries, the amount of released water decreases and the pressure drops with it. However, 200 mTorr is not a universal threshold at which every product is automatically finished. One product may be perfectly dry at 170 mTorr, while another may need the pressure to fall below 140 mTorr. Users who repeatedly process the same product therefore have an advantage. Over time, they can observe at what pressure their particular batch is reliably dry and then use this value as one of their main checkpoints.
Why you should not rely on time alone
The same product does not necessarily contain the same amount of water every time. One year may be drier and the fruit may contain less juice. A drying time of, for example, X hours may work perfectly well. The following year, however, the harvest may be much juicier and the same drying time may no longer be sufficient. The product may easily need several extra hours of drying. This is exactly where monitoring pressure is more useful than blindly repeating the drying time from the previous season. Time tells you how long the process has been running. Pressure gives you an indication of what is actually happening inside the chamber. If you have established a target pressure for a particular product, you can adapt more easily to differences between individual batches without having to estimate their water content precisely in advance.
If you are still unsure after the process has finished
After the process has finished, you can still check the result physically. In practice, it makes sense to take one of the largest or thickest pieces, ideally from the bottom tray, break it open and check the centre. If the inside is still noticeably cold or even frosty, this is a sign that some water is still present in the product. This check should only be the second line of defence. Once you stop the process and vent the chamber, you interrupt continuous drying. If you then find that the product is not yet finished, AMARU offers an additional drying function that can save the batch. Additional drying should, however, be seen exactly as that: a way to rescue an unsuccessful or prematurely stopped process. You will achieve a better result if the drying process runs continuously without interruption and you remove the product only once it is genuinely finished.
Use time and pressure together
The set drying time is important mainly as a starting point. If you process the same product repeatedly, your own experience of the pressure at which it is reliably dry will gradually become much more valuable. In practice, it therefore makes sense to monitor both: time tells you how long you have been drying, while pressure indicates whether water is still leaving the product. For repeated production, this is often more reliable than sticking to the same number of drying hours every year.
