Author: Consell Comarcal del Baix Empordà, Licence CC BY-SA 2.0
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.
Everyone knows dried milk as milk powder. Let us say at the outset that freeze-drying is purely an experiment to test our prototype and does not offer much practical use. Thanks to the low temperatures, we can discuss the better preservation of vitamins A and D. Lyophilised milk will probably offer (with proper packaging) a longer shelf life. However, to be economically viable it would need to be dried in this way on a truly large scale
Preparation
For the trial, we dried 5 litres of whole milk, 1 litre per product tray. This is just enough to cover the entire tray with a layer of about 2 - 4 mm. The milk was almost at room temperature, around 19°C.
It can certainly be recommended to pour the milk onto the trays when they are in the machine. Walking around with a quarter metre tray with a layer of liquid on it is not the best idea. When the trays are fully inserted in the machine it is a good idea to adjust the level so that there is not too thick a layer of liquid on one side.
Freezing
Milk behaves much like water - it freezes for quite a long time even with small quantities. We froze five litres for a total of 10.5 hours to get below -25°C everywhere.
Drying
The drying itself was similar to white yoghurt in the data. The continuous layer of milk on the shelves maintained the required temperature well even as the water evaporated. The process took exactly 29 and a half hours.
Result
I myself had no idea what would be left when all the water was removed from the plain milk. The assumptions about a handful of powder were not confirmed. Instead, we got a similar layer to the one we loaded, but with a reduced weight.
Rehydration
First of all, I will warn against using the resulting powder (after grinding) directly into hot coffee. I guess we can't talk about lumps, but the result is not exactly what we want in coffee. On the other hand, just a little plain room temperature water and we have milk back to the way it came out of the box.
Uses
I'll be honest - it's great to have a box of freeze-dried milk at home that I know was actually normal milk and there's nothing added. It saves me more than one visit to the grocery store. However, I can't think of a global use for it myself. The companies that can produce tons of white powder at minimal cost are the ones that have the upper hand here. Drying milk in a freeze dryer is more like a small side benefit for the owner of the freeze dryer.
