This week began with a major achievement: we managed to produce powdered milk! This might sound as if it had no connection at all to developing a reliable assay for tapeworm infection, but it is in fact crucial to our success.
Freeze drying and why we need it
What looks like a mixture between a pressure cooker and a washing machine is actually a freeze-dryer, also called lyophilisator. Admittedly, it might not be the newest model, but it is functional and Prof. Franz Bartl and Dr. Martin Heck kindly allowed us to use it to dehydrate any material almost entirely.
But why would we want to do that? Freeze-drying is a method to preserve organic material that is entirely reversable: due to the greatly reduced water content in a freeze-dried substance, microorganisms or enzymes that would normally spoil or degrade the substance are inhibited. As soon as the material is rehydrated, i.e. you add the water again, it returns to its original state. Thus, freeze-dried substances can be stored for a long time without requiring a cold chain. Originally, the method was established to transport blood samples overseas - in dried form, they could not spoil if the freezing failed. For our tapeworm assay, which we hope to be employed in even remote areas of tropical countries, it also could be a crucial advantagte: Only if the test can be stored for longer periods without a freezer nearby, it can be used in the field.
Thus, we aim to make our diagnostic tool - that is, the entire cell-free expression system with all components needed for the color reaction - storeable, by freeze-drying it directly on the membrane later used for testing. This way, the test remains functional for long periods of time even at high temperatures, as long as it is kept dry.
First steps with the freeze dryer
A first step towards this goal was to actually find a freeze-dryer which, by luck, was already at the institute. However, the machine had not been used for a while, so some testing was needed. This we did by using normal store-bought milk: freeze-drying should lead to powdered milk, clearly distinguishable from the liquid form. To this end, we froze small milk samples and put them in the machine. There, the pressure drops enough for any water to evaporate - and after some time, all we had left in our eppendorf cups was milk powder.
With the proof that this freeze-dryer still does its job perfectly fine, we even went a step further and freeze-dried a sample of the cell-free expression system. After passing through the lyophilisator, nothing seemed to remain from the few microliters of solution - at least nothing visible to the eye. But, when we rehydrated the container and added a trigger sensor, a very visible color change resulted!
Open questions for our team
A success: we have the proof that cell-free expression systems can be dryed and rehydrated while remaining functional. But that was solely one trial, and, moreover, the process is hard to follow - how can you know if there's really just 5% of the water left? While milk and its reduction to powder is certainly a way to show the concept generally works, we still need better ways to control the success of freeze drying or test system.
A first step in this direction will be to test the storeability of freeze-dried expression systems: do we still observe a visible color reaction after two weeks? After ten?
Other approaches and maybe even trouble-shooting might be needed - so if you by any chance are an expert of lyophilisation or have a freeze-dryer at home, do not hesitate to get in touch 🙂