We all hear the same warnings during hot summer days. Avoid direct sunlight or risk sun stroke, sun burn, dehydration. But what about your freeze dryer? Would the instrument benefit from the energy boost from the sun as a free external heat source or would it suffer system malfunction? In this blog post, I decide to address this question and explore the effect of direct sunlight on the freeze-drying process. See if my experimental data supports sun exposure or if the results indicate the freeze dryer should also be kept in the shade with a hat on.

I was at a park the other day and witnessed one poor mother trying to deal with her child’s temper tantrum over a push-up popsicle. At first, the popsicle was so frozen, the mum couldn’t push it out of the wrapper, so the child cried. The mother had her hands full with a sibling, so she placed the popsicle on a sunny spot on their picnic blanket to warm up. I guess she left it in the sun for too long, because the frozen treat quickly turned into juice. And so the boy cried again.

As I watched the child pouring popsicle juice all over himself, I was brought back to the lab. The mother had the right idea to use the power of the sun to speed up the melting process. So I questioned myself, can’t we do the same and use the heat of the sun to make our freeze-drying process faster at zero extra costs?

Let us consider the theory behind the idea. The sublimation process, just like the melting of a popsicle, requires the uptake of heat. This energy can be supplied via different mechanisms, such as conduction, convection and radiation. I’ve discussed these in more detail in a previous blog post.

So, in general, putting a freeze dryer in a sunny spot should be advantageous because more energy by radiation is available.

However, I had to quickly consider the obvious drawback. Too much sunlight doesn’t just ruin popsicles by melting them too fast. If too much sunlight causes the sublimation process to occur too quickly, we could run into serious problems with the freeze drying process. For example, we might cause sample collapse or even melting of the sample due to backpressure below the drying front.

There was only one way to assess the effect of sunlight on the freeze-drying process. I had to perform an experiment with and without direct sunlight!

To do this, I freeze dried solutions of polyvinyl alcohol (PVA) (5 wt % and 1 wt %) as well as Trehalose (5 wt %) under the same conditions. Briefly, I added 30 mg (150 mg) of PVA or Trehalose to 3 mL of water in 5 mL vials and froze the samples for 24 h at – 40 °C. Then I freeze dried the sample at 0.1 mbar for 3 hours in direct sunlight. If you got to see it to believe it, I did take a picture just for you:

Then I compared the results to samples of the same constitution dried at 0.5 mbar in a room without direct sunlight. I picked 0.5 mbar because this value has been proven to increase the speed of sublimation. The figures below show the temperature data obtained during the process in indirect sunlight (top graph) and in direct sunlight (bottom graph).

When the freeze-dryer was in direct sunlight, the sample temperatures increased considerably faster than when in a laboratory working environment. Samples reached temperatures above 0 degrees in the first one and a half hours and appeared dried after two and a half hours. However, once the process stopped, samples collapsed and re-dissolved. This occurrence was due to the core of the samples not drying properly. The picture below shows how the samples should have looked after freeze drying (top) compared to the damages samples after lyophilization in direct sunlight (bottom).

Direct sunlight not only had a negative influence on sample quality, but heat and radiation going through the acrylic chamber also influenced the freeze dryer itself.

Precisely, shelf temperatures increased above set point and condenser temperatures increased slightly, making the set pressure difficult to maintain.

So I’d say, save yourself some tears and tantrums and leave the sun out of the equation.

Of course, in reality, nobody is performing freeze-drying outside in the sun. But the experiments bring up a valid point.

The placement of the freeze dryer plays an important role in the smooth performance of the lyophilization process. The location of the instrument can also help extend the lifetime of the machine.

A location that provides too much direct sunlight will hinder the function of the unit by preventing the target pressure from being reached due to a condenser that is too warm. Even more, too much sun leads to heating up of the samples, causing catastrophic sample damage.

So it is best to keep yourself, your ice cream and your freeze dryer out of direct sunlight.

I admit, I thoroughly enjoyed reporting these hot experiments to you and I hope you stay cool and keep coming back for more. And in case you can’t wait, then check out what other lyophilization thoughts the sun brought me to in the past in this previous blog post.

Till next time,