Glycerin Hygroscopy Values: Results From My Experiments. AKA Putting On My Nerd Pants.

This is a short deviation from my usual writings. I'm not applying my usual rigor with citations because I'm mostly citing facts that can easily be looked up.

Preamble

I use vegetable glycerin (AKA glycerol) to dampen supplements so that they compact more tightly. This allows me to fit more in a smaller space, and also allows supplements to sink to the bottom of the stomach. For example, spinach powder is relatively light and fluffy. I'd estimate that I can fit 30% more spinach into a capsule if I add glycerin. Additionally, when glycerin is added it doesn't bounce back when compacted, therefore making it easier for me to fill pills to maximum capacity.

The Conundrum

"Why glycerin?", you may ask.Using water would provide a breeding ground for mold and bacteria. Thus, I use glycerin precisely because it is not water. High levels of glycerin can be used as a food additive to preserve food, much like sugar or salt. It lowers water activity, limiting the ability of microbes to grow.

Additionally, there is one other reason I'm using glycerin. Initially, I had tested both glycerin and propylene glycol, filling a vegetable cellulose capsule with each respectively. The capsule filled with propylene glycol dissolved, while the capsule filled with glycerin did not. Thus, I settled on vegetable glycerin.

However, I began to notice something strange. Over time, capsules to which I had added glycerin began to become stretched, and the outside of the capsule became wet. Glycerin is hygroscopic, meaning that it can absorb water from the air. As such, the fluid volume increased, expanding the capsule and overflowing into the container.

Therefore, I ran a second experiment. I combined glycerin with various amounts of water, and put it into capsules. I found that large amounts of water could be added without dissolving the capsule. The cellulose capsules could withstand at least 50% water, or 1 to 1 glycerin to water by weight. However, the larger quantities of water did warp the capsules slightly. Nevertheless, I ended up using a 1 to 1 ratio

Refinement
But the story doesn't end there. Sure, 50% water worked, but was it optimal? I wanted to get the water activity as low as possible. I always make at least a month's supply of pills, usually 3 months' worth. Thus, the more certainty that the pills will preserve, the better. So I ran a third experiment. I put 100 grams of glycerin into an open container, labeled it, set it at the back of my desk, and forgot about it. I weighed it every so often, when the fancy struck. Months later, I put it on the scale; it weighed exactly 40 grams more than the starting weight.

Thus, my new mixture of 40 parts water per 100 parts glycerin was born. However, I still needed to confirm that this was the maximum water holding capacity of glycerin. I filled a container with 50 parts water to 100 parts glycerin to see if it would continue to increase or not. As it turns out, it decreased in weight, suggesting that water evaporated.

Final Results

OK. So we have the hygroscopic capacity of glycerin. We're reasonably confident that pills will preserve. What's missing? Details. For example, how does humidity affect the hygroscopicity of glycerin? How long does it take for glycerin to absorb water? What about alternative units, and using volume instead of weight? I've taken care of that. In a final experiment, I set up a small amount of glycerin and meticulously tracked its weight over time. I converted it to various units. Note that percent is expressed as a fraction. I also looked up humidity at time of measurement, but it didn't fit neatly on the graph. You're welcome to check out the that data in the original spreadsheet. As you can see, I could lower the water content of my recipe from 40 parts per 100 to 38 or so (as seen in red: the weight ratio of water to glycerin)