Hey Sarah, I was just researching glycerin in the book ‘The Herbal Medicine-Maker’s Handbook’ by James Green. It provides a detailed description of glycerine. I’d suggest getting your hands on this book if possible!
To summarize, glycerin is the sweet fraction of fat. It is present in all true fats. Though it is sweet to taste, it is not a carbohydrate and doesn’t contain any sugar and won’t raise blood sugar.
It is made in a complex process which decomposes fats with steam pressure to separate the glycerine from the fatty acids. It is then distilled out, purified, and condensed.
On the topic of allergy, I don’t think there should be an issue. Allergies to things such as soy are generally an allergy to a protein in the food. Glycerin does not contain any proteins and so it shouldn’t contain anything to cause a reaction. However, it would be best to do a patch test on the skin of the person you’d like to give it to just in case. You can apply a small amount of glycerin on the inside of the wrist/arm and observe any response. You could also send an email to a supplier to see if they can give you a clear answer about where the glycerine is sourced from. I’d also suggest checking that it is a product that is safe for consumption (food grade).
Some information about glycerin as a solvent:
-It has about 1/2 the solvent power of pure ethyl alcohol
-It resembles water more closely than alcohol as a solvent
-It can be combined with water or alcohol
-It is not ideal for extracting resins or essential oils
-Glycerin extractions have a shelf life of 1-2 years stored in a dark, cool place.
If you need help with sourcing, I’ll do some more research, just let me know if this is helpful!