I have the following observations:
- When I put a solution of glucose (80 g/ℓ) and ammonium chloride (20 g/ℓ) in a drying cabinet at 37°C (maybe a bit warmer) a brown substance forms when all (or most) of the water has evaporated.
- When I add water to the reaction product, the substance dissolves within minutes (under shaking), but the resulting solution is brownish. From this I conclude that a reaction happened and the brown colour isn’t just the colour of some salt crystal.
- If I perform the same procedure on glucose solution or ammonium chloride solution alone, nothing similar happens, which indicates that it’s not a reaction with the air or the container. PS: If I redissolve the product of drying ammonium chloride alone, the result is acidic (pH of 5 or lower for 1 g/ℓ).
- No browning happens if I replace ammonium chloride with sodium chloride, which indicates that chloride is not responsible for the reaction.
- The same thing doesn’t happen in the presence of water. Even when the solution has already been dried to a highly viscous sludge, I still don’t observe any browning. It only happens in final stage of drying.
My questions are: What reaction is this, why does it only happen when drying, and is there some way to prevent this, e.g., with a lower temperature? I ideally want to get a dried residue containing glucose, ammonium, and chloride in my container. Just filling in the powders doesn’t help, because I cannot do this efficiently and precisely.
What I found so far is this:
- Typical browning reactions of sugar (Maillard reaction, caramelisation) require much higher temperatures or additional ingredients.
- According to the paper Reactions of Free Sugars with Aqueous Ammonia, some reactions between sugar and ammonia happen at this temperature, but with a low yield and in the presence of water.