How do I sterilize/disinfect ordinary table sugar for lab use?
I'm using the sugar in an agar and I want it to be as clean as possible. Are there any effective, conventional ways of doing this?
Autoclaving media with carbohydrates (or sugar solutions on its own) is not a good idea for two reasons:
The Maillard reaction: This occurs when you heat mixtures of amino acids and sugars. The amino group of an amino acid reacts with a carbonyl group of a sugar, fusing the two molecules together. This is a common reaction in the kitchen which is for example responsible for making the bread crust brown. It also changes flavors.
Caramellization: This is the heat induced breakdown of sugars - also leading to brown products, but much more undefined than the Maillard reaction. What products arise depends on pH, temperature and other reactands.
Both reactions may lead to products which can be inhibitory to microbe growth and should be avoided. There is also a paper which analyzed the effect of autoclaving on sugars which might be interesting to read in this context (see reference below).
So prepare a higher concentrated stock solution of your sugars, which can then be added under sterile conditions to your autoclaved and cooled down media or the agar solution (still warm enough to pour).
Sterilize this solution by filtrating it through a 0.2µM filter - depending on the volume either through syringe filters or through bottle top filters. Make sure to maintain sterile conditions.