Raffinose is a sugar found in legumes that is indigestible by humans. Which is great, because I'm looking for a sweetener that 1) won't raise blood sugar and 2) doesn't have a bitter aftertaste and isn't carcinogenic or whatever else. The question I have is: Does raffinose break down during cooking? If heat can decompose it into other sugars then it's useless for my purposes. 
In most cooked foods, it will break down.   Raffinose has a melting point of 80°C and decomposes at 118°C. So if you keep the food below 118°C, you will be OK. You can sweeten your tea with it, or use it in a custard (the big sugar molecules will interfere somewhat with the setting of the custard, but I can't predict if the effect will be noticeable at all). Applications involving simmering water should be safe, if the solution consists of water mostly. However, if you use large amounts of raffinose (e.g. you are trying to make a jam), the boiling point of the solution can be much higher than the boiling point of water.   As for baking, the raffinose will definitely decompose in the outer layers of the baked good, but this could be a very small part of the whole. The core of a properly baked cake should stay much lower than 118°C. Small items like cookies will be more problematic.   When making a substitution, you have to consider that in some recipes, sugar is more than just a sweetener. For example, I don't know if you can cream butter with raffinose the way you can do it with table sugar, and without creaming, you can't make proper cookies. Another example would be pectin: high methoxyl pectin (the one used for jams) requires high sugar concentration for gelling, so you probably can't substitute raffinose there (unless you heat it enough, because one of its decomposition products is sucrose).   Another thing to consider before eating lots of raffinose is that it is indigestible for humans and gets degraded by gut bacteria in the intestines, producing large amounts of gas - it has the same effect as eating lots of beans, for exactly the same reason.   The source for the boiling and decomposition point of raffinose is the material safety data sheet for raffinose.  