I've tried to google for an answer, but it's hard because mold has two meanings and in this case I'm using both. Some dessert got left in a silicone mold (shaping device) in the back of the fridge, and when I discovered it the dessert had gotten moldy (fungal growth). I washed the silicone as well as I could with soap and water, but it's still discolored. Is it possible to clean it enough to be food safe or do I need to throw it away?
If it is possible to clean it, what is the best way?
If the mold (shaping device) is actually silicone rubber, you should be able to kill any mold (fungal growth) quite easily.
I use silicone bread pans that are rated to 450 °F / 232°C (correction, poor memory and I never use them this hot - 500 °F/ 250 °C) in the oven, and that will kill fungal growth (and any others) in a matter of minutes or less. You could also put it into a pressure cooker, if it will fit.
Now, I also have some sort of rubber ice cube trays that are not actually silicone, and I expect those would melt into a stinky puddle and/or catch fire.
You can also run it though the dishwasher on "sanitize" if you have a dishwasher and it has that cycle option. You might look for a marking on the shaping device indicating a safe temperature range or limit (often right next to the food-safe cup and fork icon marking.)
Discoloration of silicone is common, and it isn't a safety issue. Silicone is porous, and so yours absorbed coloring, most likely from the dessert as it was left in there awhile. Cleaning it thoroughly may restore the original color, but if it doesn't it's still safe to use.
As for cleaning it just use good old soapy water, and as for the discoloration wiping some a small amount of baking soda paste (baking soda with just enough water to make a paste) and letting it sit for a few minutes before washing it out may restore the color. If it doesn't it's still usable, just maybe not that attractive.
Boil them in water!
10 minutes of boiling will do, unless you are at elevations not usually inhabitated.
This is THE method. The time-honored (used for thousands of years), traditional way to kill any microbial life. Works also for getting the smell out.
If the smell persists, a small amount of baking soda (~0.1%) in the boiling water may help.
How other possible methods may fail (they may as well work, but require more controlled process):
Chemicals (including, but not limited to, bleach, other chlorine-based disinfectants and alcohol) They may not penetrate deep enough into the material. Well, given time and shaking, they will eventually penetrate everywhere, but now you have the task to get them out. For glass, metal or porcelain the chemical disinfection is quite good, for soft and porous materials - not really.
Dry heating (in an oven) As we all know from our baking experience, the temperature over the whole container may vary a lot. On the other hand, when dry, some forms of the microbial life (e.g. spores) survive well above 100C. An oven with circulating air will perform better, but be sure to reach at least 200C for 10 minutes or more. Be also sure not to heat them above their rated temperature.
