Used frying oil + stearic acid = easily disposable goo
I really like deep frying food like doughnuts and kara age chicken but have been forgoing it for a while because of the difficulty of disposing of the oil. If you filter out any food crumbs after it cools down and/or clarify it using gelatin, you can re-use deep frying oil two or three times, but after repeated use it darkens and tends to develop an unpleasant flavour affecting the food. After that it's got to go.
You shouldn't pour it down the sink because it can contribute to fatbergs forming in the sewers. You can pour it back in the original bottle, put on the lid and put it out in the rubbish, but that means the bottle doesn't get recycled and the oil is trapped inside it in the landfill, unable to biodegrade. Restaurants have access to commercial oil removal services, which nowadays often convert the old oil to biodiesel so it gets another use, but these usually don't take oil from people's homes (kerbside oil collection may be available in some places, but certainly not where I live). I felt stink about the available options so I stopped deep-frying for a while to avoid the problem.
However, I've learned of a clever trick! You can buy stearic acid quite cheaply from the kind of stores that sell supplies for making scented soaps and candles. It's a substance sold in the form of small flakes or granules that has the property of solidifying oils (hence useful for making soaps and candles).
To solidify deep frying oil, either do this shortly after you finish cooking with it, or (if you're me) reheat some old oil you have sitting around in a bottle that you don't feel right about throwing away. It doesn't need to be super duper hot, just anywhere above about 100ºF/38ºC. For each cup of oil, add three tablespoons of stearic acid and let it dissolve (it will be pretty quick). Turn off the heat and put the pot at the back of the stove or somewhere else that it can cool down undisturbed.
The time it needs to solidify will vary depending on things like the quantity of oil and how well the pot retains heat, but I found that within a couple of hours four cups of oil in a cast-iron pot had gone cold and turned to a soft jelly consistency that could be easily scooped out of the pot and dropped in the rubbish bin. Grand.
There are, of course, packaged mixtures you can buy that do the same thing (one brand is called Fry Away) but plain stearic acid is cheaper so yay!
