For a long time, I have used citric acid as part of my wool dyeing process but it has some other uses as well.
Citric acid is a weak organic tribasic acid. It occurs naturally in citrus fruits. Here’s wikipedia’s entry on the subject. https://en.wikipedia.org/wiki/Citric_acid
If you find that, when making stewed apples or apple pie, your apple slices are discolouring before you can get them in the pan/pie dish, then add a sprinkle of citric acid to some water and drop them in there until you are ready to cook them. Drain the water off, you don’t need to rinse, and you are good to go.
Citric acid is used commercially in many foodstuffs but don’t use massive amounts of it in cooking. Small amounts are sufficient. As with most things too much of anything is not good for you.
I also discovered that it makes an amazing toilet cleaner. First of all I dissolved about three quarters of a cup of citric acid crystals in a litre of boiling water. After skooshing most of the water out of the toilet bowl with the brush, I very slowly (don’t want to crack the toilet) poured the hot water into the toilet bowl. I left it for a couple of hours, overnight would probably have been better though. Then I gave it a quick scrub with the loo brush and after flushing the toilet it was sparkling clean. Yay! I had previously struggled to remove the limescale in there, even with commercial cleaners.
I also chucked half a cup of the citric acid into my empty washing machine and put it on a hot cycle. It had previously had a musty smell and the citric acid got rid of that so now it smells nice and fresh!
Apparently you can also use it to make cheese, although I haven’t tried that yet.
Here’s a link to a couple of pages describing the process:
I am sure that there are many more uses for this stuff, please tell me if you know any of them.