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Here’s a really interesting article on how it was discovered that citrus would help. They were also able to preserve citrus and citrus juice with alcohol. They could also turn it into a concentrated syrup without too much loss of vitamin C.

From what I just read, they didn’t do this, but dried citrus, when dried at a cool temperature, retains the majority of its vitamin C.

A lot of the other light patches in this image are city lights.

More info from NASA here.