A black fungus known as aspergillus, which ironically enough is the same stuff that grows on spoiled food, produces citric acid when it metabolizes sugar in a low oxygen environment.

这是种被霉菌黑色真菌，讽刺是，它生长在变质食物上东西是，当它在低氧环境中代谢糖时会产生柠檬酸。