MCA (manufactured citric acid) is on my list of ingredients to avoid, as it seems to cause sores on the inside of my mouth. I don't get the same reaction from lemons/limes, so I figured it's the fungus contaminants.

Juices are commonly contaminated with MCA. I refer to MCA as a 'cosmetic food ingredient', which are added to change mouthfeel of a manufactured food and do not provide nutrition. Gums are the other common cosmetic food ingredient - they are added to change the mouthfeel or storage properties. Most gums are safe; I avoid all foods with carageenan, as this one supposedly has inflammatory properties.

The fine article says that Italy used to make citric acid from lemons, but that this got too expensive. I didn't realize fungus-derived citric acid has been in use since 1919.