While propionic acid is used artificially as a preservative, it is a naturally occuring byproduct of fermentation of certain kinds of carbohydrates. The articl

While propionic acid is used artificially as a preservative, it is a naturally occuring byproduct of fermentation of certain kinds of carbohydrates. The article notes the link between certain kinds of bacteria in ASD children that overproduce PPA as a byproduct.