While citric acid readily found in citric fruits, citric acid is produced on a production scale via fermentation of various sugars.
Because corn is a readily available source of carbohydrates in the US, citric acid is fermented from corn related sugars/carbohydrates using Aspergillus niger. This makes citric acid and vulnerable to natural disasters and fertilizer fluctuations.
Up Stream Materials/Building Blocks:
Carbohydrate (Corn, Molasses, Sucrose, Glucose, Fructose)
Down Stream Materials Consuming Citric Acid:
Food additive, usually for flavor or as a preservative