Answer: Mold Byproduct

It would be all too easy to think that the primary source of citric acid is citrus fruit—it is, after all, a rich natural dietary source of the nutrient—but despite how packed with citric acid some fruit might be (up to 8 percent of a lemon, by dry weight, can be citric acid), it’s not commercially viable to meet the world’s citric acid needs by extracting the compound from citrus fruit.

Although the very first industrial citric acid production, starting back in the 1890s in Italy, did extract citric acid from fruit, that process not only proved unsustainable in terms of global demand, but the outbreak of World War I led to the disruption of the industry and Italy’s hold on it.

By the early 20th century, a new method emerged when American food chemist James Currie discovered that certain strains of the fungus Aspergillus niger (a common contaminant of food that causes black mold)—seen here as you might find it in your kitchen growing on an onion—could produce large volumes of citric acid when fed a very sucrose or glucose (sugar) rich diet.

Despite scientific advances that allow for direct synthesis of citric acid (a technique invented in the 1970s), it’s far more cost effective to use mold cultures with the bulk of citric acid produced for both dietary and industrial use around the world being created by Aspergillus niger.