Citric acid is made of just three elements: carbon, hydrogen, and oxygen, arranged in a molecule with the formula C₆H₈O₇. That’s six carbon atoms, eight hydrogen atoms, and seven oxygen atoms, giving it a molecular weight of 192.12 grams per mole. But if you’re asking what citric acid is made *from* in a practical sense, the answer might surprise you: nearly all of the world’s citric acid supply, estimated at 4.4 million tons in 2024, is made by feeding sugar to a common black mold.
The Molecule Itself
Citric acid is a relatively small organic molecule built around a short carbon backbone. Three acid groups (called carboxyl groups) branch off this backbone, along with one hydroxyl group, which is essentially an oxygen-hydrogen pair. Those three acid groups are what make citric acid sour. They readily release hydrogen ions when dissolved in water, lowering the pH of whatever liquid they’re in. This is the same tartness you taste when you bite into a lemon.
The molecule is highly water-soluble, and at room temperature it forms white, odorless crystals. It occurs naturally in all living cells as part of the energy-producing cycle that keeps organisms alive, but it accumulates in especially high concentrations in citrus fruits, where it can make up as much as 8% of the dry fruit weight in lemons and limes.
Where It Originally Came From
For over a century, citric acid was extracted directly from citrus juice. In 1784, the Swedish chemist Karl Wilhelm Scheele developed a method to isolate it from lime juice, and by 1826, English producers were extracting it from lemons on a commercial scale. This worked, but it was expensive and limited by fruit supply. Producing a single pound of citric acid required enormous quantities of juice.
That changed in 1917, when an American food chemist named James Currie discovered that a black mold called Aspergillus niger could convert cheap sugar into citric acid efficiently and at industrial scale. The mold thrived in acidic environments where competitors couldn’t survive, making it ideal for large fermentation tanks. Within a few decades, microbial fermentation had almost entirely replaced citrus extraction.
How It’s Made Today
Modern citric acid production starts with a sugar source. The most commonly used raw material is sucrose, but manufacturers also use cane molasses, beet molasses, corn starch, starch hydrolysates, and various agricultural waste products. The priority is cheap, abundant carbohydrates. Molasses is particularly popular because of its low cost and wide availability.
The sugar is dissolved in water and placed in large fermentation tanks along with Aspergillus niger. The mold breaks down the sugar through a series of metabolic steps. First, enzymes on the mold’s surface split sucrose into glucose and fructose. These simple sugars get pulled into the cell, where they’re processed through the same energy pathway your own cells use. At a key step in that pathway, an enzyme combines two smaller molecules (acetyl CoA and oxaloacetate) to form citrate. Under carefully controlled conditions, including low pH, limited trace minerals, and high sugar concentrations, the mold overproduces citric acid rather than using it up for energy. The acid accumulates in the fermentation broth.
The dominant production method is submerged fermentation, where the mold grows suspended in liquid rather than on a solid surface. The entire process typically runs for several days, and industrial strains of Aspergillus niger have been optimized over decades to maximize yield.
Purifying the Final Product
Once fermentation is complete, the broth contains citric acid mixed with mold cells, leftover sugars, and various byproducts. The mold is filtered out first. The remaining liquid then goes through a purification process to isolate pure citric acid crystals.
For a higher-purity product, the acid is treated with activated charcoal or passed through ion exchange resins to remove color and impurities. It’s then crystallized. If the crystallization happens above 40°C, anhydrous citric acid forms (the dry powder you’d find in most food products). Below 36.5°C, it crystallizes as a monohydrate, meaning each molecule of citric acid holds onto one molecule of water. Both forms are widely used.
What It’s Used For
Citric acid plays several functional roles that make it one of the most widely used food additives in the world. It’s classified as Generally Recognized as Safe (GRAS) by the FDA, and it shows up in an enormous range of products.
- Acidulant: It provides sour flavor in soft drinks, candy, and canned foods. It’s the tart kick in sour gummy worms and the tang in many fruit-flavored beverages.
- Preservative: By lowering pH, it creates an environment where harmful bacteria struggle to grow, extending the shelf life of canned and jarred foods.
- Chelating agent: It binds to metal ions like iron and calcium, which prevents them from catalyzing reactions that cause food to discolor or go rancid. This same property makes it useful in cleaning products, where it dissolves mineral deposits like limescale.
- Pharmaceutical ingredient: It’s used to adjust the pH of medications, improve the flavor of chewable tablets, and act as a stabilizer in certain formulations.
It also appears in cosmetics, detergents, and water treatment systems. Its ability to grab onto metal ions while remaining safe and biodegradable makes it a versatile workhorse across industries.
Is It “Natural” or “Artificial”?
This is where things get a little blurry. Citric acid is a naturally occurring compound found in every living cell. Lemons, limes, oranges, and grapefruits are loaded with it. But the citric acid in your soft drink or snack almost certainly came from a fermentation tank, not a lemon tree. The molecule is chemically identical either way. Your body processes it the same regardless of its origin, and it’s the same compound that plays a central role in your own cellular metabolism. The production method is biological (mold eating sugar), but it happens in an industrial facility rather than on a tree.