Citric acid

Some fruits are very sour.

Lemons and limes have this sourness. It comes from a special acid. This acid is found in many fruits.

People use this acid in food. It makes candy and drinks taste good. It also helps keep food fresh. Some people use it to clean. It can even wash away hard stains. This acid is a very useful thing to have!

Citric acid is a clear, weak acid. It is found in many fruits.

Lemons and limes have a lot of it. It can make up 8% of their dry weight.

In its pure form, it looks like white crystals.

This acid is very important for living things. It helps in the citric acid cycle. This is a set of steps that makes power for cells. This cycle helps animals, plants, and bacteria.

People make a lot of citric acid every year. They make more than two million tons! Most of this comes from a special mold. This mold is called Aspergillus niger.

The mold eats sugar to make the acid.

We use citric acid in many ways. It makes candy and soda taste sour. It also helps keep food fresh. It can even clean hard water stains from glass. It is a chelating agent. This means it can grab onto metals to make them dissolve in water. This helps soaps work better in hard water.

Citric acid is a clear, weak organic acid. It is a very important substance found in many living things.

You can find it naturally in many fruits and vegetables. Citrus fruits like lemons and limes have very high amounts. In these fruits, it can make up 8% of their dry weight. This means there is about 47 grams in every liter of juice. It is also a key part of how cells work.

It acts as an intermediate in the citric acid cycle. This cycle helps animals, plants, and bacteria create energy.

In the body, citric acid helps make power for cells. This process is called the citric acid cycle or the Krebs cycle. In this cycle, a molecule called citrate synthase helps build citrate. Then, an enzyme called aconitase turns it into aconitic acid. This series of steps provides two-thirds of the energy from food. The energy is stored in a form called ATP.

This cycle is a central way that life stays active. It is a vital part of how all aerobic organisms function.

People have studied this acid for a long time. A chemist named Carl Wilhelm Scheele first isolated it in 1784. He did this by crystallizing it from lemon juice. In 1890, factories began making it on a large scale. They used juice from the Italian citrus industry at that time. Later, in 1893, C. Wehmer found that a mold could make it. In 1917, James Currie found a specific mold called Aspergillus niger.

This discovery helped companies like Pfizer start making it quickly.

Today, we make more than two million tons every year. In 2018, global production was over 2,000,000 tons. More than half of all this acid is made in China. Most of it is used to control acidity in drinks. About 20% goes into other foods and 20% goes into detergents. Some is even used in cosmetics and medicines. Most of it is made by feeding sugar to mold. The mold eats things like corn starch or molasses to grow.

We use citric acid in many parts of our lives. It is a common flavoring in soda and candy. It can also act as a preservative to keep food fresh.

It is a great chelating agent, which is a fancy way to say it grabs metals. This helps it dissolve limescale or rust from metal. It also helps soaps work better in hard water. You might see it in bath bombs or even skin peels. It is a helpful tool in both the kitchen and the lab.

Citric acid is a colorless, weak organic acid with the chemical formula C6H8O7. It is a vital substance found in nature and plays a massive role in both biology and industry.

In living things, it acts as a central intermediate in a metabolic process called the citric acid cycle. This cycle is essential for the survival of all aerobic organisms, which are creatures that use oxygen. Because it is so useful, humans produce more than two million tons of it every year. It serves many purposes, ranging from flavoring our food to helping industrial machines run smoothly.

In the natural world, citric acid is most famous for appearing in citrus fruits. Lemons and limes contain particularly high concentrations of this acid. In these fruits, citric acid can make up as much as 8% of their dry weight. This equates to roughly 47 grams per liter in their juices. The concentration varies depending on the specific type of fruit and how it was grown. For example, oranges and grapefruits have much lower concentrations, around 0.005 mol/L, compared to the 0.30 mol/L found in lemons.

Biochemically, citric acid is a key player in the citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle. This cycle is a central metabolic pathway for animals, plants, and bacteria. The process begins when an enzyme called citrate synthase catalyzes the condensation of oxaloacetate with acetyl CoA to create citrate. Next, the citrate acts as a substrate for the enzyme aconitase, which converts it into aconitic acid. This series of chemical reactions is responsible for producing two-thirds of the food-derived energy in higher organisms. This energy is eventually stored in a form called Adenosine triphosphate, or ATP.

Our understanding of citric acid has grown through many scientific discoveries. In 1784, the chemist Carl Wilhelm Scheele first isolated the acid by crystallizing it from lemon juice. Industrial production began around 1890 by treating Italian citrus juice with hydrated lime to create calcium citrate. Later, in 1893, C. Wehmer discovered that certain molds could produce the acid from sugar. A major breakthrough occurred in 1917 when American chemist James Currie found that the mold Aspergillus niger was an efficient producer. This led companies like Pfizer to begin large-scale microbial production. Today, most citric acid is made through fermentation by feeding this mold a medium of sucrose or glucose.

Modern industrial production is massive in scale. In 2018, global production exceeded 2,000,000 tons, with more than 50% of that volume coming from China. The ways we use this volume are quite diverse. Over 50% of produced citric acid is used as an acidity regulator in beverages. Another 20% is used in various food applications, and 20% is used for detergents. The remaining 10% goes toward cosmetics, pharmaceuticals, and other chemical industries.

Citric acid is also highly valued as a chelating agent. A chelating agent is a substance that can bind to metallic cations, making them soluble. This property allows citric acid to remove limescale from boilers and evaporators. It is also used to treat water, which improves the effectiveness of soaps and laundry detergents by binding metals in hard water. In the 1940s, it was even used during the Manhattan Project for the separation of lanthanides. In the kitchen, it can be used as an emulsifying agent in ice cream or to prevent sugar crystallization in caramel.

Beyond food and cleaning, citric acid has specialized roles in science and medicine. It is an alpha hydroxy acid, which makes it an active ingredient in chemical skin peels. In biological systems, it can move between different parts of a cell to regulate the production of fatty acids. It also helps regulate the size of apatite crystals in bone. Because it can act as a buffer, it is used to control pH levels in many different chemical environments. This versatility makes it one of the most important organic compounds in the modern world.