Citric acid: the chemical secrets hidden in beverages
I. What exactly is citric acid?
Citric acid, scientifically known as 2-hydroxypropane-1,2,3-tricarboxylic acid, also called citric acid, has the molecular formula C₆H₈O₇ . It ‘s naturally occurring —found in lemons, oranges, and pineapples—and is produced and consumed daily in the human body’s tricarboxylic acid cycle. Simply put, our bodies “produce” citric acid, but in very small quantities, insufficient for industrial use.
Pure citric acid is a colorless, translucent crystal, and it can also be found as a white powder. It has little smell, but a very strong, clean sour taste, unlike acetic acid which has a strange odor, or lactic acid which has a bitter taste. It has extremely high solubility; 100 ml of water at 20 degrees Celsius can dissolve 147 grams, almost in a 1:1 ratio.
There’s an interesting characteristic: calcium salts of citric acid are more soluble in cold water than in hot water. Normally, substances dissolve more at higher temperatures, but this is the opposite. Industrially, this property is used to purify citric acid—calcium is added to precipitate it, then it ‘s dissolved back in acid; this process is repeated several times to increase the purity.
Chemically, it’s a tribasic acid with three carboxyl groups, making it a stronger acid than acetic acid and lactic acid; a 1% aqueous solution has a pH of approximately 2.3. However, its most remarkable ability isn’t its acidity, but its chelating power—it can firmly bind metal ions like calcium, magnesium, iron, and copper , forming stable complexes. This ability makes it popular in the detergent, electroplating, and cosmetic industries, which will be discussed in more detail later.
One more thing to note: Citric acid decomposes when heated above 175 degrees Celsius, producing substances such as aconitic acid and itaconic acid . Therefore, temperature control is necessary during high-temperature processing, otherwise the sour taste will change.
II. Production Process
The production of citric acid is essentially a fermentation process, similar to brewing wine and vinegar, except that the strains of bacteria and raw materials used are different.
Currently, 99% of the world’s citric acid is produced using the deep fermentation method with Aspergillus niger. Aspergillus niger is a fungus that looks somewhat like the black mold on moldy bread. Under normal circumstances, the sugar it ingests is metabolized step by step through the tricarboxylic acid cycle, and citric acid is just an intermediate product that cannot be retained. But here’s where humans excel—by controlling the fermentation conditions: high sugar, low nitrogen, high oxygen, and an acidic environment, they force Aspergillus niger to only produce and not consume, secreting large amounts of citric acid into the fermentation broth.
The entire process involves approximately four steps:
The first step is raw material pretreatment. Starchy materials such as corn, cassava, and sweet potato are first crushed, then liquefied and saccharified with amylase to make glucose solution.
The second step is deep fermentation. Glucose solution is poured into a large fermentation tank, and Aspergillus niger spores are added . The tank is kept at 30 to 35 degrees Celsius with ventilation and stirring for six to eight days. It’s almost ready when the citric acid concentration in the fermentation broth reaches 10% to 15%.
The third step is extraction and purification. This step is crucial. The fermentation broth is first filtered to remove the mycelium, then calcium carbonate is added for neutralization . Citric acid reacts with calcium to form calcium citrate precipitate. The precipitate is separated, then acidified with sulfuric acid, returning it to a citric acid solution. At this point, it is a bit coarse. After ion exchange, decolorization, concentration, and crystallization, pure citric acid crystals are obtained.
The fourth step is drying and packaging. The crystals are centrifuged, dried, and packaged according to specifications, in 25-kilogram bags, before leaving the factory.
III. Uses of Citric Acid
Many people’s impression of citric acid is limited to “the acid added to beverages,” but its uses are much wider than you might imagine, going far beyond just beverages.
Let’s start with the most familiar category: food and beverages. This is the largest segment, accounting for nearly 60% of total consumption. Citric acid is undoubtedly the “number one acidulant” in food—it has a pure sour taste, a short aftertaste, and is most similar to the natural sourness of fruit. No other organic acid can replace it.
Citric acid commonly found on the market mainly comes in two types:
Citric acid monohydrate. It contains one molecule of water of crystallization and is the most commonly used, accounting for over 70%. It has good stability and does not easily clump. It is the main ingredient in carbonated drinks, juices, jellies, candies, and dairy products. If you pick up any sour-tasting beverage and look at the ingredient list, you will almost certainly find it.
Citric acid Anhydrousd. It contains no water of crystallization, resulting in higher purity, but it is more hygroscopic and 5% to 10% more expensive than its monohydrate counterpart . It is mainly used in moisture-sensitive applications, such as powdered beverages, effervescent tablets, and pills. With the rise in popularity of solid beverages in recent years, the demand for anhydrous citric acid has increased faster than that for its monohydrate counterpart .
There are also various citrates—sodium citrate, potassium citrate, and calcium citrate—which are derivative products of citric acid. Each has its own uses, and the total amount is not small.
Feed additives.
This sector has seen a particularly rapid increase in price in recent years, which many outside the industry are unaware of. Citric acid acts as an acidifier in feed. When animals are young, their stomach acid secretion is insufficient, their digestive ability is poor, and they are prone to diarrhea. Adding a little citric acid to the feed lowers the pH value of the gastrointestinal tract, which can both activate digestive enzymes and inhibit harmful bacteria such as E. coli, achieving two benefits at once.
Piglets are the biggest consumers. Piglets are most prone to problems during weaning, with high rates of diarrhea and slow growth. Adding 1% to 2% citric acid to their feed can lower the gastric juice pH from 4.5 to around 3.5, reduce E. coli counts by 60-70%, increase pepsin activity by 30%, and improve daily weight gain by 10% to 30%. Pig farmers call this “the piglet’s first taste of acid.”
Poultry also use it extensively. Adding 5 to 10 grams per kilogram to broiler feed can increase feed intake and feed conversion ratio, improve meat quality, and extend shelf life. Laying hens also benefit from it , resulting in stronger eggshells and a lower rate of broken eggs.
Aquatic products use less, generally adding 0.1% to 0.3%, mainly to adjust intestinal pH and promote mineral absorption. The effect is more obvious on fish without stomachs, such as carp and grass carp.
There’s also something called rare earth citric acid, which is a complex of citric acid and rare earth elements. It’s a new type of feed additive that can be used for pigs, chickens, fish, shrimp, cattle, and sheep. It’s essentially an upgraded version of citric acid in the feed industry.
Industrial sector
Detergents and daily chemicals are the second largest application area, accounting for 18% to 20%. It’s found in laundry powder, liquid laundry detergent, and dishwashing liquid, utilizing its chelating ability —binding calcium and magnesium ions in water, softening the water, and naturally improving cleaning effectiveness. Now that phosphorus is banned, citric acid is a good alternative to phosphates; it’s environmentally friendly and biodegradable.
Citric acid is increasingly used in cosmetics and personal care products. It belongs to the category of fruit acids (AHAs), gently exfoliating and is added to many skincare products, facial cleansers, and shampoos. It also acts as a pH adjuster, bringing the product to a slightly acidic level similar to that of the skin, making it non-irritating. Additionally, it has some antioxidant and preservative properties.
Industrial cleaning and water treatment are another major user. Citric acid is used to clean scale buildup in boilers, heat exchangers, and pipes; it’s much gentler than hydrochloric acid, doesn’t corrode stainless steel, and can even dissolve rust. It’s also used as a scale inhibitor in industrial circulating cooling water. Reverse osmosis membranes are also cleaned with citric acid solution when they get dirty.
The construction industry uses it as a concrete retarder. In summer, when temperatures are high, cement sets too quickly, making construction too slow. Adding a little citric acid can slow down the setting time, making it indispensable on construction sites.
Citric acid is used as an auxiliary agent in textile printing and dyeing , and for adjusting pH and surface treatment in electroplating. In short, it can be found wherever acidity needs to be adjusted or metal ions need to be chelated. The fastest growing segment recently is electronic-grade citric acid, used for wafer cleaning and PCB board residue removal , with a growth rate of 6% to 8%, much faster than the industry average. It requires high purity and is expensive, making it a direction for enterprise upgrades