Citric Acid

Product Introduction

Citric acid (CA), also known as citrate, has the molecular formula C₆H₈O₇. It is an important organic acid, appearing as colorless crystals that are odorless with a strong acidic taste and easily soluble in water. It serves as an acidity regulator (GB2760—2014) and is widely used as a food additive.

Natural citric acid is abundantly distributed in nature, found in plants such as lemons, oranges, and pineapples, as well as in animal bones, muscles, and blood. Artificially synthesized citric acid is produced through the fermentation of sugary substances like sucrose, molasses, starch, and grapes.

As an edible acid, citric acid enhances normal metabolic processes in the body and is harmless when consumed in appropriate doses. Adding citric acid to certain foods improves taste and stimulates appetite. In China, its use is permitted in jams, beverages, canned foods, and candies.

Production Process

a. Strain Cultivation

Citric acid is industrially produced through microbial fermentation, primarily using certain molds and yeasts. Among these, Aspergillus niger is the most competitive strain used in industry, while competitive yeast strains include Yarrowia lipolytica and *Candida lipolytica*.

• Aspergillus niger is cultured on agar, forming limited colonies that develop into dense spore structures over 10-14 days at room temperature. The colonies appear black or dark brown. To obtain strains suitable for citric acid production, methods such as acidified filter paper technique, color change zones, and single spore transplantation are employed to isolate high acid-producing and citric acid-tolerant strains, minimizing contamination from other microbes.

• Yeast cultivation for citric acid production utilizes Candida lipolytica and *Yarrowia lipolytica*. Candida lipolytica has strong lipid-decomposing abilities, with normal alkanes serving as effective carbon sources. Yarrowia lipolytica can ferment both alkanes and sugars to produce citric acid, with an optimal fermentation pH of 3.5–4.0.

b. Fermentation

Since H.A. Krebs proposed the tricarboxylic acid cycle in 1940, the fermentation mechanism of citric acid has been progressively understood. It has been demonstrated that during the biochemical conversion of sugar-based raw materials to citric acid, the pathway from sugars to pyruvic acid mirrors that of alcohol fermentation, proceeding through the Embden-Meyerhof pathway (glycolysis). Pyruvic acid is further oxidatively decarboxylated to form acetyl-CoA, which then condenses with oxaloacetate to produce citric acid, entering the tricarboxylic acid cycle.

Citric acid is an intermediate product in metabolic processes. During fermentation, significant accumulation of citric acid occurs when the activities of aconitase and isocitrate dehydrogenase are low, while citrate synthase activity is high.

The fermentation process can be divided into surface fermentation and solid-state fermentation:

• Surface Fermentation and Solid-State Fermentation: Different culture media are prepared based on various raw materials and sterilized through steaming, which gelatinizes starch and ensures sterilization. Uniform heating and proper steam flow are essential during steaming, with materials added gradually to ensure thorough processing. After cooling the sterilized materials to below 37°C, water is added, followed by inoculation and fermentation. The endpoint of fermentation is determined by measuring acidity levels regularly, ensuring that extraction occurs when acidity is at its peak to prevent citric acid degradation by bacteria.

c. Extraction

Upon completion of fermentation:

• Surface Fermentation: The fungal mat is promptly separated from the fermentation broth. The mat and fermentation trays are washed with a small amount of water, and the washings are combined with the fermentation liquid.

• Solid-State Fermentation: Citric acid is extracted by leaching with water at 80°C, typically performed 2–3 times, with the washings combined.

The fermentation broth is filtered using a filter press, and the filtrate is collected. Citric acid reacts with calcium salts and bases to form calcium citrate, which precipitates out, separating it from soluble impurities. If the acid solution contains high levels of oxalic acid, it can be precipitated at pH levels below 3 in a hot neutralized solution, allowing for prior removal of oxalates.

The neutralization endpoint is monitored using precise pH paper, maintaining pH between 6.0 and 6.8. The solution is stirred at approximately 85°C for 30 minutes to fully precipitate calcium sulfate, which is then filtered out. Calcium citrate undergoes acid hydrolysis with sulfuric acid, with the quantity determined based on citric acid content, generally not exceeding 0.2% excess. Post-hydrolysis, the acid solution is filtered.

d. Purification and Concentration

The citric acid solution is purified through decolorization and ion exchange processes to remove pigments, colloids, and metal cations such as iron, calcium, copper, magnesium, as well as anionic impurities like sulfate ions. Purification is typically conducted in chromatographic columns using GH-15 granular activated carbon and appropriate ion exchange resins.

The purified citric acid solution has a concentration of 20%–25% and must be concentrated to over 70% for crystallization. Concentration is carried out at controlled temperatures to prevent decomposition, often under reduced pressure using double or triple-effect evaporators for energy efficiency.

Concentration occurs in two stages:

1. First Concentration: The solution is concentrated and then placed in a settling tank to remove most of the gypsum through sedimentation.
2. Second Concentration: The solution is further concentrated to approximately 80% citric acid content, then immediately transferred for crystallization.

e. Crystallization

Different crystallization methods yield different forms of citric acid:

• Monohydrate Citric Acid: The 80% solution is cooled from 55°C to 40°C in a crystallizer with stirring, and seed crystals are added. The temperature is maintained below 36°C to facilitate crystallization.

• Anhydrous Citric Acid: The solution is concentrated to 83% at 60°C, cooled to 46°C, and seed crystals are introduced. The temperature is gradually reduced to 38°C to obtain anhydrous citric acid.

The crystalline slurry is then centrifuged to obtain the final citric acid product.

Efficacy and Function

1. Acidity Regulator

Applications:

1. Food Industry
2. Citric acid is the most widely produced organic acid globally through biochemical methods and is a cornerstone product in the fermentation industry. It is primarily used in the food industry as an acidulant, solubilizer, buffer, antioxidant, deodorizer, flavor enhancer, gelling agent, and color stabilizer. Citric acid is particularly important in carbonated beverages, fruit juices, lactic acid drinks, and pickled products, with demand varying based on seasonal climate changes. It accounts for about two-thirds of the total consumption of acidulants. Adding citric acid to canned fruits helps maintain or improve flavor and increase acidity in low-acid fruits (lowering pH), reducing microbial heat resistance and preventing spoilage. In candy, citric acid harmonizes well with fruit flavors. In gel foods like sauces and jellies, citric acid effectively reduces the negative charge on pectin, promoting gelation. In vegetable canning, citric acid acts as a pH adjuster, enhancing flavor while preserving quality. Its chelating properties and pH-regulating ability improve the performance of antioxidants in frozen foods, inhibit enzyme activity, and extend shelf life.

Metal Cleaning

Citric acid, an organic acid produced through microbial fermentation, is widely used in detergent production due to its specific properties and chelating ability. It demonstrates excellent performance, particularly in safety, as it is derived from food-grade microbes using grain as a raw material. Citric acid is environmentally friendly, easily degraded by microorganisms and heat, and has a strong chelating ability, especially for manganese and iron ions. Its use in detergents highlights its significant corrosion-inhibiting properties. As a milder acid compared to inorganic acids, citric acid is less corrosive to equipment, making it a safer and more reliable cleaning agent, with easy disposal of waste liquids and no harm to humans.

(1) Mechanism of Citric Acid Cleaning:

Citric acid is a safe cleaning agent with minimal metal corrosion risk. It does not contain chloride ions (Cl-), preventing stress corrosion on equipment. Citric acid can complex with Fe3+, weakening its corrosive effects. It can dissolve iron oxide and copper oxide, forming citric iron and copper complexes. Ammoniated citric acid solutions can produce highly soluble ferrous ammonium citrate and ferric citrate complexes, providing excellent cleaning results. Citric acid is mainly used to remove rust, particularly in new boilers. When mixed with amino acids, hydroxylacetic acid, or formic acid, citric acid can clean calcium, magnesium scale, and rust in boilers. When combined with EDTA, it can clean superheaters. Citric acid and its derivatives have extensive applications in chemical cleaning due to their unique physicochemical properties.

(2) Pipeline Cleaning with Citric Acid:

This is the latest cleaning technology for high-impurity hard water. It uses food-grade citric acid to soften stubborn scale, followed by microcomputer-controlled water flow and pneumatic oscillation to dislodge scale buildup inside pipes, ensuring clean and smooth water flow.

(3) Cleaning Gas Water Heaters with Blended Surfactants:

A chemical cleaner composed of citric acid, AES, and benzotriazole is used to clean gas water heaters that have been in use for years. The cleaner is injected into an inverted water heater, soaked for an hour, then drained, and the heater is rinsed thoroughly with clean water. Afterward, the water heater's outlet water temperature increases by 5°C to 8°C at the same flow rate.

(4) Cleaning Water Dispensers:

Dilute edible citric acid (in powder form) with water, and pour it into the heating tank of a water dispenser. Soak for about 20 minutes, then rinse the tank repeatedly with clean water until thoroughly clean. This method is non-toxic and effective.

Fine Chemicals

Citric acid is a type of fruit acid that accelerates keratin renewal. It is commonly used in lotions, creams, shampoos, whitening products, anti-aging products, and acne treatments. In chemical technology, citric acid serves as a reagent for chemical analysis, chromatography, and biochemistry. It is also used as a complexing agent, masking agent, and buffer solution component. When used as a detergent additive, citric acid can improve product performance by quickly precipitating metal ions, preventing pollutants from reattaching to fabrics, maintaining the necessary alkalinity for cleaning, and enhancing the performance of surfactants. It is also an excellent chelating agent and can be used as a reagent to test the acid resistance of ceramic tiles. Citric acid-sodium citrate buffer solutions are used for flue gas desulfurization, which is crucial for controlling SO2 emissions due to the high sulfur content in coal. The buffer solution's low vapor pressure, non-toxicity, chemical stability, and high SO2 absorption efficiency make it a valuable desulfurization agent.

In traditional dyeing and finishing processes, finishing typically occurs after dyeing, but the crosslinking treatment in finishing can lead to color fastness and shade issues, especially when using reducing catalysts. One-step fabric treatment can reduce costs, shorten production time, and avoid color fastness and shade issues. This method is mainly used for cotton, cotton blends, silk, wool, and viscose fibers, all of which require finishing to improve wearability. Both one-step and two-step methods have achieved good finishing results, but the one-step method is more energy-efficient, simpler, and less polluting, offering a promising development prospect. Citric acid can be used as a formaldehyde-free dyeing and finishing agent. Triethanolamine prevents citric acid from decomposing or dehydrating during fabric baking, effectively suppressing fabric yellowing.

Citric acid esters, approved by the U.S. FDA, are non-toxic plasticizers used in food packaging PVC and cellulose plastic films. Acetylated and butylated citric acid esters are used as foaming agents for methyl methacrylate polymers, stabilizers for acrylamide, initiators for polyamide adhesives, and plasticizers for PVC. Notably, butyl citrate and acetyl tri-n-butyl citrate are globally recognized as non-toxic plasticizers, offering superior compatibility, resistance to extraction, and low volatility. Acetyl trihexyl citrate and butylated tri-n-butyl citrate are used in the production of high-hygiene components like blood transfusion tubes and catheters.

Sterilization and Coagulation Processes

Citric acid, when combined with an 80°C temperature, effectively kills bacterial spores, particularly in blood dialysis machine tubing. In the coagulation process, calcium ions are essential for prothrombin activation and subsequent clot formation. Citrate ions can form a stable, non-dissociable complex with calcium ions, thereby reducing calcium ion concentration in the blood and inhibiting clot formation.

Animal Husbandry

In the tricarboxylic acid cycle, citric acid is formed from acetyl-CoA and oxaloacetate, playing a role in carbohydrate, fat, and protein metabolism within the body. Naturally occurring citric acid is found in the fruits of plants (such as lemons, oranges, and pineapples) and in the bones, muscles, and blood of animals. Synthetically, it is produced through the fermentation of sugar-containing substances like sugar, molasses, starch, and grapes. Adding citric acid to compound feed can disinfect, prevent mildew, and inhibit Salmonella and other infections in animal feed. Animals consuming citric acid can reduce the proliferation of pathogens and inhibit the production of toxic metabolites, thereby enhancing stress resistance.

(1) Increasing Feed Intake and Promoting Nutrient Digestion and Absorption

Adding citric acid to the diet can improve palatability by directly stimulating taste buds in the mouth, increasing saliva secretion, and enhancing appetite, leading to increased feed intake. Citric acid lowers the diet's pH, and after consumption, the stomach's acidity decreases, converting inactive pepsinogen into active pepsin, or directly stimulating digestive enzyme secretion. Additionally, acidic chyme entering the small intestine stimulates the secretion of enterogastrone, which reflexively inhibits gastric motility, prolongs gastric emptying time, and increases the time chyme spends in the intestines, promoting nutrient digestion.

(2) Promoting Intestinal Microflora Health

Organic acids can penetrate bacterial cell walls, creating a pH gradient between the inside and outside of the cells, inhibiting bacterial growth. Common pathogens prefer neutral to slightly alkaline pH levels, such as E. coli (pH 6.0–8.0) and Streptococcus (pH 6.0–7.5), while beneficial bacteria like lactic acid bacteria thrive in acidic environments. Citric acid lowers the gastrointestinal pH, providing favorable conditions for lactic acid bacteria and other probiotics, maintaining a healthy microbial balance in the digestive tract of livestock and poultry.

(3) Enhancing Stress Resistance and Immune Function

Immune-active cells, namely T lymphocytes and B lymphocytes, play a role in immune surveillance within the body. Research has shown that feeding broiler chickens citric acid can increase the density of immune-active cells, putting the chickens in a better immune state. This can inhibit the proliferation of intestinal pathogens and prevent the occurrence of infectious diseases.

(4) As a Mold Inhibitor and Antioxidant

Citric acid is a natural preservative. By lowering the pH of feed, it inhibits the growth of harmful microorganisms and the production of toxins, providing a significant anti-mold effect. When used as an enhancer for antioxidants, mixing citric acid with antioxidants can improve the antioxidant effect, preventing or delaying feed oxidation, increasing the stability of compound feed, and extending its shelf life.

Agriculture

In November 2022, Chinese scientists discovered during watermelon breeding that citric acid and other organic acids (which contribute to flavor) were selected during the improvement process.

Packaging, Storage, and Transportation:

Storage Conditions: This product should be sealed, protected from light, and stored in a dry, cool, and well-ventilated place, away from high temperatures.

Packaging Method: Bulk packaging is 25 kg per fiber drum; samples are 1 kg per aluminum foil bag. Custom packaging is available upon request.

Transportation Method: Express delivery or logistics. Domestic express delivery arrives within three days; logistics arrives within five days. Prices generally include domestic shipping costs.

Shelf Life: Two years.