Citric Acid

Citric acid, chemically named 2‑hydroxypropane‑1,2,3‑tricarboxylic acid, is a natural polycarboxylic acid widely existing in fruits and vegetables. It is also the most productive and widely used green organic acid manufactured by bio‑fermentation worldwide. Industrial commercial products are mainly categorized into monohydrate citric acid and anhydrous citric acid, which are selected differentially according to actual working temperature and purity requirements. The pure product appears as colorless transparent crystals or white crystalline powder, odorless, with pure and refreshing sour taste and no bitter aftertaste. It serves as the most extensively applied acidity regulator and metal chelating agent in the food, daily chemical, pharmaceutical and fine chemical industries.

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Citric acid is readily soluble in water and ethanol, soluble in glycerin, slightly soluble in acetone, and almost insoluble in non‑polar organic solvents such as ether and benzene. Its molecular structure contains three carboxyl groups and one hydroxyl group, endowing it with excellent properties including pH buffering, metal chelation, antioxidant synergism, bacteriostatic preservation and scale complex removal. It maintains stable physicochemical properties at room temperature and possesses outstanding thermal stability. Compared with organic acids such as malic acid and lactic acid, citric acid features purer sour taste, faster taste response and lower cost. It is widely used in food flavoring and preservation, industrial descaling and cleaning, daily chemical formulation, pharmaceutical preparation synthesis, environmental water treatment and other scenarios, ranking as the most versatile basic organic acid raw material. 

1. Fruit Extraction Method

Citric acid was originally extracted directly from citrus fruits such as lemons. The core technological process includes three major steps: pretreatment, extraction, separation and purification. In the pretreatment stage, lemons are cleaned and crushed, with peels, seeds and other impurities removed before being ground into pulp. Mechanical pressing or solvent extraction is adopted to separate citric acid and soluble substances from fruit juice. Finally, citric acid crystals are obtained through physical and chemical purification including heating concentration, calcium carbonate precipitation, filtration and washing.

Restricted by low natural citric acid content in fresh fruits, massive raw material loss and high production costs, this traditional preparation process has been completely eliminated by industrial fermentation technology and is no longer applied in large‑scale commercial production.

2. Chemical Synthesis Method

With acetone and carbon dioxide as core raw materials, 3‑ketoglutarate is synthesized via reaction with phenoxide as the alkaline agent in polar aprotic solvents (dimethylformamide, N‑methylpyrrolidone, etc.) at 0～60℃. The product reacts with excess hydrocyanic acid at a low temperature of 0～10℃ to form cyanohydrin intermediates, which are further hydrolyzed by excess sulfuric acid to finally produce citric acid.

This synthetic route easily generates by‑products such as isocitric acid and malic acid. Impurity removal relies on selective precipitation of calcium and barium salts, resulting in cumbersome procedures and poor purity controllability. Thus, it is rarely adopted in industrial production.

3. Biological Fermentation Method

Biological fermentation is the absolute mainstream process for industrial citric acid production, contributing to over 99% of global output. This technology uses Aspergillus niger as the exclusive fermentation strain, and low‑cost starchy and saccharide materials (corn, cassava, molasses, etc.) as carbon sources. High‑efficiency conversion of sugars to citric acid is realized via submerged liquid aerobic fermentation, featuring low cost, large output, high purity and environmental friendliness.

Raw Material Pretreatment

Starchy raw materials are purified and crushed, then prepared into 18%～22% slurry. Alpha‑amylase is added for starch gelatinization and preliminary hydrolysis at 90～100℃ to generate dextrin. Glucoamylase is subsequently added to completely saccharify and hydrolyze dextrin into glucose under conditions of 55～60℃ and pH 4.0～4.5. The saccharified liquid is filtered, decolorized by activated carbon and purified by ion exchange to remove proteins, pigments, ash and other impurities, obtaining high‑purity sugar liquid for fermentation.

Strain Expansion Culture

Aspergillus niger is cultured through a three‑stage progressive expansion system: Primary slant culture on wort agar medium at 31℃ for 4 days to obtain pure spores; Secondary triangular flask bran culture with a 1:1 ratio of bran to water, supplemented with calcium carbonate and ammonium sulfate, incubated for 96～120 hours after sterilization; Tertiary submerged expansion culture in seed tanks to cultivate sufficient high‑activity strains for formal fermentation.

Fermentation & Refining Process

The refined sugar liquid is formulated into fermentation medium, sterilized at 110～115℃ for 20 minutes and then transferred to fermentation tanks for aerobic submerged fermentation after strain inoculation. Aspergillus niger decomposes glucose into pyruvic acid via the EMP metabolic pathway, and accumulates a large amount of citric acid through carboxylation and condensation reactions. After fermentation, the broth is heated to 60～85℃ to inactivate strains and flocculate proteins, and clear filtrate is obtained after removing mycelium and suspended impurities via filtration.

Calcium carbonate or calcium hydroxide is added to the filtrate for neutralization at 80℃ until pH reaches 5.0～5.5, forming calcium citrate precipitate. The precipitate is washed with hot water for impurity removal, then acidolyzed with sulfuric acid at 60～90℃ to pH 1.5～1.8, regenerating citric acid and producing calcium sulfate by‑precipitate. After secondary filtration for residue removal, cation and anion exchange towers are used to deeply remove metal ions, sulfate radicals, chloride ions and other impurities, yielding high‑purity refined citric acid liquid.

Concentration & Crystallization

The refined liquid is concentrated under vacuum at 55～60℃ to a relative density of 1.335～1.340. Product types are classified by crystallization temperature: monohydrate citric acid crystallizes below 36.6℃, while anhydrous citric acid is obtained at temperatures above 40℃. The crystallized materials undergo centrifugal solid‑liquid separation, fluidized bed low‑temperature drying and screening classification to produce finished citric acid products.

1. Food‑grade Citric Acid

Complying with GB 1886.235-2016 National Food Safety Standard – Food Additive Citric Acid, it is a compliant organic acid additive directly applicable to food processing. Naturally present in citrus fruits, it is industrially produced by Aspergillus niger fermentation with high purity and ultra‑low content of heavy metals and harmful impurities. Classified as a GRAS (Generally Recognized As Safe) substance by the US FDA, it is widely used for food flavoring, antisepsis and fresh‑keeping in various food products.

2. Pharmaceutical‑grade Citric Acid

It meets international pharmacopoeia standards including Chinese Pharmacopoeia, USP and EP, with a purity of ≥99.8% and extremely stringent impurity control. Its core applications include regulating urine pH to dissolve calculi and reduce renal calculus recurrence risk; complexing calcium ions to inhibit blood coagulation for blood preservation; serving as a drug stabilizer and antioxidant to enhance formulation stability; and being applied in the preparation of oral rehydration salts and effervescent tablets to adjust electrolyte balance and disintegration rate.

3. Industrial‑grade Citric Acid

Executing GB/T 8269.1-2025 Quality Requirements for Organic Acids – Part 1: Citric Acid (implemented on May 1, 2026), it has a purity of ≥98% and is strictly prohibited for food, pharmaceutical and feed industries. It is mainly applied in industrial scenarios such as industrial descaling and cleaning, electroplating metal complexation and pH stabilization, textile printing and dyeing mordanting and neutralization, chemical chelating buffering, flue gas desulfurization and environmental water treatment.

4. Electronic‑grade Citric Acid

Adopting customized industrial ultra‑high purity standards, it achieves a purity of ≥99.95% with metal impurities controlled at ppb/ppt ultra‑trace levels. Manufactured via multiple ultra‑purification processes, it is a crucial fine chemical auxiliary agent for high‑end electronic manufacturing, mainly used for semiconductor wafer cleaning, PCB circuit board oxide layer removal and residual dirt cleaning of precision electronic components.

Upstream Raw Materials

Ammonia-soda process: crude salt, limestone, coal, liquid ammonia

Combined-soda process: crude salt, liquid ammonia, carbon dioxide

Trona ore process: trona ore, fuel

Soda-ash cost is highly correlated with crude-salt and coal prices; the trona-ore route is mainly affected by ore prices.

Downstream Applications

Glass Industry (largest consumption sector, >60%): float flat glass, photovoltaic rolled glass, household container glass, glass fibre. Used as flux to lower the melting temperature of quartz sand.

Detergent & Daily-use Chemicals: laundry powder, soap base, industrial cleaners, water softeners; light soda ash is predominantly used.

Chemical Feedstock: manufacturing sodium bicarbonate, sodium silicate (water glass), sodium tripolyphosphate, various sodium salts, printing-dyeing auxiliaries.

Water Treatment: pH neutralisation, water softening and heavy-metal precipitation for circulating water and wastewater systems.

Metallurgy & Mineral Processing: flux for ore flotation and desulphurisation.

Papermaking: alkali adjustment for pulping and pH control in bleaching procedures.

Food Industry: dough conditioning for flour products and acidity regulation in food processing (food-grade only).

As a core bulk green raw material in the bio‑fermentation industry, citric acid boasts a complete, mature and closed‑loop industrial chain in China with prominent scale advantages. Upstream core raw materials include corn starch, molasses, glucose, Aspergillus niger strains, industrial purified water, and auxiliary reagents such as milk of lime, sulfuric acid, activated carbon and ion exchange resins. Downstream application fields cover the whole industrial chain, including food processing, daily chemicals and cosmetics, biomedicine, industrial cleaning, textile printing and dyeing, environmental water treatment and fine chemicals.

Safety, Storage and Transportation

Citric acid is a mild natural polycarboxylic acid with non‑toxic, fully biodegradable, non‑corrosive, non‑flammable and non‑explosive properties. It is not categorized as a hazardous chemical and shall be stored and transported in accordance with ordinary general chemical goods standards. Only high‑concentration powder dust and concentrated solutions cause slight irritation to eyes and respiratory mucous membranes, with extremely low operational risks under conventional working conditions.

Storage Specifications

Store in dedicated dry, cool, ventilated and clean chemical warehouses. Keep product packages fully sealed to isolate moisture and prevent hygroscopic caking and deterioration. Stack products in independent zones and avoid co‑storage with strong alkalis and strong oxidants to prevent acid‑base neutralization and oxidative deterioration. Store food‑grade and pharmaceutical‑grade products separately to eliminate cross‑contamination risks.

Transportation Specifications

Comply with transportation standards for ordinary general chemical goods. Take full protective measures against rain, moisture, sunlight exposure, package damage and leakage to ensure sealed and stable packaging. Mixed transportation with toxic, odorous and strongly alkaline goods is prohibited. Maintain a clean transportation environment for food‑grade products to avoid contamination.

Safety Characteristics

It maintains stable physicochemical properties under sealed, dry and room‑temperature conditions, featuring non‑toxicity, environmental friendliness and natural degradability. Low‑concentration aqueous solutions are mild and non‑irritating; only powder dust causes slight mucosal irritation with no corrosion, combustion or explosion risks. Daily operation only requires dust masks, labor gloves and well‑ventilated working environment. In case of eye contact, rinse immediately with plenty of running water. No potential safety hazards exist with standardized operation.

Q: What are the core differences and selection criteria between monohydrate and anhydrous citric acid?

A: The two types share basically identical chemical composition and sour taste properties, with core differences in crystal water content, stability and applicable working conditions. Monohydrate citric acid contains crystal water, featuring fast dissolution speed and high cost performance, suitable for general scenarios such as normal‑temperature food processing, ordinary daily chemical production and conventional industrial cleaning. Anhydrous citric acid is crystal‑water free, with higher purity, high temperature resistance and superior moisture stability, applicable to harsh working conditions including high‑temperature production, high‑purity pharmaceutical preparations and precision chemical manufacturing. Monohydrate citric acid is preferred for conventional room‑temperature scenarios, while anhydrous citric acid is mandatory for high‑temperature and high‑purity harsh scenarios, and the two cannot be used interchangeably.

Q: Does moisture‑induced caking affect the use of citric acid? How to prevent it?

A: Citric acid has slight hygroscopicity and tends to absorb moisture and cake in open, high‑humidity environments. This is a physical change without chemical deterioration, and slightly caked materials can be used normally after passing re‑inspection. Effective prevention measures include keeping packages fully sealed to isolate moisture; maintaining a dry, low‑humidity, constant‑temperature and ventilated warehouse; reducing frequent warehouse opening to avoid humid air convection; and prohibiting open‑air stacking and rain exposure. Standardized long‑term storage ensures stable material properties.

Q: What are the key differences and usage taboos of industrial‑grade, food‑grade and pharmaceutical‑grade citric acid in China?

A: The three grades have identical core chemical components, differing only in impurity control precision, product cleanliness and compliance qualifications. Industrial‑grade citric acid adopts loose impurity control standards, only suitable for industrial production and strictly prohibited from contacting food, feed and human skin. Food‑grade citric acid undergoes deep impurity removal and strict heavy metal control with formal food additive compliance qualifications for various food processing applications. Pharmaceutical‑grade citric acid has the highest purity, the most stringent impurity control and optimal biosafety, applicable to high‑end pharmaceutical preparations and precision medical aesthetic scenarios.

China is the core production and export powerhouse of citric acid worldwide. Supported by mature Aspergillus niger fermentation technology, abundant biomass raw materials and a complete upstream and downstream industrial chain, China leads the globe in production capacity, product quality and cost advantages. It can fully meet customized demands for multi‑grade and multi‑specification products with international high‑end quality standards, dominating global market supply for a long time.

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