Butyric acid, chemically known as butanoic acid, is a four-carbon saturated fatty acid with the chemical formula C₄H₈O₂ and structural formula CH₃CH₂CH₂COOH. Its molar mass is 88.11 g/mol, and its CAS number is 107-92-6. As an important short-chain fatty acid, butyric acid is widely found in nature and exhibits irreplaceable application value in multiple fields such as food, feed, medicine, and chemical industry.

I. Physicochemical Properties Butyric acid possesses unique physicochemical properties, which determine its application range and storage methods. At room temperature, butyric acid is a colorless to pale yellow transparent oily liquid with a strong rancid, buttery, or sweaty odor; even very dilute solutions will have a sweaty smell. Its melting point is -5.1℃, and its boiling point is 163.7℃. It can form a binary azeotrope with water, with a water content of 81.4% and a boiling point of 99.4℃. Its relative density is 0.9577 (20/4℃), slightly lighter than water. In terms of solubility, it is soluble in water at 20℃ (approximately 9g/100mL), readily soluble in organic solvents such as ethanol, ether, and acetone, and slightly soluble in benzene. Chemically, butyric acid is a weak acid and flammable. It reacts with alkalis to form salts and water, reacts with alcohols to form esters, and undergoes a decarboxylation reaction with specific oxides at 400-500℃ to produce dipropyl ketone.

II. Natural Occurrence and Industrial Production

2.1 Natural Occurrence
Butyric acid is naturally present in various substances, with a relatively high content in dairy products such as butter and cheese. It is also an important component contributing to the unique flavor of these foods. In the human body, butyric acid is a metabolic product of beneficial intestinal bacteria (such as Clostridium butyricum) and is widely present in the digestive system. Furthermore, butyric acid can also be found in the essential oils of plants such as lemongrass and nutmeg, as well as in the sweat and digestive system of mammals.

2.2 Industrial Production Methods
Industrially, butyric acid is mainly produced through two methods. The butyraldehyde oxidation method is currently the main industrial production method. This method uses manganese acetate or cobalt acetate as a catalyst to oxidize butyraldehyde with air or oxygen, and the reaction is followed by fractional distillation to obtain butyric acid. The fermentation method uses starch or sugar as raw materials and utilizes butyric acid bacteria (such as Clostridium butyricum) for fermentation, which is suitable for producing food-grade butyric acid. In the laboratory, butyric acid can be prepared by methods such as the oxidation of n-pentanol with concentrated nitric acid.

III. Main Application Areas

3.1 Food Industry
Butyric acid plays a central role in the food industry, with three main applications: as a flavor enhancer, it can be used to blend the flavors of dairy products such as cheese and butter, requiring only trace amounts to precisely mimic the rich and mellow flavor of natural dairy products; as a food preservative, its salts (such as sodium butyrate) can inhibit microbial growth, suitable for preserving bread, pastries, pickles, and other foods, especially effective in acidic environments; as a food additive, it can be used in baked goods, beverages, candies, and other products, enhancing flavor and extending shelf life.

3.2 Feed Industry
Butyric acid and its salts (mainly sodium butyrate) serve as novel green additives in the feed industry, possessing multiple functions: they can regulate intestinal pH, inhibit the growth of harmful bacteria such as E. coli and Salmonella, while promoting the reproduction of beneficial bacteria; they can enhance livestock and poultry immunity, improve feed conversion rate, promote livestock and poultry growth and development, and can serve as a high-quality alternative to antibiotics; they can also improve the intestinal health of livestock and poultry, reducing the occurrence of intestinal diseases such as diarrhea.

3.3 Pharmaceuticals and Health Products
Butyric acid's applications in the pharmaceutical field are based on its unique physiological functions. In terms of intestinal health, sodium butyrate can be used to treat intestinal diseases such as colitis, exerting its therapeutic effect by repairing the intestinal barrier and reducing inflammatory responses. It also has anti-inflammatory effects, showing potential therapeutic value for various inflammatory diseases such as Crohn's disease and ulcerative colitis. Studies have shown that butyric acid can inhibit tumor cell growth, especially showing potential preventative and therapeutic effects against colorectal cancer. Furthermore, it can cross the blood-brain barrier, protecting the nervous system and potentially being used to treat neurodegenerative diseases.

3.4 Chemical and Industrial Applications
As an important organic chemical raw material, butyric acid has a wide range of chemical applications: it can be used to synthesize butyrate esters (such as ethyl butyrate and butyl butyrate), which have fruity aromas and are widely used in fragrances, perfumes, cosmetics, and other fields; it can be used as a plasticizer in the production of polymers such as cellulose butyrate, improving the flexibility and processing performance of plastics; in the coatings and inks industries, it can be used as a solvent and diluent; simultaneously, it is also an important raw material for synthesizing pharmaceutical intermediates such as aminobutyric acid and other fine chemical products. IV. Physiological Functions and Health Impacts Butyric acid, as a metabolic product of the human gut microbiota, plays a vital role in human health. Firstly, it is a major energy source for colonic epithelial cells, providing approximately 70% of the energy needs of intestinal cells and maintaining normal intestinal physiological function. Secondly, it promotes the proliferation and differentiation of intestinal mucosal cells, enhances intestinal barrier function, and prevents leaky gut syndrome. Thirdly, it regulates the activity of intestinal immune cells, balances pro-inflammatory and anti-inflammatory cytokines, and enhances the body's resistance to pathogens. Furthermore, it participates in the regulation of blood glucose and lipid metabolism, potentially having a preventive and ameliorative effect on metabolic diseases such as diabetes and obesity.

V. Safety and Protective Measures Although butyric acid has low toxicity, it is somewhat irritating; appropriate precautions must be taken during its use and storage. Skin contact may cause pain, redness, blisters, and burns; therefore, protective gloves must be worn, and the skin should be rinsed immediately with plenty of water. Eye contact may cause pain, redness, severe deep burns, and even vision loss; protective goggles must be worn, and the skin should be rinsed immediately with plenty of water for at least 15 minutes and medical attention sought promptly. Inhalation of its vapors may cause coughing, sneezing, headache, dizziness, and nausea; a respirator must be worn and the work environment must be well-ventilated. Ingestion may cause burning sensation, abdominal pain, shock, or collapse; avoid ingestion. If accidentally ingested, rinse mouth immediately and seek medical attention. Butyric acid is flammable and corrosive; it must be stored in a tightly closed container, away from fire and oxidizers, and with appropriate fire-fighting equipment available.

VI. Conclusion Butyric acid, as a multifunctional short-chain fatty acid, has seen its applications expand continuously, from natural dairy products to modern industrial fine chemicals, from food flavoring to pharmaceuticals and health products. With in-depth research into its physiological functions, the value of butyric acid and its derivatives in intestinal health, immune regulation, and the prevention and treatment of metabolic diseases is becoming increasingly prominent, and it is expected to play a greater role in the future biomedical field. Meanwhile, as an important industrial raw material, the green production technology and high-value-added application of butyric acid will also become an important direction for the industry's development.