**3.3 Butyric acid**

Butyric acid (C4H8O2, MW 88.106 g mol<sup>−</sup><sup>1</sup> , IUPAC Name: Butanoic acid) is a mono-carboxylic acid, and it is an oily, colorless liquid that is soluble in water, ethanol, and ether. Salts of butyric acid are known as butyrates. Butyric acid is a chemical, commonly used as a precursor to produces other substances, like biofuel [92, 93], cellulose acetate [94, 95], and methyl butyrates [96], the two last coatings, and flavors compounds, respectively. Chemical synthesis is still the primary way of production of butyric acid due to the availability of raw material [92]. But some research explores the microbial biotransformation from renewable sources like agro-industrial wastes [72]. *Clostridium tyrobutyricum* can produce butyric acid from lactose, present in milk and cheese, along with H2, CO2, and acetic acid [97]. By 2016, the butyric acid worldwide market was around 80 ktpa, with a price of US\$ 1800 per ton [98]. By 2020, global production of butyric acid is expected to reach 105 ktpa [99].
