Industrial production of Acetic acid

Acetic acid is a crucial platform substance. Only 10% of the world's production, which is used to make vinegar, is made by bacterial fermentation. It is primarily created synthetically. Acetic acid can be produced by a variety of microorganisms, some of which also incorporate CO into the process. 

Definition:

   Although it may have been preceded by certain fermented foods derived from milk, acetic acid (also known as ethanoic acid or methyl carboxylic acid) has been generated since the invention of winemaking, which goes back to at least 10,000 BC. Given that the Latin term acetum denotes sour or sharp wine, it is generally accepted that the first vinegar, which is an aqueous solution of acetic acid, was produced as a byproduct of the damaged wine. It served as a drug at first and was perhaps the very first antibiotic ever discovered. For the majority of human history, all acetic acid was produced using the same traditional method, which involved fermenting sugar to ethyl alcohol and then oxidizing it by microbes to generate vinegar. 

Acetic acid, which has the molecular formula CH3COOH, is also referred to as ethanoic acid, ethylic acid, vinegar acid, and methane carboxylic acid. Vinegar gets its distinctive smell from acetic acid, a byproduct of fermentation. In water, vinegar contains 4-6 percent acetic acid.

Two stages of fermentation were used to create this product. The process begins with anaerobic fermentation, which is the alcoholic conversion of carbohydrates into ethanol by yeasts and continues with aerobic fermentation (oxidation of ethanol into acetic acid by AAB). Before alcoholic fermentation can begin, the basic materials - starch or complex carbohydrates - need to be saccharified in order to release fermentable sugars. Due to the growing demand for vinegar, technical advancements were required to produce the product. Currently, three main techniques are used to produce vinegar: the submerged process, the generator process, and the slow surface culture fermentation, which is known as the "Orleans" or "traditional process."

Acetic Acid Production:

i. Chemical-based method:

Acetic acid as an industrial chemical is produced from fossil fuels and chemicals by three processes: acetaldehyde oxidation, hydrocarbon oxidation, and methanol carbonylation.

ii. Biology-based method

    i. Aerobic process production:

Food-grade acetic acid is produced by the two-step vinegar process. The first step is the production of ethanol from a carbohydrate source such as glucose. This is carried out at 30°C using the anaerobic yeast Saccharomyces cerevisiae. The second step is the oxidation of ethanol to acetic acid. Although a variety of bacteria can produce acetic acid, only members of Acetobacter are used commercially, typically the aerobic bacterium Acetobacter aceti at 27-37°C. This fermentation is incomplete oxidation because the reducing equivalents generated are transferred to oxygen and not to carbon dioxide:

C6H12O6 + 2CO2 + 2CH3CH2OH.

The overall theoretical yield is 0.67 g acetic acid per gram of Glucose. At the more realistic yield of 76% (of 0.67, i.e., 0.51 g per gram glucose), this process requires 2.0 pounds of sugar or 0.9 pounds of ethyl alcohol per pound of acetic acid produced. Complete aeration and strict control of the oxygen concentration during fermentation are important to maximize yields and keep the bacteria viable. Submerged fermentation has almost completely replaced surface fermentation methods. The draw-and-fill mode of operation can produce acetic acid at concentrations up to 10% wt/wt in continuous culture at pH 4.5 in about 35 hours.

2CH3CH2OH + 02 -+ 2CH3COOH + 2H20.  

    ii. Anaerobic process production:

    In 1980 another process for the production of acetic acid emerged based on anaerobic fermentation using Clostridia. These organisms can convert glucose, xylose, and some other hexoses and pentoses almost quantitatively to acetate according to the following reactions:

C6H12O6 + 3CH3COOH, 2C5H10O5 + 5CH3COOH.

Typical acidogenic bacteria are Clostridium aceticum, Clostridium thermoaceticum, Clostridium formicoaceticum and Acetobacterium woodii. Many can also reduce carbon dioxide and other one-carbon compounds to acetate.

Parameter for AAB::

pH

pH The optimum pH for the growth of AAB is 5.5 to 6.3. However, these bacteria can survive at low pH values of between 3.0 and 4.0, and some strains have been isolated from an aerated media containing acetate that could grow at pH values as low as 2.0 to 2.2. It was postulated that there are three groups of strains that might exist in vinegar production, namely, acidophilic strains that grow only at a pH value of about 3.5, acetophobic strains that only grow at pH levels higher than 6.5, and acetotolerant strains that can grow at both these values. There may be a gradual development from acetophobic to acetotolerant strains and, with prolonged exposure to low pH and high acetic acid concentrations, to acetophilic strains. This suggests the development of a gradual acid resistance in these bacteria.

Temperature

The optimum growth temperature for most AAB is 25◦C to 30◦C. The maximum temperature for the growth of A. aceti wasfound to be about 35◦C. Thermotolerant AAB that are able to grow at 37◦C to 40◦C have also been isolated. These bacteria were able to oxidize ethanol at 38◦C to 40◦C at the same rate that mesophilic strains do at 30◦C, as well as being able to oxidize ethanol more rapidly than the mesophilic strains at the higher temperatures. AAB can also be active at lower temperatures, and weak growth was observed even at 10◦C.

Production of Acetic acid

Batch fermentations in 16-L vessels with automatic pH control were conducted as a basis for predicting continuous fermentor performance at different dilution rates. Continuous fermentations in 0.3-L chemostats were performed to determine for C. thermoaceticum. Batch fermentations were conducted in pH-, temperature-, and gas-controlled New Brunswick 16-L Microferm fermentors. These units had provisions for homogeneous mixing, constant headspace flushing with CO2, gas, sterile sampling, automatic pH control, and in-place sterilization. Chemostats with a working volume of 333 mL were set up for continuous work. The units contained provisions for flushing the headspace with gas and sparging with C02 or N,, plus pH control with 10% NaOH. The headspace of the feed medium reservoir was also flushed with N, to maintain its anaerobic state.

German Method 

In 1832, German chemist Schutzenbach established the German process which is one of the quick processes (trickling method). It is also one of the oldest methods of acetic acid production. Acetic acid bacteria are Gram-negative and aerobic bacteria. The acetic acid bacteria were grown and fermented thick slim coating beech wood shavings occurs in 5000-6000 liters of wood /steel tank while the alcoholic liquid distributes of spray mechanism covered with AAB. During, the pH was maintained in the range between 2.5-3.2 and they produced at 27-30◦C of 88-90% of acetic acid from ethanol and the remaining substrate is used in biomass production. Depending on the size of the acetifier, production capacity was 70000- 100000 at 10% acidity. The acetic acid yield is approximately 1-2 kg/m^3.h. A high yield of acetic acid was produced in surface culture of 57 611 mg/L with inoculum techniques. 

Parameters: 

    (i) mash circulation, 

    (ii) cooling water through heat exchanger, 

    (iii) amount of air passed through the system.

 The advantages is it requires low space requirements, Low costs, and high acetic acid concentration. Drawbacks, high risk of clogging because of cellulose and high loss of ethanol by evaporation, and difficulty to producing AA production.

Application:

1. Antibacterial property for injuries like burns. -  Therapeutic effects
2. Used to prevent growth, spoilage and preserve. - Beverages industry
3. Food condiment and preserving meats and vegetables. - Food industry
4. Defensive effect and decrease fatality - Cardiovascular diseases
5. Used for diabetic treatment and decrease blood sugar levels. - Antidiabetic effect
6. Used in paper coatings, emulsion, resins for paints. - Petrochemical industry
7. Laboratory washing. - Machineries
8. Used in laundry detergents - Homecare
9. Managing soft tissues injury - Orthopaedic surgery

Reference:

1. Cheryan, M., Parekh, S., Shah, M., & Witjitra, K. (1997). Production of Acetic Acid by Clostridium thermoaceticum. Advances in Applied Microbiology, 1–33. doi:10.1016/s0065-2164(08)70221-1 

2. Li, P., Li, S., Cheng, L., & Luo, L. (2014). Analyzing the relation between the microbial diversity of DaQu and the turbidity spoilage of traditional Chinese vinegar. Applied microbiology and biotechnology, 98(13), 6073-6084.

3. Bayan, L., Koulivand, P. H., & Gorji, A. (2014). Garlic: a review of potential therapeutic effects. Avicenna journal of phytomedicine, 4(1), 1.