Acetic acid, fermentation inhibition

Fermentation is the name conventionally attributed to any industrial transformation that involves microorganisms. In TBV production, there are two distinct fermentations alcoholic and acetic the first is carried out by yeasts of different genera and species, the second by AAB. Usually, the two fermentations occur in the same vessel (badessa) and are performed by wild strains without any control. The results are unpredictable sometimes the alcoholic fermentation is inhibited by acetic acid produced by AAB often the acetic acid fermentation, for various reasons, does not occur. To solve the problem, a two-stage fermentation procedure has been suggested the oxidation of ethanol should always follow alcoholic... 

McLellan et al. 1999). The development of acetate-tolerant ethanol-producers is important because during acid hydrolysis of hemicellulose, acetic acid (which inhibits fermentation) is produced at a high ratio relative to fermentable sugars. 

The final product of the acetic acid fermentation, acetic acid (AcH), is weU known for its ability to inhibit growth of most microorganisms at very low... 

The breakdown of furan aldehydes leads to the formation of formic and levulinic acid. Moreover, acetic acid is formed during the degradation of hemicellulose. Partial breakdown of lignin can generate a variety of phenolic compounds (23), which also inhibit S. cerevisiae (14,15). In contrast to furan aldehydes and aliphatic acids, the toxic effect of specific phenolic compounds is highly variable (15). Different raw materials and different approaches to prepare lignocellulose hydrolysates will result in different concentrations of the fermentation inhibitors (16,17). 

There is no current commercial biologic process for the production of succinic acid. In past laboratory systems, when succinic acid has been produced by fermentation, lime is added to the fermentation medium to neutralize the acid, yielding calcium succinate (2). The calcium succinate salt then precipitates out of the solution. Subsequently, sulfuric acid is added to the salt to produce the free soluble succinic acid and solid calcium sulfate (gypsum). The acid is then purified with several washings over a sorbent to remove impurities. The disposal of the solid waste is both a directly economic and an environmental concern, as is the cost of the raw materials. Some key process-related problems have been identified as follows (1) the separation of dilute product streams and the related costs of recovery, (2) the elimination of the salt waste from the current purification process, and (3) the reduction of inhibition to the product succinic acid on the fermentation itself. Acetic acid is also a byproduct of the fermentation of glucose by Anaerobiospirillium succiniciproducens almost 1 mol of acetate will be produced for every 2 mol of succinate (3). Under certain cultivation conditions by a mutant Escherichia coli, lesser amounts of acetate can be produced (4,5). This byproduct will also need to be separated. 

Edwards, C.G., Reynolds, A.G., Rodriguez, A.V., Semon, M.J., Mills, J.M. (1999) Implication of acetic acid in the induction of slow/stuck grape juice fermentation and inhibition of yeast by Lactobacillus sp. Am. J. Enol. Vitic., 50, 204-210. 

Oenococcus is a facultative acidophilic anaerobe and grows at pH 4.8 with temperatures between 18 °C and 30 °C. It requires a rich medium supplemented with tomato juice or grape juice, and its growth is not inhibited in the presence of 10% ethanol. Glucose is fermented in lactic acid, carbon dioxide, acetic acid and ethanol (it is a heterofermenter). It converts malate into lactate and CO2 in the presence of fermentable carbohydrate. 

Using Brettanomyces claussenii, Custers showed glucose fermentation is inhibited under anaerobic conditions. Glucose was fermented more rapidly under aerobic than anaerobic conditions. He named this inhibitory effect a negative Pasteur effect". Aerobic conditions activated the fermentation of glucose to produce ethyl alcohol, carbon dioxide, and "a considerable amount of acetic acid". Only ethanol and carbon dioxide were produced under anaerobic conditions acetic acid was not. 

Wood is about 65—75% carbohydrate and has been considered as a potential source of ethanol for fuel. The carbohydrate material can be hydrolyzed to monomer sugars, which in turn can be fermented to produce ethanol. However, wood carbohydrates are expensive to hydrolyze. Hydrolysis with acids and enzymes is impeded by the crystalline structure of cellulose. Lignin interferes with processing, and hydrolytic by-products such as furfural, acetic acid, and derivatives of lignin and extractives can inhibit fermentation. Research is still being conducted on wood hydrolysis to develop a process that is economically sound. Furfural is a useful chemical feedstock and results from the dehydration of pentose sugars. It can be obtained in 9 to 10% yield from the dilute acid hydrolysis of hardwoods (75). 

Production of bulk chemicals. The production of solvents is normally characterized by a general inhibition phenomenon which has been mainly attributed to the changes in membrane permeability, or to the toxic effects on the metabolic pathway. Aqueous two-phase systems have been shown to be effective as media for the extractive fermentation of a number of solvents which include ethanol, acetone-butanol and acetic acid (3). Improved productivity has been achieved in most of the cases as compared to the conventional fermentations, which is significantly due to the elimination of product inhibition. However, there is an indication that changes in the microenvironment of the microbial cells due to the presence of non-metabolizable polymers could also contribute, in the initial phases, to the increased production. The addition of PEG and dextran to a growth medium, for instance, was shown to give increased initial ethanol yields, as a result of decrease in the chemical potential of water (8). 

P data. The inhibition consteints for etheuiol and acetone are approximately ten times greater than that for butanol, acetic acid, and butyric acid. This is indicative of the relatively low toxicity of acetone and ethanol as compared with the other fermentation products. 

---