Fermentation inhibitory compounds

One of the possible problems of using the complex mixtures is the presence of unwanted materials such as certain metal ions, particularly manganese and iron. In addition, inhibitory compounds such as acetic add may be present All need to be removed before fermentation can proceed. 

Thirdly, nutrient depletion and any inhibitory compounds do not, in general, have a large effect on the immobilised cells because the cells are fixed by immobilisation. In batch and CSTR fermentation, nutrient depletion, inhibition and accumulation of toxic by-products are major problems, but immobilised cells are usually unaffected by toxic by-products. 

Small amounts of hydrogen peroxide in raw milk can activate the lactoperoxidase-catalyzed oxidation of thiocyanate to produce a bacterial inhibitor (Hogg and Jago 1970). Inhibitory compounds resulting from oxygen metabolism can produce initially slow starter culture growth in industrial dairy fermentations if the milk has been excessively agitated. 

Helle, S., Cameron, D., et al., Effect of inhibitory compounds found in biomass hydrolysates on growth and xylose fermentation by a genetically engineered strain of Saccharomyces cerevisiae. Enzyme Microbial Technology 2003, 33 (6), 786-792. 

Chlorofusin (52) was one of the major components produced by fermentation of Fusarium sp. 22026. The DELFIA-modified ELISA was used to guide to the purification of inhibitors of the p53/MDM2 interaction from these fermentation extracts. Chlorofusin was the most abundant inhibitory compound. Titration of purified chlorofusin in the... 

Economically feasible processes for biomass conversion to ethanol requires the fermentation of the sugars generated in the pretreatment and hydrolysis steps. In agricultural residues and hardwoods, xylose constitutes about 45% of the total sugars, and therefore xylose conversion to ethanol is important for high yields. Dilute acid hydrolysis of cellulosic biomass generates inhibitory compounds such as furfural, hydroxymethyl furfural, and acetic acid [1], These inhibitors affect the ability of yeasts to ferment the hydrolyzates, and therefore a detoxification step is usually included in fermenting acid... 

D. hansenii at concentrations above 0.5 g/1, and the inhibitory effects on fermentation increased with vanillin concentration up to 3 g/1. Furfural was less inhibitory to batch fermentations, with concentrations between 1 and 5 g/1 exhibiting similar effects [23], From experiments with brewers spent grain hydrolysate (85 g/1 xylose) pretreated in various manners (raw, treated with activated carhon, concentrated, or amended with xylose), it appears that total phenolics (3.93 g/1), including vanillin and syringaldebyde, are the most inhibitory compounds to C guilliermondii fermentations. The presence of furfural (0.61 g/1) did not effect the fermentation. Sanchez and Bautista [31] found that 2 g/1 fiirfiiral inhibits oxidative metabolism and fermentation by C. guilliermondii, and it has been previously observed that furfural concentrations below 1 g/1 are not inhibitory [32-33]. Keating et al. 

Fermentation of the lignoceliulosic hydrolyzates is more difficult than the well-established processes of ethanol production from e.g. molasses and starch. Hydrolyzates contain a broader range of inhibitory compounds, where the composition and the concentration of these compounds depend on the type of lignoceliulosic materials and the chemistry and nature of foe pretreatment and hydrolysis processes. Secondly, foe hydrolyzates of hemicelluloses contain not only hexoses but also pentoses, where xylose is foe dominant sugar in foe hydrolyzates from hardwood hemicelluloses. Therefore, foe fermenting microorganism should be able to produce efoanol from foe hydrolyzates with a... 

Several of the inhibitory compounds found in hydrolyzates can be biotransformed, or, in a few cases, even be fully metabolized by yeast Conversion occurs for several of the carboxylic acids, fiirans and phenolic compounds. This suggests that continuous in situ detoxification of the hydrolyzate during the fermentation may be possible. However, diis requires a suitable mode of operation, and the bioconversion of inhibitors must be taken into account in the design of the process. 

Although acetic acid is known to be inhibitory to some strains of fermentatively growing Saccharomyces (Rasmussen et al., 1995 Fugelsang et al., 1993), it is generally believed that other inhibitory compounds are produced as well, and the final stuck fermentation represents the contribution of these multiple insults (Edwards, 1996). However, the nature of these additional inhibitors has yet to be determined. 

The causes of sluggish and stuck fermentations include fermentation at temperature extremes, nutritional deficiencies, osmoregulation, ethanol toxicity, and in low-temperature fermentations, long-term anaerobiosis. To these classically ascribed sources can be added failures in adequately preparing yeast starters as well as the presence of inhibitory compounds which may include pesticides and those produced by microorganisms. 

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