A large number of aliphatic acids are present in dilute-acid hydrolyzates originated from wood extractives, lignin degradation and sugar degradation.
- Acetic acid: a major acid constituent in hydrolyzates and is mainly produced from degradation of the acetyl group in the polysaccharides
- Levulinic acid and formic acid are products of sugar degradation
- Several kinds of fatty acids such as hexadecanoic, 9,12-octadecadienoic, oleic and octadecanoic that most likely are unmodified wood extractives, in addition to short-chain and branched aliphatic acids such as 2-methyl-2-hydroxybutanoic acid, methyl propanedionic acid and methyl botanedioic acid. These last acids are not important in terms of concentration, and thus result insignificant effects to the yeast.
The undissociated acids are harmful to the cells and inhibit cell growth. They are liposoluble and thus can diffuse across the plasma membrane into cytosol and may dissociate intracellularly. In order to maintain intracellular pH, protons must be transported across the membrane by the action of plasma membrane ATPase. This results in an increase of ATP consumption, and thereby causes lower biomass yield. In anaerobic conditions, ATP generation is achieved by the ethanol production pathway, resulting in higher ethanol yield at the expense of biomass formation. However, above critical extracellular concentration of undissociated acid, the diffusion rate of undissociated acid can exceed the transport capacity of the plasma membrane ATPase, and intracellular acidification occurs. It is found that the limit of extracellular pH at different acetic acid concentrations which allow yeast to grow. It was found that growth was possible at a pH not less than of 4.7 in cultivation containing 10 g/L acetic acid. Therefore, acetic acid is innocuous if it exists in low concentration or the cultivation is carried out at a pH higher than the extracellular pH limit.
Phenolic compounds
There are a number of phenolic compounds recognized in lignocellulosic hydrolyzates, including
- 3-methoxy-4-hydroxybenzaldehyde
- 4-hydroxyacetophenone
- vanillin
- syringaldehyde
- acetovanilone
- ferulic acid
- vanillic acid
- 4- hydroxybenzoic acid
These compounds are mainly liberated from lignin degradation in addition to aromatic wood extractives. The phenol aldehydes and phenol ketones were found as the worst inhibitors. Moreover, it was also shown the low molecular weight phenolic compounds are more toxic.
Phenolic compounds are considered to be important inhibitors due to their inhibitory effect in fermentation of lignocellulosic hydrolyzates. These compounds partition biological membranes and cause loss of integrity, hence disturb their ability to serve as selective barriers and enzyme matrices. The inhibition mechanism of phenolic compounds has not been elucidated yet.
Studies in inhibitory action of phenolic compounds have been carried out using higher concentrations than are actually present in the hydrolyzates. The water solubility of phenolic compounds is limited and depends on the composition of the liquid, which is different in hydrolyzates and the defined medium; therefore it is possible that the concentration at which the microorganism suffered has been lower. In addition, S. cerevisiae assimilates vanillin, hydroxybenzaldehyde, and syringaldehyde during fermentation, while growth has been reported on cathecol, recorcinol.
Furan compounds
Furfural and 5-hydroxymethyl furfural (HMF) have been found as further hydrolysis products of pentoses and hexoses respectively. Pentoses form furfural in high yield; but if the furfural is not removed as formed, it partially condenses into high-molecular-weight materials. By an analogous process, hexoses yield HMF which, on continued heating, yields levulinic acid and formic acid. Furfural has been reported to be a strong inhibitor for S. cerevisiae. The furfural concentration above 1 g/L was found to decrease significantly the CO2 evolution rate, the cell multiplication and the total viable cell number in the early phase of fermentation. During anaerobic fermentation, reduction of furfural to furfuryl alcohol occurs with high yields, while furoic acid is produced from oxidation of furfural during aerobic cultivation. In both cases, NADH-dependent alcohol dehydrogenase (ADH) is believed to be responsible for furfural conversion in yeasts.
HMF is chemically related to furfural and thus has similar inhibitory effects as furfural, except that it has a lower conversion rate which might be due to lower membrane permeability. It is also discovered that an addition of 4 g/L of HMF decreased the CO2 evolution rate (32%), ethanol production rate (40%), and specific growth rate (70%). However, these inhibitory effects were less than those caused by the same amount of furfural, and thus HMF cannot be considered as acutely toxic as furfural for growth and fermentation of S. cerevisiae.
The conversion rate of furfural is much faster than the conversion rate of HMF. Furthermore, HMF is converted to 5-hydroxymethyl furfuryl alcohol with a similar mechanism as it was shown in the case of furfural conversion.
3 comments:
Our solution: technologies that remove the furfural. - Originally designed to manufacture furfural, we innovated (by making them more efficient) and now we are fine-tuning them on our pilot plants for the use by the cellulosic ethanol industry. At virtually no additional costs, we create additional profits and remove the process problem you described.
Hi DalinYebo,
Thanks for sharing your solution with us about the removal of furfural. That is great: turning the byproducts from one process into a valuable product. I wonder whether you can tell us any capital cost.
Normally we just flash the blowdown treated biomass slurry. As a result, most of volitale compounds are gone and can be collected.
I'd be available to discuss costs and how we can beneficiate byproducts, etc., but would need to obtain some information from you first. Please contact me via the email address on the DalinYebo profile.
Post a Comment