Biochemical production of ethanol via enzymatic route

Currently there are three main biochemical pathways to produce bio-ethanol from lignocellulose which are based upon their specific method of lignocellulose hydrolysis.These pathways include hydrolysis by enzymes, dilute acid or concentrated acid. THe biochemical pathway utilising enzymes in the hydrolysis is an interesting one for me as it can have many different variations with regards to the required pretreatment, actual hydrolysis conditions as well as the with regards to the fermentation. Have a look at the schematic below for the route utilising enzymes to accomplish the hydrolysis.

As can be seen from the image above, this biochemical lignocellulose to ethanol process requires that the raw material firstly undergoes a pretreatment, followed by enzymatic hydrolysis, fermentation and distillation of the final product to ethanol. The pretreatment is required for the enzymes to overcome the recalcitrant nature of lignocellulose which prevents enzymes from efficiently hydrolysing lignocellulose. Many different pretreatment approaches can be followed but the main ones currently employed include steam explosion, dilute acid pretreatment and hydrothermal pretreatment to name a few. These all have the common aim of hydrolysing the hemicellulosic fraction of the lignocellulose as well as to disrupt the lignin fraction of lignocellulose which makes the cellulose fraction more susceptible to enzyme hydrolysis. The liquid stream coming from the pretreatment is usually rich in C5 (pentose sugars) while the solid fraction from the pretreatment is rich in C6 carbohydrates.

The C6 sugars are still bound up in the solid material in their carbohydrate form and need to be hydrolysed before these sugars can be fermented. Enzymatic hydrolysis is suited to this and can be performed using enzymes to reduce the long chain polymers into short sugar monomers.

A couple of variations exist as to how the fermentation is performed including, SSF, SHF and SSCoF, all of which have the advantages and disadvantages which change according to the available enzymes and microbes.

Ideally a microbe that can produce the required enzymes while fermenting the hydrolysed sugars would be the most favourable but currently this is not possible. Until this is possible economics will determine whether SHF or SSF is preferred.

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