Xylose to Acetic Acid?

In hardwood, there are about 7 acetyl groups per 10 xylose units, which can be easily cleaved under alkali conditions.

Instead of converting xylose to ethanol by direct fermentation, it may make acetic acid via fermentation of xylose to acetic acid.

2C5H10O5→5C2H4O2

Therefore, 10 moles of xylose can generate 32 moles of acetic acid because 10 moles of xylose has 7 moles of acetic acid and 10 moles of xylose can be fermented to 25 moles of acetic acid.

Y.Y. LEE reported a total 76 wt% conversion and a maximum acetate concentration of 15.2g/L (2001); Parekh and Cheryan were able to achieve final broth concentrations of 102 g/l (as HAc) with 93% conversion of the glucose feedstock using dolime (Ca(OH)2Mg(OH)2) for pH control in a fed batch fermentation with cell recycle provided by cross-flow membrane filtration.

What can we expect from yeast for bioethanol commercial production?

For biofuel production from biomass, we want to robust microorganisms. Yeast is a traditional one but we need a yeast that has the ability to ferment both C5 and C6 sugar and more product tolerance.


Many scientists cross the world have been taking efforts to engineer yeast that can improve the speed and efficiency of ethanol production, which is a critical component to economically making biofuels a significant part of energy supply.

The 1st aspect is to engineer the yeast to ferment both C6 and C5 sugars by genetic modification. One of the example is the "Purdue yeast" developed by Nancy Ho.

The 2nd aspect is to engineer the yeast that has high-ethanol-tolerance. The typical example is the one developed by by MIT scientists (see http://web.mit.edu/newsoffice/2006/biofuels.html).

New direction for biofuels?

Obama's victory means the beginning of the change in the Unite State of America, a new direction to overcome the challenge and realize the dream of prosperous. What about biofuel?

Please read the news "Ethanol and Obama":
http://www.goodfuels.org/2008/11/ethanol-and-obama/

Is it also the time to vote for bioenergy?

US presidential election is underway today. Who will get the the job of US president?

Both candidates emphasize on alternative energy. But one of them put more emphasis on bioenergy, the other more on nuclear power energy. The fate of bioenergy in the near may be decided today.

Process configuration for saccharification using enzyme

Process configuration
separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF)

SHF
Advantages:

• Run each step under optimal conditions
• Enzymatic hydrolysis at 45-50 0C and fermentation at about 30 0C
• Possible to run fermentation in continuous model with cell recycling

Disadvantages
The sugar released (glucose and cellobiose) inhibit the enzyme during hydrolysis

SSF
Advantages: Glucose produced consumed immediately by fermenting microorganisms
n avoid end-product inhibition of b-glucosidase

• Low enzyme loading
• Fast rate of hydrolysis
• High production rate
• Process integration in one reactor: cost saving
  

Disadvantages

• Inability to recycle and reuse the yeast due to its mix with lignin residue
• Formation of lactic acid
• The difference in optimal temperature for hydrolysis and fermentation
  -A compromise but improvement by the thermotolerant yeast