1. PAPER REVIEW NURHAYATI / 林海亞 N36017011
Topic : Bio-Ethanol Advisor : Prof. Jo-Shu Chang
NURHAYATI / 林海亞 N

2. Due to date : 23 November 2012
Title : Development of redox potential-controlled schemes for very-high-gravity
ethanol fermentation
Authors : Chen-Guang Liu, Yen-Han Lin, Feng-Wu Bai
Journal : Journal of Biotechnology
Year : 2011
Impact Factor : 3.045
Supporting Papers :
An innovative consecutive batch fermentation process for very high gravity ethanol fermentation with self-flocculating yeast
Optimization of nitrogen and metal ions supplementation for very high gravity bioethanol fermentation from sweet sorghum juice using an orthogonal array design
Very high gravity ethanol fermentation by flocculating yeast under redox potential controlled conditions
Simultaneous saccharification and fermentation (SSF) of very high gravity (VHG) potato mash for the production of ethanol

3. Discussion Experimental Reactor System
This study aim to enhance ethanol yield and productivity with improving cell viability that is affected by osmotic pressure and ethanol inhibition under very high gravity (VHG) fermentation condition. VHG fermentation permits excess sugars (e.g. glucose) contained in media more than 20 and/or 25% (w/v) to achieve ethanol production more than or equal to 15% (v/v). This also help to save energy consumption in downstream processes (e.g. distillation).
There are many strategies may be applied to achieve that purpose, one of them is redox potential (ORP) controlled scheme as discussed in this paper. On the other hand, several papers recommend another strategies, such as optimization of nutrients or micronutrients supplement including nitrogen requirement. (refer to supporting papers)
Experimental Reactor System
The picture illustrate the experimental apparatus for combined chemostat and aeration-controlled scheme (CCACS) operated at 150 rpm (constant agitation rate) by attaching redox potential electrode (7).
There are 3 redox potential controlled schemes had been developed in this study; aeration controlled scheme (ACS), glucose controlled feeding scheme (GCFS), and CCACS as shown in this figure.

4. Discussion Redox Potential Controlled Schemes
The initial glucose concentrations were 203±2.94, 255±2.56 and 303±4.24 g/L.
Redox Potential Controlled Schemes
1. ACS
designed for batch fermentation
maintains desired redox potential by supplementing fresh air
2. GCFS
designed for continuous fermentation
maintains fermentation redox potential by supplementing fresh glucose media
3. CCACS
also for continuous fermentation and combine both ACS and GCFS
Table below shows the effect of different initial glucose concentration and different redox potential controls using ACS.

5. Discussion
As illustrated in previous table, controlling redox potential profile at a proper level can increase ethanol production. Low initial glucose concentration results in a short redox potential controlled period, and maintaining the redox potential at a higher level extends the redox potential controlled period, but dose not increase fermentation efficiency.
GCFS utilizes the dissolved oxygen in fresh media as the sole oxidizing power to raise the redox potential, and the glucose in the media serves as a carbon and energy source for yeast growing.
The left hand figure exhibits the redox potential controlled period was extend to 21.7 h from 2.5 h (when using ACS)
And the right hand figure shows the fermentation efficiency was increased to % from 85.91%.
The GCFS can not maintain cell viability for a prolonged period of time (due to its low glucose feeding rate), then this problem can be resolved by integrating both schemes to become CCACS (consisting glucose feeding rate and aeration rate).

6. Discussion
Table above illustrates the influence of different initial glucose concentrations and different redox potential controls using CCACS (dilution rate = /h). In terms of controlling redox potential at a prescribed level, CCACS is the best operating scheme among other investigated ones.
This figure demonstrates that CCACS could also be applied to another gravity conditions ranging from ~200 to ~300 g glucose/L during ethanol fermentation.
As a result, high biomass, high cell viability, high ethanol concentration, and low residual glucose were obtained in the chemostat at 203±2.94 g/L, -50 mV with short ageing time.

7. Critical Thinking
By manipulating extracellular redox potentials, one could indirectly modulate intracellular metabolic flows to maximize ethanol production. But, however, these methods are not practical for large scale ethanol production.
To overcome this condition, we can apply the other strategies. As we known, VHG fermentation beside could enhance ethanol production due to high glucose loading, but the osmotic pressure and ethanol inhibition problems must be a big constraint. This condition can promote stuck fermentation happened, where fermentation stops earlier and still has more remaining glucose. To improve ethanol productivity under VHG condition, adding yeast extract as nutrient supplement and Mg as micronutrient in acceptable concentration (cost versus production rate) can be considered. Actually, there are many nutrients and micronutrients (or cofactor) can be used. Also, ammonium sulfate as nitrogen source in certain concentration can avoid stuck fermentation occur under this condition.
According to this thinking, I will try to supply high glucose concentration to achieve high ethanol production in my system. Then, let it works first. 