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 : 21 December 2012
Title : Characteristics of an Immobilized Yeast Cell System Using Very High Gravity
for the Fermentation of Ethanol
Authors : Hairui Ji, Jianliang Yu, Xu Zhang, Tianwei Tan
Journal : Journal of Applied Biochemical & Biotechnology
Year : 2012
Impact Factor : 1.943
Supporting Papers : -

3. Discussion Experimental Design Repeated Batch Fermentation
This study aim to present the characteristics of immobilized yeast cells continuously and evaluate both a multi-stage continuous fermentation system and repeated batch fermentation system for very high gravity (VHG) alcoholic fermentation.
Experimental Design
The repeated batch fermentation was employed with an initial sugar concentration of 280 g/L. the immobilized yeast cell system was reliable for at least 10 batches and for 28 days without accompanying the regeneration of Saccharomyces cerevisiae inside the carriers.
The multi-stage continuous fermentation was carried out in a five-stage column bioreactors with total working volume of 3.75 L. the bioreactor was operated for 26 days.
Repeated Batch Fermentation
The maximum ethanol concentration of each batch during the repeated batch fermentation increased gradually and reached g/L at the third batch.
The following figure exhibits variations of residual sugar concentration, ethanol concentration, free cell concentration, and immobilized cell concentration in carrier in repeated batch fermentation.

4. Discussion Continuous Ethanol Fermentation
In this system, the overall ethanol concentration and residual sugar concentration remained almost constant at g/L and 5 g/L on average, respectively. But as shown in that figure, the cycle times of each batch steadily increased from 48h of the third batch to 65 h of the tenth batch. As a result, the ethanol productivity decreased from 1st cycle to 10th cycle during fermentation.
Continuous Ethanol Fermentation
Based on the results of repeated batch fermentation, this system was employed by h-1 dilution rate with 65 h residence time. This figure shows that with initial sugar concentration of 280 g/L, the final ethanol concentration at the 5th reactor was kept at g/L on average, while residual sugar concentrations were kept at very low levels.
The residual sugar concentration was less than 5 g/L during the initial 15 days. It increased sharply up to 18.9 g/L after 22 days , however.

5. Discussion
During repeated batch fermentations, final ethanol concentrations remained at g/L for a period of 28 days. Although the cycle time was extended slightly on next following steps, there was no obvious fluctuation in the final ethanol concentration. The regeneration of immobilized cells contributed to this stability. As exhibited in the first figure, cells proliferated rapidly at the beginning of each batch when there was plenty of nutrition. This led to increase immobilized cells activity.
This study demonstrates that an ethanol concentration as high as 130 g/L can be achieved using combined VHG multi-stage continuous culture technologies. The ethanol concentrations increased from the first reactor to the fifth reactor with decreasing ethanol productivities. The final ethanol concentration of the fifth reactor decreased from 126 to 113 g/L after 21 days with the residual sugars concentration increasing from 8.9 to 18.9 g/L.
In brief, this study prove the feasibility for both VHG fermentation and reactor stability.

6. Critical Thinking
This study has proved both high ethanol concentration produced with very high gravity (high initial sugar feeding) and reactor stability in 26 days of fermentation. But, as discussed on this paper, this system was employed by integrating multi-stage reactor for continuous system, thus there is no extremely good results was exhibited in my opinion. Meanwhile, the ethanol productivity still very low due to they employed high residence time.