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

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 : -

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 130.12 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.

Discussion Continuous Ethanol Fermentation In this system, the overall ethanol concentration and residual sugar concentration remained almost constant at 130.1 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 0.015 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 133.41 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.

Discussion During repeated batch fermentations, final ethanol concentrations remained at 130.1 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.

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.