Transcript Repeated Batch Fermentation

PAPER REVIEW
Topic : Bio-Ethanol
Advisor : Prof. Jo-Shu Chang
NURHAYATI / 林海亞
N36017011
Due to date : 21 December 2012
Title
Authors
Journal
Year
Impact Factor
: Characteristics of an Immobilized Yeast Cell System Using Very High Gravity
for the Fermentation of Ethanol
: Hairui Ji, Jianliang Yu, Xu Zhang, Tianwei Tan
: Journal of Applied Biochemical & Biotechnology
: 2012
: 1.943
Supporting Papers :
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Discussion
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
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 multistage 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.