Handbook of plant based biofuels - Chapter 8 pot

105
8
Bioethanol from
Starchy Biomass
Part II Hydrolysis and
Fermentation
Sriappareddy Tamalampudi,
Hideki Fukuda, and Akihiko Kondo
ABSTRACT
Bioethanol, which is derived from starchy and cellulosic biomass, is becoming impor-
tant as an alternative fuel due to diminishing petroleum resources and environmental
impacts. Acid and enzymatic methods have been developed for the hydrolysis of
starchy biomass in order to release fermentable sugars. Acid hydrolysis results in the
production of unnatural compounds that have adverse effects on yeast fermentation.
In enzymatic hydrolysis of starch, the biomass has to be cooked at high tempera-
tures and large amounts of amylolytic enzymes have to be added to hydrolyze the
starchy biomass prior to fermentation. Recent advances in yeast cell surface engi-
neering developed the strategies to genetically immobilize amylolytic enzymes like
CONTENTS
Abstract 105
8.1 Introduction 106
8.2 Yeast Cell Surface Engineering: A Tool for Direct Ethanol Production
from Starch 106
8.3 Ethanol Production from Soluble Starch 108
8.3.1 Displayed Glucoamylase 108
8.3.2 Co-Displayed Glucoamylase and Amylase 109
8.4 Ethanol Production from Low-Temperature Cooked Corn Starch 111
8.5 Ethanol Production from Raw Corn Starch 113
8.6 Evaluation of Surface Engineered Yeast Strains 117
8.7 Conclusions 117
References 118

© 2009 by Taylor & Francis Group, LLC
106 Handbook of Plant-Based Biofuels
α-amylase and glucoamylase on the yeast cell surface. As a means of reducing the
cost of ethanol production, occulent and nonocculent yeast strains co-displaying
amylolytic enzymes have been developed and used successfully for direct ethanol
production from raw starch. Hence, the cell surface engineered yeast appears to have
great potential in industrial application.
8.1 INTRODUCTION
The utilization of biomass as the starting material for various chemicals and for the
production of biofuels has received considerable interest in recent years. Starchy
and cellulosic materials of plant origin are the most abundant utilizable biomass
resources. Starchy biomass has to be hydrolyzed either by enzymatic or acid hydro-
lysis to release fermentable sugars. However, acid hydrolysis results in the formation
of by-products such as levulinic acid and formic acid which have adverse effects on
yeast growth during the fermentation process (Kerr 1944). The enzymatic hydrolysis
of starchy material for ethanol production via fermentation consists of two or three
steps and requires improvement if it is to realize efcient production at low cost.
There are two main reasons for the present high cost: one is that starchy materials
need to be cooked at a high temperature (140 to 180°C) to obtain high ethanol yield
and the other is that large amounts of amylolytic enzymes, namely glucoamylase
(EC 3.2.1.3) and α-amylase, need to be added. To reduce the energy cost of cook-
ing starchy materials, previously reported noncooking and low-temperature cooking
fermentation systems have succeeded in reducing energy consumption by approxi-
mately 50% (Matsumoto et al. 1982), but it is still necessary to add large amounts of
amylolytic enzymes to hydrolyze the starchy materials to glucose.
Many researchers have reported attempts to resolve this problem by using
recombinant glucoamylase-expressing yeasts with the ability to ferment starch to
ethanol directly (Ashikari et al. 1989; Inlow, McRae, and Ben-Bassat 1988). Recom-
binant yeast that co-produces glucoamylase and α-amylase has been developed to
further improve the efciency of starch fermentation (Birol et al. 1998; De Moreas,

Astol-Filho, and Oliver 1995; Eksteen et al. 2003). Recent advances in yeast cell
surface engineering provided the tools for the display of amylolytic enzymes which
allows the utilization of yeast whole-cell biocatalyst for direct ethanol production
from starch. Moreover, integration of hydrolysis and fermentation steps by arming
yeast cells can reduce the unit operations compared to that of hydrolysis by acids
and isolated enzymes (Figure 8.1). This review summarizes the work on cell sur-
face engineering systems that demonstrated direct ethanol production from soluble
starch, low-temperature cooked starch, and raw starch.
8.2 YEAST CELL SURFACE ENGINEERING: A TOOL FOR
DIRECT ETHANOL PRODUCTION FROM STARCH
The cell surface is a functional interface between the inside and outside of the cell.
Some surface proteins extend across the plasma membrane and others are bound
by noncovalent interactions to the cell surface components. Cells have systems for
anchoring surface-specic proteins and for conning surface proteins to particular
© 2009 by Taylor & Francis Group, LLC
Hydrolysis and Fermentation of Starchy Biomass 107
domains on the cell surface. In biotechnology, the cell surface can be exploited by
making use of known mechanisms for the transport of proteins to the cell surface.
In particular, Saccharomyces cerevisiae is useful as a host for genetic engineering,
because it allows the folding and glycosylation of expressed heterologous eukaryotic
proteins and can be subjected to many genetic manipulations. Moreover, the yeast
can be cultivated to a high density in an inexpensive medium, so that the display of
enzymes on yeast cell surface has several applications in bioconversion processes.
Many glucoamylase-extractable proteins on the yeast cell surface, for example,
agglutinin (Agα1 and Aga1) and Flocculin Flo1, Sed1, Cwp1, Cwp2, Tip 1, and Tir
1/Srp 1 have glycosylphosphotidylinositol (GPI) anchors which play an important role
in the expression of cell surface proteins (Roy et al. 1991; Watari et al. 1994). GPI
anchored proteins contain hydrophobic peptides at their C-termini. After the comple-
tion of protein synthesis, the precursor protein remains anchored in the endoplasmic
reticulum (ER) membrane by the hydrophobic carboxyl-terminal sequence, with the

rest of the protein in the ER lumen. Within less than a minute, the hydrophobic car-
boxyl-terminal sequence is cleaved at the site and concomitantly replaced with a GPI
anchor, presumably by the action of a transamidase (Ueda and Tanaka 2000).
Among the GPI anchor proteins α-agglutinin and Flocculin anchors are dem-
onstrated to be suitable for the expression of hydrolytic enzymes. The molecular-
level information on α-agglutinin is utilized to target the heterologous proteins of
Starch
Cooking
Gelatinization
α-Amylase
Liquefaction
Saccharification
Arming yeast
displaying
amylolytic enzymes
(C)
Fermentation
Yeast
Ethanol
(B)
Acid hydrolysis
(A)
Glucoamylase
FIGURE 8.1 Schematic diagram of starch hydrolysis and ethanol fermentation using differ-
ent methods. (a) Acid hydrolysis, (b) enzymatic hydrolysis, and (c) arming yeast displaying
amylolytic enzymes.
© 2009 by Taylor & Francis Group, LLC
108 Handbook of Plant-Based Biofuels
biotechnological importance to the outermost glycoprotein layer of the cell wall. In
the α-agglutinin system, the C-terminal half of the α-agglutinin containing the GPI

anchor attachment signal was used to anchor the heterologous proteins on the yeast
cell surface (Capellaro et al. 1991). In the case of the occulin system two types of
cell surface display methods were developed. In one system, the C-terminal region
of Flo1p, contains a GPI-attachment signal; the second system, by contrast, attempts
to utilize the ability of the occulation functional domain of Flo1p to create a novel
surface display apparatus (Kondo and Ueda 2004).
8.3 ETHANOL PRODUCTION FROM SOLUBLE STARCH
8.3.1 d
i S P l a y e d Gl u c o a m y l a S e
Surface expression of the amylolytic enzymes was initiated by the pioneering work
of Murai et al. (1997). They reported the strategy of developing recombinant S. cer-
evisiae displaying amylolytic enzymes. The multi-copy plasmid pGA11 (Figure 8.2)
was used for the expression of glucoamylase/α agglutinin fusion gene containing the
secretion signal sequence of the glucoamylase under the control of the GAPDH pro-
moter and was introduced into the S. cerevisiae MT8-1 as host strain. The displayed
glucoamylase is from Rhizopus oryzae, an exo-type amylolytic enzyme, cleaving
α-1,4-linked and α-1,6-linked glucose effectively from starch. The anchoring of the
fusion gene on the cell wall of recombinant yeast harboring the plasmid pGA11 was
demonstrated by immunouorescence labeling of the cells with anti-glucoamylase
IgG (Murai et al. 1997; Ueda et al. 1998).
Kondo et al (2002) used occulating yeast strain YF207 for the surface expres-
sion of glucoamylase. The yeast strain YF207 is a tryptophan auxotroph with a strong
occulation ability which was obtained from Saccharomyces diastaticus ATCC60712
and S. cerevisiae W303-1B by tetrad analysis and was transformed with pGA11
GAPDH terminator
3'-Half of α-agglutinin gene
Glucoamylase gene
GAPDH promoter
pGA11
2µm

TRP1
Col E1 ori
Amp
r
Secretion signal sequence
of R. oryzae glucoamylase gene
FIGURE 8.2 Schematic representation of the expression plasmid for glucoamylase/α-
agglutinin fusion gene.
© 2009 by Taylor & Francis Group, LLC
Hydrolysis and Fermentation of Starchy Biomass 109
constructed in the previous study (Murai et al. 1997). The cell surface glucoamylase
does not show any effect on occulation ability during growth and ethanol fermenta-
tion phases. Moreover, the glucoamylase activity displayed on the surface of occu-
lent yeast strain was similar to that displayed on nonocculent yeast cells. Therefore,
the occulent yeast cells displaying glucoamylase possess both strong occulation
ability and glucoamylase activity; and hence they are considered more advantageous
in industrial processes for ethanol production from starchy materials.
The results shown in Figures 8.3a and 8.3b demonstrate that the cell-surface glu-
coamylase is effective for direct ethanol fermentation from soluble starch, because
high ethanol fermentation from soluble starch was obtained. In previous studies using
recombinant S. cerevisiae secreting glucoamylase (Nakamura et al. 1997; Briol et al.
1998) both cell growth and fermentation were performed under anaerobic or minimal
aerobic conditions; and hence over 150 h was necessary to attain ethanol concentra-
tions of 20 to 30 g/l. Ideally, a large cell mass should be obtained by high-density cell
culture under aerobic conditions and cells harvested by sedimentation were used for
the ethanol fermentation. However, in secretory expression of amylolytic enzymes,
this approach is not suitable because inoculated cells should produce a sufcient
amount of amylases before ethanol fermentation. In the study by Kondo et al. (2002),
recombinant yeast strain YF207/pGA11 displaying glucoamylase gene maintained
a high ethanol production rate (approximately 0.6 to 0.7 g l

-1
h
-1
) during repeated
utilization for fermentation over 300 h. This is attributable to high plasmid stability
during growth and fermentation phases, even though pGA11 is a multi-copy-type
plasmid, based on pYE22m. The plasmid stability in cells cultivated in YPS medium
was found to be higher than in cells cultivated with YPD medium. Since host cells
could not metabolize soluble starch, the utilization of soluble starch as the carbon
source would be a selection pressure for the yeast cells bearing plasmids. In the
case of glucoamylase-displaying yeast cells, glucose was maintained at a very low
concentration and, at the same time, a high ethanol production rate was achieved.
This might be because the recombinant yeast cells metabolize the glucose as soon
as glucose is released from soluble starch by the glucoamylase displayed yeast cells.
However, a high ethanol production rate was obtained because local glucose concen-
tration near the yeast cell surface was probably higher than that in the fermentation
medium. This low concentration of glucose in the fermentation medium is advanta-
geous in minimizing the risk of contamination.
8.3.2 co-di S P l a y e d Gl u c o a m y l a S e a n d am y l a S e
Studies show the display of only glucoamylase leads to the accumulation of insolu-
ble starch during fed-batch fermentation, because of the lack of liquefying enzyme
α-amylase. In order to overcome this problem, Shigechi et al. (2002), developed
two recombinant yeast strains co-expressing glucoamylase and α-amylase. Plasmids
for the surface expression (pAA12) and secretory expression (pSAA11) of Bacillus
stearothermophilus α-amylase were constructed and co-transformed into the occu-
lent yeast strain YF207 along with the plasmid pGA11 for cell surface display of R.
oryzae glucoamylase. The ethanol productivity by these two strains was examined
by fed-batch fermentations using soluble potato starch as substrate. The amylolytic
© 2009 by Taylor & Francis Group, LLC
110 Handbook of Plant-Based Biofuels

80
(a)
Growth
Ethanol fermentation
30
25
20
15
Ethanol concentration (g/l)
10
5
0
60
40
Starch concentration (g/l)
Glucose concentration (g/l)
20
0
02040
Time (h)
60 80
80
(b)
Growth
Ethanol fermentation
80
60
Ethanol concentration (g/l)
40
20

0
60
40
Starch concentration (g/l)
Glucose concentration (g/l)
20
0
04080
Time (h)
120 160
FIGURE 8.3 (a) Batch fermentation of starch to ethanol by YF207/pGA11. YF207/pGA11
cells were grown under aerobic conditions (2.0 ppm), harvested, and used for batch fermen-
tation. The left side of the solid line in the gure is the growth phase and the right side is
the ethanol-fermentation phase. (b) Fed-batch fermentation by YF207/pGA11. YF207/pGA11
cells were grown under aerobic conditions (2.0 ppm), harvested, and used for fed-batch fer-
mentation under anaerobic conditions. The left side of the solid line in the gure is the growth
phase and the right side is the ethanol-fermentation phase.
© 2009 by Taylor & Francis Group, LLC
Hydrolysis and Fermentation of Starchy Biomass 111
activity was detected by both the strains and ow cytometric analysis conrmed the
successful co-expression of glucoamylase and α-amylase in strains YF207/ [pGA11,
pAA12] and YF207/ [pGA11, pSAA11].
As shown in Figures 8.4a and 8.4b, both recombinant strains YF207/ [pGA11,
pAA12] and YF207/ [pGA11, pSAA11] grew faster in the growth phase than the
glucoamylase displaying yeast YF207/pGA11. The activities of glucoamylase and
α-amylase displayed on the cell surface were maintained with YF207/ [pGA11,
pAA12] during the ethanol fermentation phase, whereas in the case of YF207/
[pGA11, pSAA11] strain the secreted α-amylase was accumulated. The ethanol con-
centration produced reached 60 g l
-1

after 100 h of fermentation by both strains. But
glucose concentration is slightly higher in the culture medium of YF207/ [pGA11,
pSAA11] strain. This is probably due to the secretion of α-amylase, which decom-
poses starch in the culture medium.
In addition, the occulation ability of the yeast strain co-expressing glucoamy-
lase and α-amylase did not change during the fed-batch fermentation and was almost
the same as that of the yeast strains YF207 and YF207/pGA11. This nding sug-
gested the co-display of two amylolytic enzymes on the cell surface does not inu-
ence the occulation ability of yeast cells.
8.4 ETHANOL PRODUCTION FROM
LOW-TEMPERATURE COOKED CORN STARCH
In direct ethanol production, noncooking and low-temperature cooking fermenta-
tion systems have several advantages over conventional high-temperature cooking
(140 to 180°C) process (Matsumoto et al. 1982) because high-temperature cook-
ing requires high energy and the addition of large amounts of amylolytic enzymes.
Shigechi et al. (2000) performed direct ethanol production in a single step using
corn starch cooked at low temperature (80°C) as the sole carbon source instead of
soluble starch using yeast strains displaying amylolytic enzymes. The productivity
of ethanol from corn starch cooked at low temperature was investigated by using the
recombinant yeast strains that were developed in their previous study, that is, yeast
strains displaying only glucoamylase on the cell surface (YF207/pGA11) or yeast
strains displaying glucoamylase and either co-displaying (YF207/pGA11/pAA12) or
secreting (YF207/pGA11/pSAA11) α-amylase.
The ethanol production rate increased markedly under co-expression of glu-
coamylase and α-amylase compared with the yeast strain displaying only glucoamy-
lase (Figure 8.5). Specically, by co-displaying two amylolytic enzymes on the cell
surface, strain YF207/pGA11/pAA12 was able to produce ethanol more rapidly than
strain YF207/pGA11/pSAA11 and without time lag. These results indicated that
α-amylase, which hydrolyzes α-1,4 linkages of starch in a random fashion, plays
a very important role in efcient hydrolyzation of corn starch. It is probable that

the cooperative and sequential reaction of two enzymes is crucial for efcient uti-
lization of corn starch. Because the yeast strain YF207/pGA11/pAA12 possesses
enough glucoamylase and α-amylase activity in the initial stages of cultivation and
fermentation, it grows fast, produces ethanol without time lag, and achieves maxi-
© 2009 by Taylor & Francis Group, LLC
112 Handbook of Plant-Based Biofuels
80
(a)
80
Ethanol concentration (g/l)
60
40
20
0
60
40
20
0
04080
Time (h)
120 160
80
(b)
80
60
Ethanol concentration (g/l)
40
20
0
60

40
20
0
04080
Time (h)
120 160
Starch concentration (g/l)
Glucose concentration (g/l)
Starch concentration (g/l)
Glucose concentration (g/l)
FIGURE 8.4 (a) Fed-batch fermentation of starch to ethanol by YF207/[pGA11, pAA12].
YF207/[pGA11, pAA12] cells were grown under aerobic conditions (2.0 ppm), harvested, and
used for fed-batch fermentation under anaerobic conditions. To the left of the solid line in the
gure is the growth phase and to the right the ethanol fermentation phase. (b) Fed-batch fer-
mentation of starch to ethanol by YF207/[pGA11, pSAA11]. YF207/[pGA11, pSAA11] cells
were grown under aerobic conditions (2.0 ppm), harvested, and used for fed-batch fermenta-
tion under anaerobic conditions.
© 2009 by Taylor & Francis Group, LLC
Hydrolysis and Fermentation of Starchy Biomass 113
mum ethanol concentration (18 g/l) within the short time (36 h) of the recombinant
yeast strains.
A comparison of conventional high-temperature and low-temperature cooking
fermentation systems using the yeast strain YF207/pGA11/pAA12 co-displaying
glucoamylase and α-amylase (Figure 8.6) shows maximum ethanol concentration,
ethanol-production rate, and substrate consumption rate were almost the same in
the two fermentation systems. In high- and low-temperature cooking systems, the
yield of ethanol produced was 0.50 g per gram of carbohydrate consumed. This
corresponds to 97.2% of theoretical yield (0.51 g of ethanol per gram glucose). This
indicates that the low-temperature cooking fermentation system based on YF207/
pGA11/pAA12 is cost effective in direct fermentation of corn starch.

8.5 ETHANOL PRODUCTION FROM RAW CORN STARCH
The isolation of amylase enzyme from lactic acid bacteria Streptococcus bovis
opened new horizons for the efcient hydrolysis of raw starch (Satoh et al. 1993).
Shigechi et al. developed a novel noncooking fermentation system for direct ethanol
production from raw corn with yeast strain YF207 that co-displayed R. oryzae glu-
coamylase and S. bovis 148 α-amylase by using the C-terminal half of α-agglutinin
(pBAA1) and the occulation domain of Flo1p (pUFLA) as anchor proteins (Fig-
ures 8.7a and 8.7b).
60
50
40
30
20
10
0
010203040
Time (h)
50 60 70
0
5
10
Ethanol (g/l)
Starch (g/l)
15
20
25
30
FIGURE 8.5 Time course of anaerobic ethanol fermentation from 50 g/l corn starch cooked
at low temperature. Each group of cells was aerobically cultivated for 48 h on SDC medium,
harvested, and used in ethanol fermentation with YPS medium cooked at 80°C for 5 min.

(,). YF207; (,

) YF207/pGA11; (,). YF207/pGA11/pAA12; (,) YF207/pGA11/
pSAA11. Open and closed symbols show the starch and ethanol concentrations, respectively.
© 2009 by Taylor & Francis Group, LLC
114 Handbook of Plant-Based Biofuels
The α-amylase and glucoamylase activities conrmed the display of both
amylolytic enzymes (Table 8.1). In glucoamylase-displaying yeast strains there is
not much difference in the activities; whereas in the case of α-amylase-displaying
yeast strains, the activity is dependent on the anchor protein. The yeast strains
YF207/pBAA1 and YF207/pGA11/pUFLA which uses Flo1 anchor showed 40
times higher α-amylase activity than the yeast strains using a-agglutin anchor. It has
been reported that several α-amylases have raw starch binding abilities and that the
starch digesting domain is located in the C-terminal region (Lo et al. 2002). The two
recombinant yeast strains YF207/pGA11/pBAA1 and YF207/pGA11/pUFLA were
used in direct ethanol production from raw corn starch. The raw corn starch, which
corresponds to 200 g of total sugar per liter, was used as the sole carbon source.
As shown in Figure 8.8, strain YF207/pGA11, displaying only glucoamylase, and
strains YF207/pBAA1 and YF207/pUFLA, displaying only α-amylase, produced
almost no ethanol, while soluble sugar accumulated in the fermentation medium of
strain YF207/pUFLA due to degradation of corn starch to oligosaccharides by the
surface-displayed α-amylase. Although strain YF207/pGA11/pBAA1, co-displaying
glucoamylase and α-amylase via α-agglutinin, did produce ethanol from the raw
corn starch, the ethanol yield was low (23.5 g l
-1
) after 72 h of fermentation.
On the other hand, the yeast strain co-displaying glucoamylase and α-amylase
using α-agglutinin and Flo1P (YF207/pGA11/pUFLA) was able to produce ethanol
directly from the raw corn starch without the addition of commercial enzymes.
The concentration of raw corn starch decreased drastically during the fermentation,

as the ethanol concentration increased to 61.8 g l
-1
after 72 h of fermentation. A
60
50
40
30
20
10
0
010203040
Time (h)
50 60 70
0
5
10
Ethanol (g/l)
Starch (g/l)
15
20
25
30
FIGURE 8.6 Comparison of ethanol production between high- and low-temperature cook-
ing fermentation systems using yeast strain YF207/ pGA11/pAA12. YPS medium was cooked
at 120°C for 20 min or at 80°C for 5 min. Ethanol fermentation started with initial starch
concentration of 50 g/l. (,)
.High-temperature cooking fermentation system; (,)low-
temperature cooking fermentation system. Open and closed symbols show starch and ethanol
concentrations, respectively.
© 2009 by Taylor & Francis Group, LLC

Hydrolysis and Fermentation of Starchy Biomass 115
GAPDH
promoter
GAPDH
promoter
pBAA1
α-Amylase
gene
3'-Half of
α-agglutinin gene
(a)
(b)
2µm
Amp
r
Amp
r
s.s.
URA3
GAPDH
terminator
pUFLA
α-Amylase gene
FL-anchor gene
2µm
URA3
ColE1ori
FIGURE 8.7 Expression plasmids for cell surface display of S. bovis α-amylase. (a) Plasmid
pBAA1 for C-terminal immobilization using the α-agglutinin-based surface display system;
(b) plasmid pUFLA for N-terminal immobilization using the Flo1p-based surface display

system. s.s., secretion signal sequence of R. oryzae glucoamylase gene.
TABLE 8.1
Glucoamylase and α-Amylase Activities of Yeast Strains Carrying Different
Plasmids
Strains Glucoamylase Activity
a
α-Amylase Activity
a
YF207 ND
b
ND
b
YF207/pGA11 42.5 ND
b
YF207/pBAA1 ND
b
2.52
YF207/pUFLA ND
b
90.1
YF207/pGA11/pBAA1 45.9 2.38
YF207/pGA11/pUFLA 57.0 114
a
Both activities shown as U/g (wet weight) of cells; values are averages of three independent
experiments.
b
ND, not detected.
© 2009 by Taylor & Francis Group, LLC
116 Handbook of Plant-Based Biofuels
reduction in the particle size and the number of corn starch granules during fer-

mentation was observed by microscopy (Figure 8.9). The yield in terms of grams
of ethanol produced per gram of sugar consumed was 0.44 g/g, which corresponds
to 86.5% of theoretical yield (0.51 g of glucose consumed per gram). No glu-
cose was detected in the fermentation medium. The yeast strain YF207/pGA11/
pUFLA maintained almost the same glucoamylase and α-amylase activities during
fermentation.
200
150
100
50
0
010203040
Time (h)
50 60 70
0
10
20
Ethanol (g/liter)
Total Sugar (g/liter)
40
30
50
60
70
FIGURE 8.8 Time course of direct ethanol production via fermentation from raw corn
starch, which corresponds to 200 g of total sugar as the sole carbon source per liter using 100
g (wet weight) of cells of yeast strains S. cerevisiae YF207/pGA11 (squares), YF207/pBAA1
(triangles), YF207/pUFLA (inverted triangles), YF207/pGA11/pBAA1 (circles), and YF207/
pGA11/pUFLA (diamonds) per liter. Open and closed symbols show ethanol and total sugar
concentrations, respectively. Data are averages from three independent experiments.

Yeast
Starch particle
24 h
Yeast
Starch particle
48 h
FIGURE 8.9 Time course of starch morphology: microscopic analysis of starch granules on
treatment with the yeast cells displaying amylolytic enzymes at 24 and 48 h.
© 2009 by Taylor & Francis Group, LLC
Hydrolysis and Fermentation of Starchy Biomass 117
8.6 EVALUATION OF SURFACE ENGINEERED YEAST STRAINS
In order to evaluate the surface engineered yeast strains, Seong et al. (2005, 2006)
compared ethanol production from raw corn starch using different yeast strains.
They employed yeast strains displaying glucoamylase and co-displaying or secreting
α-amylase for the direct conversion of raw starch because the conversion of starch
to oligosaccharides by α-amylase is the rate-limiting step in the direct fermentation
of raw starch to produce ethanol by arming yeast. In their study, the nonocculent
and occulent strains that either display or secrete α-amylase were compared with
respect to their performance.
The nonocculent yeast strains secreting α -amylase (0.18 g g-dry cell
-1
h
-1
) showed
threefold higher specic ethanol production rate than the α-amylase-displaying non-
occulent yeast strain (0.06 g g-dry cell
-1
h
-1
). But the specic starch consumption

rate in the third batch of fermentation was decreased signicantly compared with the
rst two batches. The decrease in the activity was due to the removal of α-amylase
from the culture supernatant at the end of each batch, which leads to a reduction in
the productivity of α-amylase in subsequent batches. In contrast, nonocculent yeast
strains displaying α-amylase do not show signicant decrease in the specic starch
consumption rate throughout the repeated batch fermentations, whereas the specic
α-amylase activity decreased gradually. On the other hand, occulent yeast strains
secreting and displaying α-amylase also acted efciently on the raw starch with a
specic ethanol production rate of approximately 0.06

and 0.04 g g-dry cell
-1
h
-1
,

respectively. The comparatively high ethanol production rate in α-amylase-secreting
nonocculent yeast is because the diameter of the displayed α-amylase is 10
-6
m
which is three orders of magnitude lower than the secreted α-amylase with a diam-
eter of 10
-9
m (typical diameter of globular proteins) and the rate of association of the
raw starch granule is (10
-5
) with displayed α-amylase is expected to be much lower
than secreted α-amylase.
8.7 CONCLUSIONS
The yeast cells displaying amylolytic enzymes have been proved as potential bio-

catalysts for direct conversion of starchy materials to ethanol. The genes encoding
glucoamylase and α-amylase were fused with the anchor genes and were introduced
into S. cerevisiae. The yeast cells harboring these fused genes were successfully
utilized raw and low-temperature cooked starch as the sole carbon source. More-
over, occulent yeast renders the ethanol fermentation more economical because the
recovery of cells from the fermentation medium is easy to accomplish.
It was demonstrated that the specic ethanol production rate of α-amylase-
displaying or -secreting yeasts depends on the size and nature of starch granules. In
soluble or low-temperature cooked starch, yeast cells displaying α-amylase showed
higher ethanol yield than the α-amylase secretion systems since the displayed
α-amylase can access most of the small starch molecules. Raw starch yeast cells
secreting α-amylase showed better performance in batch fermentations than the
α-amylase-displaying yeast cells. But the starch consumption rate of the α-amylase-
secreting systems is signicantly decreased in the third batch. Even though the yeast
© 2009 by Taylor & Francis Group, LLC
118 Handbook of Plant-Based Biofuels
cells displaying α-amylase show low performance in batch culture, their efciency
is almost comparable to the α-amylase-secreting yeast strains during repeated batch
fermentations. In conclusion, the choice of α-amylase secretion/display depends on
the nature of the substrate and on the type of process operation.
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© 2009 by Taylor & Francis Group, LLC