The finite nature of fossil fuels combined with the environmental
problems associated with their extraction and combustion, such
as global warming and acid rain, has promoted an interest in biofuel
production. Ethanol can be produced from sucrose and from starchy
or lignocellulosic biomass [1], and numerous studies on ethanol
production from lignocellulosic biomass have recently been conducted.
The attraction of these materials for fuel production is that
they are abundant and cheaper than sucrose or starchy biomass
[1–3]. However, there are numerous limitations associated with
using lignocellulosic biomass to produce ethanol, such as the slow
rate of enzymatic degradation and high cost of enzymes [1]. As
a consequence, starch biomass is still the most commonly used
feedstock for ethanol production.
Practical ethanol production from starchy biomass such as cassava,
rice, sweet sorghum, and sweet potato have been reported
[4–7]; of these, corn is the most commonly used starchy feedstock
for bioethanol production [1]. In Japan, agricultural policies
adopted by the government have resulted in the existence of exten-
sive areas of unutilized rice fields. This has resulted in numerous
initiatives being launched to produce ethanol using high-yielding
rice that has been cultivated in these underutilized paddies [8].
Thus, in addition to developing ethanol production processes using
commonly used starchy feedstocks such as corn, it is also important
to develop cost effective and efficient processes using alternative
starchy feedstocks like high-yielding rice which are common in the
region.
Despite its efficiency as an ethanol producer, Saccharomyces
cerevisiae cannot produce ethanol from raw starch directly because
it lacks the ability to degrade raw starch into glucose. This is because
conventional ethanol production from raw starch requires the following
three steps: liquefaction of starch by heating and addition of
-amylase, enzymatic saccharification of the low-molecular liquefaction
products to glucose, and fermentation of glucose to ethanol.
The liquefaction process, which accounts for 30–40% of the total
energy used for ethanol production, combined with the large quantities
of enzymes that are required to convert the raw starch into
glucose [5,9], both contribute to making conventional ethanol production
an expensive and complex process. However, co-utilization
of commercial enzymes and/or microorganisms and use of yeast
capable of degrading raw starch can be used to reduce the costs
of ethanol production from raw starch [4–7,10]. We previously
constructed a high-performance, starch-degrading yeast capable
of direct ethanol production from purified raw corn starch by com-bining -integration and polyploidization with high ethanol yield
[11]. The polyploid characteristics of this yeast strain meant that it
is an efficient ethanol producer as well as being robust in culture.
In this study, we attempted to produce ethanol from high-yielding
rice harvested in Japan. To our knowledge, this would be first report
describing direct ethanol production from real biomass using an
amylase-expressing yeast. Importantly, the low-cost and efficient
ethanol production process described here was performed without
supplementing the growth media with nutrients such as yeast
extract or peptone.