Fermentation
By C Kohn
Agricultural Sciences
Waterford, WI
Most information is based on materials from the DOE’s Great Lakes Bioenergy Research Center, Madison, WI – www.glbrc.org
Fermentation in Society
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The process of fermentation is critical for…
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Fuel Ethanol Production
Bread making (yeast causes the break to rise)
Alcoholic Beverages
Cheese-making (bacteria produce lactic acid
which with rennet curdles the milk) and yogurt
Pickles and Sauerkraut
Additionally, fermentation is necessary for
ruminants such as cattle to break down cellulosic
vegetation.
Wisconsin has a ‘fermentation culture’.
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Many things associated with WI rely on fermentation
as a part of the process of their creation.
What is Fermentation?
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Fermentation: a form of cellular respiration done in
an environment without oxygen (anaerobic)
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Yeast and bacteria are frequently used as
fermenters
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Respiration – the complete breakdown of a source of
energy such as a sugar into CO2 and H2O in an
aerobic environment.
they consume sugars for energy and release
byproducts such as ethanol and carbon dioxide.
Fermentation is the process by which we create
ethanol from renewable plant materials.
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This is very different from transesterification, the
process by which we create biodiesel.
Transesterification is a chemical process; fermentation
is a biological process
Fermentation Components
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Fermentation consists of…
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Substrates – the substance being acted upon (usually a sugar)
Products – the substance created (ethanol, lactic acid, etc.)
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Ethanol is a waste product for the organism because it cannot
acquire any additional energy from this molecule.
Fermentation requires an organism that can break down
substrates in the absence of oxygen.
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Yeast (saccharomyces) is most often the organism of choice.
However, yeast are not the only organisms that can do this.
Many species of bacteria can also ferment.
Even your own cells can obtain energy through fermentation
(your muscles produce lactic acid when they don’t receive
enough oxygen for the demand of the physical activity)
Substrate
Products
Basic Fermentation
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So how does yeast turn sugar into ethanol?
Yeast has enzymes capable of converting a simple sugar,
called glucose, into ethanol as they extract energy from the
molecule.
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Simple sugars are easy to ferment
More complex carbs are more difficult.
Creating ethanol from sugar cane,
as they do in Brazil, is fairly
straightforward because cane juice
contains these simple sugars that
yeast can digest.
Ethanol from Starch & Cellulose
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Ethanol production becomes more difficult
when starting with corn grain or other more
complex plant materials
The long sugar molecules in starches (like corn
and potatoes) require additional steps to
complete fermentation
The fermenting organism must
have enzymes that chop the
long chains of starch into
smaller glucose units.
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This is known as saccharification
saccharification
Corn Ethanol
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Currently, the main biofuel used in the United
States is ethanol distilled from kernels of corn.
There are two methods used to convert the corn
kernel into a usable fuel:
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wet milling
dry milling.
The two different processes yield different byproducts
Steeping, the first step in the wet milling process, is
what differentiates this process from dry-milling
processes
Wet Milling
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wet milling, the kernel is
steeped before being
separated yielding a final
product of ethanol, corn
oil, corn gluten, and corn
starch which may be
modified to corn syrup.
 Generally large,
corporate owned ethanol
plants use wet milling.
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Ethanol plants owned by
farmer cooperatives
generally utilize the dry
milling process.
Dry Milling
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In dry milling, the kernel is first
ground into a fine flour
before adding water to produce
a mash.
After fermentation, the mash is
separated into ethanol and
stillage.
The stillage is centrifuged and
dried producing dried distillers
grains (DDG), a high-protein, highenergy livestock feed.
Dry milling is cheaper and
produces DDG, whilethe process
of wet milling is more flexible.
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Source: Cornell Univ.
Cellulose as a Feedstock
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Cellulose is another possible feedstock
for fermentation into ethanol.
Because cellulose makes up nearly
half of all plant biomass, cellulosic
ethanol is considered the largest
potential source of biofuel in the near
future.
Like starch, cellulose is made of long
chains of sugar
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A cellulose molecule is simply a polymer
of glucose
Polymer – a long chain of repeating
molecules.
Cellulose – a Pain in the Grass
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Unlike starch, cellulose is far harder to ferment for two key
reasons:
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1. There are fewer organisms
with enzymes that are
capable of breaking down
cellulose
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Without the right enzyme
you cannot degrade
a molecule biologically
(cellulase, hemicellulase, etc).
The kind of enzymes that
break down starch do not
work on cellulose because
of their differing bonds (right).
2. Cellulose molecules are packed into a tight crystalline
form and then wrapped up in lignin and hemicellulose.
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These two molecules sheath and protect cellulose from
degradation (next page)
Cellulose
Today’s Work – “Untangle &
Unpack”
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To access the cellulose in order to ferment it, we first
have to “unwrap” it from the other molecules and
“unpack” the crystals into their individual glucose
molecules.
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This process is called “pretreatment” and is currently the
most expensive part of the conversion process.
Researchers are looking for more efficient, less
expensive ways to easily separate the cellulose from
the lignin and hemicellulose. (unwrap)
Researchers are also working to find enzymes or
organisms that more efficiently chop the cellulose
into smaller pieces of glucose for fermentation.
(unpack)
Ruminant Fermentation
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In many ways, what we want to accomplish in regards to
fermentation of cellulosic ethanol is very similar to what the
rumen (the first stomach) of a cow already does.
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A cow’s rumen is filled with microbes that produce cellulase,
the enzyme that breaks down cellulose.
A cow’s rumen is the perfect environment for these
microbes to thrive
A cow’s rumen is a good
model for what we want
to be able to do with
cellulosic feedstocks for
ethanol
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The work of the rumen
produces far more energy
than it consumes
Rumen Advantages
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The top benefit of being a herbivore is the ability
to extract energy from cellulose in plant cell walls.
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Cellulose makes up about half of the dry weight of
stems, roots, and leaves.
A cow can meet almost all of its dietary needs from
the vegetation that surrounds it in a pasture.
Few animals are
better equipped to
obtain large amounts
of energy from cellulose.
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We would like to do the
same, but for fuel.
Bovine Fermentation
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Think about what a cow must do to break down
cellulose
Obtain the food (graze)
Grind the food to increase the surface area (chew)
Chemically break down the food (with enzymes in the
saliva such as amylase)
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This would known in chemistry as hydrolysis
Hydrolysis: a reaction with water that breaks larger
molecules into smaller ones (e.g. example breaking
cellulose into glucose).
Ferment the food in their rumen with microbes
Absorb and digest the carbohydrates in their stomachs
and intestines.
The steps in making cellulosic ethanol are very similar.