Yeasts
can grow in the presence or absence of air. Anaerobic growth, growth in the
absence of oxygen, is quite slow and inefficient. For instance, in bread dough,
yeast grow very little. Instead, the sugar that can sustain either fermentation
or growth is used mainly to produce alcohol and carbon dioxide. Only a small
portion of the sugar is used for cell maintenance and growth. In contrast,
under aerobic conditions, in the presence of a sufficient quantity of dissolved
oxygen, yeast grow by using most of the available sugar for growth and
producing only negligible quantities of alcohol.
This means that
the baker who is interested in the leavening action of carbon dioxide works
under conditions that minimize the presence of dissolved oxygen. On the other
hand, a yeast manufacturer that wants to produce more yeast cell mass, works
under aerobic conditions by bubbling air through the solution in which the
yeast is grown.
The
problem posed to the yeast manufacturer, however, is not as simple as just
adding air during the fermentation process. If the concentration of sugar in
the fermentation growth media is greater than a very small amount, the yeast
will produce some alcohol even if the supply of oxygen is adequate or even in
abundance. This problem can be solved by adding the sugar solution slowly to
the yeast throughout the fermentation process. The rate of addition of the
sugar solution must be such that the yeast uses the sugar fast enough so that
the sugar concentration at any one time is practically zero. This type of
fermentation is referred to as a fed-batch fermentation.
The
baker’s yeast production process flow chart attached below can be divided
into four basic steps. In order these steps are, molasses and other raw
material preparation, culture or seed yeast preparation, fermentation and
harvesting and filtration and packaging. The process outlined in the flow chart
takes approximately five days from start to finish.
The
basic carbon and energy source for yeast growth are sugars. Starch can not be
used because yeast does not contain the appropriate enzymes to hydrolyze this
substrate to fermentable sugars. Beet and cane molasses are commonly used as
raw material because the sugars present in molasses, a mixture of sucrose,
fructose and glucose, are readily fermentable. In addition to sugar, yeast also
require certain minerals, vitamins and salts for growth. Some of these can be
added to the blend of beet and cane molasses prior to flash sterilization while
others are fed separately to the fermentation. Alternatively, a separate
nutrient feed tank can be used to mix and deliver some of the necessary
vitamins and minerals. Required nitrogen is supplied in the form of ammonia and
phosphate is supplied in the form of phosphoric acid. Each of these nutrients
is fed separately to the fermentation to permit better pH control of the
process. The sterilized molasses, commonly referred to as mash or wort, is
stored in a separate stainless steel tank. The mash stored in this tank is then
used to feed sugar and other nutrients to the appropriate fermentation vessels.
Baker’s
yeast production starts with a pure culture tube or frozen vial of the
appropriate yeast strain. This yeast serves as the inoculum for the pre-pure
culture tank, a small pressure vessel where seed is grown in medium under
strict sterile conditions. Following growth, the contents of this vessel are
transferred to a larger pure culture fermentor where propagation is carried out
with some aeration, again under sterile conditions. These early stages are
conducted as set-batch fermentations. In a set-batch fermentation all the
growth media and nutrients are introduced to the tank prior to inoculation.
From
the pure culture vessel, the grown cells are transferred to a series of
progressively larger seed and semi-seed fermentors. These later stages are
conducted as fed-batch fermentations. During a fed-batch fermentation,
molasses, phosphoric acid, ammonia and minerals are fed to the yeast at a
controlled rate. This rate is designed to feed just enough sugar and nutrients
to the yeast to maximize multiplication and prevent the production of alcohol.
In addition, these fed-batch fermentations are not completely sterile. It is
not economical to use pressurized tanks to guarantee sterility of the large
volumes of air required in these fermentors or to achieve sterile conditions
during all the transfers through the many pipes, pumps and centrifuges.
Extensive cleaning of the equipment, steaming of pipes and tanks and filtering
of the air is practiced to insure as aseptic conditions as possible.
At the
end of the semi-seed fermentation, the contents of the vessel are pumped to a
series of separators that separate the yeast from the spent molasses. The yeast
is then washed with cold water and pumped to a semi-seed yeast storage tank
where the yeast cream is held at 34 degrees Fahrenheit until it is used to
inoculate the commercial fermentation tanks. These commercial fermentors are
the final step in the fermentation process and are often referred to as the
final or trade fermentation.
Commercial
fermentations are carried out in large fermentors with working volumes up to
50,000 gallons. To start the commercial fermentation, a volume of water,
referred to as set water, is pumped into the fermentor. Next, in a process
referred to as pitching, semi-seed yeast from the storage tank is transferred
into the fermentor. Following addition of the seed yeast, aeration, cooling and
nutrient additions are started to begin the 15-20 hour fermentation. At the
start of the fermentation, the liquid seed yeast and additional water may
occupy only about one-third to one-half of the fermentor volume. Constant
additions of nutrients during the course of fermentation bring the fermentor to
its final volume. The rate of nutrient addition increases throughout the
fermentation because more nutrients have to be supplied to support growth of
the increasing cell population. The number of yeast cells increase about five-
to eight-fold during this fermentation.
Air is
provided to the fermentor through a series of perforated tubes located at the
bottom of the vessel. The rate of airflow is about one volume of air per
fermentor volume per minute. A large amount of heat is generated during yeast
growth and cooling is accomplished by internal cooling coils or by pumping the
fermentation liquid, also known as broth, through an external heat exchanger.
The addition of nutrients and regulation of pH, temperature and airflow are
carefully monitored and controlled by computer systems during the entire
production process. Throughout the fermentation, the temperature is kept at
approximately 86 degrees Fahrenheit and the pH in the range of 4.5-5.5.
At
the end of fermentation, the fermentor broth is separated by nozzle-type
centrifuges, washed with water and re-centrifuged to yield a yeast cream with a
solids concentration of approximately 18%. The yeast cream is cooled to about
45 degrees Fahrenheit and stored in a separate, refrigerated stainless steel
cream tank. Cream yeast can be loaded directly into tanker trucks and delivered
to customers equipped with an appropriate cream yeast handling system.
Alternatively, the yeast cream can be pumped to a plate and frame filter press
and dewatered to a cake-like consistency with a 30-32% yeast solids content.
This press cake yeast is crumbled into pieces and packed into 50-pound bags
that are stacked on a pallet. The yeast heats up during the pressing and
packaging operations and the bags of crumbled yeast must be cooled in a
refrigerator for a period of time with adequate ventilation and placement of
pallets to permit free access to the cooling air. Palletized bags of crumbled
yeast are then distributed to customers in refrigerated trucks.