Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
PROCESS FOR THE PRODUCTION OF YEAST
BACKGROUND OF THE INVENTION
Yeasts are the most important group of microorganisms of commercial value
worldwide.
They have been used by humans for thousands of years.
The total quantity of yeast production when brewing and fermenting and in food
production
amounts to millions of tons per year. Yeast production in the European
Community has a
turnover of 800 million Euros and secures more than 8,000 jobs, supplier
industries not even
counting.
The most important yeast from a commercial point of view is saccharomyces
cervisiae.
However, there are numerous less common yeasts with a potential for
technological
applications. Kluyveromyces lactis, for example, is an important strain of
yeast. These
microbes are used in the large-scale production of chymosin (rennin) in
fermenters; this
rennin that replaces the conventional form obtained from slaughtered animals
is widely used
today in cheese production.
Saccharomyces yeasts are generally rated as safe ("GRAS"). They produce two
important
metabolites-ethanol and carbon dioxide. Ethanol is used for the production of
alcoholic
beverages, as a fuel and solvent. Carbon dioxide is used as a leavening agent
for bread
dough, for producing carbonated beverages, as an inert gas when preserving
foods and as a
solvent for extractions. Yeast itself is used as a flavor enhancer in foods
and as a nucleotide
source in the production of substitutes for breast milk. In addition, yeast is
an important
source of vitamin B in human and animal nutrition. Sterile yeast extract is
used in the
cultivation of industrial mold fungus cultures for enzyme production or for
producing starter
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cultures. Feed yeast is used in cattle and horse feed for stabilizing an
anaerobic environment
in the rumen. Yeast modified by genetic engineering is used for producing
proteins such as
insulins that are applied in human medicine.
Yeast is always produced from a pure yeast culture that has been isolated by
selection,
breeding, or genetic modification with regard to its power in the intended
application. When
producing pure yeast cultures, the dual principle of using a bacteriologically
flawless sample
for initial inoculation and maintaining this purity over the entire yeast
production cycle.
Baker's yeast (saccharomyces cervisiae) has its origin in top-fermentation
brewer's yeast. A
pure yeast culture is obtained by selecting certain yeast strains and their
properties and by
subsequent cell multiplication on a nutrient substrate of molasses and various
additives.
Baker's yeast, for example, is characterized by high dough raising power and a
low content
of gluten-degrading enzymes. While a particular yeast strain is, of course, a
trade secret
proprietary to the yeast producer, the technological process of yeast
multiplication is
generally known.
Multi-stage cultivation is common in bioengineering to produce a large scale
of pure culture
biomass. In a one-stage cultivation process in which a large substrate volume
is inoculated
with a small quantity of the respective microorganisms, sufficient biomass
multiplication
would take a long time. This would have several disadvantages: A longer dwell
time in
which the yeast creates an optimum environment for yeast growth (reduced redox
potential,
concentration of growth stimulators), a high risk of contamination with
undesirable
organisms, and economic inefficiency.
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Yeast utilizes a wide range of carbohydrates and sugars. But there is as yet
no type of yeast
known that is capable of utilizing all the sugars available. The top-
fermentation strain
saccharomyces cerevisiae typically utilizes glucose, fructose, mannose,
galactose,
saccharose, maltose, maltotriose, and raffinose. Saccharomyces cerevisiae and
related
strains, however, cannot utilize C5 sugars such as ribose, xylose, arabinose,
or cellobiose.
The most common substrate for the cultivation of yeast is an aqueous solution
of 8 - 10% of
molasses. Molasses contains about 50% sugar. The molasses is clarified to
separate any
sludge and sterilized using high-pressure vapor. It is then diluted with water
and kept in
buffer vessels until it is needed for the fermentation process. The solution
is then set to a pH
value of about 4.5 to 5.0 using inorganic acids and enriched with minerals and
vitamins of
the B group that are needed for yeast production. Required nutrients and
minerals include
nitrogen, potassium, phosphate, magnesium, and calcium with traces of iron,
zinc, copper,
manganese, and molybdenum. The substrate is typically supplied with nitrogen
by adding
ammonium salts, ammonia liquor, or anhydrous ammonia. Phosphates and magnesium
are
added as phosphoric acid or phosphate and magnesium salts. V itamins (biotin,
inosite,
pantothenic acid, and thiamine) are also required for yeast growth.
The yeast cultures are aerated to achieve as great a biomass yield as
possible. Yeasts are
highly developed monocellular fungi. They are facultatively anaerobic: In the
presence of
air, they produce carbon dioxide and water from sugar and oxygen. This
metabolic process is
called respiration and produces a great amount of energy (ATP). Glucose is
completely
oxidized by respiration, and all the biochemical energy it contains is
released.
Glucose + Oxygen -> Carbon Dioxide + Water + Energy (38 ATP)
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This energy is used by yeast for keeping up vital functions and for
synthesizing biomass, i.e.
for yeast growth. A few other substances besides oxygen and sugar are required
for
producing biomass, in particular, nitrogen.
In the absence of oxygen, yeast can still produce energy from sugar to keep up
its vital
functions. The anaerobic metabolism of yeast has been defined as fermentation
by Louis
Pasteur. During fermentation, carbon dioxide and ethanol are produced from
sugar. Glucose
oxidation is incomplete. Ethanol contains a quantity of energy so that only a
small portion of
the biochemical energy contained in the glucose is released:
Glucose -> Carbon dioxide + Ethanol + Energy (2 ATP)
Accordingly, the multiplication of yeast cells is very limited in anaerobic
conditions.
Industrial yeast production is based on the aerobic process. Air is blown
through the solution
in which the yeast is grown to create aerobic conditions.
The problem the yeast producer is facing is not as simple as just supplying
air during the
fermentation process. If the sugar concentration in the growth medium exceeds
a minor
amount, the yeast will produce some alcohol even if the air supply is
sufficient or abundant
(Crabtree effect). This problem can be solved by adding the sugar solution to
the yeast
slowly over the entire fermentation process. The rate of adding the sugar
solution must be
selected so that the yeast uses up the sugar fast and the sugar concentration
virtually is zero
at any given point in time. This type of fermentation is called fed-batch
fermentation.
Yeast is cultivated in several stages when producing baker's yeast. For
example, yeast could
be transferred from a test-tube culture in substrate with 50 ml,
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then I liter, 10 liters, 40 liters, 400 liters, 4 m3, 10 m3, and finally 200
m3. The initial stage of
yeast growth takes place in the lab. A portion of the pure yeast culture is
mixed with
molasses wort in a sterilized flask and left to grow in for 2 to 4 days. The
entire contents of
the flask is then used to inoculate the first fermenter in the purified
culture stage. Purified
culture fermentations are discontinuous fermentations in which the yeast
multiplies in 13 to
24 hours. One to two fermenters are used in this stage of the process.
But for the fact that there are no precautions for sterile aeration and
aseptic transfer to the
next stage, purified culture fermentation is basically a continuation of the
flask fermentation
step. All growth media and nutrients are introduced to the vessel prior to
inoculation.
The next fermentation stage is a stock culture fermentation. The contents of
the intermediate
fermenter is pumped into the stock fermenter that is designed for continuous
aeration. This
stage is called inoculating or budding yeast fermentation since the yeast is
separated by
centrifuging from the major portion of the fermenter liquid after fermentation
is complete,
which yields a stock culture for inoculating the next stage. Aeration is
powerful, and
molasses and other nutrients are added in steps. The liquid from this
fermenter is typically
divided into several portions for pitching the concluding shipping yeast
cultivation process.
Alternatively, the yeast can be separated by centrifuging and stored for
several days before
use in the concluding shipping yeast cultivations.
The introduction of the fed-batch process into yeast production marks the
beginning of
modern yeast industry. Fed-batch production was introduced in Germany in 1915,
and
today's yeast production is still relying on the fed-batch process. In a fed-
batch fermentation
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process, molasses, phosphoric acid, ammonia and minerals of yeast are added at
a controlled
rate. The rate is selected so that the yeast is fed just that quantity of
nutrients and sugar
needed to maximize multiplication and prevent the production of alcohol.
At the start of each batch, a relatively considerable quantity of process
water is transferred to
the fermenter together with the budding yeast. The optimum quantity of sugar
to be added
per gram of yeast and per hour is predefined. Molasses is added to the
fermenter at a rate that
corresponds to the predetermined quantity of sugar to be added.
When making baker's yeast, a basic supply of 16 to 18 g/l is common. Nitrogen
and
phosphorus can either be fed at a specific rate or intermixed with the process
water prior to
adding molasses. The quantities are determined by the weight ratio of sugar,
nitrogen, and
phosphorus. The yeast culture has ripened after 8 to 16 hours of cultivation.
2.5 g of nitrogen
and 5.0 g of phosphorus or 0.3 g of nitrogen and 0.5 g of phosphorus per 100 g
of sugar are
required for yeast production.
The cultivation temperature is typically between 25 C and 35 C, the pH value
is determined
by the quantities of nitrogen added. Budding yeast is produced at a pH value
between 4.5
and 5.0; shipping yeast is produced at a pH value between 5.0 and 7Ø
The concluding shipping yeast production has the highest degree of aeration,
and molasses
and other nutrients are added in steps. Considerable quantities of air must be
supplied to the
concluding shipping yeast production process so that these vessels are often
started in a
staggered pattern to reduce the size of the air compressors. The final
fermentation stages take
around 11 to 15 hours. After adding all the molasses required to the
fermenter, the liquid is
aerated for another 0.5 to 1.5 hours to allow further ripening of the yeast,
which makes the
yeast
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more stable for refrigerated transport. This method also ensures that small
quantities of
ethanol are removed by respiration.
At the end of the fermentation process, the fermenter broth is centrifuged
off, washed with
water and re-centrifuged, yielding a yeast milk with a solids concentration of
about 18%.
The yeast milk is cooled down to about 8 C and stored in a separate,
refrigerated yeast milk
container made of stainless steel. The yeast milk can directly be loaded onto
tankers and
carried to the customers that have a respective yeast milk handling system.
Before 1825,
yeast was only available as yeast milk. Tebbenhof introduced pressed yeast in
1825. Yeast
milk production became popular again when wholesale bakeries for large-scale
output
developed.
Alternatively, yeast can be centrifuged, and the solid yeast can be further
concentrated using
a filter press or rotary vacuum filter. A filter press will yield a filter
cake containing 27 to 32
percent of solids. A rotary vacuum filter will yield a filter cake containing
about 33 percent
of solids.
The filter cake is than intermixed in mixers with small quantities of water,
emulgators, and
flux oils to become the final product. Emulgators are added in the production
of pressed
yeast to give the yeast a white, creamy appearance and to inhibit the
formation of water
stains on the yeast cake. A small quantity of oil, typically soy bean or
cottonseed oil, is
added to enhance extrusion of the yeast through nozzles into endless strips of
yeast cake.
These strips are cut, and the yeast cakes are packaged and cooled down to a
temperature of
less than 8 C whereupon they are ready for transport on a refrigerated truck.
100 kg to 250
kg of budding yeast and one ton of molasses are required to produce one ton of
pressed yeast
with a dry-matter content of 27% (Y27).
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When producing active dry yeast, pressed yeast is molded into cylinders that
are dried in a
fluidized-bed drier. Active dry yeast can be stored at room temperature.
The molasses substrate is replaced with starch substrates when producing
organic yeast.
Wheat, corn, or potato starch are dextrinized into a sugar solution using
technical enzymes.
Organic baker's yeast produces less carbon dioxide since the final product
contains about
30% of starch.
Fed-batch fermentations are not completely sterile. It is inefficient to use
pressure vessels for
ensuring the sterility of the huge volumes of air needed in these fermenters
or for creating
sterile conditions for all transfer processes through numerous lines, pumps,
and centrifuges.
The equipment is thoroughly cleaned, pipes and containers are charged with
steam, and the
air is filtered to ensure conditions are as aseptic as possible. However, it
is almost impossible
to avoid a certain form of microbial contamination, and the presence of
bacteria such as
leuconostoc, pediococcus, aerococcus, bacillus, lactococcus and e.coli have
been reported.
The fed-batch stages are cultivated for a limited period of time (10 to 20
hours). The
relatively short cultivation period and a large yeast inoculum are to prevent
foreign
organisms from growing into a visible infection. Presently, controlling
infection seems to be
limited to physical methods such as the use of wash separators for removing as
many
bacteria as possible with the yeast wort due to the difference in density when
harvesting
yeast cells.
Another preventive measure occasionally proposed is to reduce the pH value but
optimum
yeast growth can only be achieved at a pH value around 6. Sometimes sodium
chloride is
used to support the preservation of pressed yeast over a longer period of
time.
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Controlling infection in yeast production is a must for ensuring the purity of
the yeast
culture. Bacterial contamination is particularly interfering when yeast is
used in fermentation
processes such as in beer or ethanol production. In this case, the yeast could
be the source of
infection for the fermentation process. Yeast producers are fully aware that
the number of
bacterial contaminations per gram of yeast milk, pressed yeast, or active dry
yeast is a
decisive factor determining the quality of the yeast.
This invention provides a method of controlling bacterial contamination during
the
production and storage of yeast.
U.S. patent publication 6,326,185 B1 describes a method of decontaminating
brewer's yeast
used in fermentation so that this yeast can be used in subsequent
fermentations. The yeast is
brought into contact with tetrahydroiso-a acids up to a final concentration of
40 ppm while
the pH value of the mixture is set to about 2.0 to 2.6. The mixture is then
kept at a specific
temperature over a specific period of time.
U.S. patent publication 1,477,132 relates to a yeast composition, in
particular a yeast
composition for rice fermentation, and a production method therefor. Cooked
rice is
intermixed with hop juice. The rice is treated with yeast so that the yeast
can grow. Hop
bitter acid is added to prevent oxidation, degradation and other toxic
metabolic processes
caused by bacteria.
WO 2004/072291 A2 relates to the use of hop acids in bioethanol production for
the
inhibition of microorganisms. This application only mentions in general that
the method for
controlling microorganisms using hop bitter acid can also be applied when
multiplying yeast.
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It is the problem of this invention to provide a method for the commercial
production of
yeast in which the activity of microorganisms can be prevented or at least
decreased in a
relatively cost-efficient manner.
It is another problem of this invention to provide a method for the commercial
production of
yeast in which physical methods of decreasing the activity of unwanted
microorganisms in
yeast can be avoided or at least be simplified.
It is another problem of this invention to provide a method for the commercial
production of
yeast in which remedial building work, such as pressure vessels at the stage
of fed-batch
production of the yeast, can be avoided or at least be simplified.
It is another problem of this invention to provide a method for the commercial
production of
yeast in which the required intensity of cleaning the equipment can be
reduced.
It is another problem of this invention to provide a method for the commercial
production of
yeast, in particular shipping yeast, in which the use of chemical antifoaming
agents can be
reduced or avoided.
BRIEF SUMMARY OF THE INVENTION
According to the invention, yeast is brought into contact with a sufficient
quantity of an
auxiliary processing agent that is either a natural substance or derived from
a natural
substance for inhibiting or at least decreasing the activity of microorganisms
during the
commercial production of budding and/or shipping yeast. This measure can
effectively
replace other expensive measures aimed at inhibiting or at least decreasing
the activity of
unwanted microorganisms.
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The auxiliary processing agent that is either a natural substance or derived
from a natural
substance is selected from at least one member of the group including hop
bitter acid,
colophonium, and myristic acid or derivatives thereof. All these acids have a
bactericidal
effect.
It is preferred that the auxiliary processing agent that is either a natural
substance or derived
from a natural substance is added to the carbohydrate substrate for
cultivation prior to adding
the yeast culture.
It is preferred that the auxiliary processing agent that is either a natural
substance or derived
from a natural substance is added prior to any heat treatment of the
substrate. It is preferred
that the auxiliary processing agent that is either a natural substance or
derived from a natural
substance is added continuously.
In addition, or alternatively, the auxiliary processing agent that is either a
natural substance
or derived from a natural substance is added before the step of producing
budding yeast.
Since the budding yeast is washed before it is transferred to the process step
of producing
shipping yeast, hop bitter acid and, in particular, tetrahydroiso-a acid wit
its bitter taste, can
surprisingly be used without any negative effect on the taste of the finished
yeast product.
In addition, or alternatively, the auxiliary processing agent that is either a
natural substance
or derived from a natural substance is added to the step of producing shipping
yeast.
In addition, or alternatively, the auxiliary processing agent that is either a
natural substance
or derived from a natural substance is added to the step of producing yeast
milk.
Depending on the pH value of the respective production step, specific acids or
derivatives
thereof are provided for the best effect.
It is preferred that the auxiliary processing agent that is either a natural
substance or derived
from a natural substance is added discontinuously, in other words, as a shock
dose,
preferably at the beginning of the production step. The concentration
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of the auxiliary processing agent that is either a natural substance or
derived from a natural
substance is higher at the beginning of the production step and decreases over
time. If
microorganisms are introduced to a production step by a contaminated nutrient
or
contaminated equipment and would thus interfere with the yeast production
process, such
activity by microorganisms is prevented from the start, and the yeast
production process will
neither be impaired by the initial contamination nor by contamination
introduced later. Since
the auxiliary processing agent that is either a natural substance or derived
from a natural
substance is added as a shock dose at the beginning of the production step,
its concentration
is diluted in the course of the yeast production process so that the bitter
taste of the hop bitter
acid is decreased or completely avoided.
This is particularly true if the production process of budding yeast is a fed-
batch method.
The concentration of the auxiliary processing agent that is either a natural
substance or
derived from a natural substance decreases in the course of the fed-batch
process.
The auxiliary processing agent that is either a natural substance or derived
from a natural
substance is preferably selected from at least one member of the group of hop
bitter acids
that includes alpha acids, beta acids, rho-iso-a acids, iso-a acids,
hexahydroiso-a acids,
tetrahydroiso-a acids, and hexahydro-beta acids.
It is preferred that tetrahydroiso-a acids or beta acids are added alone or in
combination
during the process step of producing budding yeast as the auxiliary processing
agent that is
either a natural substance or derived from a natural substance. Tetrahydroiso-
a acids are the
most effective hop compound at the pH value of 4 to 5 that is typically found
when
producing budding yeast. This is why it is most useful to add tetrahydroiso-a
acids in this
process step.
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The initial concentration of tetrahydroiso-a acids is in the range from 1-
1000 ppm,
preferably 10 - 100 ppm, preferably 20 - 60 ppm, and most preferably 15 - 30
ppm. This
concentration provides the bactericidal effect. On the other hand, the hop
bitter acid is
sufficiently diluted throughout the production process that it does not impair
the taste of the
yeast.
The final concentration of tetrahydroiso-a acids in the treated container is
in the range from
- 20 ppm, preferably 3 - 15 ppm, and most preferably 2 - 10 ppm, which is
sufficient for
inhibiting bacteria growth.
Beta acids are preferably added as an auxiliary processing agent that is
either a natural
substance or derived from a natural substance to the step of producing
shipping yeast. It has
been found that beta acids contribute to the prevention of yeast cell
flocculation caused by
electrochemical interaction with bacteria. In addition, beta acids are best
qualified for use in
the shipping yeast production step because they are most effective at a pH
value of 5 to 7, the
typical pH value at the step of producing shipping yeast. It has also been
determined that
beta acids have an antifoaming effect in the production step of shipping
yeast. The use of
beta acids that are added as an auxiliary processing agent that is either a
natural substance or
derived from a natural substance in the step of producing shipping yeast
therefore helps to
reduce or avoid the use of chemical antifoaming agents.
The initial concentration of beta acids is in the range from 1- 1000 ppm,
preferably 10 - 100
ppm, and most preferably 40 - 60 ppm.
The final concentration of beta acids is in the range from 8 - 300 ppm,
preferably 3 - 30 ppm,
and most preferably 1- 12 ppm.
Alternatively or in addition, colophonium and/or myristic acid may be added as
an auxiliary
processing agent that is either a natural substance or derived from a natural
substance during
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the production of shipping yeast. These acids reach their best performance at
a pH of 5.0 to
7.0 as well.
For best results, the initial concentration of colophonium and/or myristic
acid is in the range
from 1- 1000 ppm, preferably 5 - 500 ppm, and most preferably 10 - 100 ppm.
It has been found that colophonium and/or myristic acid can also be added to
the
carbohydrate substrate at the beginning of the yeast production process.
It is preferred that the pH value during this step of producing shipping yeast
does not drop
below 5.0, more preferably not below 6Ø
DESCRIPTION
Embodiments of the method according to the invention are described below with
reference to
the figures. Wherein:
Fig. 1 shows an example of the production of baker's yeast on molasses
substrate;
Fig. 2 represents a diagram showing the inhibition of bacteria growth in MRS
broth at
pH 5 using tetrahydroiso-a acids and beta acids.
Fig. 1 shows a diagrammatic view (flowchart) of the production process of
baker's yeast. A
mixture of sugar cane and beet molasses is sterilized by heat treatment with
high-pressure
steam, and its pH value is set by acidification with sulphuric acid. The
molasses is clarified
to remove any sludge. Clarified, sterile molasses is used as so-called
carbohydrate substrate
to produce yeast biomass.
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According to a first embodiment of the invention, hop bitter acids, in
particular,
tetrahydroiso-a acids, are added to the clarified sterile molasses (see line A
in Fig. 1) to
prevent any activity by gram-positive bacteria, especially in the subsequent
processes. The
substrate typically is an aqueous solution containing 8 - 10% molasses. The
molasses
contains about 50% sugar.
The clarified sterile molasses is introduced to the purified culture fermenter
in which the
molasses is intermixed with process water and purified culture.
According to another embodiment of the invention, hop bitter acids, in
particular,
tetrahydroiso-a acids, are added to the purified culture fermenter (see line B
in Fig. 1) to
prevent or at least decrease the activity of unwanted microorganisms.
The mixture is then transferred from the aqueous molasses solution and the
purified yeast
culture to a budding yeast fermenter. In the budding yeast fermenter, the
solution is set to a
pH value of about 4.0 to 5.0 and enriched with minerals and vitamins of the B
group that are
needed for proper yeast growth. Required nutrients and minerals include
nitrogen,
potassium, phosphate, magnesium, and calcium with traces of iron, zinc,
copper, manganese,
and molybdenum. The substrate is typically supplied with nitrogen by adding
ammonium
salts, ammonia liquor, or anhydrous ammonia to the input material. Phosphates
and
magnesium are added as phosphoric acid or phosphate and magnesium salts.
The yeast cultures in the budding yeast production process are also aerated to
achieve a
maximum biomass yield.
According to another embodiment of the invention, hop bitter acids are
introduced to the
budding yeast fermenter (see line C in Fig. 1) to prevent or at least decrease
the activity of
unwanted microorganisms.
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These acids are preferably added as a shock dosage at the beginning of the
budding yeast
fermentation process.
The final concentration of the hop bitter acids, in particular, tetrahydroiso-
a acids, is between
and 10 ppm. It has been determined that tetrahydroiso-a acids are most
effective in
suppressing or at least decreasing the activity of microorganisms in the
budding yeast
production step. Even small quantities of tetrahydroiso-a acids are sufficient
to achieve the
desired result. The budding yeast production process is carried out as a fed-
batch process in
which molasses, phosphoric acid, ammonia, and minerals are added to the yeast
at a
controlled rate. Thus the initial concentration of hop bitter acids decreases
in the course of
the fed-batch production process. As a result, the hop bitter acid
concentration in the
fermenter decreases and any bitterness of the final product that might be
caused by the hop
bitter acid is surprisingly prevented after the washing step.
The budding yeast is then washed in the budding yeast wash separators where
hop bitter
acids, i.e. tetrahydroiso-a acids, are partially removed, which in addition
prevents any
bitterness in the final product.
The budding yeast is then stored and prepared for the production of shipping
yeast in
shipping yeast fermenters. According to another embodiment of the invention,
hop bitter
acids, in particular, beta acids, are added in the step of producing shipping
yeast (see line D
in Fig. 1). Since the pH value in the production step of shipping yeast is in
the range from 5
to 7, beta acids are most effective here. The use of beta acids in the
shipping yeast
production process also provides an antifoaming effect. As a result, the use
of chemical
antifoaming agents can be decreased or even avoided.
In addition, adding beta acids has the advantageous effect that flocculation
of yeast cells that
is often caused by electrochemical interaction with bacterial yeast cells
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can be prevented since beta acids prevent adhesion of such unwanted
microorganisms to the
surface of the yeast cells.
After completion of the shipping yeast fermentation process, the shipping
yeast is washed
and further processed into yeast milk, pressed yeast, or eventually dry yeast.
According to another embodiment of the invention not illustrated in Fig. 1,
auxiliary
processing agents that are either a natural substance or derived from a
natural substance may
be added to yeast milk to fight any activity of unwanted microorganisms as may
be caused
by re-infection with the goal to extend the storage quality of the shipping
yeast product.
It goes without saying that the invention covers, according to the invention,
the addition of
hop bitter acids in the steps A, B, C, and D in combination (A, B, C and D),
as individual
measure (e.g. C only), or in partial combination (such as C and D).
Alternatively or in addition to hop bitter acid, colophonium and/or myristic
acid can be
added as an auxiliary processing agent that is either a natural substance or
derived from a
natural substance in the shipping yeast production step (see line D in Fig.
1).
Fig. 2 reveals the inhibiting effect of tetrahydroiso-a acids at a
concentration of about 5 ppm
on the activity of bacteria. It has been found that beta acids become less
effective as
compared to tetrahydroiso-a acids at a lower pH value, which means that about
triple the
amount of beta acid as compared to tetrahydroiso-a acid has to be applied to
achieve a
similar efficiency.
