2. INTRODUCTION TO BAKER’S YEAST
The earliest production of Baker’s yeast was accomplished by a Dutch process in 1781
by utilizing grain mash as raw material.
Later on sugar solutions and aeration were used in Germany. The process was further
improved to avoid ethanol fermentation and to improve biomass yield.
Currently Baker’s yeast worldwide production is at the level of a millions tons per
year. The process is based on aerobic cultivation of Saccharomyces cerevisae on
carbohydrates.
3. Now a days the most widely used organism for baking is Saccharomyces cerevisae
commonly known as Baker’s Yeast.
One of the most important characteristic of Saccharomyces cerevisae that provides
this species a leading edge above other species is the special property of this species
to generate CO2 in the dough which is commercially used in Baking process.
4. Specially selected strains of Saccharomyces cerevisae are employed for the
production of Baker’s yeast.
The properties of these strains are:-
1) Stable physiological characteristics.
2)Vigorous sugar fermentation in dough called dough raising power
3) Cellular dispersion in water.
4) Good keeping quality without autolysis.
5)Rapid growth and high yield in a fermentor.
6)Maintenance of a pleasing appearance on storage.
Stock cultures of these strains are carefully maintained, and single cell
isolations are often employed to retain cultural consistency of these
characteristics.
5. PRODUCTION OF BAKER’S YEAST
The production medium for baker’s yeast contains a mixture of Cane
molasses and beet molasses and various salts, including ammonium
and phosphate salts, cornsteep liquor may be added to supply organic
nitrogen compounds.
A mixture of cane molasses and beet molasses is commonly
employed to supply the yeast with “bios”factors for growth.
Bios factors include Biotin, Pantothenic acid, and of these, Biotin is
the most critical for yeast growth that is why mixture of cane
molasses and beet molasses is used because the beet molasses is
deficient in this vitamin and the blackstrap cane molasses also at
times are deficient in vitamins particularly in Pantothenic acid and
Inositol. Thus to supply all the “bios” factors for growth a mixture
of both the molasses is used.
6. Procedure of production
A material balance for baker’s yeast formation can be written as follows:
Sucrose(200g)+NH3(10.3g)+O2(100.5g)+Minerals(7.5g)→Biomass(100g)+CO2(
140g)+H2O(78g)
Typical yields are about 0.5 gram of cells per gram of substrate and
1g of cells per gram of oxygen.
To prepare the fermentation medium ,the molasses and cornsteep
liquor are adjusted to pH 4.5to5, heated, filtered, and diluted to
provide a concentration of 0.5 to 1.5%sugar.This treatment lowers
the level of microbial contamination and clarifies the molasses.
The sugar content of the medium is purposely kept low in order to
favor cell multiplication and to inhibit ethanol production because
high carbohydrate concentrations may provoke ethanol production
even in aerobic conditions and ethanol production is inhibitory to
cell growth and reduces biomass yield ,this effect is popularly
known as the Crabtree effect.
7. To avoid the Crabtree effect the carbohydrate concentration in the bakers yeast
production is kept low so the whole process is done in Fed batch mode.
The fermentation temperature is maintained at 300 C or less, and the culture broth pH
is maintained between 4 and 5 to aid in controlling the growth of bacterial
contaminants.
At initiation of the fermentation, the culture is aerated, with the aeration rates then
increasing during the next 8 to 12 hours as the fermentation progresses.
Intermittent feeding of continuously sterilized nutrients starts when the initial
concentration of nutrient s are depleted. The feeding rates of nutrients are adjusted to
maximize growth rate.
Dissolved-oxygen monitoring is utilized to evaluate the metabolic activity of yeast
and is the basis for feed back control systems for nutrient addition.
At the end of the 8-12 hour fermentation period ,the aeration rate is decreased, and the
sugar and ammonia additions are dropped. The latter conditions then are imposed on
the yeast cells for a period of approximately 1 hour to allow maturation of the cells
before harvest
8. Growth of yeast results in a highly viscous culture broth.Consequently, mechanically
agitated fermentors are preferred over airlifts or aerated columns.
Typical reactors have a working capacity of 50 m3 to350 m3 with a height-to-
diameter ratio of 3.Vigrous aeration and agitation are required to provide oxygen for
biomass production.
Reactors are aerated in the bottom by perforated horizontal pipe spargers.
Stirred tanks utilized for yeast production typically have oxygen transfer rates of 1
mole O2/1h
Temperature is controlled by cooling coils, and pH is controlled by addition of acid
or base.
9. Harvesting of Baker’s Yeast
After 20 to 30 h of culture, the broth is centrifuged for the separation of yeast cells from the
broth. The cells are washed several times to remove inert solids.
Centrifugal separations results in a high light colored cream with up to 22% solids from yeast.
The cream is stored in agitated tanks in 2-40C,and part is used for seeding additional
fermentations .
Bakers yeast can be sold in the form of cream, in compressed form(30% yeast solids),or
dried(95% yeast solids).
Filters press or rotary vacuum filters are used to concentrate the cream. The filtered yeast may
be mixed with emulsifiers prior to being extruded into yeast cakes or packaged in large paper
bags.
Fresh baker’s yeast may be marketed as free flowing particles by adding modified starch or
micronized cellulose.
10. Various forms and uses of Yeast
Food and feed yeast-
1) Various yeasts are nutrionally good sources of protein and Vitamin B. Therefore
,yeast is commercially produced for animal feed, and it is possible that yeast may
gain greater acceptance as human food since two-thirds of world population is
underfed.
2) Saccharomyces cerevisae grown on a molasses medium can be used as food and
feed yeast, but the economics often are poor. Better economics however are
associated with Torula yeast, also known as Candida utilis or Torulopsis
utilis,because of its ability to assimilate a wide variety of carbon and nitrogen
compounds, including pentoses, under simple fermentation conditions. Candida
arborea and Oidium lactis also have been utilized to produce feed yeast on a
commercial scale.
