cell overview cerevisiae

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Saccharomyces cerevisiae

in bakery industries

Name : Rinanti Pritanova

ID Number : 1-4209-026

Food Technology 4

Life Sciences

Swiss German University


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1. Introduction

The aim of this paper is to discuss about Saccharomyces cerevisiae , both about its characteristic and also

about its production process in food industries. Actually, there are many kind of purpose in using

Saccharomyces cerevisae in food industries, but this time the common used in bread making that will beexplained. The fermentations focus on a maximum biomass yield with limited ethanol production,

favouring the oxidative or aerobic metabolic yeast pathway. Bakers usually use yeast as a leavening

agent in the rising of dough for baking. A secondary contribution of yeast to bread is flavouring and

aroma.

2. Cell Overview

2.1 Name

The binomial name of this micro bacterium is Saccharomyces cerevisiae. "Saccharomyces " derives

from Latinized Greek and means "sugar mold" or "sugar fungus", saccharo- being the combining form

"sugar-" and myces being "fungus". Cerevisiae comes from Latin and means "of beer".Other names for

the organism are :

S. cerevisiae (short form of the scientific name)

Brewer's yeast, (though other species are also used in brewing)

Ale yeast

Top-fermenting yeast

Baker's yeast

Budding yeast

Figure 2.1 S. cerevisiae under DIC microscopy
http://en.wikipedia.org/wiki/Latinhttp://en.wikipedia.org/wiki/Greek_languagehttp://en.wikipedia.org/wiki/Yeast#Beerhttp://en.wikipedia.org/wiki/Baker%27s_yeasthttp://en.wikipedia.org/wiki/Baker%27s_yeasthttp://en.wikipedia.org/wiki/Yeast#Beerhttp://en.wikipedia.org/wiki/Greek_languagehttp://en.wikipedia.org/wiki/Latin


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The scientific classification for Saccharomyces cerevisiae is :

Kingdom : Fungi

Phylum : Ascomycota

Subphylum : Saccharomycotina

Class : Saccharomycetes

Order : Saccharomycetales

Family : Saccharomycetaceae

Genus : Saccharomyces

Species : S. cerevisiae

2.2 Type

Saccharomyces cerevisiae is a eukaryotic organism. It is usually spherical, oval or cylindrical in shape and

a single cell of S. cerevisiae is around 8m in diameter, around 10 m long by 5 m wide. Each cell has a

double-layered wall (thick-walled), which is permeable to certain substances and in this way food

material is taken into the cell and metabolites leave it. It reproduces by a division process known

as budding, or in some cases by division (fission). It is one of the most intensively

studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the

model bacteria.

Figure 2.2 Saccharomyces cerevisiae . Numbered ticks are 10 micrometres apart
http://en.wikipedia.org/wiki/Buddinghttp://en.wikipedia.org/wiki/Eukaryotehttp://en.wikipedia.org/wiki/Model_organismhttp://en.wikipedia.org/wiki/Molecular_biologyhttp://en.wikipedia.org/wiki/Cell_biologyhttp://en.wikipedia.org/wiki/Escherichia_colihttp://en.wikipedia.org/wiki/Escherichia_colihttp://en.wikipedia.org/wiki/Escherichia_colihttp://en.wikipedia.org/wiki/Bacteriahttp://en.wikipedia.org/wiki/Bacteriahttp://en.wikipedia.org/wiki/Escherichia_colihttp://en.wikipedia.org/wiki/Cell_biologyhttp://en.wikipedia.org/wiki/Molecular_biologyhttp://en.wikipedia.org/wiki/Model_organismhttp://en.wikipedia.org/wiki/Eukaryotehttp://en.wikipedia.org/wiki/Budding


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2.3 Usage

As it was said before, S. cerevisiae is most useful yeast owing to its use since ancient times in

baking and brewing. It is the microorganism behind the most common type of fermentation. This

species is also the main source of nutritional yeast and yeast extract .

2.4 Isolation

The yeast is believed that it was originally isolated from the skins of grapes (one can see the yeast as a

component of the thin white film on the skins of some dark-colored fruits such as plums; it exists among

the waxes of the cuticle) .

2.5 Life Cycle

There are two forms in which yeast cells can survive and grow: haploid and diploid. The haploid cellsundergo a simple life cycle of mitosis and growth, and under conditions of high stress will, in general,

die. The diploid cells (the preferential 'form' of yeast) similarly undergo a simple life cycle of mitosis

and growth, but under conditions of stress can undergo sporulation, entering meiosis and producing a

variety of haploid spores, which can proceed on to mate.

2.6 Cell Cycle

Growth in yeast is synchronised with the growth of the bud, which reaches the size of the mature cell by

the time it separates from the parent cell. In rapidly growing yeast cultures, all the cells can be seen to

have buds, since bud formation occupies the whole cell cycle. Both mother and daughter cells can

initiate bud formation before cell separation has occurred. In yeast cultures growing more slowly, cells

lacking buds can be seen, and bud formation only occupies a part of the cell cycle. The cell cycle in yeast

normally consists of the following stages G1, S, G2, and M which are the normal stages of mitosis.

Although most yeasts reproduce only as single cells, under some conditions some yeasts can form

filaments.

3. Production Overview

3.1 Substrate Used

S. Cerevisiae primarily uses molasses as its growth substrate the main constituent of which is the

disaccharide sucrose. Sucrose is composed of the two monosaccharides glucose and fructose. Glucose

degradation can proceed via two distinct pathways, depending on the availability of oxygen within the
http://en.wikipedia.org/wiki/Bakinghttp://en.wikipedia.org/wiki/Brewinghttp://en.wikipedia.org/wiki/Fermentation_(biochemistry)http://en.wikipedia.org/wiki/Nutritional_yeasthttp://en.wikipedia.org/wiki/Yeast_extracthttp://en.wikipedia.org/wiki/Wax#Vegetable_waxeshttp://en.wikipedia.org/wiki/Plant_cuticlehttp://en.wikipedia.org/wiki/Haploidhttp://en.wikipedia.org/wiki/Diploidhttp://en.wikipedia.org/wiki/Haploidhttp://en.wikipedia.org/wiki/Biological_life_cyclehttp://en.wikipedia.org/wiki/Mitosishttp://en.wikipedia.org/wiki/Diploidhttp://en.wikipedia.org/wiki/Cell_growthhttp://en.wikipedia.org/wiki/Stress_(medicine)http://en.wikipedia.org/wiki/Meiosishttp://en.wikipedia.org/wiki/Sporeshttp://en.wikipedia.org/wiki/Microbiological_culturehttp://en.wikipedia.org/wiki/Mitosishttp://en.wikipedia.org/wiki/Mitosishttp://en.wikipedia.org/wiki/Microbiological_culturehttp://en.wikipedia.org/wiki/Sporeshttp://en.wikipedia.org/wiki/Meiosishttp://en.wikipedia.org/wiki/Stress_(medicine)http://en.wikipedia.org/wiki/Cell_growthhttp://en.wikipedia.org/wiki/Diploidhttp://en.wikipedia.org/wiki/Mitosishttp://en.wikipedia.org/wiki/Biological_life_cyclehttp://en.wikipedia.org/wiki/Haploidhttp://en.wikipedia.org/wiki/Diploidhttp://en.wikipedia.org/wiki/Haploidhttp://en.wikipedia.org/wiki/Plant_cuticlehttp://en.wikipedia.org/wiki/Wax#Vegetable_waxeshttp://en.wikipedia.org/wiki/Yeast_extracthttp://en.wikipedia.org/wiki/Nutritional_yeasthttp://en.wikipedia.org/wiki/Fermentation_(biochemistry)http://en.wikipedia.org/wiki/Brewinghttp://en.wikipedia.org/wiki/Baking


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to ammonium ions. They can also use most amino acids, small peptides, and nitrogen bases as a

nitrogen source. Histidine, glycine, cystine, and lysine are, however, not readily used. S. cerevisiae does

not excrete proteases, so extracellular protein cannot be metabolized.

Yeasts also have a requirement for phosphorus, which is assimilated as a dihydrogen phosphate ion,

and sulfur, which can be assimilated as a sulfate ion or as organic sulfur compounds such as the amino

acids methionine and cysteine. Some metals, like magnesium, iron, calcium, and zinc, are also required

for good growth of the yeast.

The yeast culture is grown up to a stage where it is large enough to start the process in the factory.

Thousands of litres of yeast are produced in specially designed fermenters, fed with a molasses broth.

When growth is complete the fermented yeast is separated from the medium in which it was grown,

resulting in a creamy suspension of concentrated active yeast. Molasses is a by-product of sugar refining

and contains about 50% sugar.16 It serves as the source of carbon and energy for the process. It is

supplemented with a number of nitrogenous compounds and vitamins such as biotin16, which are

required for the proper and efficient growth of the yeast cells.

3.3 Operating Mode

In bread making, the flour used contains starch, protein and an enzyme amylase. The flour is mixed with

water to form dough. Amylase digests the starch into glucose. Lack of O 2 inside the dough causes the

yeast to respire anaerobically. As a result glucose is fermented to alcohol and CO 2 the CO produced

causes the dough to rise and because of this, cavities appear. Alcohol produced evaporates during

baking. One yeast cell can ferment approximately its own weight of glucose per hour, giving rise to large

volumes of CO 2.

The enzymes which are formed by the yeast cells and act as biological catalysts in the fermentation

process are Maltase, Invertase and Zymass complex. Maltase has the ability to convert maltose, which is

produced by starch degradation by alpha- and beta-amylases, to glucose and acts when the supply of simple sugars has been exhausted. Invertase converts sucrose to glucose and fructose, while the activity

of the zymass complex results in the conversion of glucose, fructose and other simple sugars into carbon

dioxide and ethanolAs described previously, it is carbon dioxide which causes the dough to rise.
http://en.wikipedia.org/wiki/Ammoniumhttp://en.wikipedia.org/wiki/Ionshttp://en.wikipedia.org/wiki/Amino_acidshttp://en.wikipedia.org/wiki/Peptideshttp://en.wikipedia.org/wiki/Histidinehttp://en.wikipedia.org/wiki/Glycinehttp://en.wikipedia.org/wiki/Cystinehttp://en.wikipedia.org/wiki/Lysinehttp://en.wikipedia.org/wiki/Proteaseshttp://en.wikipedia.org/wiki/Yeastshttp://en.wikipedia.org/wiki/Phosphorushttp://en.wikipedia.org/wiki/Sulfurhttp://en.wikipedia.org/wiki/Sulfatehttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Calciumhttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Calciumhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Sulfatehttp://en.wikipedia.org/wiki/Sulfurhttp://en.wikipedia.org/wiki/Phosphorushttp://en.wikipedia.org/wiki/Yeastshttp://en.wikipedia.org/wiki/Proteaseshttp://en.wikipedia.org/wiki/Lysinehttp://en.wikipedia.org/wiki/Cystinehttp://en.wikipedia.org/wiki/Glycinehttp://en.wikipedia.org/wiki/Histidinehttp://en.wikipedia.org/wiki/Peptideshttp://en.wikipedia.org/wiki/Amino_acidshttp://en.wikipedia.org/wiki/Ionshttp://en.wikipedia.org/wiki/Ammonium


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Figure 3.1 CO 2 produced by the yeast causes dough to rise

Commercial yeast production starts in the laboratory where a small quantity of a yeast culture is

injected into a closed flask containing a sterile solution of molasses, ammonium salts to provide a source

of nitrogen, and phosphate, necessary for yeast development and reproduction. The yeast culture is

made up of a particular yeast strain, which is normally kept on an agar slant. An enormous number of

strains of Saccharomyces cerevisiae exist, many of which have already been selected for baking. The

closed flask that the culture is injected into is kept at a constant temperature and the yeast grows

vigorously for 12 hours. It is then transferred to a larger flask containing a further solution of molasses

and nutrient material and more growth takes place. The transfer process is repeated again until a largeenough culture of yeast is obtained to start the main yeast production process in the factorys large

fermentation vessels. Fermentation vessels for yeast production range from 40,000 to 200,000L. The

progressive increase of fermentor size used is known as scale-up.

Until this stage the yeast cultures have been grown in the absence of air; this is known as anaerobic

fermentation. Anaerobic fermentation is, however, inefficient in terms of yeast growth, and subsequent

stages of yeast production take place with sterile air being blown/sparged through the growing yeast

cultures; this is known as aerobic fermentation. The reason why the early stages of yeast production

take place in the absence of air is to favour the growth of yeast cells instead of other organisms, such as

bacteria, which may gain access to the culture, since these would also grow rapidly and could decrease

the efficiency of the process and affect the final yeast quality. A small amount of alcohol is produced

during the early stages, which inhibits the growth of foreign organisms.
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The fermentation process continues with air being blown through the yeast cultures and molasses

solution and nutrients being added continuously, at a constantly increasing rate that is directly

proportional to the yeast cell population. By maintaining this supply level just sufficient for the amount

of yeast present, together with an adequate supply of air, maximum yeast cell reproduction takes place

with the minimum production of alcohol as indicated previously. At the end of the first stages of yeast

growth about 12 tonnes of yeast is produced and this is known as seed or mother yeast. The seed yeast

is divided into portions and these are used to start other fermentations. These fermentations are carried

on as before with increasing addition of air, molasses solution and nutrients, and each 3 tonnes of seed

yeast produces about 11 tonnes of the final bakers yeast.

Throughout the whole fermentation process stringent checks are carried out to ensure that yeast

growth and quality are maintained, so that the final 40-50 tonnes of bakers yeast are of the same quality

and have the same characteristics and properties as the original few milligrams of pure yeast culture

that started the process. At the end of the fermentation stage the yeast is present as a suspension of

cells in a dark brown liquid containing the residues of the molasses. The yeast is removed from the

fermentation liquid by a process of washing and separating in centrifugal separators, signaling the end

of the fermentation and beginning of the downstream processing stage

The fermentation of bakers yeast is strongly directed towards maximum biomass production, no

byproducts such as ethanol are desired and so the fermentations are sectioned to obtain this max

biomass.

Figure 3.3 The diagram of yeast fermentation


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3.4 Yield

The whole processing will be yielded :

Biomass

2 ethanol

CO2

H2O

The bakers yeast production mainly desire biomass as the product, and however the additional

production of ethanol is unwanted and the production of CO 2 is needed to make the dough rise.

3.5 Downstream Processing

Downstream Processing can be defined as the stages of processing that take place after the

fermentation or bioconversion stage.The yeast broth which is produced by fermentation, containing

approximately 5% solids, can be manipulated into two main types of bakers yeast product and an

additional intermediate saleable product. These are cake yeast, granular yeast and cream yeast, each of

which requires a downstream process to arrive at the desired product.

3.5.1 Cream Yeast

Cream yeast is not typically termed a bakers yeast product but is relevant a s it represents a major stepin the process and is a marketable product itself. At the end of the fermentation, the fermentor/yeast

broth is concentrated using a series of combined centrifugation and washing steps, into a yeast cream

with a solids concentration of approximately 20%. The yeast is then cooled to approximately 4C, an

ideal temperature to restrict the growth of any contaminating mesophilic microorganisms. The cooled

yeast cream is stored in a stainless steel cream tank, which is insulated and equipped with agitators and

cooling pipes, effectively preventing heat exchange with the surrounding atmosphere, keeping the

cream at 4C. Following storage either of two pathways can be followed. The first involves the

preparation for sale of the cream yeast itself. Cream Yeast is basically the liquid product and can

therefore be transferred into sterile tanks/containers and distributed to bakeries, where it is used to

produce yeast based products. The advantage of this is that it excludes any human handling and

therefore reduces the risk of contamination by handling, however due to its high (water) volume,

transport costs can be expensive. For this reason, distribution is generally confined to a particular area.


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3.5.2 Granular Yeast

Granular Yeast, also known as Instant Dried Yeast, is a form of compressed yeast. Stored cream/liquid

yeast is passed through a filter, usually a filter press or rotary vacuum filter, which removes water

increasing its solids content to approximately 30%. Salt may also be added to the cream yeast prior to

filtration to aid the removal of water. The filtered yeast is then dried using fluid-bed dryers. As the yeast

is dry it generally does not require refrigeration as the low water content reduces the risk of microbial

contamination. Emulsifiers and oils can be added at this point to texturize the yeast and aid the cutting

process. As the name implies, granular yeast is crumbled into granules, the granulation process being

carried out by a granulator. Granular broths are typically used to make restoring drinks to serve in a cup;

the practicality of granular products coming both from their instantly soluble nature and the fact that

they are easily measured.

3.5.3 Cake Yeast

The filtered and dried yeast can alternatively be used to make cake yeast. Cake yeast is another form of

compressed yeast and can be categorized as active dry yeast. It Page 9 of 12 differs from granular yeast

in that rather than granulation, the dried yeast is extruded or cut into blocks/cakes. Similar to granular

yeast cake yeast also contains about 30% solids (70% water). The composition of solids may vary

depending on the growth rate of the yeast as lower growth rates give lower protein, lower activity,

higher carbohydrate, and higher stability.12 Both types of compressed yeast are then packaged,

typically vacuum packed to reduce the risk of contamination by aerobic bacteria, and distributed to

wholesalers or traders. The shelf life of Active Dry/Cake Yeast and Instant Dry/Granular Yeast at ambient

temperature is 1 to 2 years.

3.6 Idea of Improvement

The idea of improvement on this bacteria is to try to invent this bacteria that could produce CO2 morerapidly. Research that has been carried out by the Tokyo laboratory of Yeast and Fermentation showed

that new strains of bakers yeast that produce CO2 more rapidly, are more resistant to stress or produce

proteins or metabolies that can modify bread flavor, dough rheology or shelf-life. So, the development

of baking industries could be wider and have more variety whether it is in ther flavor, texture or shelf-

life.


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The other improvement is to recycle the waste of bread and product return from bakery industries. The

waste of bread can be watering into a mash and trying to extract desirable components, for example like

liquid sour, ethanol or carbon dioxide. But this action can only be undergo in sterile conditions.

3.7Comment on the production overview

After I knew the detail process about the production process of bakery industries using saccharomyces

cerevisiae bacteria, I think this is very interesting because we could learn how really small organism

could help and plays an important role in food industries. Additionally, it has many advantages not only

in one food product, but also has function in several food products.

4. References

http://en.wikipedia.org/wiki/Saccharomyces_cerevisiae

http://www.microbiologybytes.com/video/Scerevisiae.html

http://www.dcu.ie/~oshead/BE401/lectures/pres4382018852bea.pdf

http://primephysique.com/Portals/46981/images/alcohol%20fermentation%20process.gif

http://www.desktopclass.com/education/9th-10th/genetic-engineering-and-biotechnology-lesson-

20-1.html
http://en.wikipedia.org/wiki/Saccharomyces_cerevisiaehttp://www.microbiologybytes.com/video/Scerevisiae.htmlhttp://www.dcu.ie/~oshead/BE401/lectures/pres4382018852bea.pdfhttp://primephysique.com/Portals/46981/images/alcohol%20fermentation%20process.gifhttp://www.desktopclass.com/education/9th-10th/genetic-engineering-and-biotechnology-lesson-20-1.htmlhttp://www.desktopclass.com/education/9th-10th/genetic-engineering-and-biotechnology-lesson-20-1.htmlhttp://www.desktopclass.com/education/9th-10th/genetic-engineering-and-biotechnology-lesson-20-1.htmlhttp://www.desktopclass.com/education/9th-10th/genetic-engineering-and-biotechnology-lesson-20-1.htmlhttp://primephysique.com/Portals/46981/images/alcohol%20fermentation%20process.gifhttp://www.dcu.ie/~oshead/BE401/lectures/pres4382018852bea.pdfhttp://www.microbiologybytes.com/video/Scerevisiae.htmlhttp://en.wikipedia.org/wiki/Saccharomyces_cerevisiae

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