Yeast is a single-celled fungus that is used for fermentation. It was first observed microscopically in 1680 and in 1857 Pasteur proved it was responsible for alcoholic fermentation. Yeast is used to make bread rise through fermentation, producing carbon dioxide and ethanol. It is also used to make beer, wine, nutritional supplements, probiotics, industrial ethanol, and is an important model organism for scientific research.

1. Yeast
Saccharomyces cerevisiae

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Introduction
•In 1680, Dutch naturalist Anton van Leeuwenhoek first microscopically
observed yeast.
•In 1857, French microbiologist Louis Pasteur proved in the paper "Mémoire
sur la fermentation alcoolique," that alcoholic fermentation was conducted by
living yeasts and not by a chemical catalyst.
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What are Yeasts
• Yeasts are eukaryotic , single-celled
microorganisms classified as members of the
fungus kingdom.
• 1,500 species are currently identified( 1% of
all fungal species).
• Yeast sizes vary greatly typically measuring 3–
4 µm in diameter
• The most common mode of vegetative
growth in yeast is asexual reproducing by
budding.
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• Yeasts do anaerobic (in absence of
oxygen) cell respiration (fermentation)
to produce ethanol and carbon dioxide
from sugar (glucose).
• C6H12O6 → 2 C2H5OH + 2 CO2
Fermetation process
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Importance & Uses of yeast
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Yeasts have a lot of economic value to mankind for example :
• Bread making (Baking)
• Alcoholic beverages
• Nutrition and Probiotics
• Industrial ethanol production
• Scientific research

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• Records of using yeast date back to the
ancient Egyptians, being used in baking as a
leavening agent, where it converts the
fermentable sugars present in dough into the
gas carbon dioxide.
• Yeast not only helps bread dough rise, but it
also aids in developing and strengthening
proteins in the dough and adds to its flavor.
• Yeast may be best known as the agent in
bread that makes it rise. When you add yeast
to your flour and water mixture, it breaks
down the large starch molecules into simple
sugars. During this process, the yeast creates
carbon dioxide and ethyl alcohol, which make
the air bubbles that causes the bread dough
to grow.
Bread Making
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Beer & Wine making
• production of beer is by steeping a starch source in water
and fermenting the resulting sweet liquid with yeast.
• The type of yeast used will affect the type and flavor of
beer produced.
• Commonly used Yeast is S. Cerevisiae .
• In Wine making fermentation can be from naturally
occurring yeasts on the skin of the grape or using a
starter culture of Saccharomyces cerevisiae ‘in which case
the juice is inoculated with populations of yeast’’.
Brewing
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Nutrition & Probiotics
• Yeast is used in nutritional supplements,
especially those marketed to vegans.
• It is often referred to as "nutritional
yeast" when sold as a dietary
supplement yaest is usually S.cerevisiae.
• It is an excellent source of protein and
vitamins, especially the B- complex
vitamins
Probiotics
• Some probiotic supplements use the
yeast S. boulardii to maintain and
restore the natural flora in the
gastrointestinal tract.
Nutrition

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Industrial ethanol production
• In the industry of fermentation , yeast is used not only in
food industries to make bread, wine and beer, but also in
non-food industries, such as the Biofuel industry, to
produce ethanol.
• Bioethanol is the principle fuel used as a petrol substitute
for road transport vehicles which is a clear colourless liquid,
biodegradable, low in toxicity and causes little
environmental pollution .

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Scientific research
• Yeast are Model organisms in genetics as it is one of the simplest eukaryotic
• Studying the biology of yeasts has enabled scientists to work out the connections
between genes and proteins, and the functions they carry out in our cells by studying
their homologues in the yeast also the study of fundamental cellular processes such
as the cell cycle, DNA replication,Recombination,cell division and metabolism.
• Genetic manipulation in yeast is easy and cheap compared to similar experiments in
more complex animals
• Various yeast species have been genetically engineered to efficiently produce various
drugs, About 20% of biopharmaceuticals are produced in S. cerevisiae, including
insulin, vaccines for hepatitis, and human serum albumin

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