What is Yeast?
Fungi have been used for thousands of years to modify foods and beverages.
Bread made without yeast fungi is flat.
The addition of yeast to flat bread dough causes the dough to rise during baking.
The result is the soft texture we associate with bread.
Yeasts are used in different cultures to make other modified foods.
Yogurt, beer, and wine were invented in Europe and the Middle East.
Saki, soy sauce, miso, tempeh, ont-jom and similar products were invented in the Far East.
The distinctive flavors and textures of Camembert, Brie and blue cheeses are due to fungi.
The soft texture of bread results from Baker's yeast.
The yeast converts small amounts of sugars in the dough to carbon dioxide and alcohol.
Carbon dioxide trapped in the dough makes it rise.
The alcohol evaporates in the oven.
Fermentation is the name of the process that the yeast uses to make carbon dioxide.
Three things are required for fermentation.
First, fungi or bacteria must be present.
Second, no oxygen must be present.
Third, an organic compound must be present.
How fast fermentation occurs depends on temperature.
The starting compounds determine the products.
Sugar produces carbon dioxide and alcohol.
Other products are made by changing the fungus and starting compounds.
These products include vitamins, hormones, antibiotics, enzymes, acids and alcohol.
The yeast experiment page shows fermentation in action.
Yeast added to soda produces carbon dioxide that blows up the balloon.
The balloon keeps out oxygen.
If oxygen is present then the balloon stays limp.
If sugar is missing then the balloon does not inflate.
If the balloon stays limp, then fermentation is not happening.
Biotechnology, one of the newest businesses, is based on fermentation.
About 200 chemicals are made.
Most of the chemicals are antibiotics.
The most money is made from antibiotics and other drugs.
Fizzy Fermenting
More alcohol is made than any other chemical.
It is an important solvent.
It is also the starting point for making other chemicals.
The need to reduce gasoline use has increased the need for more alcohol.
Most is used to make gasohol that contains 10% alcohol.
Even more alcohol will be needed to make E85 fuel that is 85% alcohol.
E85 is used in flex-fuel vehicles to save even more gasoline.
To produce that much alcohol requires scaling up the process.
The pop bottle used in the yeast experiment becomes a fermenter.
Tubes for adding nutrients and paddles for stirring are added.
A computer is used to check and control conditions.
Equipment is added to extract products.
Other equipment is needed to dispose of excess heat and waste.
A large plant spreads over acres and needs rail and road service.
The world's largest fermenter was 200 feet high, 25 feet in diameter and held 736,300 gallons.
It was used to produce yeast for food.
What happens inside the pop bottle (fermenter) when yeast is grown?
At the start, oxygen and nutrients are plentiful.
Once the yeast is added, yeast cells start to grow and multiply.
Growth is slow in stage 1;
then increases rapidly in stage 2.
Growth doubles every hour in stage 3.
Growth and nutrient supply are in balance during this stage.
At the end of this stage, there are so many cells that there isn't enough oxygen or nutrients.
Growth slows down as a result (stage 4).
Growth is very limited during stage 5.
Oxygen is absent or nearly gone.
Growth declines during stage 6 when nutrients are depleted.
Different chemicals are made by the yeast in each stage of growth.
Growing cells make carbon dioxide, enzymes, amino acids and some vitamins (stages 1, 2, 3).
Enough carbon dioxide is made to put the fizz in soft drinks.
When nutrients are unbalanced (stages 4 and 5) normal growth can't occur.
Some chemicals are made; some are not made.
Some chemicals that are steps in making the final products in normal growth build up.
Antibiotics and some vitamins are released when growth stops (stage 5).
In stage 5 there isn't enough oxygen for normal respiration.
Fermentation starts and produces less energy than respiration.
Cells survive but growth stops.
Cells release alcohol and lactic acid during stage 5.
Carbon dioxide is not made.
Different fungi make different products in stage 5.
Rhizopus stolonifer, bread mold, makes a dye called carotene.
Cheddar cheese is dyed with carotene to make it orange.
Penicillium chrysogenum makes penicillin.
Penicillin was the first miracle drug.
More than 600,000 tons of citric acid (vitamin C) made by the fungus Aspergillus niger is used every year in soft drinks.
Experiments
To experiment with fungi, mycologists often need to grow them.
Simply allowing bread to become moldy is not an experiment.
An Experiment is the test of an idea.
Often, this idea is expressed in the form of the question:
What if...?
What happens when...?
What kind of effect...?
Experiments are designed to use the methods and materials that will give the most complete and accurate answer to an inquiry.
Designing Growth Experiments
Fungi break down and absorb organic material for their nourishment, so any experiment must first provide them with food.
Oxygen and moisture are also necessary.
A material for the growth of fungi for experiments is called a medium.
Most commercially prepared media for growing fungi are extracts of plant materials like potatoes.
A medium that is specially prepared to contain only the exact nutrients required by one species of fungus is called a "Minimal medium".
The choice of growth medium depends on the question that is being asked.
If the question is "What kinds of fungi grow naturally on bread?" the choice of medium is simple.
You could just put a slice of bread in a plastic bag, close it to retain moisture and await mold growth.
However, observing only one slice of bread would not make an effective experiment.
Your chosen slice may not have any mold spores on it, or contain spores of all the species present in the loaf.
It might be too dry to allow growth.
You would have to use a number of bags to account for all reasonably possible growth failures and successes.
The slices of bread would be replicates.
Replicates allow the treatment to be repeated often enough to allow you to determine if they results are significant or the product of random chance.
You will also need to decide how to record your results.
Do you identify each species of mold by its scientific name, or do you just describe them (fluffy red colonies, white fuzzy spots, blue-grass velvet, etc.)?
A more complicated question requires the design of a more complicated experiment.
At first glance, "What effect does the preservative in some breads have on mold growth?" seems as if it could be answered with a loaf of bread and some plastic bags, like the first experiment.
However, the best experiment on the effect of a preservative on mold growth would use two loaves of bread.
These loaves would be identical in preparation and ingredients, except for the presence or absence of a preservative.
The bread without the preservative would be the control and the bread containing the preservative would be the treatment.
Replication of both treatment and control gives the experimenter a way to understand the effect of substance by showing what happens when it is both present and absent.