Unit, 3, AQA, Microorganisms, (Part 1)

Here are my notes paraphrased from CGP biology book on microorganisms.

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Willliam Matcham
Biology 3 ­ Microorganisms
Food and drink from Microorganisms
Microorganisms such as bacteria cause changes in food. Often these changes can be bad, but sometimes
we can use them to our advantage. Microorganisms such as yeast are used as part of the production of
some foods, including cheese and bread.
The Theory of biogenesis has been developed over the years
1. People used to believe that life could somehow spontaneously generate from non-living material.
2. But evidence showed that this was impossible. The evidence of the time suggested that living
organisms are created from other living organisms through reproduction ­ this is the theory of
biogenesis.
3. Here is how ideas changed with time to fit the available evidence:
Before 1765, it was thought that substances
in food changed into microbes, which was
what caused the food to go off. A scientist
called Lazzaro Spallanzani boiled two sets of
broth to kill all microbes in the food, then
sealed one of the flasks and left the other
one open. The open one went off
This showed that microbes got into the food
from the air, but people thought this meant
that air simply kicked off the reaction in the
broth. The theory that `fresh' air caused
substances in food to change into microbes
was disproved by Theodor Schwann in 1837.
He proved that meat would not go off in air,
provided that the air was heated first to kill
the microorganisms.
A more conclusive experiment was
conducted by Louis Pasteur in 1859.
He heated broth in two flasks, both of which
were left open to the air. However, one flask
had a curved neck, so that the bacteria would
settle in the loop and not get into the broth.
The broth in the flask with the curved neck
stayed fresh, proving that it was the
microbes and not the air causing it to go off.
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Most Cheese is made using Bacteria:
1. A culture of bacteria is added to milk.
2. The bacteria produce solid curds in the milk.
3. The curds are separated from the liquid whey.
4. More bacteria can be added here to the curds.
5. The lot is left to ripen for a period of time.
6. Moulds are added to give blue cheese its colour and taste.…read more

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5. The sugary solution is fermented by yeast, turning the sugar into alcohol.
In wines, the yeast uses the natural sugar in grape juice as the energy source.…read more

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