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Biology 3 (ii) -
Microorganisms…read more

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Food and Drink from Microorganisms
The theory of biogenesis has been developed over the years. People used to think that life could
spontaneously generate from non-living material. Evidence supported the theory that living things are
created from other living organisms ­ this is the theory of biogenesis. The theory was changed to fit the
available evidence:
Before 1765 it was believed that substances in food were changed into microbes, which caused the food to
go off. A scientist called Lazzaro Spallanzani boiled two sets of broth to kill the microbes, then sealed one
flask and left the other open. Only the open one went off. This showed that microbes got into the food
from the air but opponents just thought that it meant air from outside the flask was necessary to start the
change. The theory that `fresh' air caused substances in food to change into microbes was disproved by
Theodor Schwaan in 1837. He showed that meat would not go off in air, provided the air was heated first to
kill microorganisms. A more conclusive experiment was carried out by the famous scientist Louis Pasteur in
1859. He heated broth in two flasks, both of which were left open to the air. However, one of the flasks had
a curved neck so that bacteria in the air would settle in the loop, and not get through to 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.
Most cheese is made using bacteria. Here's what happens: 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
are sometimes added to the curds, and the whole lot is left to ripen. 5) moulds are added to give blue
cheese its colour and taste.
Yoghurt is made using bacteria. Bacteria are used to clot milk during the manufacture of yoghurt. 1) The
milk is often heat treated first to kill off any bacteria, then cooled. 2) a starter culture of bacteria is added.
The bacteria ferment the lactose sugar (present in the milk) to lactic acid. 3) the acid causes the milk to clot
and solidify into yoghurt. 4) Sterilised flavours (e.g. fruit) are sometimes added.…read more

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Using Yeast
Yeast is a single-celled fungus. A yeast cell has a nucleus, cytoplasm, a vacuole, and a cell
membrane surrounded by a cell wall.
Yeast can respire with or without oxygen. This is the anaerobic equation (process
Yeast can also respire aerobically producing more energy to grow and reproduce:
Yeast is used in dough to produce nice, light bread. The yeast converts sugars to carbon
dioxide and some ethanol. It is the carbon dioxide that makes the bread rise. As the carbon
dioxide expands, it gets trapped in the dough, making it lighter.
Yeast is used to make beer. 1) beer is made from grain ­ usually barley. 2) the barley grains
are allowed to germinate for a few days, during which the starch in the grains are dried in a
kiln. This process is called malting. 3) the malted grain is mashed up and water is added to
produce a sugary solution with lots of bits in it. This is then sieved to remove the bits. 4) Hops
are added to the mixture to give the beer its bitter flavour. 5) The sugary solution is then
fermented by yeast, turning the sugar into alcohol.…read more

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Microorganisms in Industry
Microorganisms are grown in fermenters on a large scale. A fermenter is a big container full of liquid culture
medium which microorganisms can grow and reproduce in. The fermenter needs to give the microorganisms the
conditions they need to grow and produce their useful product. (see page 85 for fermenter diagram). 1) food is
provided in the liquid culture medium. More can be pumped in if needed. 2) air is piped in to supply oxygen to the
microorganisms. 3) the microorganisms need to be kept at the right temperature. The microorganisms produce
heat by respiration, so the fermenters must be cooled. This is usually done with water in a water-cooled jacket. The
temperature is monitored by instruments. 4) the right pH is needed for the microorganisms to thrive. Instruments
will monitor this. 5) sterile conditions are needed to prevent contamination from other microorganisms. 6) the
microorganisms need to be kept from sinking to the bottom. A motorised stirrer keeps them moving around and
maintains an even temperature.
Mycoprotein ­ food from fermenters. Mycoprotein means protein from fungi. It's a type of single-celled protein.
Mycoprotein is used to make meat substitutes for vegetarian meals ­ Quorn. A fungus called Fusarium is the main
source of mycoprotein. The fungus is grown in fermenters, using glucose syrup as food. The glucose syrup is
obtained by digesting maize starch with enzymes. The fungus respires aerobically, so oxygen is supplied, together
with nitrogen (as ammonia) and other minerals. It's important to prevent other microorganisms growing in the
fermenter. So the fermenter is initially sterilised using steam. The incoming nutrients are heat sterilised and the air
supply is filtered.
Penicillin is made by growing mould in fermenters. Penicillin is an antibiotic made by growing the mould Penicillium
chrysogenum in a fermenter. The mould is grown in a liquid culture medium containing sugar and other nutrients.
The sugar is used up as the mould grows. The mould only starts to make penicillin after using up most of the
nutrients for growth. Alexander Fleming discovered Penicillin accidently in 1928. A culture of bacteria became
contaminated with a mould. This mould wiped out areas of bacteria. No one took much notice of Fleming's findings
until the Second World War, when the huge number of injuries made it important to find something that would
heal infected wounds.…read more

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Fuels from Microorganisms (part 1)
Fuels can be made by fermentation of natural products ­ luckily enough, waste products can often be used.
Fermentation is when bacteria or yeast break sugars down by anaerobic respiration.
Ethanol is made by anaerobic fermentation of sugar. Yeast make ethanol when they break down glucose by
anaerobic respiration (see equation at the bottom). Sugar cane juices can be used, or glucose can be
derived from maize starch by the action of carbohydrase (an enzyme). The ethanol is distilled to separate it
from the yeast and remaining glucose before it's used. In some countries, e.g. Brazil, cars are adapted to
run on a mixture of ethanol and petrol ­ this is known as `gasohol'.
Biogas is made by anaerobic fermentation of waste material. Biogas is usually 70% methane (CH) and 30%
carbon dioxide (CO). Lots of different microorganisms are used to produce biogas. They ferment plant and
animal waste, which contains carbohydrates. Sludge waste from, e.g. sewage works or sugar factories, is
used to make biogas on a large scale. It's made in a simple fermenter called a digester or generator. Biogas
generators need to be kept at a constant temperature to keep the microorganisms respiring away. There
are two types of biogas generators ­ batch generators and continuous generators. Biogas can't be stores as
a liquid (it needs too high a pressure), so it has to be used straight away ­ for heating, cooking, lighting, or
to power a turbine to generate electricity.
Fuel production can happen on a large or small scale. Large-scale biogas generators are now being set up in
a number of countries. In some countries, small biogas generators are used to make enough gas for a
village or a family to use in their cooking stoves and for heating and lighting. Human waste, waste from
pigs, and food waste can be digested by bacteria to produce biogas. By-products are used to fertilise crops
and gardens.…read more

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Fuels from Microorganisms (part 2)
Batch generators make biogas in small batches . They're manually loaded up with waste, which is left to
digest, and the by-products are cleared away at the end of each session.
Continuous generators make biogas all the time. Waste is continuously fed in, and biogas is produced at
a steady rate. Continuous generators are more suited to large-scale biogas projects.
(see diagram) Any generator needs to have an inlet for waste material to be put in, an outlet for the
digested material to be removed through and an outlet so that biogas can be piped to where it is
When biogas generators are being designed, the following factors need to be considered: COST ­
continuous generators are more expensive than batch ones, because waste has to be mechanically
pumped in and digested material mechanically removed all the time; CONVENIENCE ­ batch generators
are less convenient because they have to be continually loaded, emptied and cleaned; EFFICIENCY ­ gas
is produced most quickly at about 35°C. If the temperature falls below this the gas production will be
slower. Generators in some areas will need to be insulated or kept warm, e.g. by solar heaters. The
generator shouldn't have any leaks or gas will be lost; POSITION ­ the waste will smell during delivery,
so generators should be sited away from homes. The generator is also best located fairly close to the
waste source.
Biofuels are a `greener' alternative to fossil fuels. The carbon dioxide released into the atmosphere was
taken in by plants which lived recently, so they're `carbon neutral'. The use of biofuels doesn't produce
significant amounts of sulphur dioxide or nitrous oxides, which cause acid rain. Methane is a
greenhouse gas and is one of those responsible for global warming. It's given off from untreated waste,
which may be kept in farmyards or spread on agricultural land as fertiliser. Burning it as biogas means
it's not released into the atmosphere. The raw material is cheap and readily available. The digested
material is a better fertiliser than undigested dung ­ so people can grow more crops. In some
developing rural communities women have to spend hours each day collecting wood for fuel. Biogas
saves them this drudgery. Biogas generators act as a waste disposal system, getting rid of human and
animal waste that'd otherwise lie around, causing disease and polluting water supplies.…read more

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