GCSE Biology AQA Unit 3 - Microorganisms in food production.

How microorganisms are used in the production of: 

  • Yoghurt
  • Cheese 
  • Bread
  • Alchoholic drinks

And a brief overview on Yeast and Lactobacillus Bulgaris.

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  • Created by: Annie
  • Created on: 14-05-11 12:36


Some bacteria (eg Lactobacillus), when given a suitable energy source of sugar, respire anaerobically.

Lactobacillus breaks down the sugar in milk for energy. This sugar is called lactose;

Lacose -> Latic acid + energy

The presence of latic acid lowers the PH of the milk. 

This affects the proteins in the milk and they start to coagulate, forming clumps.

These clumps are knowns as curds, and the remaining liquid is known as whey. 

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Yoghurt is made using the bacteria Lactobacillus Bulgaris. 

First the Milk is heated to about 70*C and then cooled before the bacteria is added. This kills any other microorganisms in the milk.

They need to be killled because otherwise, if they were allowed to grow, they would contaminate the yoghurt and produce un-wanted waste substances. 

A starter culture of Lactobacillus Bulgaris is then added to the milk, and left for a few hours.

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Sometimes the enzyme- rennen, is added to the milk with Lactobacillus Bulgaris. This acts on the proteins which makes them coagulate even more than they would do with just the bacterium.

The curds and whey are then seperated. 

The curds are then usually pressed and made into cheese. The type of cheese made depends on factors such as: 

  • type of milk
  • which bacterium is used
  • amount of salt
  • Temperature the bacteria worked at
  • length of time the cheese was left to ripen
  • conditions in which the cheese was left to ripen
  • whether the curds were pressed or not etc.
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Yeast can respire with or without oxygen (aerobically or anaerobically). However aerobic respiration, like in humans, releases a lot more energy.

Glucose + oxygen -> carbon dioxide + water + energy

When yeast respire anaerobically, this is called fermentation.

Glucose -> ethanol + carbon dioxide + energy

Yeast feed Saprophytically- which means they secrete enzymes which break down energy sources around them eg starch, from large molecules into smaller ones. These molecules then diffuse into the cell.

Yeast also produces best at around 40*C.

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When yeast respires it produces Carbon Dioxide. When this happens is a mixture of flour and water (dough) the bubbles of gas get trapped and cause the bread to rise. The flour contains starch, amylase, and protein. 

The starch is the energy source for the yeast. The amylase digests the starch into sugar. The yeast then absorbs the sugar. (However water is required for the amylase to work on the starch).

Amylase is the most important protein but gluten is also important as it forms the sticky stretchy threads as the yeast works on the dough, and so helps to trap the carbon dioxide- which helps the bread to rise.

The mix is then heated at high temperatures because:

  • It kills the yeast
  • breaks down the waste product ethanol
  • it alters the remaining gluten and starch to make a firm textured bread.
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Usually yeast respires Anaerobically. When it does it produces ethanol and Carbon dioxide, this process is called alcoholic fermentation.

Beer is made from grain- usually Barley.

The barley grains are allowed to germinate for a few days, during which the starch in the grains is broken down into sugar by enzymes. The grains are then dried in a kiln. This process is called MALTING.

The malted grain is mashed and wateris added to produce a sugary solution with lots of bits in. These bits are then sieved out.

Hops are then added to give the beer it's bitter flavour.

Yeast is then added, and the sugary solution is fermented by yeast, turning the sugar into alcohol.

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