Module Three 13.5 How are microorganisms used to make food and drink?

Microorganisms are used to make food and drink:

There are many different types of micro-organisms.

Bacteria are very small single celled organisms. There is no nucleus. The single circular chromosome is effectively 'naked' in the cytoplasm. There are often many smaller circular pieces of DNA in the cytoplasm known as plasmids. They often contain genes for antibiotic resistance.

Not all bacteria are harmful. Some are actually useful.

• The bacteria Lactobacillus is added to milk to make yoghurt. The bacteria feed on the lactose sugar found in milk and produce lactic acid. This turns the milk slightly sour which in turn clots the milk to produce natural yoghurt.
  
  

  This process relies on the growth of bacteria - usually mixed cultures including Lactobacillus species - on milk, and the conversion is due to anaerobic respiration by the bacteria. The milk is usually skimmed milk - "low fat"- often concentrated with extra "milk solids".

  As the bacteria respire anaerobically, they produce lactic acid:

  lactose ­­­(milk sugar)  = lactic acid + energy

  The acidic conditions (pH 3.7-4.3) cause milk proteins to be coagulated, which provides the thickened texture of yoghurt. Other chemical products from the bacterial growth, e.g. ethanol (acetaldehyde) contribute to the flavour.

  Keeping qualities
  As the yoghurt is cooled, the bacterial growth rate is reduced and the product can keep for about 10 days at 5 °C. The normal numbers of bacteria are about 10⁸ (100,000,000) per gram. After some time, acid slowly released by the bacteria gradually kills them and causes the proteins to separate into curds and whey.
• Acetobacter is used to convert alcohol into vinegar.
• Some cheeses are ripened using enzymes from bacteria they are added to warm milk; curds are produced that are more solid than yoghurt; these curds are separated from the remaining liquid part of the milk (whey);

Bacteria can multiply very quickly under the right conditions, often doubling in number every 20 minutes.

Yeast

When yeast respires it produces ethanol (alcohol) and carbon dioxide.

For making beer:

• the starch in barley grains is broken down into a sugary solution by enzymes in the germinating grains, in a process called malting;
• the sugary solution is extracted then fermented;
• hops are then added to give the beer flavour.

In wine-making the yeast uses the natural sugars in the grapes as its energy source.

Yeast is also used in bread making as it gives off carbon dioxide gas which makes the bread rise, giving it a light and fluffy texture and the ethanol evaporates.

Bread making

When yeast respires it makes carbon dioxide gas. When the gas is made in the dough, it gets trapped and so tiny bubbles form inside the dough. This makes the bread rise. 'Knocking back' gets rid of large bubbles which can develop. When the dough is baked the bubbles expand. The yeast dies in the baking process and ethanol is evaporated.

Using micro-organisms to make food and drinks

How Yoghurts are Made

How Stuff Works - Beer Making

How Stuff Works - Bread Making