Edexcel IGCSE Biology · Spec 5.5-5.8
Microorganisms and Food
How microorganisms are used to make food and drink, including yoghurt and bread, using fermenters.
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Microorganisms and Food
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Explained
Yeast, bacteria, and controlling a fermenter
Two microorganisms make two familiar foods, and each does it by respiring. Getting the products right, and in the right order, is most of this topic.
Bread
Enzymes in the yeast first convert the starch in the flour into sugars such as maltose and glucose.
The yeast then respires those sugars, producing carbon dioxide. The gas is trapped in the stretchy dough, forming bubbles, and the dough rises.
Yeast respiring anaerobically also produces ethanol, which evaporates during baking. That is why bread does not make you drunk, and it is a mark worth taking if the question asks what happens to the ethanol.
Yoghurt
The milk is first pasteurised, heated to somewhere between 70 and 95 degrees Celsius, to kill any microorganisms already present that would compete or spoil it.
It is then cooled to about 40 to 45 degrees before the Lactobacillus bacteria are added. Cooling matters twice over: the bacteria would be killed by the hot milk, and this range is the optimum temperature for their enzymes.
The bacteria convert lactose, the sugar in milk, into lactic acid. That lowers the pH, which makes the milk proteins clot and set, giving yoghurt its texture, and the acidity also prevents other bacteria from growing, which preserves it.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for the bread question credits five points, of which three are needed: that enzymes in the yeast convert starch in the flour to sugars, that the yeast cells respire, that carbon dioxide is produced, that it is trapped in the dough so the bread rises, and that the ethanol produced evaporates. It allows either aerobic or anaerobic respiration.
The starch to sugar step is the one usually left out. Yeast cannot respire starch, so something has to convert it first, and saying so is a mark.
For yoghurt the mark scheme is specific about temperatures: milk heated at 70 to 95 degrees or pasteurised, to kill any microorganisms, then cooled and kept at 40 to 45 degrees, so the Lactobacillus added can survive and have the optimum temperature for their enzymes.
On the pH part it wants that milk is about neutral to begin with, that lactic acid is produced, that this makes the pH drop, and that this stops other bacteria growing or clots the milk. The examiner report on that paper notes that learners found this item more difficult and that only the best responses explained the drop in pH and its two consequences.
Two consequences, not one. The acid both sets the yoghurt and preserves it, and each is worth stating.
Industrial fermenters
A fermenter is a large vessel that grows microorganisms under controlled conditions, and each control exists for a reason.
- A water jacket removes the heat released by respiration, keeping the temperature at the optimum for the enzymes. Too hot and they denature.
- pH is monitored and corrected, again to keep the enzymes working at their optimum.
- Nutrients are added so growth is not limited by a shortage of food.
- Sterile air is bubbled in when the process is aerobic. Some processes need no oxygen, and then it is deliberately excluded.
- Paddles stir the contents, distributing microorganisms, nutrients, heat and oxygen evenly and stopping the culture settling at the bottom.
- Everything is sterilised first, and aseptic technique is used when adding the culture, so that no unwanted microorganism gets in and competes or contaminates the product.
If asked why any one of these is used, give the control and then the reason. Stirring so that conditions are the same throughout; cooling so that the enzymes are not denatured. The reason is always the second half of the mark.
Spec 5.5-5.8
What you need to know
- Describe the role of yeast in making bread
- Describe the role of bacteria in making yoghurt
- Explain how an industrial fermenter works
Active recall
Quick check
Answer each question before opening the answer.
What is the role of yeast in bread and in brewing?
Yeast respires anaerobically: the carbon dioxide makes bread rise, and the ethanol is used in brewing.
What is a fermenter used for?
Growing microorganisms under controlled conditions to make products such as yoghurt, penicillin or mycoprotein.
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