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Use of Biological Resources

Food production — Edexcel Linear - OL Revision Notes & Topical Questions

10 min read 12 parts Lesson 5 of 8 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

Using Microorganisms: Yeast, Yoghurt and Industrial Fermenters — EDEXCEL LINEAR - OL notes: Explain how yeast makes bread and how to investigate its fermentation, how Lactobacillus makes yoghurt, and how fermenters control conditions.Microorganisms can be used to make foods.

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Pasteurisation
  • Mycoprotein

Part 1 of 5Yeast and bread making

Microorganisms can be used to make foods.

1
Yeast is a single-celled fungus that can respire aerobically and anaerobically.
2
In bread making, yeast enzymes break down flour starch into sugars, which the yeast respires.
3
It respires aerobically at first, then anaerobically when oxygen runs out, producing ethanol and carbon dioxide.
4
The carbon dioxide is trapped in small air pockets and makes the dough rise.
5
During baking the ethanol evaporates, so bread contains no alcohol, and the high temperature kills the yeast so respiration stops.

Putting the stages in order for an exam: flour starch is broken down to sugars by yeast enzymes; yeast respires aerobically, using up oxygen; it then respires anaerobically; ethanol and carbon dioxide are made; carbon dioxide inflates the dough; baking evaporates ethanol and kills the yeast. The word equation for fermentation is glucose to ethanol plus carbon dioxide (with a small release of energy), and it is called anaerobic because no oxygen is used.

Part 2 of 5Practical: anaerobic respiration in yeast

Anaerobic respiration in yeast: oil excludes oxygen and carbon dioxide is detected in limewater
Anaerobic respiration in yeast: oil excludes oxygen and carbon dioxide is detected in limewater

Anaerobic respiration in yeast is called fermentation (glucose to ethanol and carbon dioxide). It is important in bread and alcoholic drinks. To investigate temperature:

1
Mix yeast with sugar solution (the glucose source) in a boiling tube.
2
Add a layer of oil to keep oxygen out so only anaerobic respiration occurs.
3
Connect the tube with a capillary tube to a second tube of limewater.
4
Place the tube in a water bath at a set temperature and count the bubbles of carbon dioxide in a fixed time (for example 2 minutes).
5
Repeat at other temperatures.

More bubbles per minute means a faster rate. The rate rises towards the optimum temperature because enzyme activity increases, but beyond the optimum the enzymes denature and carbon dioxide production slows and stops. CORMS: change temperature; same yeast species; repeat at each temperature; measure bubbles in a set time; standardise sugar concentration, volume and pH and the mass of yeast.

Part 3 of 5Bacteria and yoghurt

Bacteria are useful because they make complex molecules and reproduce rapidly. Yoghurt is made using Lactobacillus:

1
Sterilise all equipment to remove unwanted bacteria and chemical contamination.
2
Pasteurise the milk at 85 to 95°C to kill other bacteria, which would compete for lactose or spoil the taste.
3
Cool to 40 to 45°C and add Lactobacillus.
4
Incubate for several hours while the bacteria ferment lactose to lactic acid, which sours and thickens the milk.
5
Stir and cool to 5°C to stop the bacteria; add flavours and fruit and package.

The lower pH also prevents growth of harmful microorganisms, so yoghurt keeps longer than milk.

Why both heat treatments?

Bacteria and yoghurt
The high temperature of pasteurisation kills unwanted bacteria,
  • but if the milk were not cooled to 40 to 45°C before adding the culture it would kill Lactobacillus too.
Cooling to 5°C at the end slows enzyme activity so the yoghurt does not become too sour.
This shows how temperature is used at each stage for a different purpose.

Part 4 of 5Industrial fermenters

Industrial fermenter with temperature control, air supply and stirring
Industrial fermenter with temperature control, air supply and stirring

Fermenters are large containers for culturing bacteria and fungi, used for yoghurt, mycoprotein and antibiotics such as penicillin, and for GM microorganisms. Conditions can be controlled to give large quantities of the right organism.

ConditionWhy and how
Aseptic precautionsSteam cleaning kills contaminants so only the wanted microorganism grows
NutrientsSupply respiration substrates and building materials for growth and reproduction
TemperatureProbe and water jacket keep the optimum for enzymes
pHMonitored with a probe and adjusted with acid or alkali
OxygenSupplied for aerobic respiration
AgitationPaddles mix microorganisms, nutrients, oxygen, temperature and pH evenly

Part 5 of 5Worked examples

  1. Explain why bread dough rises. [3]3 marks

  2. In the yeast investigation, why is oil added? [1]1 mark

  3. Explain why yoghurt lasts longer than milk. [2]2 marks

  4. Give two reasons a fermenter is stirred and cooled by a water jacket. [2]2 marks

Key terms

TermMeaning
FermentationAnaerobic respiration in yeast making ethanol and carbon dioxide.
LactobacillusBacterium that turns lactose into lactic acid to make yoghurt.
PasteurisationHeating milk to kill unwanted bacteria.
FermenterVessel for culturing microorganisms in large amounts.
AsepticFree from contaminating microorganisms.
MycoproteinProtein food made from fungus grown in a fermenter.

Avoid these mistakes

1
Saying bread contains alcohol — the ethanol evaporates when baked.
2
Using high temperature to speed yeast respiration indefinitely — enzymes denature.
3
Saying milk is pasteurised to add bacteria — it kills unwanted ones.
4
Forgetting aseptic precautions in fermenters.

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