Fermentation and food biotechnologyIB MYP Biology: Revision notes

Q: What is fermentation?

Fermentation is the use of microorganisms such as yeast and bacteria to break down sugars without oxygen (anaerobic respiration), releasing energy and making useful products. Microorganisms are used because they grow quickly, are cheap to feed and make products we want.

Section 1

What is fermentation?

Fermentation is the use of microorganisms such as yeast and bacteria to break down sugars without oxygen (anaerobic respiration), releasing energy and making useful products. Microorganisms are used because they grow quickly, are cheap to feed and make products we want.

The type of product depends on the microorganism:

Yeast (a fungus) ferments glucose to ethanol and carbon dioxide: glucose → ethanol + carbon dioxide ( $\mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2}$ )
Lactic acid bacteria ferment lactose to lactic acid
Other anaerobic bacteria ferment waste to methane

Key termsfermentationanaerobic respirationyeast

Section 2

Yeast in bread and brewing

Bread: yeast is mixed with flour, sugar and warm water. The yeast ferments the sugar and makes carbon dioxide, which forms bubbles in the dough so that it rises. The alcohol evaporates when the bread is baked, and the heat kills the yeast.

Brewing: yeast ferments sugar from grains such as barley, or from fruit juice, to make ethanol in beer and wine. The carbon dioxide escapes through an airlock, which lets gas out but keeps air and microorganisms out.

Conditions needed: a warm temperature (around 30-40 °C, the optimum for yeast enzymes), a supply of sugar, and little or no oxygen. Too hot and the enzymes denature and the yeast dies.

Key termsairlockoptimum temperature

Common mistake

Fermentation is not the same as aerobic respiration. Fermentation happens with little or no oxygen and makes ethanol and carbon dioxide in yeast.

Section 3

Bacteria in yoghurt and cheese

Yoghurt: milk is heated to kill unwanted microorganisms, then cooled to about 43 °C and live Lactobacillus bacteria are added. The bacteria ferment the milk sugar (lactose) to lactic acid. The acid lowers the pH, which makes the milk proteins clump together, so the milk thickens and tastes sour.

Cheese: lactic acid bacteria are added to milk and sour it. Enzymes then make the milk separate into solid curds and liquid whey. The curds are pressed and left to ripen into cheese.

Key termslactic acidlactosecurds

Section 4

Biofuels and biogas

A biofuel is a fuel made from living material that can be replaced.

Bioethanol: yeast ferments sugar from crops such as sugar cane to make ethanol, which is mixed with petrol.
Biogas: anaerobic bacteria in a sealed digester ferment animal manure, sewage and plant waste to make methane, which is burned for cooking, heating and electricity.

Advantages: renewable; the carbon dioxide released when they burn was taken in by plants as they grew, so they are nearly carbon neutral; waste is used up.

Disadvantages: growing crops for fuel takes land that may be needed for food or forest; biogas production is slow in cold weather.

Key termsbiofuelbioethanolbiogascarbon neutral

Section 5

Industrial fermenters

For large-scale production, microorganisms are grown in huge stainless steel tanks called fermenters. The conditions are controlled so that the microorganisms grow as fast as possible:

Temperature: a water jacket or cooling coils remove the heat that the microorganisms release
pH: monitored and adjusted
Nutrients: sugar and minerals are supplied
Oxygen: supplied through bubbles if the microorganism needs it (not for anaerobic fermentations)
Stirring: paddles mix the contents evenly
Sterile: equipment is cleaned and sterilised so that unwanted microorganisms do not grow

Key termsfermentersterile

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Fermentation and food biotechnology

A bakery in Lisbon makes bread by mixing flour, sugar, yeast and warm water into a dough. The baker leaves the dough in a warm place for an hour. The dough doubles in size and has a slightly sweet smell of alcohol. It is then baked in a hot oven.
Explain why the baker uses warm water, not cold water or boiling water, to mix the dough.2 marks
A dairy in Istanbul makes yoghurt. Milk is heated to 85 °C, cooled to 43 °C and then a small amount of live bacteria (Lactobacillus) is mixed in. The mixture is kept at 43 °C for six hours in a closed container. By the end the milk has thickened and tastes sour.
Explain why the milk thickens and tastes sour during the six hours.2 marks
A student investigates how temperature affects fermentation by yeast. She mixes 5 g of yeast with 100 cm³ of glucose solution in a flask and joins the flask by a delivery tube to an upturned measuring cylinder full of water. She measures the volume of gas collected in 10 minutes. She repeats the experiment at 15 °C, 25 °C, 35 °C, 45 °C and 65 °C. The volumes of gas collected were 4 cm³, 18 cm³, 41 cm³, 30 cm³ and 0 cm³ respectively.
State a testable hypothesis for this investigation and give a scientific reason for it.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).