Using anaerobic respiration: urwagwa, bread, ikivuguto and biogas – Cellular Respiration (Senior 3 Biology) | Ganzaa

Cellular Respiration

Lessons

Senior 3 · Biology

Cellular Respiration

4 lessons

Lesson 4 of 4

Using anaerobic respiration: urwagwa, bread, ikivuguto and biogas

25 min

The same organism, two products

A brewer making urwagwa and a baker making bread in Kigali use the same living organism, yeast, and the same process, anaerobic respiration. The brewer wants the ethanol and lets the carbon dioxide escape; the baker wants the carbon dioxide and lets the ethanol escape. Long before anyone knew yeast was alive, people across Rwanda and the world were using its respiration, and that of bacteria, to make and keep food.

Definition

Biotechnology: using living organisms, or the substances they make, to produce useful products.

Yeast: a single-celled fungus that respires aerobically when oxygen is present and anaerobically, making ethanol and carbon dioxide, when it is not.

Lactic acid bacteria: bacteria that respire anaerobically, turning sugars into lactic acid; they are used to make fermented milk and to preserve animal fodder.

Biogas: a fuel gas, mainly methane, made when bacteria break down dung and plant waste without oxygen.

Urwagwa: banana beer

Urwagwa is the banana beer of Rwanda and Burundi. It is made from ripe East African Highland bananas, and ground sorghum is added as a source of wild yeast. Figure 1 sets out the steps, with the biology of each.

Figure 1: Making urwagwa, and the biology behind each step.

The covered container matters: once the yeast has used up the oxygen dissolved in the juice, it must respire anaerobically, which makes the ethanol. If air could get in freely, the yeast would respire aerobically and make little alcohol, and bacteria could turn the ethanol into vinegar. Fermentation also makes heat, so the brew stays warm, and the carbon dioxide makes it fizz. Sorghum beer, ikigage, is made in a similar way from sorghum.

Worked example 1: how much alcohol?

Banana juice contains 90 g of sugar per litre. Assume all of it is fermented as glucose: $\mathrm{C_6H_{12}O_6} \rightarrow 2\,\mathrm{C_2H_5OH} + 2\,\mathrm{CO_2}$ . What mass of ethanol forms in one litre? (Relative formula masses: glucose 180, ethanol 46, carbon dioxide 44.)

From the equation, 180 g of glucose gives $2 \times 46 = 92\ \text{g}$ of ethanol and $2 \times 44 = 88\ \text{g}$ of carbon dioxide. Check: $92 + 88 = 180$ . ✓
90 g is half of 180 g, so the ethanol is $\frac{90}{180} \times 92 = 46\ \text{g}$ .
In reality the yield is lower: some sugar is used for the yeast's growth, and fermentation stops before all the sugar is used. (Illustrative figure for the sugar.)

Bread

Flour, water, a little sugar and yeast are mixed into a dough and left in a warm place. The yeast respires, first aerobically and then anaerobically as the oxygen in the dough runs out. The carbon dioxide forms bubbles that are trapped by the stretchy protein (gluten) in the dough, so the dough rises. In the hot oven the yeast is killed, the bubbles expand and set to give bread its light texture, and the small amount of ethanol evaporates.

Figure 2: Volume of the same dough kept at two temperatures (illustrative data).

Worked example 2: reading Figure 2

(a) Calculate the mean rate at which each dough rose in the first 30 minutes. (b) Explain the difference. (c) Suggest why the dough at 35 °C rises more slowly after 40 minutes.

(a) At 35 °C: $\frac{95 - 50}{30} = \frac{45}{30} = 1.5\ \text{cm}^3$ per minute. At 20 °C: $\frac{61 - 50}{30} = \frac{11}{30} \approx 0.37\ \text{cm}^3$ per minute.
(b) Yeast respiration is controlled by enzymes, which work faster at 35 °C, close to their optimum, than at 20 °C; more carbon dioxide is made each minute.
(c) The yeast is using up the sugar in the dough, and the dough cannot stretch for ever, so the rise slows.

Ikivuguto, silage and biogas

Ikivuguto and yoghurt. Lactic acid bacteria in milk respire anaerobically, turning the milk sugar, lactose, into lactic acid. The acid gives the sour taste, makes the milk proteins thicken, and keeps many harmful bacteria from growing, so fermented milk keeps longer than fresh milk.
Silage. Grass or maize stems are packed tightly and sealed so that no air can enter. Lactic acid bacteria ferment the sugars, and the acid preserves the fodder, which can be fed to dairy cows in the dry season.
Biogas. Cow dung, water and plant waste are fed into a sealed tank, a digester. Bacteria break them down without oxygen and release biogas, mostly methane, which is piped to the kitchen as a cooking fuel. The leftover slurry is a good fertiliser. A family with a digester needs less firewood, which also protects trees and keeps smoke out of the kitchen.

Anaerobic microbes can also cause harm: they spoil food that is not stored properly, and some make poisons. And ethanol is a drug: as you learnt in Senior 1, alcohol damages the developing brain and body, and it is not for school pupils.

Warning

Common mistakes. (1) Saying bread rises because of the alcohol: it rises because of the carbon dioxide. (2) Saying yeast is a bacterium: it is a single-celled fungus. (3) Saying ikivuguto is made by yeast: its fermentation is mainly by lactic acid bacteria, making lactic acid.

Anaerobic respiration by yeast: fermentation, FuseSchool. Watch for the two products of yeast fermentation and which one a baker and a brewer each want.

Tip

Memory hook: "Bakers want the bubbles, brewers want the booze, milk wants the acid." Bread: CO₂. Urwagwa: ethanol. Ikivuguto: lactic acid.

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Practice: uses of anaerobic respiration

Name the organism that ferments urwagwa, and state what the ground sorghum adds.
Why must the brew be kept covered while it ferments?
Explain why bread dough rises, and what happens to the yeast and the ethanol in the oven.
Name the product of anaerobic respiration that preserves ikivuguto, and the type of organism that makes it.
Give two benefits to a farming family of a biogas digester.
From the equation for fermentation, what mass of carbon dioxide forms when 45 g of glucose is fermented completely?
A baker leaves dough in a cold room overnight and finds it has hardly risen. Explain.
Factory-bottled banana beer is heated before bottling. Suggest why, and what would happen to a sealed bottle that was not heated.

Show answersHide answers

Yeast; the sorghum brings wild yeasts.
So that the yeast, once the oxygen is used up, respires anaerobically and makes ethanol; air would let it respire aerobically and let bacteria spoil the brew.
Yeast respires and makes carbon dioxide, which forms bubbles trapped in the stretchy dough. In the oven the yeast is killed and the ethanol evaporates.
Lactic acid, made by lactic acid bacteria.
For example: a free cooking fuel (less firewood to collect or buy), and slurry that can be used as fertiliser; also less smoke in the kitchen.
180 g of glucose gives 88 g of carbon dioxide, so 45 g gives $\frac{45}{180} \times 88 = 22\ \text{g}$ .
At low temperature the yeast's enzymes work very slowly, so it makes little carbon dioxide.
Heating kills the yeast and stops fermentation. In an unheated sealed bottle the yeast would go on making carbon dioxide, building up pressure until the bottle might burst, and the drink would keep changing.

Key points

Yeast fermentation: urwagwa and ikigage use the ethanol; bread uses the carbon dioxide.
Lactic acid bacteria make ikivuguto and yoghurt and preserve silage; bacteria in a digester make biogas (methane) from dung.
Fermentation conditions: no oxygen, warmth, sugar; heat kills the yeast and stops it.

1The same organism, two products

2Urwagwa: banana beer

3Bread

4Ikivuguto, silage and biogas

The same organism, two products

Urwagwa: banana beer

Bread

Ikivuguto, silage and biogas