IGCSE Biology / Using microorganisms
Beer, wine and yoghurt
Edexcel International GCSE Biology (4BI1), chapter 21: Using microorganisms, specification point 5.7. Free lesson.
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Beer and yoghurt are made by different microorganisms respiring anaerobically. Brewing produces ethanol, while yoghurt production uses lactic acid to thicken milk.
How barley starch becomes sugar
Beer brewing starts with malting. Barley grains germinate, which activates the enzyme amylase. Amylase breaks the stored starch into maltose, a sugar that yeast can ferment. The grains are then dried and crushed so the sugars can dissolve into warm water. Hops are added for flavour, then the mixture is boiled to kill unwanted microorganisms. The liquid must cool before yeast is added, because hot liquid would kill the yeast.
How yeast makes beer
Yeast ferments the sugar in a sealed vessel. This keeps oxygen out, so yeast respires anaerobically and produces ethanol and carbon dioxide. If yeast respires aerobically, it produces carbon dioxide and water instead, so no ethanol is made. Brewing takes place at about 20 to 30 °C, the suitable temperature range for the yeast. The beer is filtered to remove yeast and often pasteurised to stop further fermentation.
How wine differs from beer
Wine uses the same basic idea as beer: yeast ferments sugar anaerobically to produce ethanol. Grapes already contain sugar, so the malting step used to release sugar from barley is not needed.
How Lactobacillus makes yoghurt
Yoghurt is made using Lactobacillus, a bacterium, rather than yeast. The wanted product is lactic acid. The milk is pasteurised first to kill unwanted microorganisms, then cooled to about 40 °C, which is the optimum temperature for the bacteria. Lactobacillus respires anaerobically, using lactose in the milk and producing lactic acid. The acid lowers the pH. This makes milk proteins denature and coagulate, so the milk thickens into yoghurt. Flavourings or fruit are added, then the yoghurt is cooled and packaged. The thickening is controlled milk curdling.
How the two processes compare
Both processes start by killing microorganisms already present, then adding a chosen culture. Both use anaerobic respiration, but the microorganism, sugar, product and temperature differ. Beer: yeast, a fungus, ferments maltose from barley or sugar in grapes. The wanted product is ethanol, and the temperature is about 20 to 30 °C. Yoghurt: Lactobacillus, a bacterium, uses lactose in milk. The wanted product is lactic acid, and the temperature is about 40 °C.