Food Production
Use of Biological Resources
Yoghurt is made from milk using the bacterium .
- Milk is first heated (pasteurised) to kill unwanted microorganisms, then cooled to about 40 °C, and a culture of Lactobacillus is added.
- The bacteria respire the milk sugar lactose anaerobically, producing .
- The lactic acid lowers the pH of the milk. The acidity makes the milk proteins clot (coagulate), thickening the milk into yoghurt and giving it its sharp, tangy taste.
- The acidic conditions also help preserve the yoghurt by stopping the growth of many spoilage and harmful microorganisms.
Common exam question
Describing how Lactobacillus turns milk into yoghurt
Question: Describe how Lactobacillus makes yoghurt from milk, complete a passage on yoghurt production, or explain why rising acidity measures yoghurt production (2–7 marks).
Asked in 3 of the 23 papers, with the same credited chain each time. Milk is heated (pasteurisation; sterilisation also scores) so the bacteria in it are killed, then cooled before Lactobacillus (Streptococcus also scores) is added. The bacteria respire the milk sugar lactose anaerobically, producing lactic acid (lactate is accepted), which lowers the pH and makes the milk protein clot (coagulate), thickening the milk. The three-mark describe version credits any three of respiration, lactose, lactic acid and clotting, so the sugar and the acid are separate marks, and "the milk becomes acidic" counts for the acid. Where rising acidity measures yoghurt production, the two marks come from any two of anaerobic respiration, by the bacteria, and producing lactic acid; with less oxygen the bacteria respire more anaerobically, so the acidity rises faster.