Yoghurt is made by adding a culture of Lactobacillus bacteria to warm milk and keeping the mixture at a suitable temperature. As the bacteria feed on the mi...

(a) Type of microorganism [1]. Yoghurt is made using bacteria (Lactobacillus).

(b) How the pH changed [2]. Reading Fig. 10.1, the pH fell throughout the four hours, becoming more acidic: it dropped from about \(6.6\) at the start to about \(4.4\) after four hours. One mark for the direction (pH falls / becomes more acidic), one for supporting it with values read from the graph.

(c) Why the pH falls [3]. The bacteria feed on and respire (ferment) the sugar in the milk; this produces lactic acid; as the acid builds up in the mixture it lowers the pH. Any three of these linked points score. The cause is the acid the bacteria make, not the bacteria themselves.

(d) Why the milk thickens [2]. The lactic acid makes the dissolved milk protein coagulate (clot); the clotted protein turns the liquid milk into a semi-solid, so it thickens. One mark for coagulation of the protein by the acid, one for the resulting thickening.

(e) Sugar used by the bacteria [1]. Lactose, the sugar naturally present in milk.

(f) Why the milk is heated first [2]. Heating kills any unwanted or harmful microorganisms already in the milk; this stops them spoiling the yoghurt or competing with the added Lactobacillus culture. One mark for killing existing microorganisms, one for the reason.

(g) Why yoghurt keeps longer than fresh milk [1]. The low pH (acidic conditions) inhibits the growth of the decay and spoilage bacteria that would turn milk bad, so yoghurt lasts longer.