A student makes ginger milk curd using two different methods. Both methods use milk and ginger juice. Ginger contains a protease enzyme called zingibain, which cleaves milk proteins, causing them to coagulate and set into a firm curd.
Method A: Warm milk is mixed with fresh ginger juice. It is left for 10 minutes. The milk sets into a firm curd.
Method B: Warm milk is mixed with ginger juice that has been boiled and then cooled. It is left for 10 minutes. The milk remains liquid.
Explain why the milk in Method A sets, but the milk in Method B remains liquid. Use your knowledge of enzymes.
The graph below shows the rate of an enzyme-catalysed reaction as the concentration of substrate is changed.
Draw an X on the graph to show where approximately 50% of the enzymes' active sites are occupied with substrate. Explain your decision.