Why does the yeast population not keep on growing and spoil the cider?
Yeasts require an abundance of oxygen for growth. The fermentation process actually takes a day or two to start, because the yeast feeds off the oxygen present in the juice until this is gone. Sometimes food spoils because of unwanted fermentation. [If you are unlucky, for example, you could buy a pot of yoghurt that has gone off because of fermentation. You can usually notice that there might be a spoilage problem because the lid is raised due to the carbon dioxide which is pressing on it from inside the pot. When you open the lid, it goes pop as the gas is released.] When the alcohol concentration reaches about 10% the alcohol damages the yeast and fermentation stops. You can study the growth pattern of yeast cultures.
