Thermophilic and Mesophilic Cheese Making Starters: in a way these two types of starters are the same, they contain bacteria that are used to inoculate your milk, to ripen it and to add bacteria for the later stages of cheese making.
Why two types?
Cheese making is all about temperature and for simple soft cheeses, crumbly cheeses, cream cheeses and cottage cheeses the temperature needed for curd formation is around 28-32 C (82 to 86 F).
However, hard cheeses need a higher temperature, often over 38 C (100 F). Unfortunately the bacteria in many starters will not withstand this temperature.
Consequently we use mesophilic starters for soft cheeses that need a lower temperature process and thermophilic starters for harder cheeses that require things a little hotter.
The Ripening of Milk
If you are going to make a cheese, adding the starter can be said to be the most important stage. Giving the starter time to work before the addition of rennet is important, because the bacteria in the starter builds up the acidity of the milk. This process is called ripening.
Adding starter and rennet at the same time will result in longer clotting time and reduced curds. Usually a starter is added and a couple of hours later, rennet is added. Many cheese makers use pH strips to test the acidity of the milk before adding rennet.