The Principles of Cheesemaking

Milk is the starting point for cheesemaking and using fresh, pure milk is critical. Unpasteurized milk is widely believed to produce the best tasting cheeses.  The pasteurization process, which involves heating the milk to kill germs, can sometimes affect the delicate enzymes in the milk.  Pasteurization may inhibit the formation of cheese curds and affect the taste of the final product.   Unpasteurized milk can be purchased from family farms that raise goats, sheep or cows.  If that isn‘t feasible, any type of  whole, 2% or skim milk may be used.
In cheese making, caseins (also known as milk proteins) are transformed by a starter culture into curds, forming the basis of cheese.  In this process, the lactose in milk is changed to lactic acid.   This can be loosely compared to functioning of yeast in beer making.   Several types of starters are used in cheese making and are chosen based on the type of cheese.   For example, some are better for making mozzarella, others for feta or camembert. Starter cultures help shape the taste, feel and scent of the finished cheese.
At the next step in the process, when the pH reaches the right level, rennet is added to the mixture.  Rennet helps the starter culture to create the curds.  Basically, it’s the «curdling» of the milk.  Next, the curds are processed depending on the desired final product.  Some cheeses such as cheddar are pressed into a mold.  Others are salted and aged.  Cheeses like Swiss require additional cooking of the curds.   Some only require salting.   Fresh cheeses such as ricotta can be eaten at this time as they are not aged.  
Aging and ripening yields a firmer cheese.  Time, temperature, humidity, bacteria and salt all affect the ripening process.  At this point, molds may develop which also contribute to the flavor of the cheese. 
As you can see, milk, the choice of starter cultures, rennet and the concentration and aging processes all help determine the finished cheese.  While creating a good cheese is a series of trial and error, they are many general concepts that will help you get started. Your patience and attention to detail will be rewarded with delicious homemade cheeses to share with family and friends.