Cheese Making, Basic Principles
The basic principle involved in making all natural cheese is to coagulate or curdle the milk so that it forms into curds and whey. As anyone knows who has left milk un-refrigerated for a period, milk will curdle quite naturally. The milk sours and forms into an acid curd.
The least sophisticated cheeses are the fresh, unripened varieties typified by Cottage Cheese. These are made by warming the milk and letting it stand, treating it with a lactic starter to help the acid development and then cutting and draining the whey from the cheese. The cheese can then be salted and eaten fresh. This is the simplest, most basic form of cheese.
Today's cheese making methods help the curdling process by the addition of a starter (a bacterial culture which produces lactic acid) and rennet the coagulating enzyme which speeds the separation of liquids (whey) and solids (curds).
Cheese cultures are necessary to inoculate the milk with friendly bacteria. These bacteria serve two functions, 1st they cause the milk to become more acidic aiding its coagulation, and 2nd they help develop the flavor of the cheese. Cheese cultures are divided into two basic types mesophilic and thermophilic. These terms describes at the temperature the culture thrives at.
Mesophilic (from the Greek words meso - meaning intermediate and philic - which means loving) cultures thrive around room temperatures but cannot survive at high temperatures. Examples of cheeses made with these bacteria are Cheddar and Gouda.
Thermophilic (from the Greek words thermo - meaning heat and philic - which means loving) cultures require a higher temperature and used when the curd is cooked to as high as 132F. Examples of cheeses made from these bacteria are Swiss and Italian cheeses.
Professional quality cultures can be bought from a cheesemaking supply company and are usually available in a freeze dried form. A home-spun method is to use cultured buttermilk as a mesophilic starter or fresh yogurt as a thermophilic starter.
After acidification, coagulation begins. Coagulation is converting milk into curds and whey. As the pH of the milk changes, the structural nature of the casein proteins changes, leading to curd formation. Essentially, the casein proteins in the milk form a curd that entraps fat and water. Although acid alone is capable of causing coagulation, the most common method is enzyme coagulation. The physical properties of enzyme-coagulated milk are better than that coagulated purely with acid. Curds produced by enzyme coagulation achieve a lower moisture content without excessive hardening.
Enzymes used to coagulate milk come from a number of sources: animals, plants, and fungi. The traditional source of enzyme is rennet. Rennet is a preparation made from the lining of the fourth stomach of calves. The most important enzyme in rennet is chymosin. Today, most chymosin is a recombinant product made possible by genetic engineering. Until 1990, the only source of rennin was calves. Around 1990, scientists created a system to make chymosin that doesn't require calves. Using genetic engineering, the gene for chymosin was cut from a calf cell and inserted into the genomes of bacteria and yeast. The microbes make an exact copy of the calf chymosin. Microbes replicate and grow rapidly, and can be grown continuously. Thus, the supply of rennet is assured. Approximately 70% of the cheese made in the U.S. is coagulated using chymosin. The chymosin made by the yeast cells is the same as that made by the calf cells.
