I'm Jim Tedder with the VOA Special English Agriculture Report.

Last week, we heard how cheese maker Jonathan White in New Jersey uses traditional methods to make his products. An important part of the process is natural fermentation of milk. The fermentation is caused by bacteria and other organisms. But the chemical changes of fermentation are not enough to make most cheeses.

About two thousand five hundred years ago, sheep farmers made a discovery. They found that a part of the stomach of a young sheep or cow could turn milk into a solid, called curd. Later, cheese makers found that they could place the stomach in salt water to separate the substance responsible for curdling milk. It is called rennet. Rennet can also be made from plants.

The discovery of rennet has been called the first effort in biotechnology.

Jonathan White explains that rennet solidifies milk quickly. This keeps fermentation from producing too much acid and saves milk sugars. He usually puts the rennet into the milk around the time of his mid-day meal. By the time his meal is done, the rennet has separated the solid curd from the remaining milk liquid, called whey.

Some whey is always saved to put in the next day's cheese-making.

The curd is cut in pieces to speed the curdling process. When the curd has hardened enough, Mister White removes it from the vat and puts it in containers that form the cheese. After this, the cheese must air-dry and settle.

Mister White carefully controls fermentation. He does so by controlling the amount of salt, the amount of acid produced by fermentation and the amount of water in the cheese. These three things, with the addition of time, all influence the taste of the cheese.

Mister White does not pasteurize his milk. He says this permits him to control fermentation better.

Pasteurization is a heat treatment. Milk is usually heated to about seventy-two degrees Celsius for at least fifteen seconds, then cooled. Pasteurization does not kill all organisms, but reduces their number.

Jonathan White says cheese made with unpasteurized milk must be aged at least sixty days.

He and his wife, Nina, say they enjoy sharing their knowledge with people interested in cheese-making and farming. Their Bobolink Dairy is on the Web at cowsoutside.com.