In 1857, Louis Pasteur realized that heat treatment would destroy unwanted microbes. Shortly after this, the pasteurization of milk began in Europe and America. By 1940, this process had become well established as dairy herds became larger, milk traveled farther, and larger milk processing plants and cheese factories held milk longer.

Fresh milk naturally contains healthy bacteria which inhibit the growth of undesirable and dangerous organisms. Pasteurization removes many of these friendly bacteria along with the bad. Without these friendly bacteria, pasteurized milk is actually more susceptible to contamination. Currently, there are several different approaches to pasteurizing milk:

Thermization, or heat treatment, is a low temperature (145F) and short time (15 seconds) pasteurization that has the lowest impact on natural bacteria and enzymes in milk and is commonly practiced in Europe.

Pasteurization can take one of two forms:

Low Temperature (145F) Long Time (30 min.) referred to as vat or batch pasteurization.

High Temperature (162F) Short Time (15 sec.) referred to as HTST (high temp short time).

Lately, dairies have been extending the pasteurization temperatures above 172F. This destabilizes the whey proteins, which, in turn, keeps the calcium from bonding and making a good curd. (If your curd forms as a loose mass or something looking like Ricotta, your milk source has probably been heated above 172F.)

Ultra-pasteurization (UP) is a range of milk processing temperatures from 191-212F, for varying times. According to Food and Drugs 21 CFR131.110e(1)ii, all milk fitting this definition must be labeled as ultra-pasteurized.

However, the law states that this label need only be 1/2 the size and gives no designation as to where it should be on the package. So, 'buyer beware' here. We have had many customers write in who have purchased what they thought was regular whole milk to get home and discover a tiny UP on the package.

We do not recommend using UP milk to make cheese (especially Mozzarella). You can see the visual difference in how UP and UHT milks work (compared to good milk) in the cheese making process 

Ultra High Temperature Sterilization (UHT) is milk that has been heated to (280F) for a short time (2 seconds). It is basically devoid of any bacteria whatsoever. The shelf life of this milk is 60 days, compared with the 18 days of lower temperature pasteurization.