Control location and activity of plasmin components in dairy products.

Plasmin, the major native milk serine proteinase, is an important contributor to proteolytic breakdown in milk and dairy products. Plasmin activity has beneficial proteolytic effects during the ripening of certain cheeses. Plasmin breaks down casein. It reduces the time needed to ripen cheese, which would have enormous economic benefits for cheese manufacturers.

Plasmin exists predominantly in fresh milk in its inactive form, plasminogen, which is converted to active plasmin by plasminogen activators. It is not clear how these plasmin system components are affected by whey proteins and bacteria in milk, but this is important to understand as we seek to reduce the time needed for cheese ripening.

The whey fraction obtained from the cheesemaking process is used to make whey protein concentrates and isolates, which are used as functional ingredients in different foods. Plasmin activity in whey protein products may cause a breakdown of food proteins that cause beneficial or detrimental effects to a product's quality.

Plasmin appears to be a contributor to age gelation in ultra-high-temperature (UHT) milk. Factors that affect the location and activity of plasmin components are economically relevant to dairy processors. Researchers have identified several factors that affect the location and activity of plasmin components in milk, cheese, whey and milk model systems. These include:

* Heifer stage of lactation and lactation number;

* Proteases produced by psychrotrophic bacteria;

* Various cheesemaking conditions; and

* Thermal processing conditions.

Each of these factors holds potential with regard to its ability to control plasmin components in milk, cheese, whey and model systems. Research generally focuses on the native milk enzyme plasmin, and the factors that control its activity and location, as well as its effect on the quality of dairy products.

Further information. S. Suzanne Nielsen, Department of Food Science, Purdue University, 745 Agricultural Mall Dr., West Lafayette, IN 47907; phone: [phone]; email: [e-mail].