Research into milk's inherent antimicrobial system

The dairy industry's desire to extend the shelflife and optimize the safety of its inherently-perishable products is driven by the economics of a geographically-expanding market. Pasteurization, along with temperature control and new packaging techniques, are among the primary approaches used to ensure the quality, safety and shelflife of dairy products, along with aseptic processing, modified atmosphere packaging and bacteriocins.

In addition, there's been a fair amount of research into applying milk's own nonimmunological proteins that have antimicrobial properties. These include lactoperoxidase, lactoferrin and xanthine oxidase. These proteins are involved in complex systems that cause microorganisms to become inactivated. Lactoperoxidase is naturally present in cow's milk and forms an antimicrobial system with hydrogen peroxide and thiocyanate.

The mode of bacterial action of lactoperoxidase entails a major change in the cytoplasmic membrane of various microorganisms. Such gram-negative bacteria as E. coli are more readily killed and lysed by lactoperoxidase than gram-positive bacteria. This is probably due to the bacteria's cell wall composition and thickness.

Most of the U.S. foodborne disease outbreaks related to cheese products have involved high-moisture cheeses. Some researchers have investigated using lactoperoxidase to overcome post-pasteurization contamination by gram-negative pathogens. Research showed significant destruction of E. coli in raw milk. But when the milk was pasteurized and made into high-moisture cheese, the lactoperoxidase was inactivated, enabling surviving E. coli to grow.

The lactoperoxidase system isn't approved for use in the U.S. because its activation requires the addition of a thiocyanide compound, which is recognized as unsafe for infants and children. But it's been recommended for use in developing countries where there is not sufficient refrigeration to protect raw milk when it's transported from the farm to the processing plant.

Meanwhile, lactoferrin, an iron-binding protein in milk, inhibits the growth of bacteria that have high-iron requirements, such as coliforms. But it has no effect on bacteria that need little iron. Xanthine oxidase is an enzyme associated with the fat globule membrane in cow's milk. It produces hydrogen peroxide, which is bactericidal by itself or can be used to activate the lactoperoxidase system.

Further information. Amy Skovsende, Dairy Management Inc., 10255 W. Higgins Rd., Suite 900, Rosemont, IL 60018; phone: [phone]; fax: [phone]; URL: http://www.dairyinfo.comm.