Natural or formulated antimicrobials.

Among the approaches employed in making foods safe is the use of chemical agents exhibiting antimicrobial activity. These chemicals may be either synthetic compounds intentionally added to foods or naturally occurring, biologically derived antimicrobials. These substances may exhibit antimicrobial properties in the foods in which they normally are found or may be used commercially as additives--hurdles--to other foods requiring preservation.

Consumer perception that industrially synthesized food antimicrobials may be associated with potential toxicological problems has generated interest within the industry in using naturally occurring compounds. Yet commonly used antimicrobials, such as organic acids that are routinely produced in large quantities through chemical synthesis, also are found naturally in many foods.

The extraction of these and other antimicrobials from natural sources can be complex and expensive. Nevertheless, synthetic agents may be considered less desirable by a segment of the consuming public. Sources of natural antimicrobials include herbs, spices, fruits, milk, eggs and lactic acid bacteria.

The selection, manufacture and commercial application of a proper antimicrobial is challenging because of the complexity of food, the variety of factors influencing preservation and the complex chemical and sensory properties of natural antimicrobials. Naturally occurring antimicrobials could be useful as individual factors or hurdles in multifactor food preservation systems. Natural antimicrobials that have found food uses include egg-white lysozyme, hydrogen peroxide, ethanol, natamycin and nisin.

You should take a number of factors into consideration if you're using natural antimicrobial substances in food products:

* The physical and chemical properties of the antimicrobials, such as their chemical reactivity, solubility, dissociation constant (pKa) and toxicity;

* The composition and properties of the food to be preserved; and

* The types and numbers of microorganisms to be controlled.

Ideal antimicrobials should not contribute to the development of resistant strains. Consider their modes of action:

* Their reaction with the cell membrane causing increased permeability and loss of cellular constituents;

* The inactivation of essential enzymes; and

* The destruction or functional inactivation of genetic material.

While no antimicrobial is able to preserve a product that is grossly contaminated, the use of combinations of preservatives and processes is likely to remain an important strategy in preservation efforts aimed at meeting consumer demand for wholesome and safe products.

Further information. V.K. Juneja, Microbial Food Safety Research, USDA-ARS-Eastern Regional Research Center, 600 E. Mermaid Ln., Wyndmoor, PA 19038; phone: [phone]; fax: [phone]; email: [e-mail].