Antimicrobial Compounds

Food antimicrobials are chemical compounds added to or present in foods for the purpose of retarding microbial growth or killing microorganisms. The major targets for antimicrobials are bacteria, molds, and yeasts that are either pathogenic or cause spoilage of foods. The effectiveness of food antimicrobials against viruses and parasites carried by foods is less well characterized. Food antimicrobials are sometimes referred to as food preservatives; however, the latter include food additives that are antimicrobials, antibrowning agents, and antioxidants. Under normal use conditions, food antimicrobials are bacteriostatic or fungistatic rather than bactericidal or fungicidal. The former indicates inhibition of growth of cells while the latter indicates killing of a population. Bacteriostasis is often reversible. Because food antimicrobials are generally static in nature, they will not preserve a food indefinitely. Depending on storage conditions, the food product eventually spoils or becomes hazardous. In addition, food antimicrobials are normally not capable of concealing spoilage of a food product. Rather, the food remains wholesome during its extended shelf life. Food antimicrobials are often used in combination with other food preservation procedures such as heat or refrigeration.

The effectiveness of food antimicrobials depends on many factors, including those related to the target microorganisms, characteristics of the food product, the storage environment, and processing of the food (1). Microbial factors that affect antimicrobial activity include inherent re sistance of a microorganism, initial number, growth stage, cellular composition (eg, gram reaction), previous exposure to stress, and injury. Important factors affecting activity associated with the food product include pH, oxidation reduction potential, and water activity. pH is the most important factor influencing the effectiveness of many food antimicrobials. Antimicrobials that are weak acids are most effective in their undissociated or protonated form because they are able to penetrate the cytoplasmic membrane of a microorganism more effectively in this form. Therefore, the pKa value of these compounds is important in selecting a particular compound for an application. The lower the pH of a food product, the greater the proportion of acid in the undissociated form and the greater the antimicrobial activity. Storage factors affecting antimicrobial activity include time, temperature, and atmosphere. Processing of foods may lead to shifts in microflora and reduction in microbial numbers. Most of the preceding factors influence microbial inhibition or inactivation in an interactive manner.

Cellular targets of food antimicrobials may include the cell wall, cytoplasmic membrane, metabolic enzymes, protein synthesis, or genetic systems. The exact mechanisms for most food antimicrobials are not known or are not well defined. These compounds likely have multiple targets with concentration-dependent thresholds for inactivation or inhibition. A given target is important in an inhibitor's overall mechanism only when its sensitivity is within the range of the antimicrobial concentration that inhibits growth (2).

Following are discussions of compounds that are used traditionally in foods as antimicrobials including salt (sodium chloride), organic acids, nitrites, parahydroxybenzoic acid esters, sulfites, and dimethyl dicarbonate. In addition, selected compounds that are approved as food additives for other uses but have antimicrobial activity as well as compounds that occur naturally in animals, plants, and from microorganisms and have a potential for use as food antimicrobials are addressed.

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