Food preservatives can be extrinsic (intentionally added), intrinsic (normal constituent of food), or developed (produced during fermentation) (Potter and Hotchkiss, 1995; Jay, 1996). Factors affecting preservative effectiveness include: (1) concentration of inhibitor, (2) kind, number, and age of microorganisms (older cells more resistant), (3) temperature, (4) time of exposure (if long enough. some microbes can adapt and overcome inhibition), and (5) chemical and physical characteristics of food (water activity, pH, solutes, etc.). Preservatives that are cidal are able to kill microorganisms when large concentrations of the substances are used. Static activity results when sublethal concentrations inhibit microbial growth.
Some examples of inorganic preservatives are sodium chloride (NaCl), nitrate and nitrite salts, sulfites, and sulfur dioxide (SO2). NaCl lowers water activity and causes plasmolysis by withdrawing water from cells. Nitrites and nitrates are curing agents for meats (hams, bacons, sausages, etc.) to inhibit C. botulinum under vacuum packaging conditions. Sulfur dioxide (SO2), sulfites (SO3), bisulfite (HSO3), and metabisulfites (S2O5) form sulfurous acid in aqueous solutions, which is the antimicrobial agent. Sulfites are widely used in the wine industry to sanitize equipment and reduce competing microorganisms. Wine yeasts are resistant to sulfites. Sulfites are also used in dried fruits and some fruit juices. Sulfites have been used to prevent enzymatic and nonenzymatic browning in some fruits and vegetables (cut potatoes).
Nitrites can react with secondary and tertiary amines to form potentially carcinogenic nitrosamines during cooking; however, current formulations greatly reduce this risk. Nitrates in high concentrations can result in red blood cell functional impairment; however, at approved usage levels they are safe (Nitrite Safety Council, 1980; Hotchkiss and Cassens, 1987). Sulfiting agents likewise can cause adverse respiratory effects to susceptible consumers, particularly asthmatics (Stevenson and Simon, 1981; Schwartz, 1983). Therefore, use of these two classes of agents is strictly regulated.
A number of organic acids and their salts are used as preservatives. These include lactic acid and lactates, propionic acid and propionates, citric acid, acetic acid, sorbic acid, and sorbates, benzoic acid and benzoates, and methyl and propyl parabens (benzoic acid derivatives). Benzoates are most effective when undissociated; therefore, they require low pH values for activity (2.5–4.0). The sodium salt of benzoate is used to improve solubility in foods. When esterified as parabens, benzoates are active at higher pH values. Benzoates are primarily used in high-acid foods (jams, jellies, juices, soft drinks, ketchup, salad dressings, and margarine). They are active against yeast and molds, but minimally so against bacteria. They can be used at levels up to 0.1%.
Sorbic acid and sorbate salts (potassium most effective) are effective at pH values less than 6.5 but at a higher pH than benzoates. Sorbates are used in cheeses, baked or nonyeast goods, beverages, jellies, jams, salad dressings, dried fruits, pickles, and margarine. They inhibit yeasts and molds, but few bacteria except C. botulinum. They prevent yeast growth during vegetable fermentations and can be used at levels up to 0.3%.
Propionic acid and propionate salts (calcium most common) are active against molds at pH values less than 6. They have limited activity against yeasts and bacteria. They are widely used in baked products and cheeses. Propionic acid is found naturally in Swiss cheese at levels up to 1%. Propionates can be added to foods at levels up to 0.3%.
Acetic acid is found in vinegar at levels up to 4–5%. It is used in mayonnaise, pickles, and ketchup, primarily as a flavoring agent. Acetic acid is most active against bacteria, but has some yeast and mold activity, though less active than sorbates or propionates. Lactic acid, citric acid, and their salts can be added as preservatives, to lower pH, and as flavorants. They are also developed during fermentation. These organic acids are most effective against bacteria.
Some antibiotics may be found in foods, although medical compounds are not allowed in human food, trace amounts used for animal therapy may occasionally be found. Bacteriocins, which are antimicrobial peptides produced by microorganisms, can be found in foods. An example of an approved bacteriocin is nisin, which is allowed in process cheese food as an additive. Some naturally occurring enzymes (lysozyme and lactoferrin) can be used as preservatives in limited applications where denaturation is not an issue. Some spices, herbs, and essential oils have antimicrobial activity, but such high levels are needed that the food becomes unpalatable. Ethanol has excellent preservative ability but is underutilized because of social stigma. Wood smoke, whether natural or added in liquid form, contains several phenolic antimicrobial compounds in addition to formaldehyde. Wood smoke is most active against vegetative bacteria and some fungi. Bacterial endospores are resistant. Activity is correlated with phenolic content. Carbon dioxide gas can dissolve in food tissues to lower pH and inhibit microbes. Developed preservatives produced during fermentation include organic acids (primarily lactic, acetic, and propionic), ethanol, and bacteriocins. All added preservatives must meet government standards for direct addition to foods. All preservatives added to foods are GRAS.