Food preservation techniques had been employed by humankind since as far back as 3000 BC when ancient Sumerians used salt to preserve meat and fish, dry skins of animals, and make butter and cheese. Techniques of pickling, preserving, mummification, wine making, and fermenting were common in early civilizations of China, Greece, Rome, and Babylon, though they may not have understood the principle behind these techniques. Since Louis Pasteur discovered the role of microorganisms in spoilage of food, many new methods of preservation have been discovered and perfected through the years. These methods include freezing, smoking, canning, boiling, pasteurization, dehydration, and pickling. Preservation of food is done to retain the natural appearance and characteristics of victuals while it is also increasing its shelf life for storage (Jay, Loessner, and Golden 3-4). Food preservatives are any natural or synthetic ingredients added to food to prevent undesirable chemical changes due to microbial action (Russell, and Gould 1-42). This paper discusses the mechanism of action of various commonly used food preservatives.
Food preservatives may be natural or chemical. Natural food preservatives are sugar, salt, vinegar, oil, and alcohol. Salt and sugar effectively slow down the speed of the microbial growth. Salt helps in preservation of food by causing osmosis. When food products are exposed to salt, moisture is drawn out from the cells; as microorganisms need water to survive, they cannot grow and multiply in the absence of moisture. Therefore, salted food stays unspoiled for a long period of time. Sugar also draws out water from the cells by osmosis and works in the similar way as salt. Vinegar, on the other hand, contains citric acid, which is a chelating agent that disrupts the action of enzymes; hence, it helps in food preservation (Montville, and Matthews 337-347).
Chemical preservatives are extremely effective for an extended shelf life and are of three main types: antimicrobial agents, antioxidants, and chelating agents (Montville, and Matthews 337-347)
Antimicrobial agents are generally bacteriostatic compounds as they only inhibit the growth of microorganisms rather than killing them. Commonly used antimicrobial agents include benzoates (like benzoic acid, sodium benzoate), sodium and potassium sorbates, and nitrites (like sodium nitrite). Different antimicrobial compounds act at metabolic targets on a microorganism; for instance, some of them may disrupt the cell wall or the cell membrane; others interfere with the action of enzymes or cause genetic changes. Weak organic acids, such as benzoic acid and sorbic acid, inhibit the growth of bacterial and fungal cells. Sorbic acid also inhibits the germination and growth of bacterial spores. These preservatives are effective at low pH environment as they are in an undissociated state at low pH and can freely cross the cell membrane of a microbial cell. Once inside the cell, they encounter a higher pH and dissociate into charged anions and protons, which cannot cross the cell membrane. Thus, there is a build-up of toxic anions inside the microbial cell, which causes disruption of metabolic activity and, consequently, slows down the growth rate of the microbe. Therefore, benzoates and sorbates are used to preserve acidic foods, like cold drinks, juices, jams, jellies, wines, cheeses, and pickles (Russell, and Gould 1-42).
Nitrites are generally used as preservatives in meat products. Bacteriostatic effect of nitrite preservatives increases with a decrease in pH and is particularly strong below a pH of 6.0. Nitrites act on microbial enzymes, genes, cell walls, cell membranes and bind essential nutrients. Nitrite preservatives disrupt the integrity of microbial cell walls and membranes, thus increasing cell permeability and allowing entry of other toxic substances. Nitrites also attack enzymes and proteins required for DNA synthesis, thereby interfering with gene regulation and cell reproduction (Russell, and Gould 1-42).
Commonly used antioxidants include vitamin E, vitamin C, sulfites (sulfur dioxide), butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and tertiary butylhydroquinone (TBHQ). Deterioration of food can be caused by oxidation. It may be auto oxidation of unsaturated fatty acids or enzyme catalyzed oxidation. Auto oxidation produces free radicals as by-products, which are responsible for the off odors and bad flavors characteristic of rancid food. Free radical scavengers like BHA, BHT, TBHQ, and tocopherols combine with the free radicals slowing the rate of auto oxidation. BHA is used to preserve foods like butter, meat, beer, beverages, potato chips, and nut products; BHT is used for fats and oils, and sulfites are added in beer, wine, and dried food; vitamin E is used largely for fruits and vegetables (Adams, and Moss 98-317).
Some specific enzymes may also cause oxidation of food (known as enzyme catalyzed oxidation) and the various by-products of such reactions cause deteriorative changes in food. Enzymes like phenolases cause oxidation of the amino acid tyrosine. This reaction leads to browning of fruits and vegetables, like potatoes and apples, after cutting. Antioxidants like vitamin C, sulfites, and ascorbic acid bind free oxygen and inhibit this reaction (Adams, and Moss 98-317)
Chelating agents are compounds that bind and inactivate metal ions. They can form several bonds with a single metal ion. The ones commonly used as preservatives are EDTA (ethylenediaminetetraacetic acid), citric acid, and polyphosphates (Montville, and Matthews 337-347).
Traces of metal ions in food (iron, calcium, copper, cobalt) accelerate spoilage of food products and cause oxidation, discoloration, and turbidity. Metal ions act as catalysts for oxidation. Chelating agents bind these molecules, thereby increasing the efficiency of antioxidants and preventing oxidation of foods. For instance, EDTA and polyphosphate are generally used to preserve canned seafood as the magnesium in seafood can react with ammonium phosphate for producing glossy crystals. EDTA binds Ca2+ and Mg2+ ions, thereby increasing the cell susceptibility of gram-negative organisms, as these cations are essential in maintaining a functional cell membrane. EDTA also enhances the action of lysozyme (Montville, and Matthews 337-347)
A discussion of food preservatives is incomplete without a mention of biopreservatives. Biopreservation involves the use of microorganisms or their products to prevent degradation of food. The most widely used biopreservatives are lactic acid bacteria (LAB). Some LAB species produce certain antimicrobial proteins called bacteriocins. Nisin is a type of bacteriocin, which is most commonly used as a food preservative (Boye, and Arcand 373-374). Nisin has antimicrobial action against many gram-positive organisms. It works by increasing the permeability of bacterial cell membrane and causing efflux of many essential cell components, thus causing bacterial death (Adams and Moss 98-317).
Several countries across the world are facing food shortage; therefore, food security is a major global challenge. People cannot afford to let food go to waste; therefore, preservatives are necessary in extending the shelf life of food. Despite the fact, health concerns dictate the need to reduce chemicals from humans’ daily diet. Although chemical preservatives are effectively used in preservation of food articles, all chemicals have side effects. Preservatives used in food are deemed fit for human consumption by the FDA, but they can still have adverse effects on health. Future research in food preservation technology needs to focus on finding newer, more effective natural preservatives to decrease dependence on synthetic ones.