How do food preservatives work? In a recent issue of Chemical & Engineering News food scientist Hassan Gourama explains how a variety of different chemicals can be used to keep food safe and palatable.
Chemical preservatives cannot take the place of stronger preservation methods, like commercial sterilisation, which kills most bacteria and enzymes, but they can be used effectively to delay spoiling and to stop the growth of harmful micro-organisms.
Preservatives can be grouped into three general types: antimicrobials that block growth of bacteria, moulds or yeasts; antioxidants that slow oxidation of fats and lipids that leads to rancidity; and a third type that fights enzymes that promote the natural ripening that occurs after fruits or vegetables are picked.
Sulphur dioxide works in all three areas and is part of a larger group called sulphites, compounds found in numerous foods. A small percentage of people are allergic to sulphites (see FN.com headline news today on new food labelling rules). Some vinegars, fruit juices and dried fruits contain sulphites.
Sulphites block the growth of microbes by interrupting the normal functioning of their cells, according to Gourama, who is based at Pennsylvania State University.
Propionates are antimicrobials that help keep bakery products fresh. Propionic acid occurs naturally in apples, strawberries, grains and cheese. It works against bread moulds and spores of a bacterium that leads to a condition called "rope" that makes bread inedible. Benzoates, another weak antimicrobial, fight fungi, working best in areas at a low acid level, where there are few bacteria. Benzoates are found naturally in cranberries.
Nitrites and nitrates most commonly are used to preserve meats and block deadly botulism bacteria. These preservatives also give cured meat its fresh pink colour because without nitrates or nitrites meat turns brown. The only potential problem with nitrites, Gourama claims, is that they react with amino acids to form nitrosamines, cancer-causing agents. The levels used in cured meat are low and are not of concern, he added in the article.
Antioxidant preservatives stop the chemical breakdown of food when products are exposed to the air. Unsaturated fatty acids in oils and lipids are especially susceptible to oxidation and will take on a rancid flavour and odour as a result.
A third group of preservatives attacks enzymes in food that cause fruits and vegetables to over-ripen after harvest. One enzyme, for example, causes apples and potatoes to turn brown soon after they are cut or peeled. Acids such as ascorbic acid (vitamin C) and citric acid retard this process by making the pH level uncomfortably low for the enzyme.
As a clear indication of the current consumer trend towards healthy food products, Gourama claims that today there are many food scientists searching for more preservatives in natural products. New antimicrobials have even been found in the micro-organisms themselves, he concluded.
