Microbiology

Food Spoilage

Spoilage by growth of molds, bacteria, and yeasts limits the time that food can be stored and still be safe and palatable. The environmental conditions optimal for reproduction vary with the type of microorganism. Foods no longer serve as suitable hosts for microorganisms when they are preserved properly. Alterations in moisture level, pH, temperature, and solute concentration are ways of preserving foods.

In addition to the economic losses represented by spoiled food, health hazards may develop while foods are stored unless they are preserved effectively. Salmonellosis, streptococcal infection, staphylococcal poisoning, and botulism are some of the illnesses that can result from contaminated foods.

Freezing protects foods by reducing the rate at which viable microorganisms reproduce by holding the foods below the freezing point of water. Blanching is done prior to freezing vegetables to inactivate enzymes and thus retard deteriorative changes during storage. Browning in sensitive fruits can be avoided by coating them with sugar, a sugar syrup, lemon or other acidic juice, or a solution of ascorbic acid. Specific foods, such as egg yolks, salad  dressings, and starch-thickened mixtures, require use of ingredients selected to overcome textural problems that would otherwise occur during frozen storage. Freezing should be done at a very rapid rate to create numerous small ice crystals, thus minimizing the damage to cell walls. Commercial techniques are more effective at accomplishing rapid freezing than are those available in the home.

Canning is somewhat laborious, but this rigorous heat treatment and sealing in airtight containers enable canned foods to be stored at room temperature for a very long time. Time and temperature of processing must be controlled carefully to ensure that spores of C. botulinum or other microorganisms that may be present are killed.

Low-acid foods such as vegetables and meats must be processed in pressure canners to reach a high enough temperature. High-acid foods (pH 4.5 or lower) can be processed safely in a water bath canner. Inadequate processing can result in flat-sour spoilage, sulfide spoilage, hydrogen swells, and botulism, as well as other less common problems.

Some commercial food processors use non-thermal processing techniques to preserve food. These include irradiation, high-pressure processing, and pulsed electric field processing. Gamma rays from cobalt-60 and beta particles generated by machines can be used to irradiate food. This method of preservation is quite costly, but it is very effective in preventing deteriorative changes for prolonged periods. At the present time, the FDA has authorized irradiation to a limited extent because of the need for additional research to determine safe
conditions for treating many foods by this method. Controlled and modified atmosphere are storage techniques used to maintain optimal quality and reduce food waste.

Why does food get moldy?

Fungal spores are everywhere. They germinate and grow into colonies on any surface that provides enough food, oxygen, and moisture. Fresh bread, cheese, fruits, and vegetables are all perfect for fungal growth. Often, a fruit remains mold-free until its protective skin is punctured, but fungi quickly take over once they gain access to the moist interior.

Perhaps this would be a better question: Why don’t some foods get moldy?

Humans have devised many ways to keep perishable foods fresh. Refrigeration dramatically slows the rate of fungal growth. Salt and sugar, in sufficiently high concentrations, also retard mold growth by limiting the fungus’s ability to take up water by osmosis. Dried foods are preserved in the same way. Cooking and pickling prevent spoilage by damaging microbial enzymes.

One additional method is to add chemical preservatives to foods. Organic acids such as sodium benzoate are common food additives that inhibit mold growth by disrupting fungal cell membranes. Many processed foods are so laden with preservatives that their shelf lives extend foryears, a remarkable accomplishment in a world full of hungry microbes.

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