Food is considered spoiled when an undesirable change in the colour, flavour, odour or texture has occurred. Foreign substances in food products make foods undesirable. It is a gradual process and is a natural phenomenon; it occurs at varying rates depending on the storage temperature, kind of food involved, kind of microorganisms present, packaging materials used, food additives used and method of preservation.

There are seven main factors that must be controlled to limit microbial spoilage of food. These being the source of microorganisms, the amount of moisture available, the atmosphere in which the food is packaged, pre-cooking or special processing to kill microorganisms, the time period, the temperature at which the food is held and the use of preservative agents. This discussion continues from Part 1--Biotalk 21 April 2003.

Pre-cooking or Special Processing
There are a number of processes that can reduce the microbial load of a food before it is packaged. The major types are cooking, or partial cooking, irradiation, drying and aseptic processing. With the exception of aseptic processing, each of these processes can dramatically reduce the number of viable microorganisms in a food. Their effectiveness will depend on the type of food, the initial bacterial load and the conditions of the process used. Partial cooking must be carefully controlled in case there are bacterial spores present, the heating step may, in fact, cause the spores to become vegetative bacteria and initiate growth during the cooling step or during the shelf life of the product. Aseptic handling is designed to ensure that no additional microorganisms are added during the food processing step.

Time
Some organisms grow faster than others. Under ideal conditions, certain bacterial populations can double in 5 to 7 minutes; others require hours. Bacteria that reproduce most quickly will dominate. The longer the time that a food product remains between the temperatures of 5 oC and 60 oC the more likely that microbial growth will occur. This is especially important following cooking where a rapid cooling step is essential.

Temperature

As stated above the "magic" numbers for food temperatures are less than 5 oC and greater than 60 oC. That is, the major steps are cooking and chilling temperatures. However, the time the food is held at the cooking temperature is an integral part of the control mechanism. Probably the single most important factor in preventing microbial food spoilage is temperature. Generally speaking, the cooler the food can be kept, the longer shelf life it will have. A good "rule of thumb" to use: To double the shelf life of a food that needs refrigeration, lower the temperature 8 oC, or for every 8 oC decrease in storage temperature, food will last twice as long. Remember, however, that some foods such as tomatoes and lettuce will freeze or damage at 0 to 0.5 oC. In general the closer you can get to 0 oC the better. It is also important to note that some bacteria are able to grow and survive at low temperatures. The major group of significance is the psychrotrophs, this includes pathogens such as Listeria monocytogenes, and some strains of Escherichia coli, Bacillus cereus and Clostridium perfringens. The bacteria Brochothrix thermosphacta and Pseudomonas species are the major causes of food spoilage in chilled foods.

Preservatives
Preservatives are food additives that prolong the shelf life by protecting against deterioration by microbes. The addition of weak acids lower the pH of a food matrix and can limit microbial growth as can a variety of chemicals. Both the chemicals that can be used and the foods that are allowed to contain preservatives are controlled by legislation.

The discussion will continue with preservatives in the next BioTalk Food Newsletter.

Sources: Spoilage of Processed Foods: Causes and Diagnosis. AIFST Inc. NSW Branch, Food Microbiology Group. Food and Spoilage and You,
George Schuler, William Hurst, Estes Reynolds and James Christian; revised by P.T. Tybor. Extension Food Science. University of Georgia College of Agricultural & Environmental Sciences.