(Archived in Digital Commons, University of Nebraska-Lincoln)
- Intro to Microorganisms in Food
- Symptoms of Foodborne Illnesses
- Food Microbiology: Bacteria, Yeasts and Mold
- Growth Factors of Microorganisms
Foods contaminated with pathogenic microorganisms usually do not look bad, taste bad, or smell bad. It is impossible to determine whether a food is contaminated with pathogenic microorganisms without microbiological testing. To avoid potential problems in foods, it is very important to control or eliminate these microorganisms in food products.
Diseases which result from pathogenic microorganisms are of two types: infection and intoxication.
- Foodborne infection is caused by the ingestion of food containing live bacteria which grow and establish themselves in the human intestinal tract.
- Foodborne intoxication is caused by ingesting food containing toxins formed by bacteria which resulted from the bacterial growth in the food item. The live microorganism does not have to be consumed.
For a foodborne illness (poisoning) to occur, the following conditions must be present:
- The microorganism or its toxin must be present in food.
- The food must be suitable for the microorganism
- The temperature must be suitable for the microorganism
- Enough time must be given for the microorganism
to grow (and to produce a toxin).
- The food must be eaten.
The most common symptom associated with foodborne illnesses is diarrhea. Each pathogenic microorganism has its set of characteristic symptoms.
The severity of the foodborne illness depends on the pathogenic microorganism or toxin ingested, the amount of food consumed (dose), and the health status of the individual. For individuals who have immunocompromised health conditions, or for the aged, children, or pregnant women, any foodborne illness may be life-threatening.
Bacteria, yeasts, and mold are microorganisms associated with foods. The individual microorganism cannot be seen without the aid of a microscope. The size of these microorganisms are measured in microns (1 micron is 1/1000 of a millimeter or U25,40A of an inch). More than a thousand microorganisms in a cluster are barely visible to the eye.
Microorganisms may be classified into three groups according to their activity:
- Beneficial microorganisms may be used in the process of making new foods. Cheese is made with microorganisms which convert the milk sugar to an acid.
- Spoilage microorganisms cause food to spoil and are not harmful to humans. A spoilage microorganism is responsible for souring milk.
- Pathogenic microorganisms are disease-causing microorganisms. The living microorganism or a toxin (microbial waste product) must be consumed to cause symptoms associated with specific pathogenic microorganisms.
Microorganisms can be found virtually everywhere. Bacteria and molds are found in the soil and water. Yeasts are found mainly in the soil. Plant and animal food products support the growth of microorganisms. Bacteria have been detected on plants and animals; molds are usually found on fruits and vegetables; yeasts are generally found on fruits. Many bacteria are part of the normal microflora of the intestinal tracts of man and animals.
- Microorganisms may be transferred from soil and water to plants and animals.
- Raw food stuffs contain microorganisms which may be transferred to processed foods by careless handling.
- Food handlers with poor hygiene practices may transfer microorganisms to food.
- If suitable conditions exist, some of these microorganisms may grow to create a public health concern.
- Specific bacterial species (pathogenic microorganisms) are the main causes of foodborne illnesses in humans.
All microorganisms require moisture, a food source, enough time, and suitable temperatures to grow and multiply.
Microorganisms are composed of about 80% water which is an essential requirement for microorganisms to grow. Moisture requirements vary for each species of microorganism. In general bacteria need more water than yeasts. Yeasts require more water than molds to grow. If water is not available for microorganisms in a food product, the microorganisms may remain but will not grow and multiply.
Certain components in foods will make water unavailable for microorganisms (and thus can inhibit growth).
Salt & Sugar
Salt and sugar added to foods "tie" up water and lower the water activity. When enough salt or sugar is added to a food, the water activity will be lowered to a level that will prevent microorganisms from growing.
- In general, bacterial growth is inhibited by the addition of 5-15% salt. Yeasts and molds can tolerate up to 15% salt.
- To inhibit mold growth, 65-70% sugar must be added. The addition of up to 50% sugar will inhibit bacteria and yeast growth.
Some microorganisms are tolerant of certain conditions.
- Halophilic (salt-liking) microorganisms require salt to be present for the organism to grow.
- Osmiophilic ( sugar-liking) microorganisms, usually yeasts, grow best at high concentrations of sugar.
- Xerophilic (dry-liking) microorganisms can grow with limited moisture.
Microorganisms need a source of nutrients to grow and multiply.
Microorganisms need time to grow and multiply. Under favorable conditions (enough moisture and food available with the desired temperature), cell division (reproductive growth) may occur every 20 to 30 minutes. The time for a microbial cell to double is called the generation time.
Microorganisms grow best within certain temperature ranges. Bacteria are classified into three groups, depending on the temperature at which the bacteria grows best.
- Psychrophilic (cold-liking) bacteria (responsible for food spoilage in refrigerators, grow rapidly at room temp.)
- Growth range 32-77°F
- Optimum temperature 68-77°F
- Mesophilic (middle-liking) bacteria
- Growth range 68-110°F
- Optimum temperature 68-113°F
- Thermophilic (heat-liking) bacteria
- Growth range 113-158°F
- Optimum temperature 122-131°F
Other factors affecting growth:
- Varying requirements for Oxygen (aerobic vs. anaerobic bacteria, e.g.)
- pH - acidity or alkalinity (most microorganisms prefer a pH near neutral [pH = 7.0])
- Darkness vs. Light (Ultraviolet light is lethal to microorganisms)
The bacteria which cause foodborne illness in humans grow best at body temperature (98.6°F - mesophilic bacteria). See more about illness-causing bacteria, Hepatitis A virus and parasites ?
For more information contact: Julie Albrecht, Ph.D., R.D., UNL Extension Food Specialist
Frazier, W.C. and Westhoff, D.C. 1988. Food
Microbiology 4th Edition. McGraw-Hill Inc., New York, N.Y.
IAMFES. 1991. Procedures to Implement the
Hazard Analysis Critical Control Point System. International
Association of Milk, Food and Environmental
Sanitarians, Inc. Ames, Iowa.
IFT Scientific Status Summary. 1988. Bacteria
Associated with Foodborne Diseases. Food Technol.
Ryser, E.T. and Marth, E.H. 1989. "New foodborne
pathogens of public health significance. Am. J.
Diet. Assoc. 89:948-956.
USDA-FSIS. 1989. FSIS Facts, Preventable
Foodborne Illness. Bulletin # FSIS-34. United States
Department of Agriculture, Food Safety Inspection
Service. Washington, DC.
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