Wednesday, August 20, 2014

Food Microorganisms & Food Poisoning - I

Microbes in Foods
Nature uses microorganisms to carry out fermentation processes, and for thousands of years mankind has used yeasts, moulds and bacteria to make food products such as bread, beer, wine, vinegar, yoghurt and cheese, as well as fermented fish, meat and vegetables. Currently, more than 3500 traditionally fermented foods exist in the world. They are of animal or vegetable origin and are part of our daily life. Alcoholic drinks are not the only fermented drinks; cocoa beans and coffee grains are fermented after harvest in order to develop their typical flavour profiles. Moreover, fermented products have geographical specificities: in Europe, cheese and bread are widely consumed. In Africa, products manufactured from fermented manioc play a key role in the diet and in Asia, products derived from soy or fish are consumed on a daily basis. Fermentation is one of the oldest transformation and preservation techniques for food. This biological process allows not only the preservation of food but also improves its nutritional and organoleptic qualities (relating to the senses; taste, sight, smell, touch). A well conducted fermentation will favour useful flora, to the detriment of undesirable flora in order to prevent spoilage and promote taste and texture.

The first realization that microorganisms were involved in food production processes was in 1837, when scientists discovered the role of yeast in an alcoholic fermentation. Later, when the world renowned French chemist and biologist Louis Pasteur was trying to explain what happened during the production of beer and vinegar in the 1860es, he found that microorganisms were responsible. However, it wasn’t until after the Second World War that the food industry began to develop the biotechnological techniques we rely on today to produce a wide variety of better, safer foods under controlled conditions.

Food safety is a major focus of food microbiology. Pathogenic bacteria, viruses and toxins produced by microorganisms are all possible contaminants of food. However, microorganisms and their products can also be used to combat these pathogenic microbes. Probiotic bacteria, including those that produce bacteriocins, can kill and inhibit pathogens. Alternatively, purified bacteriocins such as nisin can be added directly to food products. Finally, bacteriophages, viruses that only infect bacteria, can be used to kill bacterial pathogens. Thorough preparation of food, including proper cooking, eliminates most bacteria and viruses. However, toxins produced by contaminants may not be liable to change to non-toxic forms by heating or cooking the contaminated food.

Food Poisoning
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. Pathogenic microorganisms can be transmitted to humans by a number of routes. 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:
  1. The microorganism or its toxin must be present in food,
  2. The food must be suitable for the microorganism to grow,
  3. The temperature must be suitable for the microorganism to grow,
  4. Enough time must be given for the microorganism to grow (and to produce a toxin).
  5. The food must be eaten.
Symptoms of Foodborne Illness
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 immune-compromised health conditions, or for the aged, children, or pregnant women, any foodborne illness may be life-threatening.

Food Microbiology and Foodborne Illness
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 is measured in microns (1 micron is 1/1000 of a millimeter). 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. 

Growth Factors of Microorganisms
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 grow 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). 

Frazier, W.C. and Westhoff, D.C. 1988. Food
Microbiology 4th Edition. McGraw-Hill Inc., New York, N.Y.

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