SOURCES OF MICROBIAL CONTAMINATION TO FOODS
The microbial flora of foods usually consist of
1).The microorganisms associated with the raw material,
2).Those that are acquired during handling and processing and
3).Those that survive treatment during processing and storage.
These microorganisms do not arise by spontaneous generation but must have contaminated the food at one point or the other. The microbiologist is concerned with knowing the sources of contamination to foods because by knowing the sources, it becomes easy to control the contamination and keep the microbial load in the food as low as possible. By doing this, a longer shelf life can be achieved for the food and the chances of microbial illness when the food is ingested is reduced. With food preservation techniques, it becomes easier to control and eliminate chances of contamination when the source of the contamination is known.
Potential sources of contamination are the following:-
Soil, Water, Air, Humans, Plants and Animals, Processing Equipment, Ingredients, Product to Product and Packaging materials. Sometimes it may be difficult to say which organism came from a particular source.
SOIL:-
Soil is a natural habitat of many microorganisms which is usually in very high numbers. The microbial density is greater near the soil surface and reduces as you go deeper into soil. The types and the numbers of microorganisms vary with the type of soil, (eg:- sandy or clay soil) as well as with the environmental conditions.
In general, soil is not a good medium for the growth of microorganisms but the microorganisms tend to survive.
Microbial growth in the soil is limited to areas of organic matter. These areas include areas of the root of plants, plant debris falling onto soil, animal carcasses, and fecal deposits by animals and dead microorganisms. Wherever any of these are present in soil, there will usually be a higher number of microorganisms present. The microorganisms in soil are usually in some form of resting stage or in some reduced form of metabolic activity. The spores of Bacillus and Clostridium are quite prevalent in some soils. Generally, bacteria usually outnumber the other types of microorganisms. Those that are common to foods and also found in foods include species of Acinetobacter, Alcaligenes, Arthrobacter, Bacillus, Clostridium, Corynebacterium, Micrococcus , Flavobacterium and some yeasts and moulds.
Microorganisms in the soil can contaminate tubers or root crops by direct contact. Crops growing near the ground level are also contaminated by dirt blown by the wind or by the rain splashed onto the soil in which they are growing. Food crops such as cabbage, lettuce, beans, etc become victims of direct contamination by soil microorganisms. The microbial numbers and types on the crops are influenced by the degree of contamination of the soil in which they are grown.
Mechanical harvesting also increases the amount of soil contamination. Research has found that hand harvested crops have a lower number of soil microorganisms compared to crops that are mechanically harvested. Cereal crops are contaminated mostly during mechanical harvesting. Marine sediments have a high number of soil microorganisms and the numbers are usually higher nearer the shore than as you go deeper into the water. These sediments serve as a source of microorganisms for water and for fish and shellfish. During the trawling of fishnets along the bottom of the sea or rivers, the sediment is disturbed so that it contaminates the fish or shellfish that are caught in the nets.
WATER
Water is a potential source of microbial contamination to foods. Rain contains microorganisms it picks up from the air. As rain lands on the soil, it is further contaminated by soil microorganisms.
Water contacts food during production, harvesting and processing. If the water used for irrigation of fruits and vegetables is sewage effluent, then the fruits and vegetables can become potential health hazards.
Seafoods are harvested from water. Microorganisms in the water contaminate the surface, gills,and intestinal tract of fish and shellfish. The occurrence of fecal coliforms inside fish is a direct reflection of the pollution level of the water environment. Potential pathogens in the drinking water of animals can be a health hazard to the animal and to people working with the animal. During harvesting, water may be used to hydro-cool the vegetables. If this water is already polluted with sewage or animal wastes, this becomes a source of inoculation of microorganisms to the vegetables. Furthermore, water redistributes organisms already on the vegetables from a higher level of contamination to other parts of the vegetables so that the entire vegetables become contaminated. Since many vegetables and fruits are eaten raw, the use of untreated water to wash these foods can serve as a vehicle of transmission of pathogenic microorganisms. Ice is used to cool and maintain coolness for fresh foods. There are microorganisms that can still survive inside the ice especially if the water used to form the ice was already contaminated. As the ice melts, the organisms associated with it are passed onto the food and the organisms associated with the food are also passed back into the melted ice water. For this reason, the reuse of melted ice water for foods is never a recommended nor an acceptable procedure.
Water in a food processing plant can be a source of microorganisms for contaminating food. Water is the NUMBER 1 raw ingredient in any food industry. In a food industry, water enters into the processing of most foods in the following ways:-
(i) Cleaning of equipment and processing areas
(ii) Washing of the food
(iii) Conveying the food
(iv) An ingredient of the food.
The safety of water that is used in foods is of major importance. Even if water is considered safe for drinking, it may not be acceptable for food processing. This is because the concept of water safety arises from the water being free from pathogenic organisms. It may however still contain food spoilage organisms that are not pathogenic to humans. For instance, psychrotrophs are responsible for the spoilage of dairy foods and refrigerated milk and milk products. A food processor in the dairy industry should therefore scrutinise the water not only for pathogens but for psychrophilic and psychrotrophic organsims that can spoil its product.
In the poultry industry, water is used for the scalding of the birds prior to de-feathering. Research has shown that dirt from the head, body, feathers, feces and feet contaminate the scald water. Even though the temperature of the scalding water is high enough to kill many pathogens or spoilage, organisms, not all microorganisms are killed. These surviving thermophilic organisms are able to contaminate the internal portions of the birds through the lungs and vascular system if the birds are still taking their last breath and the heart is still pumping before the scalding takes place.
In food canning operations, water is used to cool the cans of food immediately after heat processing. This is done to sharply drop the temperature of the can and its contents so that the thermophilic organisms which may have survived the heat treatment process are not given a chance to survive or grow. If left to air cool, chances are that these surviving thermophiles may germinate. Due to the heat and expansion of the metal, the seams and the seals of the cans are under stress and leakage can therefore occur. It is therefore important that the cooling water be maintained with as few microorganisms as possible. Since no further processing occurs after cooling, any contamination of the food by microorganisms in the cooling water can grow inside the food product to cause spoilage or if pathogenic, may cause food borne illness
Water is also used as a direct ingredient for mixing the food. In this case water therefore becomes a direct source of microbial contamination to the food.
AIR
There is no normal or natural microflora of air. Micro-organsims found in the air come from various sources. Generally, mould spores are more prevalent than other microorganisms. The main source of microorganisms to the air is decaying plant material near the ground surface. Mold spores especially are picked up by gusts of wind near the ground surface and disperses them into the air.
The types of microorganisms in the air are often associated with then type of activity going on in that area. Human beings shed aerosols and organisms during coughing or sneezing. The air in a classroom for instance will therefore be heavily contaminated by organisms coming from talking, sneezing, coughing which are all the activity going on in that area. In an enclosed space, the microbial population of the air is directly proportional to the number of persons there, their activity and the rate of air circulation. Outdoors, humans may not contribute significantly to the microbial population of the air, but in an enclosed environment such as a food processing area, their contribution is significant. The microbial flora of the air in a food processing environment reflects the sanitary condition of the plant. Food plant sanitary practices therefore advise to take a sample of the microbial flora of the air in a food processing environment from time to time.
Bacteria and viruses from enteric matter become airborne when a toilet is flushed. These microorganisms settle out on the surfaces throughout the washroom. Sanitary practices especially in food processing industries therefore instruct the washing of hands most especially after using the washroom. Yeasts are predominant in the air around bakeries and breweries. Aerosols are produced in food processing plants by spray washing or by spray cooling of food or by high pressure sprays used in cleaning. Workers also produce aerosols in the working areas. Operation of equipment, movement of equipment can cause turbulence of wind currents that increase the microbial load in the air. In clean areas of a food processing plant, the numbers of microorganisms in the air can be quite few. However in dirty areas or areas for example areas where live animals are handled or raw products are brought into the processing operation, the microbial load can be quite high.
Air is therefore subjected to a number of sources of microorganisms and the numbers or types present are dependent upon many factors such as the tendency to settle and maintain viability. Because of all these factors, the contamination of exposed food is easy and food is constantly subjected to airborne contamination until it is sealed in a package.
However, the effect that air has on food microflora is dependent on the level of contamination of the air and the time of contact that the food had with the air.
HUMANS
A human embryo develops in a relatively sterile environment. However at birth, the baby is subjected to a massive invasion of microorganisms with the skin being the most available part of the body for colonization.
Many microorganisms are however merely transient on the skin while others become established as normal, permanent flora.
The skin is never free from microorganisms. The dirtier the skin, the greater the contamination. Normal human skin contains a relatively stable microflora but the numbers and types vary from person to person, site to site and day to day. Washing the skin removes most of the transient microorganisms, but it is practically impossible to remove all of the normal flora. Staphylococcus aureus is found more on the hands and the face than on other parts of the body. The organism is associated with the nose and is spread when people handle their faces and noses. S. aureus is also associated with infections such as acne, pimples and boils. It is therefore easy for microorganisms to be passed from humans to foods simply by handling.
The hands are subject to contamination by considerable number of bacteria, most of which are unable to multiply on the hands and usually die. However, these transient bacteria can be passed onto food products when they are handled. Workers in the meat and poultry processing plants have been found to harbour an unusually high number of E.coli and Salmonellae on their hands.
Hair covering the skin is also a potential source of microorganisms to foods. Hair acts as a carrier to retain and shed microorganisms. Sideburns and moustaches are not permitted to be worn in a food processing environment. This is because these hairy growths serve as carriers of microorganisms and a continuous source for disseminating S.aureus.
Apart from this, the hair itself may fall off as hair strands to contaminate the food. Whenever a hair strand is found in food, it is likely that associated microorganisms with that hair strand would also have contaminated the food. This is why good manufacturing practices in food processing environments must insist on the practice of hair covering and removal of all other hairy growths such as beards or moustaches that cannot be covered.
Carelessness of human beings is an important factor in humans being a source of microbial contamination to foods. Failure to properly clean and sanitize equipment, failure to wash the hands, working and handling food when harbouring an infection, poor personal hygiene, lack of care in handling food are all the careless things that people do that increase the microbial load of foods.
EQUIPMENT
With the increase in technological industrialization, more machines were developed to process foods and do the work that humans would have normally done. There has therefore become more contact of food with machines and equipment. Equipment may be cleaned and sanitized but this does not mean that it is sterile. Even on washed, visibly clean surfaces, the survival and growth of microorganisms is still possible. Visibly clean surfaces may still have food debris that can serve as nutrients for microbial growth. However, the potential for survival and growth increases with improperly cleaned or sanitized equipment. Therefore, if food residues are visible on any food processing equipment, it is sure that a potentially high microbial level will occur. Equipment that have many joints or poorly soldered joints have a potential for high microbial build up. Food debris that get stuck in such joints will most certainly serve as nutrients for microbial build up. This then serves as a source of contamination when food contacts these surface. Small pieces of equipment such as knives, cutting boards etc are used in food processing. Metal processing equipment do not support the growth of micro-organisms. It has no natural or normal microflora. Stainless steel is more expensive than other material as food process equipment material. However, because it does not support the growth of microorganisms, is easily cleaned and is also durable, stainless steel is the best choice of material in manufacturing food processing equipment. In terms of product quality, the use of stainless steel as a food processing equipment cannot be overemphasized.
PRODUCT TO PRODUCT
(NO DATA ON THIS CHECK LATER, LECTURER GAVE NO DATA)
INGREDIENTS
Although ingredients may constitute a small part of the total food, they nonetheless add a substantial number of microorganisms. Spices or seasonings are often the high source of microorganisms. Spices are parts of plants such as dried seeds, buds, fruit or flower parts, barks, roots usually of tropical origin. These seasonings and flavours may contain bacteria over 108/gm. Dried pepper can be contaminated with bacteria and especially fungi. Aspergillus flavus is found in spices and have the potential to produce aflatoxins.
Thermophilic bacteria usually in the form of spores are added to foods with ingredients such as spices, starch, flour and sugar. Thermophilic spores become important when considering canned foods. The higher the level of heat resistant spores, the greater the chance that some may survive the heat treatment given to canned foods, and become a potential spoilage problem.
Since ingredients are an important source of microorganisms, food processors must establish acceptable microbiological limits for these substances and ensure that if they are used in food manufacture, adjustments of the process will be made to take care of any excessive bacterial load.
PACKAGING
Packaging is a potential source of food contamination. However, compared to the number of microorganisms on foods, the microbial flora of most food packaging materials is low. For instance, disposable bottles usually have a lower microbial count than returnable reusable bottles.
Plastics used for packaging food products may have attracted dust and microorganisms during the manufacture of the plastic. Product to product contamination is also possible via packaging. For instance, if the same packaging material used for poultry is also used for vegetables, the containers can be contaminated with Salmonellae which now become transferred to the vegetables.
Packages serve as a protective covering to limit or prevent microbial contamination. However, packaging does not prevent microbial growth. It is important that preparation of the food for packaging is designed to limit or prevent microbial contamination prior to packaging.
Furthermore, packaging must be durable enough to maintain integrity during storage and distribution.
PLANTS AND ANIMALS
Feed for animals and food for plants also serve as a source of potential contamination of food.
Microorganisms in animal feed can contaminate the feet, the hair, and the feathers of the animals.
Consumption of the feed also adds organisms to the digestive tract. These organisms that survive the rigours of digestion, when excreted can contaminate the exterior portions of the animal.
When animal or human wastes are used as fertilizers, pathogens from these sources can contaminate the plant and associated foods. When vegetables are eaten raw, this becomes a source of food borne illness.
Plants are contaminated by microorganisms from several sources such as water, air, fertilizers, etc..
Microbial flora on plant surfaces vary with the kind of plant. Various bacteria can adhere to and colonize the surface of leaves. Flowers of fruits harbour yeasts such as Saccharomyces, Hansenula, torulopsis, Candida, Rhodotorula etc… The surface and the interior tissue of plants can also become contaminated.
Animals also harbour the types of organisms that are found in their environment. Animals harbour organisms in their gastrointestinal tract, nasal passages, on their skin, feet, hair or feathers. When animals such as rats or mice are allowed to come into contact with food, these organisms can be passed on from the animal onto the food.
Flies much more so than any other common insect are a serious potential carrier of disease producing organisms. Flies pick up organisms on their hairy legs or feet. They then contaminate exposed food by walking over the food or leaving their excreta on the food. Flies spread Salmonellae, Shigella, Vibrio, Escherichia and other potential disease causing organisms.
Milk has also been contaminated from the udder of an infected cow.
Milk of a healthy cow is sterile. However, the teat canal of the cow is sometimes microorganism laden and these organisms now contaminate the milk during the milking process.
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