1. INTRODUCTION
The purpose of studying abattoir microbiology is to ensure that the number of micro-organisms on the meat is as low as possible by the time it leaves the abattoir. For this purpose some knowledge of micro-organisms, how they behave and how to control them, is needed.
Definition
Microbiology is the study of micro-organisms that are small living creatures; exist as single cells or cell clusters and that are mostly free-living. (Microbial cells are thus distinct from animals or plant cells that are unable to live free in nature but can exist only as part of organisms consisting of many cells.)
Food microbiology is the study of small living creatures found in foods.
2. TYPES OF ORGANISMS
A number of small organisms are important in foods such as:
• Bacteria
• Yeasts and moulds (fungi)
• Viruses
• Protozoa
3. IMPORTANCE OF BACTERIA
Bacteria are by far the most important group encountered in red meat and poultry production and are of great concern from the standpoint of both food spoilage and foodborne disease. The fungi are less important but do cause some problems. Viruses and protozoa are of concern but more difficult to pick up in routine tests. The emphasis in this overview will be on the bacteria.
4. ELEMENTARY BACTERIOLOGY
The size, shapes and habits of bacteria
Bacteria are exceedingly small individual single celled organisms that cannot be seen with the unaided eye. They are observed under a microscope. By the time bacterial colonies can be seen with the naked eye, they consist of many (more than a billion) cells per colony. Bacteria are therefore present on surfaces in the abattoir and on the meat without our being aware of them unless they are so many that we can actually see them. By this time meat will be slimy and obviously spoilt.
Different types of bacteria differ in shape and size. They can be round, oval; rod shaped etc. and can be larger or smaller. Usually the shape and size of bacteria tells the non-specialist hardly anything about the type of organism one is dealing with.
Bacteria are everywhere except when they are deliberately excluded or destroyed. By practising good abattoir hygiene and slaughtering techniques, the number of bacteria on carcasses can be kept low or even reduced.
Requirements for Bacterial Growth
It is important to remember that, like all animals, bacteria require moisture and certain nutrients in order to grow. Meat is rich in nutrients and water and can support good growth of a variety of bacteria. In addition, there are many other environmental factors that influence the ability of bacteria to survive and grow, such as temperature, the gasses in the atmosphere around them (gaseous atmosphere), acidity (pH), etc. It is the manipulation of these growth requirements that helps us to control micro-organisms in the abattoir.
Bacterial growth
Method of Bacterial Growth (Multiplication)
When we speak of bacterial growth, we are really referring to bacterial multiplication. Bacteria multiply by the process of binary fission, which is 1 parent cell divides to produce 2 daughter cells (one generation); each daughter cell divides to produce 2 additional cells and so on. For example, if we assume that we have 1000 bacteria per gram of meat, and that all the bacteria multiplied as above, they would reach 1 000 000 (also written as 1x106) bacteria per gram in 10 generations.... (1 000; 2 000; 4 000; 8 000; 16 000; 32 000;
64 000; 128 000; 256 000; 512 000; 1 024 000).
From this follows that both the number of bacteria initially present as well as the number of multiplications they undergo determine the number of bacteria eventually present on the meat.
Both initial numbers and growth of bacteria on carcasses must be kept low during abattoir processing.
Generation time, i.e., the time necessary to produce a generation, is an important consideration in abattoirs. Some types of bacteria have a very short generation time; only a matter of minutes, while others have a generation time of hours. An important factor influencing the generation time of bacteria is temperature. The use of refrigeration to lengthen the generation time of unwanted bacteria (e.g. spoilage bacteria) and therefore slow down their multiplication is a common and most important method of extending the shelf-life of perishable products like meat.
Growth Cycle
The growth cycle of bacteria consist of a short period of little or no growth (lag phase), then the population increases rapidly (the phase of logarithmic growth or log phase) toward a maximum and reaches a plateau (stationary phase), and then decreases (death phase). The lag phase is a period of adjustment during which there is considerable cellular activity but little or no cell division. Providing a less than desirable environment for multiplication such as refrigeration usually prolongs the length of the lag phase. During the lag phase, cell division occurs rapidly at a fairly constant rate. During the stationary phase, a balance between cell division and cell death maintains the maximum number of live cells. Cell death during the phase of death is caused by many factors, the most prominent of which is the accumulation of the cell’s own products. An example would be, in the case of fermented meat products, the accumulation of lactic acid.
GROWTH CYCLE
Temperature and Bacterial Growth
Most bacterial species have a minimum, maximum, and optimum temperature for growth.
• Psychrotrophic bacteria – capable of growth at commercial refrigeration temperatures. Most psychrotrophic bacteria grow best at 15-25°C (59-77°F) and at slower rates under refrigerated storage. These are especially significant because many of the common red meat and poultry spoilage bacteria are psychrotrophs.
• Mesophylic bacteria – grow best at temperatures between 20°C and 45°C (68-113°F). Bacteria growing in the gastro-intestinal tract of the live animal are mostly mesophiles.
• Thermophilic bacteria – grow best at temperatures above 45°C (113°F). Many will not grow below 40°C (104°F).
• Thermoduric bacteria – capable of surviving mild heat treatments such as pasteurisation.
4.2.4 Gaseous Atmosphere and Bacterial Growth
• Aerobic bacteria grow only in the presence of oxygen.
• Anaerobic bacteria grow only in the absence of oxygen.
• Facultative anaerobic bacteria grow in the presence or absence of oxygen.
• Micro-aerophilic bacteria grow in an atmosphere containing less oxygen than in air.
The fact that bacteria have varying gaseous atmospheric requirements is of great significance in the red meat and poultry industries. Meat is often wrapped in plastic that is readily permeable to oxygen. Here we expect the growth of psychrotrophic aerobes. However, on vacuum packaged meat products other bacteria, such as micro aerophilic bacteria or psychrotrophic facultative anaerobes, will grow better. The special plastic material used for vacuum packaging is not permeable to oxygen.
5. GROUPS OF BACTERIA
As already stated, bacteria are present in all natural environments. We breathe in countless bacteria even in the purest mountain air. Most of those are not harmful, some are used by man and a small minority are undesirable.
Bacteria used in foodstuff production
Some bacteria are used to produce the desirable body, texture and flavour in foods. Examples of these are fermented sausages, pickles, cheese, yoghurt, sauerkraut, vinegar, souring of sorghum beer and sourdough bread. Yeasts are used to produce ordinary bread and alcoholic beverages such as wine, barley and other beers.
Spoilage bacteria
This group includes bacteria that will cause deterioration of foods through breakdown of the food constituents and/or accumulation of undesirable end products of bacterial metabolism. Poultry and meat even if produced under hygienic conditions and stored under refrigeration, will ultimately become unacceptable due to the growth of psychrotrophic spoilage bacteria.
Food-Borne Pathogens
Those bacteria that is capable of causing illness in persons consuming the food. Some of the more important examples from meat include Salmonella, Campylobacter, Yersinia enterocolitica, Staphylococcus aureus, Listeria monocytogenes, Clostridium perfringens, and Clostridium botulinum (Clostridium botulinum is particularly dangerous because it produces one of the most powerful known toxins).
6. BACTERIAL SPORES
A few bacteria (e.g. the genera Bacillus and Clostridium) are capable of producing a body called a spore inside the cell (functionally spores resemble plant seeds). Bacterial spores are more resistant to adverse conditions such as heat, chemicals, dehydration, and irradiation than the cell itself. Thus, the cell may be killed but the spore may survive.
Among these spore forming bacteria are:
• Typical spoilage bacteria of heat processed canned foods such as the flat sour bacteria (Bacillus steareothermophillus) and
• Some are capable of causing food-borne disease, such as Clostridium botulinum, Clostridium perfringens, and Bacillus cereus.
7. BACTERIA ISOLATED FROM RED MEAT AND POULTRY
Members of the following bacterial genera can with greater or lesser regularity and some only rarely, be recovered from red meat, poultry and their products:
Acinetobacter Hafnia
Aeromonas Lactobacillus
Alcaligenes ListeriaArthrobacter Microbacterium
Bacillus Micrococcus
Bronchothrix Moraxella/Acinetobacter groep
Brucella Mycobacterium
Campylobacter Pediococcus
Clostridium PseudomonasCorynebacterium Salmonella
Enterobacter Staphylococcus
Escherichia Streptococcus
Flavobacterium Yersinia
8. MICROBIOLOGY OF MEAT PRODUCTION
The deep muscle tissues of healthy, slaughtered livestock contain few, if any, micro-organisms. However, their exterior surfaces (hide, hair, skin, feathers,) are naturally contaminated with a variety of micro-organisms as are their gastro-intestinal tracts. From the moment of slaughter, each processing step subjects the carcass to opportunities for contamination with micro-organisms from the exterior surfaces, utensils and equipment and, most importantly, from the gastro-intestinal tract.
Cutting of carcasses also involves the use of utensils and equipment and transfers micro-organisms to the cut surfaces. Theoretically removal of the skin should expose the sterile surface of the muscle but in practice the extra handling seems to contribute significantly to the bacterial load on the surfaces. This happens with meat production where the skin is removed early in the slaughtering process (e.g. beef, mutton, lamb, ostrich, and goat) or where the skin is removed later on (e.g. some pork cuts, skinned chicken portions).
There is ample opportunity to contaminate the exposed tissues of the carcass with micro-organisms from:
• exterior surface of the animal
• contents of the gastro-intestinal tract
• equipment and utensils
• workers garments and hands
• the abattoir itself (e.g. air, floor drains, water drip from ceiling)
• water (and if used, ice)
• food additives (e.g. spices for value added products)
Therefore, we need to control this opportunity for contamination by:
Using properly cleaned equipment.
Ensuring that the abattoir is properly cleaned/sanitised.
Use hygienic methods of dressing that control contamination. Clean utensils at appropriate intervals during the process.Apply a high standard of personal hygiene.
Meat with a good shelf-life has 102-104 organisms per cm2. To put the numbers of organisms associated with some sources of contamination into perspective: The exterior surfaces (hide, hair, skin, feathers) of healthy, live animals and birds are naturally contaminated with large numbers of a variety of micro-organisms. In a study of live cattle, 107 organisms were found per cm2 of hide. The soil (ground) is also a major source of micro-organisms and has comparable numbers (107) of bacteria per gram of soil. Faeces are about 100 x
more contaminated and have an APC and “coli forms” of about 109 and 108 per gram of faeces, respectively. All of these can therefore serve as sources of microbial contaminants of the meat.The hide, fleece or skin of the animal is known to be a major source of carcass contamination (pathogens and spoilage bacteria). Special care should be taken to avoid contact with the meat. Removal of hides or fleece should be carried out in a manner that avoids contact between the outside of the skin and the carcass. When the surface of the hide touches the surface of meat during removal, can cause transfer of significant numbers of organisms to the meat surface. Likewise, hands and equipment that touch the outside of the hide can serve to transfer organisms to the meat and should not come into contact with the underlying carcass meat before thorough cleansing.Since it is extremely difficult to obtain clean meat from dirty animals or birds, it is important that only relatively clean animals are presented for slaughtering. The cleanliness of livestock depends on husbandry, weather and climate (rainy, dry), methods of transport (stress causes defaecation and urination) and holding conditions at the abattoir. Cattle from feedlots may carry more faecal bacteria and less soil organisms than those from pastures. The modern trend is that excessively dirty animals should not be slaughtered until action has been taken to clean them. Also, strategies should be developed to reduce the number of such animals presented for slaughtering.From the figures quoted above, it is clear that under normal conditions, the heaviest and potentially the most dangerous load of bacteria is in the animal’s digestive tract. Already a small volume of material from the intestinal tract can contaminate the carcass with sufficiently high numbers of “coli forms’” to cause problems so that rupturing of the intestines or spillage of the intestinal content would cause severe contamination of the carcass. It is essential that great care be taken during evisceration to keep the viscera intact.In addition to the skin, the gastro-intestinal and respiratory tracts, urine and milk are other important animal sources of contamination. Meat handling and preparation involves contact with knives, hands and clothing of workers, processing equipment, (e.g. saws, hooks, boning tables, conveyers) and water used to wash carcasses, hands and equipment. Airborne spread of particles and aerosols will also occur in the abattoir. All of these factors can lead to the transmission of potentially hazardous organisms and contamination of carcasses. To minimise contamination, it is logical that attention should be paid to sanitation of all equipment (e.g. knife-sterilizers), well-chlorinated water, personal hygiene, hand-washing facilities near worker stations as well as the other methods of hygienic slaughtering.An important point to remember is that microbes firmly attach to meat and skin. This process is not yet well understood but it appears to become irreversible with time – the longer organisms remain on the meat the more difficult it becomes to remove them. In poultry processing, the contact period between the meat surface and contaminating organisms is reduced by washing carcasses at intermediate points during processing before attachment occurs. The principle should not be applied to larger carcasses because too much wetting spreads rather than removes contamination. In fact, when small volumes of faeces, intestinal contents, mud or soil are spread over the carcass by rinsing, the clean areas of the carcass can become quite heavily contaminated. This is the reason why carcasses should not be rinsed. Wet carcasses also tend to spoil more rapidly - especially if wet and warm. Un-split carcasses should never be washed and split carcasses should only partially washed under lowest pressure possible.With any delay between consumption or further processing, it is essential to cool the carcass. As far as the microbiological quality of the carcass is concerned, fast chilling is indicated to restrict microbial growth. However, too rapid chilling can lead to cold-shortening of pre-rigor muscle and a loss of tenderness. With these conflicting requirements, optimal conditions for chilling must be a compromise. During chilling, contamination may occur by carcasses touching one another, by contact with dirty floors and walls, by splashing if cleaning is carried out in a loaded chiller and from the air, especially if the filters are not regularly cleaned.Cutting of carcasses also involves the use of utensils and equipment that transfer micro-organisms to the cut surfaces. This happens with meat production where the skin is removed early in the slaughtering process (e.g. beef, mutton, lamb, ostrich, goat) or where the skin is removed later on (e.g. some pork cuts, skinned chicken portions).The main challenge to the meat industry in relation to hygiene is to minimise external contamination of meat with micro-organisms during all stages of the production chain.......................................................................................................................