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5. Basic methods of quality control

The quality of meat and meat products is defined by the following criteria:

• palatability (typical texture and consistency, juiciness, good flavour);
• proportion of lean meat to fat;
• freshness and adequate conservability of the products;
• absence of harmful micro-organisms or substances; and
• appropriate (preferably minimal) use of additives and meat extenders.

The different criteria need different methods of quality control, such as:

• organoleptic evaluation
• physical test methods
• chemical analysis
• microbiological examination

According to the accuracy needed, the control method applied can be simple or more complicated and different auxiliary technical devices must be used.

In order to inform consumers and meat processors about the quality of meat and meat products, simple and fast control methods are best suited in many cases, although exact details on residues, toxins and special food components can only be obtained through specialized laboratories.

Basic methods for quality control must involve little or no equipment and obviously sensory evaluation will be most important. Some physical tests, however, can easily be performed using simple instruments such as thermometers, manometers, scales, etc.

By contrast, chemical and microbiological tests are more complicated. These methods not only require standard equipment but also skilled and experienced personnel to do the tests and to interpret the results.

The following mainly refers to the basic methods of quality control used in connection with the handling and processing of meat. These control methods can easily be applied for meat products processed with simple meat preservation techniques.

ORGANOLEPTIC EVALUATION

Organoleptic evaluation consists in describing the attributes of food, in this special case of meat and meat products, that can be perceived by the sense organs. The attributes to be evaluated are appearance, colour, texture and consistency, smell and taste.

Appearance

The way meat looks, either as a carcass or as boneless meat cuts, has an important impact on its objective or subjective evaluation. Grading is an objective evaluation method in this context. Traditional methods of carcass grading after slaughter involve the aspect of beef or pork sides, poultry carcasses, etc. Skilled graders are able to classify different carcasses by checking the size, the volume of muscular tissue, fat layers, etc. Although in modern grading procedures more and more technical equipment has been incorporated, visual methods are still in use. They can be of special value in most developing countries where no extremely sophisticated methods are needed.

The way the consumers or the processors check the appearance of meat is subjective. Differences will be registered in the relation of lean meat and fat including the degree of marbling or in the relation of bones and lean meat. Furthermore, unfavourable influences can be detected such as unclean meat surfaces, surfaces too wet or too dry, or unattractive blood splashes on muscle tissue.

Processed meat, on the other hand, can roughly be evaluated by its appearance according to the different raw materials of which the product is composed and where the use of some components is exaggerated (for instance too many particles of visible fat or connective tissue, etc.). Special product treatments (for instance chilling, freezing, cooking, curing, smoking, drying) or the kind and quality of portioning and packaging (casings, plastic bags, cans) will be recognized by evaluating the appearance.

Colour

Under normal circumstances the colour of meat is in the range of red and may differ from dark red, bright red to slightly red; but also pink, grey and brown colours may occur. In many cases the colour indicates the type and stage of the treatment to which the meat has been subjected, as well as the stage of freshness.

In judging meat colour, some experience is needed to be able to distinguish between the colour which is typical for a specific treatment or which is typical for specific freshness. Furthermore, meat deriving from different species of animals may have rather different colours, as can easily be seen when comparing beef, pork and poultry meat.

The natural colour of fresh meat, except poultry meat, is dark red, caused by the muscle pigment, myoglobine. Fresh meat surfaces which have been in contact with the air for only a short period turn into a bright red colour because of the influence of the oxygen in the air. Oxygen is easily aggregated to the myoglobine and drastically changes the colour of the meat surfaces exposed to it. On the other hand, in the absence of oxygen, for example in meat cuts packaged in impermeable plastic bags, meat surfaces remain or become dark red again. The same conditions generally prevail in the interior of meat cuts which are not reached by oxygen. Changes from dark red to bright red are therefore typical and are normal reactions of fresh meat.

Meat which is in the process of losing its freshness, however, no longer shows a bright red colour, even when intensively exposed to the air, because of the partial destruction of the red meat pigment which results in a grey, brown or greenish colour. Once these conditions occur the consumer has to decide, after carefully checking the appearance, together with testing smell and taste, whether the meat has to be discarded as a whole or whether use can be made of some parts which so far have not been altered.

Remarkable changes in the meat colour occur when fresh meat has been boiled or cooked. It loses its red colour almost entirely and turns to grey or brown. The reason for this is the destruction of the myoglobine through heat treatment. On the other hand, it has long been known that after pickling (curing) fresh meat with curing ingredients (nitrite), the meat colour remains red during longer storage periods, after ripening, drying and even after intensive heat treatment. Obviously the original meat colour has not been conserved, but a chemical reaction has taken place during the curing process transforming the unstable pigment of the fresh meat into a stable red pigment. This is the typical colour shown in sausages of all types, raw and cooked hams, corned beef, etc.

It should also be noted that cured products have a longer shelf-life than fresh meat because of the conserving effect of the curing salt. However, cured products will also deteriorate under unfavourable conditions, cooked cured products sooner than raw cured products. Cured products with a decreasing keeping quality can be recognized when the red colour becomes pale or changes to grey or green.

Texture and consistency (tenderness and juiciness)

Meat prepared for the consumer should be tender and juicy. Meat tenderness depends on the animal species from which the meat originates. Lamb, pork and poultry meat are sufficiently tender after slaughter, but beef requires a certain period of maturation to achieve optimal eating quality.

Texture and consistency, including juiciness, are an important criterion, still neglected by many consumers, for the eating quality of meat. Often consumers do not know that the eating quality of meat can be upgraded by ripening, especially in the case of beef and similar meats. There is also a great deal of consumer negligence in how to prepare meat. It should be cooked to become sufficiently tender, but cooking should not be too intense otherwise the meat becomes dry, hard and with no juiciness.

The simple way to check the consistency of foods is by chewing. Although this test seems easy, in practice it is rather complicated. Taste panelists need experience, particularly when the different samples have to be ranked, for example which sample is the toughest, the second toughest or the most tender.

The texture is of less importance in meat products, such as cured or canned products, sausages, etc., because they are either made of comminuted meat and/or meat which has undergone heat treatment or long maturation periods and will therefore generally be tender. On the other hand, inadequate processing methods (too intensive cooking, curing, comminuting) may cause losses in the desired consistency and juiciness, and the best way to check this is by chewing.

Smell and taste (aroma and flavour)

These characteristics are related to each other to a certain extent because they have to be evaluated together for the reliable determination of a product's flavour. The smell of fresh meat should be slightly acidic, increasing in relation to the duration of the ripening period because of the formation of acids such as lactic acid. On the other hand, meat in decomposition generates an increasingly unpleasant odour owing to substances originating from the bacterial degradation of the meat proteins, such as sulphur compounds, mercaptane, etc.

The freshness of meat is generally indicated by its smell together with its appearance and colour. Sorting out deteriorated meat is mandatory from the point of view of the product's palatability. It is also important because of the fact that high bacterial contamination of meat in decomposition could be accompanied by food-poisoning bacteria(pathogens), which have a deleterious impact on consumers' health. On the other hand, the best fresh meat can also be heavily contaminated with food-poisoning bacteria because these micro-organisms do not cause organoleptic alterations by destruction of meat proteins. Food poisoning can therefore only be avoided by proper hygienic meat handling. The flavour of fresh meat can also be checked by putting small samples (approx. 10 pieces of 1 cm³ each) in preheated water of 80°C for about five minutes (boiling test). The odour of the cooking broth and the taste of the warm meat samples will indicate whether the meat was fresh or in deterioration or subject to undesired influences, for instance rancidity of the meat fat, any a typical meat flavour due to the feed and the sex (boar taint) of the animal or treatment with veterinary drugs shortly before slaughter.

When processing the meat, the smell and taste of the meat products can differ a great deal owing to heat treatment and the use of salt, spices and food additives. Every meat product has its typical smell and taste, and the test person should know about it. Changes in these qualities indicate the use of improper raw materials or a deterioration of the meat product during storage.

Experience is required to become acquainted with the typical flavour (smell and taste) of foods. Only four basic taste components--sweet, sour, bitter and salty--will be perceived by the taste buds. These receptors are small papillae located in certain areas of the tongue. However, the overall flavour consists of smell and taste produced by the meat components and influenced and covered by spices and those compounds produced by ripening or heat treatment. Flavour test panelists should be aware of these special cases. Panelists should not smoke or eat spicy meals before starting the test and should rinse their mouth frequently with warm water during the test.

Sensory evaluation plays an important role in the examination of meat and meat products. Not only does scientific sensory evaluation with skilled panelists using special test programmes and point systems give reliable results, but useful results can also be obtained in a simple way at the consumer level. For the average consumer sensory evaluation is the only way to decide whether or not he or she should buy or eat a certain product.

In developing countries consumers do not receive sufficient information and training on this point, although it is often the only means available for quality control. Sensory evaluation is easy to understand and to perform. What is needed is a basic knowledge of the composition of foods and their typical texture, colour and flavour.

PHYSICAL TEST METHODS

Physical test methods focus either on the actual condition of meat and meat products, or on the conditions around the product, for example in storage rooms, packages, etc. Equipment will be needed for all these tests which is easily applicable and resistant to utilization under the conditions of practical meat handling and processing.

Temperature

Storage of meat and meat products requires low temperatures to make sure that the growth of micro-organisms will be retarded (chilling between-1 to +4°C) or inhibited (freezing preferably in the range of -18 to -30°C).

Cooking of meat requires high temperatures (starting from a temperature of about 55°C needed for denaturation, but generally higher temperatures are applied, up to 100°C).

Canning of meat requires temperatures above 100°C, and for sterilized products where all micro-organisms are inactivated, at least 121°C.

These examples demonstrate the importance of different temperatures for different purposes and the necessity of exact temperature measurements using thermometers or temperature recorders.

Glass thermometers should not be used in direct contact with meat because they may break, leaving undesired fragments in the meat, but they are useful when permanently fixed to walls of chillers or production rooms or to cooking equipment or autoclaves for easy checking of the relevant temperatures.

Mechanical bimetal-thermometers, utilizing the extension or contraction of a bimetal spiral under various temperatures, are not very accurate and not sufficiently shock-resistant for practical work in meat industries. Nevertheless, they are widely used and can serve for rough estimates.

Electrical thermometers (Fig. 33), consisting of a sensor and a battery-powered electronic instrument, are well suited for meat industries. The sensor functions as a semiconductor. Under different temperatures, differences in the electric conductibility of the sensor are produced. The temperature which the sensor takes up by contact to the surrounding media (water, air, meat, etc.) produces a certain voltage in the electric system. This voltage is registered and displayed as a digital reading of the actual temperature on the instrument.

FIG. 33.
Electrical thermometer with digital display and two sensors for measuring air temperature (left) and the temperature of meat, liquids, etc. (right).

Advantages of modern electronic thermometers are:

• no glass or other parts that can easily break;
• the sensor can be easily pushed deep into the meat, as well as into frozen meat, and is also heat-resistant under sterilization temperatures;
• display of the correct temperature within seconds;
• no frequent calibration necessary;
• a wide range of temperatures can be covered using one instrument (the temperature range of the instruments recommended for use in meat industries should be between +140°C and -40°C); and
• accurate temperature measurement, also in decimals.

Humidity

In some special field of meat processing and storage, air humidity is of importance.

In cutting rooms the humidity of the air should be below the level which would cause vapour condensation on the surfaces of the meat being deboned and cut. Vapour condensation may enhance bacterial growth.

Storage chillers for fresh meat require a balanced air humidity that does not cause wet surfaces on the meat with resulting accelerated bacterial growth, but on the other hand keeps evaporation losses low. The relative humidity recommended for this special purpose lies in the range of 70 percent.

Chambers for the maturation of raw hams or dry sausages of the salami type require controlled air humidity, starting from 90–95 percent and after a certain period finalizing the process at 70–75 percent relative humidity. This procedure is important for the balanced drying and ripening of the products. Suitable instruments (hygrometers) for the exact measurement of relative humidity are therefore needed.

In simple but less accurate hygrometers a hair or special synthetic fibre is connected with a pointer and, according to the contraction of the hair or fibre under dry conditions and its extension under wet conditions, the pointer indicates the actual relative humidity on an appropriate scale.

A more accurate way for humidity control is the psychrometric system. These instruments use a dry and a wet sensor to define the ambiental temperature. The temperature indicated by the wet sensor will always be lower, in this case, because of evaporative cooling. The drier the air, the more intensive evaporative cooling will be. Using both temperature values (dry and wet temperature), the value of the relative humidity is determined in practical work using a table for easy calculation.

A modernized psychrometric system which uses electronic devices is available. In this case the humid sensor has not actually to be kept wet, but consists of hygroscopic material with altering electrical resistance. The relative humidity calculated from the temperatures delivered by the sensors by means of a micro-processor is directly displayed on the instrument (Fig. 34).

Water activity (a_w)

Water activity is the free water available for microbial growth in a food product. Free water is that part of the water content that can be eliminated from the product in the form of water vapour. Hence, the term "water activity" is defined as the ratio of the water vapour pressure measured in the product and the pressure of a saturated water vapour atmosphere at the same temperature. This physical definition is used in connection with a number of meat products whose keeping quality depends on their water content. Micro-organisms need a certain degree of moisture to be able to grow on foods. The minimum moisture content necessary for microbial growth varies with the single species of micro-organisms and can be expressed in terms of minimum water activity required.

The lowest a_w-values permitting growth of spoilage organisms are:

• normal bacteria 0.91
• normal yeasts 0.88
• normal moulds 0.80
• halophilic (NaCl-tolerant) bacteria 0.77.

The keeping quality of dried meats and meat products without refrigeration depends on their water activity. Dried meat such as biltong, charque, etc. reaches a sufficiently low water activity to be shelf-stable. However, water activity should decrease as fast as possible as slow drying could cause deterioration in a prolonged phase of the process with a still high water activity. The situation is more complicated in the case of products which cannot be dried too intensively such as dry sausages or raw hams. The water activity of these products is relatively low but would still allow the growth of some undesired micro-organisms. Under these circumstances an appropriate shelf-life has to be ensured by the combination of several inhibiting factors, i.e. water activity, content of salt and curing ingredients and the acidity of the product.

Information on the water activity of certain products can be important for further handling, packaging and storage. Simple methods for the determination of water activity are therefore useful.

FIG. 34.
Electronic psychrometer (hygrometer) and sensor (right) for direct measurement of the relative air humidity.

As water activity refers only to the water available for microbial growth in a product, the chemical analysis of the total moisture content is of limited value since it would also include the water bound by the proteins. The proper way to determine water activity is to measure the humidity of the remaining air in a hermetically closed small cabinet which is to a certain extent filled with the product sample. After a short time a hygroscopic equilibrium between the sample and the surrounding air will be reached. Thus, the humidity determined in the air is equivalent to the humidity available in the product and water activity can be calculated.

For the measurement of air humidity under these conditions, the same principles apply as previously described. Simple devices utilize the extension or contraction of hairs or synthetic fibres, and more sophisticated and more expensive systems use electronic devices.

The sample is placed in the bottom part of the tin and then the lid of the tin that contains the device to measure the humidity is hermetically screwed on. After two hours, hygroscopic equilibrium is reached in the can and the reading of the instrument corresponds to the actual water activity of the product, provided the test has been carried out at a temperature of exactly 25°C. If this temperature cannot be maintained, corrective calculations will be necessary.

Some examples for values of water activity (a_w) of different products are shown:

fresh raw meat	0.99–0.98
cooked ham	0.98–0.96
frankfurter-type sausages	0.98–0.93
liver sausage	0.97–0.95
raw cured ham	0.96–0.80
dry sausage (salami type)	0.96–0.70
dry meat	0.75–0.50

A certain number of micro-organisms are inhibited at a_w 0.95, but other species are still able to grow. At a_w 0.92 all bacteria groups are inhibited, but the growth of moulds and yeasts is still possible.

Airtight closure of cans

For shelf-stable canned meat products two aspects are important from the microbiological standpoint. During sterilization, micro-organisms and their spores have to be inactivated and the can must be hermetically sealed to avoid further contamination of the product after sterilization.

Invisible small perforations of the tinplate or small defects after the closure of the lid will inevitably lead to a recontamination by penetrating bacteria and after a short time spoilage of the canned product will occur. Cans should therefore be checked from time to time for these defects.

A simple method is available for this purpose. Using an air-pump with a special device to penetrate the tinplate, the air is pumped into a closed but empty can (Fig. 35). The internal pressure built up in the can can be controlled by a manometer connected with the air pump. When dipping the inflated can into water it can easily be seen whether the can is hermetically sealed and if not where the cause for the permeability is, either in the prefabricated body (side wall and bottom) of the can or in the area of the lid seam. In the latter case the function of the can-closing equipment in the processing plant has to be thoroughly checked.

Weight differences

The high water content of meat (approx. 70 percent) and meat products (which varies from 70 percent to 10 percent) causes weight differences owing to evaporation losses or drip losses that occur during handling, processing or storage.

Unpackaged meat and meat products are especially subject to considerable evaporation losses. During chilling of warm carcasses evaporation losses of 1 to 2 percent cannot be avoided but further evaporation losses of chilled or frozen meat should not occur when suitable storage conditions (not too dry) or suitable packaging (plastic bags, containers, boxes) are employed. However, some drip losses of packaged meat cannot be avoided.

During meat processing weight losses of meat by cooking, frying or other heat treatment can be registered and reach high values (up to 30 to 35 percent). These losses are unavoidable.

On the other hand, some meat products require weight losses by evaporation to reach their specific keeping quality, for example raw hams, dry sausages or dried meat. In this case, water activity as previously described plays an important role.

Information on weight losses in meat handling and processing is important from the economic and technological point of view. Weight losses can easily be determined using scales of different types, such as suspension scales for carcasses or batches of products and horizontal scales for packages or portions.

Salt concentration in brines

In addition to dry curing methods (dry salt and curing ingredients on the meat), brines are also used for pickling and curing the meat. Brines contain salt and in most cases also sugar and nitrite dissolved in water. With this curing process, meat is either immersed into a brine or the brine is injected into the meat with special devices. In both cases salt is a limiting factor for the sensory quality of the products; in other words salt is needed but should not exceed 2.5 to 3 percent in cooked cured products and 4.5 to 5 percent in raw cured and dried products.

To comply with these requirements, simple methods for testing salt concentration in brines are necessary. For this purpose salimeters have proved to be a useful piece of equipment. Salimeters are densimeters, the graduation showing salt concentrations. Salimeters are dipped into the brine and according to a lower or higher salt content they sink deeper or less deep into the brine. The reading of the salimeters at surface level indicates the salt concentration of the brine. The various technologies of meat curing use brines with NaCI-concentrations in the range of 8 to 22 percent.

FIG. 35.
Mechanical instrument to prove airtight closure of cans.

OTHER PHYSICAL TEST METHODS

The physical test methods which have been described can easily be performed since the use of the technical equipment necessary is not too complicated. Other physical test methods exist too, for example, light intensity measurement, colour measurement or texture and consistency measurements of meat and meat products. These tests require rather complicated and expensive instruments and skilled technical personnel. For routine work, criteria such as light, colour, texture and consistency can be evaluated in a satisfactory way by using the corresponding sensory test methods.

Chemical analysis

Chemical characteristics of foods are related to the product itself and refer primarily to the content of specific substances, which are important from the point of view of keeping quality, flavour, nutritional value, etc., or which may also represent harmful residues.

The test methods necessary are generally complicated and need sophisticated equipment. However, there are also some simple and quick methods for chemical testing with sufficient accuracy which can be applied in the daily routine work such as pH-measurement, moisture/fat/protein determination and various screening methods utilizing test paper strips.

pH-measurement

The pH-value or acidity of meat is important in relation to the meat's microbiological and keeping quality. In the live animal the pH-value of the muscular tissue is about 7.0 to 7.1. Very soon after slaughter a drop in the pH-value is observed and after several hours (24 hours or less) the pH-value reaches its lowest level of about 5.6 to 5.8. The increasing acidity is because of the post-mortem formation of lactic acid from glycogen, a sugar-like substance stored in the live animal's muscles for energy supply.

In meat lactic acid causes a decrease in pH-value which is favourable for keeping quality (low pH inhibits bacterial growth) and for flavour (acidity is one of the components of meat flavour). However, the pH of meat is not uniform either in different carcasses or in different muscles of one carcass. Physiological oscillations do not greatly harm meat quality but abnormal reactions in meat are of great economic, hygienic and technological impact.

There are two types of abnormal reaction with regard to the pH in meat. First the pH-value may drop too fast and second it may not reach the normal low level several hours after slaughter, but remain in the range of 7.

Both abnormalities can easily be detected by pH-measurement in the meat. A too fast pH-value decrease is evident, when one hour after slaughter low pH-values in the range of 5.6 to 5.8 are already reached. This phenomenon occurs only in pigs and the meat remains pale, soft and exudative (PSE). Because of its paleness and wetness (low water-holding capacity), this meat should not be used for ham and sausage manufacture (gives dry, tasteless products).

An insufficient decrease of the pH-value, which occurs both in pork and beef, is of hygienic significance because of the lack of building up a certain degree of acidity and suppressing microbiological growth. This meat also remains close to pH-value 7 after several hours, and is dark, firm and dry (DFD). It should not be used for meat and meat products which have to be stored over a longer period, such as vacuum-packed meat cuts, dry sausages of the salami-type or cured raw hams. However, it is well suited for cooked meat products because of its extremely good water-holding capacity.

It can be seen from this that the pH-measurement is of particular importance for the selection of the raw material for meat processing purposes. Hence, portable electric pH-meters are widely distributed and utilized in the meat industry (Fig. 36).

The pH is measured on meat surfaces or in the meat itself, in the latter case by pushing the sensor into the muscle or by means of an incision using a knife. The sensor consists of a glass electrode filled with an electrolyte (solution of KC1) and a sensitive glass membrane attached at the top.

Through the membrane the difference in the hydrogen-ion concentration, which corresponds to the acidity of the meat, is detected and digitally displayed on the attached instrument.

pH-measurement on meat can easily be performed but the following points must be considered:

• the electrode sensor must be completely filled with the electrolyte;
• the instrument must be adjusted daily (calibrated) using two buffer solutions with pH-values 4 and 7;
• after each measurement the electrode must be cleaned using distilled water;
• before each measurement the temperature of the meat, meat product, etc. must be checked and the instrument adjusted accordingly.

Moisture/fat/protein determination

Information about the moisture, fat and protein content is essential for the evaluation of the quality of different meats and meat products. Determination methods have changed a great deal in this field in recent years. Revolutionary techniques were introduced using X-rays, infra-red radiation or microwaves in automatic equipment for quick analyses of moisture, fat and protein. These modern methods are time-saving, the results are delivered within minutes or seconds and high numbers of samples can be tested. However, the equipment is expensive and therefore not suitable for small industries. For routine controls, where not necessarily highly accurate but reliable results on moisture, fat, protein and anorganic components (ash) are needed, cheaper and less complicated methods can be applied. A specially designed laboratory scale, together with some other devices, is required. After homogenizing and weighing the sample, it is fast dried using an infrared beam (or a microwave oven if available). The weight difference is equivalent to the product's water (moisture) content. The fat is then dissolved using a fat-extracting liquid and removed together with the liquid. The solvent is evaporated. The weight of the residue represents the fat content of the sample. Finally, the sample is charred in a muffle furnace and the weight of the residue is the ash content. Since the sum of the percentages of moisture, fat, ash and protein must be 100, and since the percentage of moisture, fat and ash is known, the protein content in percent is calculated as follows: 100% minus the percent of moisture, fat and ash. This method is not precise, but it is fast, provides useful results about the composition of meat and meat products and can be applied without high costs.

FIG. 36.
Portable electric pH-meter with sensor (glass electrode). The glass electrode is protected by a removable cover of flexible synthetic material in order to avoid breakage and keep the diaphragm of the sensor humid.

For chemical evaluations a number of screening methods are also available using different test papers. The results are indicated by changes of the colour of certain areas on the paper strips. These test papers are used for pH-measurement, screening of the nitrite content and even for the screening of some harmful residues such as antibiotics. pH-measurements on meat with test strips are negatively influenced by the meat pigment making the colour determination often difficult and the pH-determination not very accurate.

MICROBIOLOGICAL EXAMINATION

These control methods cannot be carried out without laboratory equipment, because they require sample preparation under sterile conditions, incubation of the samples under constant temperatures and sufficient microbiological knowledge on the part of the personnel involved to interpret the results. However, the application of microbiological methods is the only way to obtain information about the hygienic status of places, equipment and foods. It is true that unclean conditions will always indicate high microbiological contamination and one could argue that a thorough cleaning-up rather than a further microbiological analysis would be needed in those cases. But there could also be the need of detecting the source of permanent contamination (for example through the water, movement of personnel, raw material delivered, etc.) or of food poisoning bacteria. Under these circumstances microbiological examinations can often be very helpful and solve immediate problems.

Some methods suitable for routine work should be mentioned.

Trigger methods

Microbiological culture media in special small moulds are lightly pressed against walls, equipment (knives, machines), meat surfaces or hands of personnel. The micro-organisms adherent to these objects are absorbed by the surface of the culture media, and after adequate incubation (one to two days at 30 to 37°C), microbial colonies can be identified and counted on the media. Each one of the colonies grown during incubation corresponds to one micro-organism which was on the object tested.

Instead of culture media a special sterile strip of cellotape together with a trigger can be used for taking samples from surfaces (Fig. 37). After that the cellotape is laid on a culture media for incubation. This procedure allows the utilization of one culture medium for the incubation of different samples at the same time (Fig. 38). However, there is one disadvantage with the trigger system. In the case of high bacterial contamination of the surfaces, tested bacterial colonies will grow very densely together and can no longer be counted.

Swab method

Surface contamination related to a certain area can be sampled using a sterile swab. After rubbing the swab gently along the surface to be tested (Fig. 39), it is suspended on the surface of a culture media. In contrast with the trigger method, bacterial contamination can be spread over the whole surface (Fig. 40) which is important in the case of high contamination. Thus the samples can always be evaluated since the single colonies are not grown together (Fig. 41). However, the method lacks some accuracy since bacteria may remain in the swab.

FIG. 37.
Trigger and sterile cellotape for microbiological sampling of the meat surface.

FIG. 38.
Culture medium with various fields after incubation of different samples taken using the technique shown in Fig. 37.

FIG. 39.
Sampling microbial contamination on a defined surface area marked by sterile template with sterile swabs.

FIG. 40.
Transfer of the sample taken with swab on to the surface of the culture medium.

FIG. 41.
Bacterial colonies grown from one cell each on the culture medium after the incubation period.

FAO TECHNICAL PAPERS

FAO ANIMAL PRODUCTION AND HEALTH PAPERS:

1. Animal breeding: selected articles from World Animal Review, 1977 (C* E*F* S *)

2. Eradication of hog cholera and African swine fever, 1976 (E * F * S *)

3. Insecticides and application equipment for tsetse control, 1977 (E * F *)

4. New feed resources, 1977 (E/F/S *)

5. Bibliography of the criollo cattle of the Americas, 1977 (E/S *)

6. Mediterranean cattle and sheep in crossbreeding, 1977 (E * F *)

7. Environmental impact of tsetse chemical control, 1977 (E * F *)

7 Rev. Environmental impact of tsetse chemical control, 1980 (E * F *)

8. Declining breeds of Mediterranean sheep, 1978 (E * F *)

9. Slaughterhouse and slaughterslab design and construction, 1978 (E * F * S *)

10. Treating straw for animal feeding, 1978 (C *, E *, F *, S *)

11. Packaging, storage and distribution of processed milk, 1978 (E *)

12. Ruminant nutrition: selected articles from World Animal Review, 1978 (C * E * F * S *)

13. Buffalo reproduction and artificial insemination, 1979 (E **)

14. The African tripanosomiases, 1979 (E * F *)

15. Establishment of dairy training centres, 1979 (E *)

16. Open yard housing for young cattle, 1981 (E * F * S)

17. Prolific tropical sheep, 1980 (E * F * S *)

18. Feed from animal wastes: state of knowledge, 1980 (E *)

19. East Coast fever and related tick-borne diseases, 1980 (E * S *)

20/1. Trypanotolerant livestock in West and Central Africa, 1980 Vol. 1 - General study (E * F *)

20/2. Trypanotolerant livestock in West and Central Africa, 1980
Vol. 2 - Country studies (E * F *)

20/3. Le bétail trypanotolérant en Afrique occidentale et centrale
Vol. 3 - Bilan d'une décennie, 1988 (F *)

21. Guidelines for dairy accounting, 1980 (E *)

22. Recursos genéticos animales en América Latina, 1981 (S *)

23. Disease control in semen and embryos, 1982 (E * F * S *)

24. Animal genetic resources - conservation and management, 1981 (E *)

25. Reproductive efficiency in cattle, 1982 (E * F * S *)

26. Camels and camel milk, 1982 (E *)

27. Deer farming, 1982 (E *)

28. Feed from animal wastes: feeding manual, 1982 (E *)

29. Echinococcosis/hydatidosis surveillance, prevention and control: FAO/UNEP/WHO guidelines, 1982 (E *)

30. Sheep and goat breeds of India, 1982 (E *)

31. Hormones in animal production, 1982 (E *)

32. Crop residues and agro-industrial by-products in animal feeding, 1982 (E/F *)

33. Haemorrhagic septicaemia, 1982 (E * F *)

34. Breeding plans for ruminant livestock in the tropics, 1982 (E * F * S *)

35. Off-tastes in raw and reconstituted milk, 1983 (E * F * S *)

36. Ticks and tick-borne diseases: selected articles from World Animal Review, 1983 (E * F * S *)

37. African animal trypanosomiasis: selected articles from World Animal Review, 1983 (E * F *)

38. Diagnosis and vaccination for the control of brucellosis in the Near East, 1983 (Ar * E *)

39. Solar energy in small-scale milk collection and processing, 1983 (E * F *)

40. Intensive sheep production in Near East, 1983 (E * Ar *)

41. Integrating crops and livestock in West Agrica, 1983 (E * F *)

42. Animal energy in agriculture in Africa and Asia, 1984 (E/F *)

43. Olive by-products for animal feed, 1985 (Ar * E * F * S *)

44/1. Animal genetic resources conservation by management, data banks and training, 1984 (E *)

44/2. Animal genetic resources: cryogenic storage of germplasm and molecular engineering, 1984 (E *)

45. Maintenance systems for the dairy plant, 1984 (E *)

46. Livestock breeds of China, 1985 (E *)

47. Réfrigération du lait à la ferme et organisation des transports, 1985 (F *)

48. La fromagerie et les variétés de fromages du bassin méditerranéen, 1985 (F *)

49. Manual for the slaughter of small ruminants in developing countries, 1985 (E *)

50. Better utilization of crop residues and by-products in animal feeding: research guidelines - 1. State of knowledge, 1985 (E *)

50/2. Better utilization of crop residues and by-products in animal feeding: research guidelines - 2. A pracital manual for research workers, 1986 (E *)

51. Dried salted meats: charque and carne-de-sol, 1985 (E *)

52. Small-scale sausage production, 1985 (E *)

53. Slaughterhouse cleaning and sanitation, 1985 (E *)

54. Small ruminants in the Near East: Vol.I 1986 (E *)
Selected papers presented at Tunis Expert Consultation

55. Small ruminants in the Near East: Vol II 1986 (Ar * E *)
Selected papers from World Animal Review

56. Sheep and goats in Pakistan, 1985 (E *)

57. Awassi sheep, 1985 (E *)

58. Small ruminant production in the developing countries, 1986 (E *)

59/1. Animal genetic resources data banks, 1986 (E *)
1 - Computer systems study for regional data banks

59/2. Animal genetic resources data banks, 1986 (E *)
2 - Descriptor lists for cattle, buffalo, pigs, sheep and goats

59/3. Animal genetic resources data banks, 1986 (E *)
3 - Descriptor lists for poultry

60. Sheep and goats in Turkey, 1986 (E *)

61. The Przewalski horse and restoration to its natural habitat in Mongolia, 1986 (E *)

62. Milk and dairy products: production and processing costs, 1988 (E * F * S *)

63. Proceedings of the FAO expert consultation on the substitution of imported concentrate feed in animal production systems in developing countries, 1987 (E *)

64. Poultry management and diseases in the Near East, 1987 (Ar *)

Availability Janurary 1990
Ar - Arabic
C - Chinese
E - English
F - French
S - Spanish
* Available
** Out of print
*** In preparation

The FAO Technical Papers are available through the authorized FAO Sales Agents or directly from Distribution and Sales Section, FAO, Viale delle Terme di Caracalla, 00100 Rome, Italy.

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Source: https://www.fao.org/3/x6932e/X6932E05.htm

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