Previous Page Table of Contents Next Page

SIMPLE TEST METHODS FOR MEAT PRODUCTS

The application of quality control on a regular basis is regarded as necessary for all types of meat plants. Although small meat plants will not have special quality control (QC) staff and laboratories this should not impede regular quality and hygiene control.

Guidelines are provided hereunder on how to organise and implement quality control measures, that can be performed by skilled staff without a specialized laboratory. Although the described test methods are simple and will in some cases only provide approximate results, they may help to improve the quality of products and quality consciousness of staff and good manufacturing and hygienic practice in general.

In this chapter simple methods of

Sensory evaluation

Sensory evaluation is a common and very useful tool in quality assessment of processed meat products. It makes use of the senses to evaluate the general acceptability and quality attributes of the products.


Fig. 416: Areas of the tongue where taste buds and reception areas for different tastes are located

In the simplest way of sensory testing, the meat processor, possibly assisted by other staff, will test a product’s colour, smell, taste and texture upon manufacture. In a more sophisticated approach a team of trained panelists can be used in order to make the results as objective as possible. For this purpose it is useful to have an appropriate testing room available with lights, temperature and seating arrangements with individual testing compartments so as not to distract the members of the panel (Fig. 417).

As an ideal arrangement the panel is composed of ten well trained panelists usually employed in the meat processing plant. If ten panelists are not available, a smaller panel can also produce good results provided the panelists are knowledgeable at sensory testing. It is obvious that for reliable test results the panelist need relevant instructions and some experience in the food sector. Only people with good sensory capability, should be chosen in order to find out differences in colour, texture, flavour and taste.


Fig. 417: Sensory evaluation panel

All panelists must use proven and identical test methods in order to make their results comparable. Each panelist involved in such tests is given a scoresheet, where they mark their findings. Scoresheets of the team of panelists are evaluated and a test result for each individual product is produced based on multiple observations.

Common test methods used in sensory evaluation are:

  1.  Paired comparison test for simple difference where two coded samples are presented to the panelists for evaluation on simple difference (Fig. 418).
  2.  Triangle test where three coded samples are presented at the same time, two are identical and the third is odd and the panelist is asked to identify the odd sample (Fig. 419).
  3.  Hedonic scale rating test or acceptability test where samples are tested to determine their acceptability or preference (Fig. 420).

Fig. 418: Score sheet for Paired-Comparison Test for Simple Difference

Fig. 419: Score sheet for Triangle Test

Fig. 420: Score sheet for hedonic-scale rating test


* Note: Numbers in parentheses are to be assigned during data analysis and are not to appear in the score sheet.

Tests for simple difference and the triangle test are very useful methods for quality control and product development. Newly formulated products can be evaluated by determining if a simple difference exists between the new products developed and the old ones. Similarly, the hedonic scale rating can be used for internal factory testing, and this method is also suitable for market research by determining the consumer’s acceptance or preference for certain products.

Physical test methods in meat processing

With physical test methods important parameters such as temperature, acidity (pH), water activity (aw) and water binding capacity can be determined. Other physical parameters are light intensity and mechanical testing for texture. All routine physical testing can be carried out with portable instruments.

Electronic thermometer (Fig. 421, 422, 423)

Temperature measurement with thermo-elements/thermocouples is based on the thermo-electrical effect. The following is the physical principle:

In a closed circuit composed of two (amalgamated) metals (e.g. Ni and CuNi) an electric current is generated, if the welding points of the two metals are exposed to different temperatures.

The electronic thermometer functions according to this principle. On one welding point of the thermocouple the reference temperature is taken. The other welding point is the tip of the metallic thermo-sensor of the instrument, which is exposed to the temperature to be measured. Both welding points are of different temperature, which generates the electric current within the system. The electric tension (voltage) is equivalent to the temperature difference between the two points and can directly be translated into the temperature reading on the instrument.


Fig. 421: Compact digital thermometer

The welding point for the reference temperature is located in the instrument. For the correct functioning of the system, the reference temperature must be at a constant level. The temperature of 0°C is taken for reference. Even though the instrument is exposed to various temperatures, the reference temperature is electronically set constantly at 0°C regardless the ambient temperature.


Fig. 422: Electronic thermometer (digital)

Fig. 423: Electronic thermometer. Measurement of temperature of meat batter during comminuting

Important temperature control points are:

Non-contact infrared temperature measurement

Physical principle: Each object which has a temperature higher than absolute zero (= -273 Kelvin), emits energy in the form of infrared radiation. The energy released can be measured through special optical sensors and directly indicated as the temperature of the object.

With infrared thermometers, temperatures are measured without direct contact of a temperature sensor with the medium to be tested. Infrared thermometers have a built-in laser pointer. The light spot emitted by the instrument onto the surface to be tested indicates the area of measurement. Due to this principle, the system allows the measurement of the surface temperature only, not of the internal temperature.

Infrared thermometers are well suited for screening tests, e.g. temperature of incoming meat or of goods in chillers and freezers. The method can also be applied for moving objects. In meat processing, it can be used for measuring the temperatures in frying pans, ovens etc. For temperature measurements of frozen goods in boxes, measurements on the outside of the box are not reliable and the boxes must always be opened. Surfaces with strong light reflection will also provide inaccurate results.

For exact temperature measurements in meat and meat products, e.g. for temperature testing within internal plant control systems such as HACCP, electronic temperature measurement remains the method of choice.


1) Temperature control carried out with specific systems to be inserted in cans or autoclaves, portable electrical thermometer are not suitable for this specific purpose.

Data-logger for temperature measurement

These are electronic instruments without a monitor but with built-in device for saving data. The data-logger measures temperature within certain periods (e.g. every 10 or 30 minutes etc.) and saves it. Data-loggers can be used in refrigerated rooms. For this specific purpose they can be equipped with sound alarm systems in the event of exceeding temperature limits. Another application is for measurements within foods, such as frozen goods or even during the sterilization of canned goods. The data saved in the logger can be evaluated using a computer.

pH meters (Fig. 424, 425)

Portable instruments are battery driven and have glass electrodes. The pH–value in meat and meat products can be measured by direct contact between the sensitive diaphragm of the electrode and the meat tissue. Through the diaphragm differences in electrical load between the meat and electrolyte solution (e.g. Potassium chloride KCl) inside the glass electrode are measured and directly indicated as the pH-reading. In raw fresh meat, it is recommended to spray small amounts of distilled water onto the tissue at the point of measurement (prior to inserting the electrode), because the operation requires some fluidity in the sample and the glass electrode should be thoroughly wet. The amount of water necessary will not appreciably alter the pH. For accurate pH readings the pH-meter should be calibrated before use and adjusted to the temperature of the tissues to be measured. The electrode must be rinsed with distilled water after each measurement.


Fig. 424: Portable pH-meter for direct measurement in meat

Fig. 425: pH meter. Inserting glass electrode in meat tissue

The pH is a measure of the acidity or alkalinity in solutions or water containing substances. pH values lower than 7 are considered acidic, while pH values higher than 7 are considered alkaline. A pH of 7 indicates neutrality. pH values are related to the concentration of hydrogen ions (H+) in the substance.

Typical pH values for meat and meat products are:

Product

pH value (range)

Meat mixes in jelly + vinegar added

4.5 to 5.2

Raw fermented sausage

4.8 to 6.0

Beef

5.4 to 6.0

Pork

5.5 to 6.2

Canned meats

5.8 to 6.2

Curing brines

6.2 to 6.4

Blood sausages

6.5 to 6.8

Muscle tissues, immediately after slaughter

7.0 to 7.2

Blood

7.3 to 7.6

pH measurement is useful for:

Hygrometers (Fig. 426)

Hygrometers measure the relative humidity and are used in production and storage rooms of the meat industry.

Recommended values for the relative humidity are:

Meat boning and cutting rooms

45% to 60%

Meat packaging rooms

45% to 60%
Chilling rooms
85% to 95%

Storage / ripening rooms for meat

70% to 85%

Ripening rooms for raw fermented ham and sausages

80% to 95%
(depending on the
stage of ripening)

Relative humidity is the ratio of the existing (absolute) vapour pressure of water in air to the saturation vapour pressure (= maximum amount of water vapour that can be held) in air of the same temperature


Fig. 426: Hygrometer. Measurement of rel. humidity in meat chiller

Example:

Air at 20°C can hold 17g of water vapour per m3 (water vapour content at saturation). If the water (vapour) present in the air of 20°C at a given moment is 9g per m3 (absolute water vapour content), the calculation for the relative humidity is as follows:

or

AW - meter (mechanical instrument) (Fig. 427, 428)

Besides sophisticated electronic instruments used for industrial production and research, there are simple mechanical instruments available for the measurement of the water activity (aw) of meat products under practical conditions.

Water activity1 is the term for the amount of free (not chemically or physically bound) water, which is available for the growth of microorganisms. This information is particularly important, as higher amounts of free water favour the growth of microorganisms, while lower amounts (drier products) result in less microbial growth. Bacteria usually require at least aw 0.91 and fungi at least aw 0.71.

The amount of free water in a product is equivalent to the air humidity produced by a product sample in a small enclosed system. This is the principle of the simple aw-measurement method (Fig. 427, 428). The product sample is placed inside a hermetically closed small can-like container (Fig. 428). Through evaporation an equilibrium of humidity in the small airspace above the product and the humidity of the sample is build-up and this is directly measured by means of a hygrometer built into the lid of the instrument. Pure water (representing 100% free water) is equivalent to aw-value of 1, all other food samples have lower aw-values than 1 depending on their free water content.


1) aw is defined as the vapour pressure of water of a substance divided by that of pure water at the same temperature.

Table 17: Typical aw in meat products (left) and limiting aw for the growth of microorganisms (right).

Product

aw range

Microorganisms

aw

Fresh meat

0.99 (0.99 to 0.98)

Pseudomonas
0.93

Most bacteria between
aw 0.91 – 0.96

Cooked ham

0.97 (0.98 to 0.96)

E. coli
0.93

Raw-cooked sausages

0.97 (0.98 to 0.93)

Salmonella species

0.91-0.95
Liver sausages

0.96 (0.97 to 0.95)

Listeria
0.93
Blood sausages

0.96 (0.97 to 0.86)

Cl. botulinum types

0.91-0.95

Raw-fermented ham

0.92 (0.96 to 0.80)

Cl. perfringens

0.93-0.95

Raw-fermented sausages

0.91 (0.96 to 0.70)

Bacillus species

0.90-0.95
Dried meat

0.70 (0.90 to 0.60)

Lactobacillus
0.90
Staph. aureus
0.86-0.90
Most yeasts
0.87-0.90
Most moulds
0.80-0.85

Areas for aw-control:


Fig. 427: Set of two simple awmeters enabling simultaneous measurements of two samples

Fig. 428: Aw-meter with product sample to be tested, lid (has to be attached) with built-in hygrometer

Water holding capacity (WHC)

WHC plays a role in meat to be used for further processing (see page 7). It is also important in meat batters, which have to undergo heat treatment. Low WHC results in separation of jelly and/or fat during heat treatment (see page 131). The WHC can be measured using a glass compressorium (Fig. 429), where the sample of meat or batter is compressed onto a water absorbing sheet of paper. The larger the water infiltrated area on the paper, the poorer is the WHC of the meat/batter (Fig. 430).


Fig. 429: Compressing meat samples for determination of water holding capacity

Fig: 430: Low water binding (above), good water binding (below)

Lux – meter (Fig. 431)

This instrument is used to test and, if necessary, to adjust the intensity of artificial light in working places. During meat processing, normal working places should have at least a light intensity of 300 Lux and quality control / meat inspection places 500 Lux.


Fig. 431: Lux-meter Light sensor and monitoring unit in one portable instrument

Instruments for texture measurement

Sensory testing (chewing) is normally sufficient to test tenderness/toughness or homogenous/fibrous structure of meat and meat products. If more objective results are desired, special instruments for texture measurement can be employed. The instrument shown in the photo, measures the shear-force necessary to cut through meat/meat products (Fig. 432, 433). Comparative texture measurements are usually taken from same tissues or products which were submitted to different treatments such as ripening, cooking etc.


Fig. 432: Instrument for texture measurement

Fig. 433: Measurement of shearforce

Simple methods of chemical analysis
(Protein, fat, water, ashes)

Chemical analyses to determine the content of protein, fat, water and minerals (ashes) of processed meat products (see also table 1) are carried out to establish the nutritive and economic value of the products. Samples of the meat product are finely ground and weighed accurately for each respective chemical analysis.

The determination of the moisture content (or water content) is done by drying an appropriate amount of the sample. The difference in weight between the fresh and dried samples represents the water content. For rapid determination of moisture content a microwave oven is useful (Fig. 434).

The protein content is determined at laboratory level by using the Kjeldahl method (Fig. 435), where meat products are digested by acid to obtain the nitrogen compounds and then distilled and titrated to determine nitrogen quantitatively, with which the protein component can be calculated. In a simplified approach protein is not chemically determined, but can be calculated (approximately) as the remaining component, after water, fat and ashes content has been determined and subtracted from 100%. This simple mathematical method should only be applied for pure meat and meat products as it is not accurate for highly extended products containing non-meat ingredients such as grains, starches or vegetables (see page 81). In the case of using meat extenders and/or fillers, the result reflects the organic non-fat component (protein and carbohydrates in %) of the product.

Determination of the fat content is the most complicated component of simple meat and meat product analysis, as analytical equipment (Soxhlet apparatus, Fig. 436) is needed. Samples for fat analysis are semi-dried before being subjected to ether-extraction using the Soxhlet apparatus. After complete extraction, the fat is obtained by evaporating and recovering the ether.


Fig. 434: Microwave oven
(for water)

Fig. 435: Kjeldahl distilling apparatus
(for protein)

Fig. 436: Soxhlet extraction apparatus
(for fat)

Fig. 437: Muffle furnace
(for minerals)

The defatted samples are then used for ash analysis by subjecting it to a temperature of +600°C in a muffle furnace for two hours. The weight of the ash is used to calculate the minerals content in % (weight of ash, divided by total sample weight, multiplied by 100).

Sampling and analytical procedures

Sampling of Meat and Meat Products

Step I. Grind the cold meat sample, minimum weight 500 gms. Use food grinder with 3mm plate opening.

Step II. Mix rapidly at a cold temperature.

Step III. Keep ground sample in glass or similar containers which are air and liquid tight.

Step IV. Ready for analysis. If any delay occurs, chill the sample to inhibit decomposition.

Step V. Weigh the sample as rapidly as possible to minimize loss of moisture.

Moisture Analysis
(Microwave Drying)

General:

Samples are dried in a microwave oven and the loss of weight upon drying is expressed as percent moisture content.

Application:

This method may be used to determine the moisture content of fresh meat, semi-processed meat, meat mixes and processed meat products.

Equipment:

Mincer with 6mm plates or heavy duty food processor.

Balance with at least 0.1g sensitivity.

Microwave oven with 600-700 watt microwave energy output, turntable and time accurate to 15 seconds.

Desiccators with silica gel.

Beaker

Filter papers, 7cm diameter or open weave disposable kitchen cloth.

Silicon carbide (carborandum) finely ground.

Sand or salt.

Approximate Drying Times for Sample Sizes of Meat

Sample size

Approximate Drying Time

3 x 10g

3.5 – 4.5 min.

3 x 25g

7.5 – 9.5 min.

2 x 50g

8.5 – 11 min.

Weight of beaker plus filter paper

=

A

Weight of beaker plus filter paper + sample
        (before drying) in grams

=

B

Weight of beaker plus filter paper + sample
        (after drying) in grams

=

C

Method:

  1. Prepare the sample by mincing or chopping as described in sample preparation.
  2. Preheat the oven
  3. Dry the beakers and filter papers by heating them in a microwave oven for one minute.
  4. Determine the heating time necessary to completely dry the samples in the microwave oven.
  5. Weigh an empty beaker plus filter paper. Weigh about 10 grams of sample in the beaker. For meat samples, spread the samples into a thin layer around the lower wall of the container with spatula or spoon. Place the filter paper over the top of the beaker and fold to close and accurately weigh the beaker plus filter paper.
  6. Place the samples in the preheated oven. The samples should be spaced at equal distances around the turntable.
  7. Cool the samples in a desiccator and accurately weigh the beaker plus dried samples plus filter paper.
  8. Repeat drying until constant weight is obtained.

Crude fat determination using samples dried from the microwave oven:

  1. Get the weight of the dried sample.
  2. Put the dried sample inside the filter paper and fold to close.
  3. Place the dried sample inside the soxhlet extraction tube connected to the soxhlet flask.
  4. Pour enough ether into the extraction tube.
  5. Extract for 10 hours, at 3-4 drops per second.
  6. After extraction, take out the defatted sample from the extraction tube and air dry the sample for traces of ether. Dry further in an oven at 100°C and cool in a dessicator. Weigh the defatted cooled samples to constant weight.

Computation:

Ash determination:

  1. The defatted sample is placed in a constant weight porcelain crucible with cover.
  2. The crucible is then placed in a muffle furnace, and at a temperature of 600°C the sample is ignited for two hours.
  3. After ignition the crucible is placed in the oven to bring down the temperature for about 30 minutes, then cool in a dessicator for another 30 minutes.
  4. The sample is then weighed to constant weight.

Computation:

Protein content determination:

Calculation of the approximate protein content for pure meat and meat products:

% Protein = 100% - (%water + % ash + % fat)

This calculation is not applicable for meat products that were extended.

Microbiological sampling and testing

The purpose of microbiological testing is to determine the degree of bacterial contamination on surfaces of equipment, tools, premises as well as in meat and meat products. This testing can be done qualitatively as microbiological screening, for example by contact such as using an impression plate or quantitatively by determining the exact number of microorganism per sample unit (in cm2 or grams) by using the swab or the destructive method. Quantitative testing can be either determination of the entire contaminating flora, also called “total plate count” or determination of a specific group of microorganisms out of the entire flora, also called “selective plate count”.

a) Contact method (Fig. 438)

The microbiological culture medium is put in direct contact with the surface of equipment or tools to be tested (Fig. 438 a,b). Microbes contaminating the surface are removed from there as they adhere to the sticky culture medium. The culture medium containing the microbes from the test surface is incubated e.g. at 30°C for 2 days. Each bacterium grows as a bacterial colony visible to the naked eye. Colonies can be counted to allow assessment of the degree of contamination.

Advantage

:

Simple procedure, can be carried out without laboratory.

Disadvantage

:

In case of heavy contamination, colonies may overgrow/overlap and individual colonies are difficult or impossible to distinguish. Result in this latter case would be “heavy contamination”, but conclusion on the exact degree is not possible.

New commercially available systems for the contact plate technique facilitate the application and provide more accurate results. The culture medium is attached to a plastic chip, which has a flexible hinge for better handling. The test chip is placed in a fitting sterile plastic tube upon surface testing and incubated (Fig. 438c). Results are available after 24 hours when using incubation temperatures of 35-37°C. Areas not accessible for the direct impression, e.g. inside equipment, can be tested by using a swab, and the impurities gathered are transferred to the culture medium (Fig. 438d). The method allows approximate quantitative microbial testing by comparing the test results with reference microbial growth schemes provided by the supplier.


a) Impression on test surface (purple colour). Direct transfer of bacteria from test surface to culture medium (in petridish)

b) Testing of surface of meat processing equipment (above). Below left: Contact plate before impression. Below right: Contact plate after impression and incubation

c) Test chip for contact method in action. Plastic tube (behind) to cover test chip upon use

d) Transfer of sample from swab to culture medium (from area not directly accessible with test chip)

Fig. 438: Contact plate (impression plate) method

In this context, it should be mentioned, that rapid control systems, which provide immediate results on the cleanliness of surfaces, are based on detection of metabolic substances (ATP or NAD) originating from bacterial growth. The systems, which were developed to check the immediate effect of sanitation in large food industries, are of less relevance for small to medium industries and can be costly.

b) Swab method (Fig. 439, 440)

Contaminating bacteria are removed from the surface to be tested by using a sterile swab. Standardization by using a reference square area is needed (e.g. by sterile metal frame) (see Fig. 439). Microorganisms collected by the swab technique are rinsed off with sterile water (see Fig. 440). The microbial content of the liquid is tested.

Advantage : Even in case of heavy contamination, the number of microorganisms can be determined by applying dilution techniques (see page 335).

Disadvantage : Part of the contaminating flora may not be recovered, in particular in case of uneven rugged surfaces, e.g. meat.


Fig. 439: Swab method. Bacterial collection with swab on cutting board.

Fig. 440: Swab method. Bacteria to be rinsed off with sterile water, transfer of solution on culture medium.

c) Destructive methods (for use on meat/meat products)

A standardized sample is cut out (“destructive”) from the surface of meat or meat products, for example by using a sterile knife and metal frame (Fig. 441, 442). The sample received, which has a defined surface area, is further standardized by removing tissue from the bottom layer until a standardized weight (e.g. 10g) (Fig. 443) is achieved.

Advantage

:

The testing includes all microorganisms present in the sample. Samples can be exactly standardized according to surface area (cm2) or weight (g). The sample comprises not only superficial contamination, but also microorganisms from the interior of meat/meat products.

The meat sample is homogenized in sterile water by using laboratory equipment (“Stomacher”) (Fig. 444). Transfer of homogenized solution on culture medium is by dilution techniques (see page 335).


Fig. 441: Determination of standardized sample

Fig. 442: Cutting out meat sample from carcass

Fig. 443: Trimming/weighing of meat sample

Fig. 444: Homogenizing meat sample (in “Stomacher”)

Microbiological Analysis

a) Total Plate Count (using nutrient agar)

For determination of the number of viable or living microorganisms in a sample.

  1. Meat sample (10 grams meat + 90 ml sterile distilled water or 0.1% peptone water). Homogenize in stomacher. First dilution.
  2. Transfer 1 ml from first dilution (101) to second test tube (Test tube contains 9 ml. of sterile distilled water) (2nd dilution or 102) then from second test tube transfer 1ml to the third tube (3rd dilution or 103) and so on up to the 4th or 6th dilution.

  1. Inoculate sample.

Pipette 1 ml from 3rd dilution and transfer to the sterile petridish, also from the 4th dilution to another sterile petridish depends upon how many dilutions are desired (see below and Fig. 445).

The inoculation is usually done according to the spread plate method. The diluted sample is released from the pipette onto the solidified agar and spread on the surface by means of a sterile bent glass stick. The alternative is the pour plate method, where the sample is first put into the Petri dish and 15 ml agar (liquefied in a water bath at 44-46°C) are poured into the plate afterwards. Agar and sample are thoroughly mixed by rotating the Petri dish.

  1. Incubate for 12 to 24 hours at 35 to 37°C, alternatively 24-48 hours at 30°C.
  2. Results
  3. Count all colony forming units (CFU), including those of pinpoint size (Fig. 446). Select spreader-free plate.

    1. normal plates 25-250 counts
    2. plates with more than 250 colonies for all dilution - too numerous to count
    3. plates with no CFU. Report as less than 1 times the corresponding dilution used.

Inoculation of sample and reading of results


Fig. 445: Inoculation of sample

Fig. 446: Reading of results from Petri dish

b) Selective Plate Count

The total plate count is a good indicator for the overall bacterial load of meat and meat products. Critical hygienic dimensions are reached when the total number of bacteria on fresh meat lays between 10,000 (1,0x104) and 100,000 (1,0x105) per g (see also page 353). However, the total number does not allow any conclusions on the nature of the microorganisms, i.e., if the bacteria are harmful or harmless.

Therefore, practicable microbiological standards should, in addition to the total plate count, always include the number of hygienically sensitive microorganisms, which can be used as an indicator for specific hygienic risks. These microorganisms are selected out of the total number of bacteria. This can be done using selective bacterial culture media, which contain chemical additives that suppress the growth of all bacteria except the group of microorganisms that shall be detected and used as indicator bacteria.

The indicator bacteria most commonly used is the group of Enterobacteriaceae. Enterobacteriaceae are part of the intestinal microflora, i.e. they are present in high numbers in the faeces of humans and animals. Most importantly, harmful food poisoning bacteria belong to this group e.g., pathogenic E.coli and Salmonella. If larger numbers of Enterobacteria are found in food, there is the probability of massive contamination with dirt or even faecal material with all the consequences, in particular presence of food poisoning bacteria.

The number of Enterobactericeae should not exceed 100 per cm for the criterion “good microbiological standard” (see also page 353). The selective culture medium used for the determination of Enterobactericeae is the Violet Red Bile Agar (VRB), which contains Crystal violet and bile salt for the inhibition of all other bacteria (Fig. 447).

Other commonly used selective culture media are Lactobacilli MRS Agar for the isolation of Lactobacillus (Fig. 448), BAIRD-PARKER Agar for the isolation of Staphylococcus aureus (Fig. 449), XLT4 for the isolation of Salmonella (Fig. 450) and Mc Conkey Agar for the isolation of moulds (Fig. 451) (see also page 356, 357, 359).

Microbiological detection kits are now on the market, which deliver screening results without the need of a laboratory. Such kits are particularly designed for the detection of pathogens such as Salmonella, Listeria or E. coli O157 H7. They indicate presence or absence of bacteria by change of colour on test strips submerged in a liquid suspension of materials to be tested.


Fig. 447: Selective medium for Enterobacteriaceae
(blue colonies)

Fig. 448: Selective medium for Lactobacillus
(small white colonies)

Fig. 449: Selective medium for Staphylococcus aureus
(yellow colonies)

Fig. 450: Selective medium for Salmonella species
(black colonies)

Fig. 451: Selective medium for moulds
(diffuse grayish colonies)

Previous Page Top of Page Next Page