quality tests on milk

Chemical & Microbiological Testing Procedures In Dairy Quality Assurance

To ensure that you have exceptional quality of dairy products, you must ascertain the quality of the raw material. Chemical tests for dairy products is one way of ensuring maintenance of consistent quality throughout the value chain.

At every stage, you must ensure that the standard tests are carried out on the milk samples to be sure the quality meets the desirable standard.

The quality management begins at the farm. The condition of the animal affects the quality of milk produced for processing.

As such, the animal must be very healthy, the animal feeds must be of high quality, the shed must be clean and the health of the person tending to the animals must be sound.

All these will ensure that they do not transmit zoonotic diseases to other milk consumers through the milk.

One will need to carry out various tests to ascertain the quality of milk.

They include:

Organoleptic tests: these are tests you carry out using your senses such as sight, smell, and temperature. Tasting of raw milk is generally discouraged. Chemical, physical, and microbiological tests are necessary to ascertain the quality of milk.

Various chemicals and reagents are involved in the quality assurance tests carried out on milk. We will look at the reagents needed for the dairy quality assurance tests, what they are used for, and how they are prepared.

Precaution!

Take a lot of care when handling any of these reagents as they are extremely corrosive. Ensure you have the appropriate personal protective equipment.

Reagents for Chemical Tests

1. Chromic Acid

Chromic acid is used for descaling glassware. It is prepared from potassium dichromate powder and sulphuric acid.

Reagents and apparatus

  • Top loading balance
  • 1000 ml volumetric flask
  • 1000 ml measuring cylinder
  • Potassium dichromate powder
  • Concentrated sulphuric acid

Corrosive reagents!

Preparation Instructions

  • Weigh 92gms of potassium dichromate and transfer into a 1000 ml volumetric flask.
  • Add 250mls of distilled water while shaking to dissolve the dichromate.
  • Add concentrated sulphuric acid slowly and carefully to the solution in the 1000mls mark.
  • Immerse the flask under cold water during addition, as the process is exothermic.
  • Label the reagent with the name and date of preparation.

2. Sodium Hydroxide N/9 (NaOH)

 N/9 (normal 9) Sodium Hydroxide is used for the determination of acid strength. It is prepared from sodium hydroxide pallets.

Apparatus and reagents

  • 50 ml burette
  • 10 ml volumetric pipette
  • 1 Lt volumetric flask
  • Conical flask
  • Top loading balance
  • Glass funnel
  • Sodium hydroxide- analytical reagent
  • N/9 oxalic acid

Preparation Instructions

  • Weigh 4.5gms of sodium hydroxide pellets using a top loading balance
  • Transfer the pellets into a 1 Lt volumetric flask
  • Add about 250mls of distilled water to dissolve the pellets
  • Make up solution to volume using distilled water
  • Take 10 ml of the above solution and transfer into a conical flask using a volumetric pipette add 2-3 drops of phenolphthalein indicator.
  • Fill 50mls burette with N/9 oxalic acid. Run the acid into the sodium hydroxide until endpoint. This changes solution from purple to clear. Initially the oxalic acid is run rapidly but slowly towards the end to detect the end point.
  • Record the volume of oxalic acid used. If more than 10mls of oxalic acid is used the NaOH is more concentrated.

Calculations and Standardization

  • If 12 ml of oxalic acid is used, every 10 ml of NaOH requires 2 ml of distilled water.
  • The volume remaining in the flask of NaOH is 990mls, divide 990 with 10 and multiply by 2. Thus amount of water to add is 198mls.
  • Add water and titrate 10mls of NaOH with 10 ml of oxalic acid, they should neutralise at a ratio of 1:1
  • Transfer the re-agent into amber bottles or a suitable closable container, labelled with name and date of preparation.

3. Potassium Iodide (KI) 5%

5% potassium iodide solution is used for the detection of hydrogen peroxide in milk. It is prepared from potassium iodide granules.

Apparatus /reagents

  • potassium iodide granules
  • distilled water
  • Top loading balance
  • 100 ml volumetric flask
  • Spatula, opaque material

 Preparation Instructions

  • Weigh 5 gm of potassium iodide and transfer into a 100 ml volumetric flask
  • Add distilled water while shaking to dissolve the granules and the solutions made up to the mark.
  • Label the reagents with date, name and who prepared it.
  • The bottle is covered with opaque material as light affects this reagent.

4. Starch Solution 2%

2% starch solution is also used for detecting hydrogen peroxide in milk. It is prepared from starch powder.

Apparatus and reagents

  • Starch powder
  • Distilled water
  • Top loading balance
  • 100 ml volumetric flask

Preparation Instructions

  • Weigh 2 gm of starch powder and transfer into a 100 ml volumetric flask
  • Add distilled water while shaking to disperse the powder up to the 100 ml mark
  • Label the reagent with name and date of preparation.

5. Hydrochloric Acid N/10

Hydrochloric acid solution (of the right concentration) is used for caustic strength determination in CIP detergents (lye). It is prepared from analar grade hydrochloric acid

Apparatus and reagents

  • Hydrochloric acid
  • Distilled water
  • Chilled water in a water bath
  • 1 litre volumetric flask
  • 10 ml pipette
  • 1 ml pipette
  • Pipette filler

Preparation Instructions

  • Pipette 9 ml of analar HCl and transfer into 1 lt volumetric flask
  • add 250 ml of distilled water while shaking under chilled water
  • Make up to the 1 lt mark using distilled water. The flask is stoppered and the reagent transferred into a suitable container.
  • Label the reagent with name and date of preparation.

6. Phenolphthalein Indicator 2.5%

Phenolphthalein indicator is used to mark the end of an acid-base titration. The indicator solution is prepared from phenolphthalein powder

Apparatus and reagents

  • Top loading balance
  • 100 ml volumetric flask.
  • Phenolphthalein powder
  • Concentrated ethanol (95-96%)

Preparation Instructions

  • Weigh 2.5gms of phenolphthalein powder and transfer into100 ml volumetric flask.
  • Add concentrated ethanol (95-96%) while shaking to dissolve and after dissolution it is made upto 100 ml mark.
  • Add N/9 sodium hydroxide solution until further drop of sodium hydroxide causes a faint pink colour to appear.
  • Phenolphthalein indicator is colourless in acidic solutions and purple in alkaline solution.
  • Label the reagent with name and date of preparation.

7. Ethanol 80% v/v

Used in the alcohol test of milk. Prepared from industrial grade ethanol.

Reagents and Apparatus

  • Ethanol Absolute (96% – 99%)
  • Distilled Water
  •  Measuring Cylinder( 1000ml)
  • Alcoholmeter
  • Thermometer

Preparation Procedure

  • Ascertain the Ethanol concentration first.
  • Prepare solution from the established concentration.
  • If the initial concentration of Ethanol is 96 %, then to make 80 %;
  • Put 833.33 of Ethanol in a measuring cylinder then top up to 1000 ml. the formula below shall apply;
  • x/y x   100, (where x is the required alcohol percentage divide by y which is the initial ethanol percentage i.e. 80/96 x 100 = 83.33).
  • Confirm the Alcohol concentration by using an Alcoholmeter at 200C

8. Sulphuric Acid 

Used in Gerber/Butterfat Test. Prepared from the industrial grade sulphuric acid.

Apparatus and Reagents

  • Volumetric Flask
  • volumetric flask 1000 ml
  • Measuring cylinder 1000 ml
  • Thermometer
  • Acid Hydrometer
  • Sulphuric Acid Technical Grade
  • Distilled Water in a trough
  • Chilled Water

Preparation Procedure

  • Measure 100 ml of Distilled water using a measuring cylinder
  • Put the distilled water into a volumetric flask
  • Add Sulphuric slowly while immersing the flask in chilled water in a trough
  • Make up solution to 1000 ml mark with Sulphuric Acid
  • Let the Acid cool down to 200C
  • Check the density of the Sulphuric Acid using the Acid Hydrometer
  • The Acid should be 1.815 ± 2 g/ml at 200C

Important Note:

If the density is below 1.815 ± 2 g/ml at 200C, add more Sulphuric Acid.

Add distilled water if the Acid density is above 1.815 ± 2 g/ml at 200C.

9. Phosphatase Test Reagent

 Used in carrying out the phosphatase test in pasteurized milk.

Reagents and Apparatus

  • Sodium Carbonate
  • Sodium Hydrogen carbonate
  • p-Nitrophenyl Disodium Phosphate
  • Distilled Water
  • Weighing Balance
  • 1,000 ml Volumetric Flask
  • 100 ml Volumetric Flask
  • Parchment paper
  • Spatula

Procedure for Buffer Preparation

  • Weigh 3.5 g of Sodium Carbonate using a spatula onto parchment paper.
  • Transfer the sodium carbonate in to a volumetric flask.
  • Weigh 1.5 g of Sodium Hydrogen Carbonate using a spatula onto parchment paper.
  • Add the sodium hydrogen carbonate into the flask containing the sodium carbonate.
  • Add Distilled water into the flask while shaking it for the content to dissolve.
  • Top up to the 1,000 ml mark.

Procedure for Substrate Preparation

  • To make the substrate, weigh 0.15g of p-Nitro-phenyl Disodium- Phosphate on parchment paper and put in to a 100 ml volumetric flask.
  • Add the buffer (4.6 above) while shaking up to the 100 ml mark.
  • You must prepare fresh substrate every week and discard the old solution.
  • Label the reagent indicating the name and date of preparation.

10. Oxalic Acid N/9

Used for standardization of N/9 Sodium Hydroxide.

Apparatus and reagents

  • Oxalic Acid
  • N/9 Sodium Hydroxide
  • 5% Phenolphthalein Indicator
  • Distilled Water
  • 10 ml pipette
  • 1000 ml volumetric flask
  • Beaker
  • Funnel
  • Weighing balance
  • Spatula

Preparation Procedure

  • Weigh 7 grams of Oxalic Acid on parchment paper
  • Put the Oxalic Acid into a 1000 ml volumetric flask
  • Add about 250 ml of distilled water to dissolve the Oxalic Acid
  • Make up solution to 1000 ml mark using distilled water
  • Take 5 ml of the above solution and transfer into a beaker. Using a dropper add 2-3 drops of phenolphthalein indicator.
  • Titrate against N/9 Sodium Hydroxide to the end point.
  • Record the volume of Sodium Hydroxide used. If it is more than 5 ml of NaOH, then the Oxalic Acid has a very high level of concentration.

 Standardizing the Reagent

  • If you use 6 ml of N/9 NaOH, then every 5 ml of N/9 Oxalic Acid requires 1 ml of distilled water.
  • The volume remaining in the flask of Oxalic Acid is 995 ml, divide 995 by and multiply by 1: thus amount of distilled water to add is 199 ml.
  • Add distilled water and titrate 5 ml of N/9 Oxalic Acid with 5 ml of N/9 NaOH, it should neutralize at a ratio of 1:1
  • Transfer the re-agent into amber bottle or a suitable container, labeled and indicate the name, concentration and date of preparation.

Reagents for Microbiological Testing Procedures

Microbiological testing is a crucial part of the quality assurance process because it anchors the safety of the food product. Every product that passes through the plant must be attested to have high standards that the consumers expect. It is important to conduct a series of microbiological testing procedures on dairy products to ensure that the final product meets the required standards by passing all the stringent measures required for a satisfactory product.

Here is the list of microbiological testing procedures applicable to dairy products:

1. Standard Plate Count

The purpose of this work instruction is to ensure that the number of colony forming units (CFU) per millimetre or per gram of the original sample is determined correctly. A defined test portion or series of decimal dilutions of the sample are mixed with culture media in Petri dishes and incubated. The number of colony forming units (CFU) per millilitre or per gram of the original sample is calculated from the number of colonies counted on selected dishes

Scope

This work instruction is applied to the inter-laboratory testing of milk, milk products and additives.

Reagents and equipment for media preparation

  • Analytical balance
  • Spatula
  • Filter papers
  • Autoclave
  • Distilled water
  • Water bath at 370c
  • Tryptone Glucose Extract Agar
  • Ringers solution

Reagents and equipment for preparation of plates and analysis of sample   

  • Sterile Petri dishes 90 – 95mm diameter
  • Sterile pipettes of 1ml and 10ml
  • Volumes of 9mls of sterile quarter strength ringers solution in sterile universal bottles
  • Sterile Petri dishes
  • Tryptone Glucose Agar, Oxoid

Microbiological Testing Instructions

Preparation of standard plate count media

  • Suspend 24g of Tryptone Glucose Extract Agar in 1 litre of distilled water. Bring to boil to dissolve completely.
  • Dispense into bottles and sterilize by autoclaving at 1210c for 15 minutes.

Preparation of quarter strength ringer’s solution

  • Dissolve 1 tablet in 500ml distilled water. Sterilize by autoclaving at 1210c for 15minutes and let cool.

Microbiological Testing Method

  • Mix the sample thoroughly by shaking the sample container 25 times in 10 seconds over a 30 cm arc. You may then shake the sample mechanically on a mixer. Ensure that the interval between mixing and pipetting does not exceed 3 minutes.
  • Transfer 1 ml of milk aseptically into 9 ml of diluents and mix thoroughly by orbital share or manually either mechanically or by aspirating the pipette 10 times
  • Transfer 1 ml from the first dilution aseptically with a fresh pipette to a further 9ml of diluents aseptically with a fresh pipette to a further 9ml of diluents and mix. Further dilutions can be prepared by transferring 1ml of each successive dilution to further 9ml diluents using a sterile pipette in each case.
  • Transfer 1ml of each dilution chosen for assay using another sterile pipette into a labelled Petri dishes starting with the most dilute of the dilutions chosen
  • Add 15ml of the tempered melted medium aseptically to each inoculated Petri dish. N/B the  temperature of the media should not exceed 460c due to the possible damaging effects on the micro flora of the sample.
  • Mix immediately after pouring by 5 to and fro movements followed by 5 circular clockwise movements followed by 5 to and fro movements at right angles to the first set followed by 5 circular anticlockwise movements.
  • Allow the Petri dishes to stand on a clean horizontal surface until the medium sets, invert and transfer to the incubator
  • Incubate the Petri dishes at 370c for 48hrs after which you select the Petri dishes with counts between 10 and 300 colonies for counting
  • If more than one dilution has counts in the range of 10 to 300, use each dilution to calculate the number of organisms per millilitre of sample.

Control for the test

For each bottle of agar, pour one ml in sterile Petri dish and incubate as for the samples. If colonies are observed in these controls the test results should be considered with caution. In a case where more than 10 colonies are observed, the test results are void.

Calculations of results

Calculate the number of colonies per ml in each dilution having between 10 and 300 colonies per plate. Read the results and express the answer as number of colony forming units per ml (or g)

2. Determination of Yeasts and Moulds

The purpose of this work instruction details the enumeration of yeasts and moulds in milk and milk products. Here, we show the methods for the detection and enumeration of yeast and moulds

Requirements for this microbiological testing procedure

  • Quarter strength sterile ringers solution
  • Malt Extract Agar; Oxoid
  • 10% lactic acid to adjust pH to 3.5
  • Incubator at 22-250c

Yeasts and Moulds Testing Instructions

Preparation of Malt Extract Agar

  • As per the manufacturer’s instructions on the tub
  • Add 1 ml of lactic acid 10% to each 100ml of sterilized medium at 50-550 The medium must not be heated after the addition of acid, as the gelling properties of the medium will be lost
  • Mix well before pouring.

Yeasts and Moulds Testing Procedure

  • Prepare serial tenfold dilutions of the sample in 9ml. sterile quarter strength ringers solution
  • Pipette 1ml from each dilution onto a sterile Petri dish, add 15ml of Malt Extract Agar and mix by swirling the plate
  • Allow to solidify
  • Invert and incubate the plates at room temperature or 300c incubator for 5 days
  • Count plates containing 10-150 colonies. If many yeast are present, plates with 150 colonies are usually readily countable
  • Report in colonies/ml
  • Prepare slides of the colonies, identify and record what organism is present.

If more than one dilution plate has been counted, calculate the number of CFU/ml for each plate counted. Mean the result and express as colony forming units/ml (CFU/ml)

3. Detecting Coliforms Through Microbiological Testing

This method instructs on how to conduct an experiment to check for the presence or absence of both faecal and non-faecal coliforms in milk and milk products. Presence of Coliforms in dairy products is suggestive of unsanitary conditions or practices during production, processing or storage.

Re-agents and equipment

  • Violet red bile agar, Oxoid
  • Quarter strength ringer’s solution
  • Sterile Petri dish
  • Sterile 1ml pipette
  • Incubator at 370c
  • Water bath at 450c

Coliforms Testing Instructions

Preparation of violet red bile agar

  • Suspend 38.5 g of violet red bile agar in one litre of distilled water.
  • Bring to the boil to dissolve completely.
  • N/B no further sterilization is necessary. Cool to 450c in a water bath.

Coliforms Testing Method

  • The specimen/sample to be plated should be diluted to avoid medium overgrowth. If the colonies lie too closely together in the medium they will become uncharacteristic and difficult to count or identify. The most suitable number of colonies is 15-150 per Petri dish.
  • Prepare a tenfold dilution series in quarter strength sterile ringers solution for milk and milk products and in peptone water for ingredients and swabs transfer 1ml of the original solution of 1ml from the chosen dilution with a sterile pipette into a sterile Petri dish
  • Pour 15ml of violet red bile agar (VRBA) at 450c into each petri dish and swirl to mix.
  • After the media has solidified, invert the Petri dishes and place in an incubator at 370c for 24 hrs.
  • Count the colonies of coliform bacteria, if necessary by a counter or manually marking the underside of the plate with a marker pen. Only petri dishes containing 10-150 colonies per plate should be counted.

Calculation of the results

  • Count the number of coliform organisms per plate noting the dilution of the sample on the plate.
  • Calculate the number of CFU/ml for each plate. If more than one plate was counted, get the mean of the results.

4. Sampling of Milk and Milk Products for Microbiological Testing and Analysis

This is the outline for the procedure for sampling milk from the silos, in process products, and finished products from the stores for microbial analysis.

Sampling Equipment

  • Sampling bottles
  • Cool box (insulated)
  • Gel-packs
  • Ladles
  • Burner

Sterilization of sampling equipment

  • Sterilize the sampling equipment for bacteriological testing by autoclaving at 1210c for 15-20 minutes
  • The sampling bottles/bags shoud have caps/closed adequately
  • Containers and closures should be sterilized and dry
  • Containers shall be of a material, which adequately protects the sample during handling, storage and in transit

Sampling technique for bacteriological testing purposes

Depends on the purpose for which sampling is done and the type of the products being sampled.

  • Random sampling from the stores
  • For sampling of milk from tanks with nozzles, wipe the nozzle with 70% ethanol or surgical spirit before sampling; let the milk run for some few seconds before taking a sample
  • Take some sample into the sterilized bottle swirl and drain. Repeat this. Take a sample for analysis and tightly cap the bottle.

Preservation of samples for bacteriological testing

  • Bacteriological samples should not have preservatives
  • Hold the samples at low temperatures 0-50 The cool box used to store these samples should ensure the temperatures range is maintained.
  • Transfer products to refrigerator as soon as possible
  • Microbiological analysis should start not later than 24hrs after sampling

5. Isolotion and Enumeration of Thermodurics in Raw Milk

This work instruction covers isolation and enumeration of thermoduric bacteria in raw milk. The purpose of this work instruction is to ensure the enumeration and detection of bacteria which survive exposure to pasteurization temperatures. Thermodurics microorganisms are generally gram positive rods, often spore forming and gram positive cocci.

Reagents

  • ¼ strength sterile ringer’s solution
  • Sterile Petri dishes
  • Water bath at 640c
  • Incubator at 370c
  • Tryptone Glucose Extract Agar

Microbiological Testing Instructions

  • Preparation of standard plate count media
  • Suspend 24 gm of Tryptone Glucose Extract Agar in I litre of distilled water. Bring to the boil to dissolve completely. Dispense into bottles and sterilize by autoclaving at 1210c for 15minutes.
  • Preparation of quarter strength ringers solutions
  • Prepare as per manufacturer’s instructions; sterilize by autoclaving at 1210c for 15minutes

Microbiological Testing Procedure

  • Aseptically take 10mls of well agitated milk aseptically and heat for 35minutes in a water bath at 650c
  • Cool the sample and prepare 10 fold dilutions in a quarter strength ringers solution
  • Add 1ml of the diluted sample to 15ml of cooled standard plate count agar. Pour into a sterile Petri dish and incubate at 370cfor 48hrs
  • Count the plates having between 10-300 colonies per plate, note the dilution

Calculations

Thermoduric count (organisms) per ml of milk = colony count x dilution factor. If more than one plate was counted, calculate the number of CFU/ml and then the results.

6. Wash-up and Sterilization in the Oven for Microbiological Testing Purposes

This work instruction covers glass wares used in micro biology analysis. The purpose of this work instruction is to ensure proper cleaning and sterilization of the glassware used in bacteriological analysis.

Glassware sterilization

Wash with warm soapy water and rinse thoroughly with cold tap water so as to remove any soap residue. Put in wire baskets and dry in a hot air oven.

Sterilization procedure (hot air oven)

  • Before sterilization all glassware must be clean and dry. Wrap glassware in grease paper, aluminium foil or put in sterilization tins and cover well with cotton wool.
  • Apply sterilization tape to indicate complete sterilization and place in a hot air oven
  • Set the oven temperatures at 1700C and maintain temperature of 1700C for 2 hours. Check the sterilization tape indicator. If ok, turn off the oven and allow the glassware to cool gradually.

7. Decontamination of Benches, Equipment and Rooms After Microbiological Testing

This work instruction covers bacteriological facilities likely to bring out cross contamination. Decontamination usually means making equipment and waste free from infectious agents

Instructions

  • Wipe down the benches with 70% alcohol or surgical spirit at the beginning of work, during work and at the end of the day.
  • Disinfect all accessible parts of the equipment with 70% alcohol.
  • Take swabs inside of the incubators with 70% alcohol after removing and incubating the plates.

Cleaning the Floors

  • Dilute powder disinfectant detergent in a bucket of clean water.
  • Thoroughly scrub the floor and rinse well.
  • Ensure that you clean the floor at least twice a day and immediately in case of spillages.

8. Disposing Laboratory Waste After Microbiological Testing

The purpose of this work instruction is to ensure all specimens, cultures and other materials used in the microbiological laboratory are made noninfectious before being discarded or leave the factory. The instruction covers how to dispose the wastes from the microbial plates after incubation. Paper towels and tissues used to wipe benches and equipment and to dry hands. Disposable gloves, Glass Pasteur pipettes, slides and covers slip used scalpel blades, scissors, knives, forceps etc.

Instructions

  • Place the infected material in the sterilizing boxes in the autoclave
  • Ensure the water level in the autoclave is okay.
  • Close the autoclave properly and start heating.
  • Open the air vent until steam starts to come out through the control valve.
  • Close the control valve and let the autoclave attain 1210c then continue heating for 15 minutes.
  • Allow the autoclave to cool down and the pressure to cool down to 00
  • Open the autoclave, remove the material and discard through the normal waste material disposal system.

Waste Water and Processing Water Analysis

The quality of process water must be maintained at very high standards, otherwise, there will be uncontrollable contamination of the food being processed at the plant. Waste water emitted from the plant must be handled properly to avoid contaminating the environment. The quality of such water must be determined to facilitate proper handling.

1. Chlorine Content Determination

This work instruction is to ensure process water is assessed for chlorine content in order to determine its suitability for use in production

Equipment and reagents

  • Housman paleis test DPD tablet number 1 and 3
  • Comparator disc number 3/40 A& 3/40 B

Work Instructions

  • Rinse a cell with the sample water
  • Pour enough sample water to cover the tablet and add one tablet of DPD 1
  • Shake and allow to disintegrate then make up to 10mls
  • Mix and match at once and take reading on comparator
  • This is free residual chlorine
  • For total residual chlorine add to the cell one DPD 3 tablet and mix and match at once then take reading on comparator disc.
  • Combined residual chlorine is the difference between free residual chlorine and total residual. Enter the records on water quality book

P.S.: \\

Chlorine added into water reacts first with hydrogen sulphide and the organic matter up to the break-point level, what remains is therefore the amount that will destroy the microorganisms.

Recommended Standards

Free residual chlorine of 0.2 – 0.5 ppm is recommended for potable water.

2. Bacteriological Tests on Waste Water and Processing Water

The purpose of this work instruction is to ensure proper analysis of water.

Reagents and equipment

  • 125ml bottles
  • 25mls universal bottles
  • Water samples of at least 200mls
  • Mackonkey broth purple CM5a
  • Durham tubes (big and small)

Water sampling method

  • Label each bottle with the laboratory sample number
  • Mix the sample of water thoroughly by inverting the bottle at least 10 times
  • Inoculate the bottles of the sterile broth as follows
  • Add 50ml of water to the bottle containing 50ml (double strength) of MacConkey broth
  • Add 10ml of water to each of the five Universal bottle containing 10ml of broth (double strength)
  • Add 1ml of water to each of the five bottles containing 5ml of broth (this is for untreated water samples single strength)
  • Mix the contents of each bottle
  • Incubate the inoculated broths in water bath at 440c for 24 hours with the bottle loosely capped.
  • After incubation, examine and account tubes that have produced both acid and gas
  • Read the results from the probability tables.

Probability Tables for Estimating the MPN of Faecal Coliform Bacteria

Untreated water samples

Note: – the most probable numbers from 0-20 are correct to the nearest unit above 20 are correct to the nearest 5

Probability Tables for Estimating the MPN of the Faecal Coliform Bacteria in Waste Water

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