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ESTABLISH MENT OF REGIONAL REFERENCE CENTRE

FOR MILK PROCESSING AND MARKETING.

QUALITY CONTROL MANUAL

Raw Milk and Milk Products

Apia, April 2000

Teuvo V.A. Siirtola

GCP/SAM/007/FRA.


Table of contents

1. BACKGROUND

2. QUALITY CONTROL

2.1 Introduction

2.2 Quality control laboratory

2.3 Milk sampling

2.3.1 Sampling from milk can

3. QUALITY CONTROL PRACTICES

3.1 Platform tests

A. Organoleptic tests

B. Lactometer test

C. Alcohol test

D. Clot-on- boiling test

3.2 Quality control during milk processing

3.2.1 Sampling

4. TESTING METHODS

4.1 Organoleptic test

4.1.1 Application to raw/liquid milk

A. Abnormal colour/consistency

B. Abnormal smell and/or taste

C. Chemical changes

D. Off-flavours from feeds

E. Absorption of off-flavours from air, milk containers etc.

4.1.2 Application to m ilk products 32

4.2 Lactometer test (Lactodensimeter test, Specific gravity)

4.3 Alcohol test

4.4 Clot-on-boiling test

4.5 Titratable acidity

4.5.1 Preparation of reagents

4.6 Phosphatase test (IDF Method)

5. MAINTENANCE OF LABORATORY EQUIPMENT

5.1 Routine sanitising of all equipment

5.2 Simple sterilisation methods of equipment

6. SAFETY IN LABORATORY

7. SCOPE OF QUALITY CONTROL

7.2 Transport of milk from farm to Milk Collecting Point/Centre or to Milk Plant

7.3 Milk Collection Point/Centre, Milk Processing Plant

7.4 Transport of milk products

7.5 Distribution and resale

1. BACKGROUND

The use of milk and milk products as human food has got a very long history. The milk - as it is meant to be the first and sole food of offspring of mammals - is more or less a complete food as it contains in a balanced form all the necessary elements for building up and maintaining of human and animal body. In addition it contains immunoglobulins which protect the newly born against number of diseases. Milk has also various properties, which make it easy to convert it into number of different milk products or to use it as ingredient for other food items. Various cultures have their own traditional ways to use milk and to prepare different milk products.

Milk is also a perishable product. Milk is an ideal media for micro-organisms and as it is a liquid it is very easily contaminated and invaded by bacteria. Thus milk can transmit diseases of microbial origin to people from sick animals and/or people carrying certain diseases and contaminating the milk with pathogenic bacteria during its handling.

The milk is practically sterile in the upper part of the udder of a healthy milk animal. The closer the teats milk is located the more microbes are found. Milk is contaminated with bacteria mainly during milking. It may be possible to milk animals such a clean way that milk only contains 500 to 1 000 bacteria per ml. Usual total bacteria count in milk is up to 50 000 per ml after milking but sometimes the count is one or several millions - indication of poor hygiene during milking and milk handling or/and diseased milk animal.

Milk becomes usually sour after keeping it at ambient temperature after milking. How soon this does happen depends on quantity and quality of contamination and on milk temperature. The increased acidity (decrease in pH) in milk is commonly related with the development of lactic acid by microbiological fermentation (Lactic acid bacteria) of lactose.

There are also other types of micro-organisms which do attack other components of milk than lactose; proteins (Proteolytic bacteria) and milk fat / lipids (Lipolytic bacteria). Nevertheless, as result of microbial activity the quality of milk deteriorates and if used for processing results in poor quality milk products. Therefore only using fresh milk of tested quality as raw material can produce high quality milk products.

For above reasons the dairy industry strictly controls the quality of incoming milk from dairy farmers. If the milk quality does not fulfil the set minimum quality standards, it is rejected which means economical loss to the farmer. Most of the countries have implemented special laws and regulations concerning composition and hygienic quality of milk and milk products to protect consumers and the public health.

On the other hand the dairy organisations and governments in turn usually help dairy farmers through extension services to produce good quality milk. Animal health and AI services are also provided to the farmers.

The most crucial aspects from the point of view of maintaining high quality of fresh milk are listed below: - good animal health status - hygienic milking and milk handling on farms - cooling of milk immediately after milking if possible (e.g. using cold water) - transport of milk to milk processing plant as soon as possible after milking, maximum within two hours - protect the milk containers/cans from dust and direct exposure of sun Maintaining of good quality of milk is benefiting all; farmers, milk processing plants and consumers. As result the milk producer will receive full value for his produce, the processing losses of milk processing plant because of poor quality raw milk are minimised and the consumer can relay on the quality of the milk products.

The above brief background with special emphasis to raw milk quality was given for better understanding of the quality control aspects and the methodology this manual includes. The manual contains quality control aspects and methods, which were considered relevant at the time of its preparation. Simple but reliable quality testing methods were chosen which use locally available inexpensive chemicals and equipment commonly used for laboratory work.

The Quality Control Manual prepared by FAO Project; AG: DP/UGA/84/023, Kampala, Nov. 1988 was used as source of methodology.

Apia, April 2000

Teuvo V.A. Siirtola

2. QUALITY CONTROL

2.1 Introduction

Quality control includes a wide range of and scope of activities to be carried out regularly. The ultimate task of quality control is to provide the plant management and persons responsible for quality with information of fulfilment of previously defined quality criteria and standards at every stage of process as early as possible. Early warning about failures and inability to meet quality standards at any stage of processing helps correct the situation in time and decreases losses caused by irrelevant quality of the end product. The planning and establishment of quality control for a milk plant includes the following:

  • Organisation and staffing of the quality control activity within the dairy plant Training the quality control personnel
  • Definition of quality standards taking into account also the legal requirements for raw materials, ingredients, packaging materials, processing line and equipment, end products, storage, handling during distribution etc.
  • Laboratory and methods for testing and analysing for the quality parameters including organoleptic, physical, chemical and microbiological methods - Sampling methods and schedules.
  • Alarm limits (maximum/minimum) for quality parameters - Recording and reporting systems.

2.2 Quality control laboratory

The task of a quality control laboratory is to organise and carry out all the practical work included in the quality control activities of a milk processing plant. The laboratory has to be equipped with all necessary apparatus, testing and analysing methods and staff in order to facilitate the above-mentioned function.

The laboratory should be able to control the following:

  • Raw milk quality during collection and at reception
  • Various quality aspects related to processing
  • Quality of finished milk products
  • Quality of milk products during storage/distribution
  • Cleanliness and hygiene at the milk processing plant

Clear and regular recording of test results as well as regular and timely reporting to the personnel responsible for processing and the plant management.

2.3 Milk sampling

It is very important that the sampling of milk is always made in correct and hygienic way so that the milk sample will represent the whole lot of milk (e.g. the whole contents of milk can) and that neither the milk nor the milk sample will be contaminated during sampling. Sampling of liquid milks always involves thorough agitation before the sample is taken in order to make the contents of a milk container as homogenous as possible for obtaining a representative sample. Too vigorous agitation should, however, be avoided because air bubbles if dispersed in milk will change its physical properties and disturb analysing. Very clean and for microbiological purposes even sterile device and sample bottles made of materials (stainless steel, glass, plastic of appropriate quality) which will not affect the milk nor tests are used.

The milk samples need to be taken special care of. If the samples have to wait for analysing or this need to be transported to a laboratory the samples should be cooled down and kept under refrigerated conditions during subsequent storage and transport. Using potassium dichromate tablets or solution (10% water solution, 1 ml to 100-ml milk) can also preserve samples for physic-chemical analyses. It is clear that before analysing each sample needs to be carefully agitated once again.

Sample bottles and test tubes should be clearly marked or labelled so that each sample can be easily identified and related to the original milk container.

2.3.1 Sampling from milk can

A. Agitate the milk with at least ten full lengths of plunger or dipper and immediately after this take the sample of required size into a sample bottle and close it. In order to make sure that a sample will well represent the whole contents of milk can take the half of the required sample from the lower portion and another half from upper part of the milk can.

B. If no plunger is available, agitate the milk as well as possible with a dipper having a handle long enough for doing this. Take half of the sample from the lover portion and half from the upper portion of milk can.

C. Sampling from a smaller milk container can be done after turning the container upside down not less than ten times before sampling. This can be done providing that container in question can be closed well. This method of agitation of milk applies also to milk samples in a laboratory before testing.

 

3. QUALITY CONTROL PRACTICES

These methods vary from rapid platform tests to more sophisticated and time consuming analysing methods including, e.g. microbiological quality tests.

However, the scope and quality of quality control and methodology depend greatly on the size of the enterprise, volume of production and the product mix. Small dairy plants producing only one product or so and starting with limited production capacity have to relay on less sophisticated quality control methods. These should, anyhow, be reliable and cover the most essential quality criteria like overall hygienic quality of milk and end products as well as hygienic level of operations.

The quality control needs standards and methodology used should be under continuous evaluation and development according to increases in and diversification of production and improvement of processing methods.

3.1 Platform tests

Platform tests or milk reception tests are the commonly used names for the tests carried out by the persons responsible for raw milk collection and/or reception. The tests in question are rapid quality control tests - organoleptic tests being of crucial importance - whereby the milks of inferior or questionable quality can be screened out before the milk leaves its original container and is mixed with bulk milk during milk collection and/or reception. This is of crucial importance from the point of view of processing and quality of end products, because one single lot of milk of poor quality can spoil the rest of the milk it is mixed with. Application of platform tests does not directly involve laboratory analysis of raw milk samples but in suspected cases a sample from milk should be taken to the laboratory for further inspections for quality. This lot of milk should be taken aside and not mixed with bulk milk in order to verify its quality. In case the milk does not pass the quality tests and does not comply with previously defined quality standards it should be rejected. It is also very important that the milk producer in question will be contacted in order to find out the reasons for spoilage of milk and in close co-operation with him to find ways how to improve the situation in the future and how to eliminate this problem.

At milk reception sites - during milk collection and reception at milk plant - the platform tests can be applied as follows:

A. Organoleptic tests

The appearance of the surface of the milk and the lid is observed and inspected instantly after removing the lid of incoming milk can or container. Any abnormal colour of the milk, visible dirt and particles, changes in viscosity etc. are observed. Any abnormal smell is noticed by inhalation of air standing above the milk in the upper part of the milk can. B. Lactometer test

If the milk appears during organoleptic inspections to be too thin and watery and its colour is "blue thin" it is suspected that milk contains added water. Lactometer test serves as a quick method for determination of adulteration of milk by adding water. The lactometer test is based on the fact that the specific gravity of whole milk, skim milk and water differ from each others.

B. Alcohol test

In case there is any reason to suspect that milk is sour, alcohol test is used as platform test for rapid determination of elevated acidity of milk. Anyhow, if the result of alcohol test indicates too high acidity in milk a sample from milk is to be taken to the laboratory for further testing of titratable acidity.

Alcohol test is based on fact that the proteins in milk, which has become sour, e.g. as result of lactic acid formation by bacteria become susceptible to alcohol precipitation.

If the mixing of equal quantities (e.g. 2 ml) of milk and 68% alcohol in a test tube results in coagulation of proteins it indicates that milk is sour. This milk is not fit for any processes applying pasteurisation, because the proteins in milks having increased acidity have also loosed the stability to the temperatures used for pasteurisation.

For above reasons it is recommended that alcohol test is applied to every and each incoming milk can and container if the milk is to be pasteurised.

C. Clot-on- boiling test

Also, this test is used for rapid testing of increased acidity of milk. As stated above heating will precipitate proteins of milk if it is sour. This method is slower than alcohol test but very useful where and when an alcohol test is not available.

This test is performed simply by heating small amount of milk in a test tube over a flame or immersed in boiling water for five minutes. The result can be seen immediately. If no coagulation occurs, it indicates that milk can stand heating operations at the time of testing.

3.2 Quality control during milk processing

One of the most important tasks amongst the quality control is to control and follow up regularly the fulfilment of quality standards at every stage of process flow in order to guarantee the best possible quality of end products. In this order the quality control activities include the following:

  • Quality of all raw materials, ingredients, additives, packaging materials, etc. used in processing;
  • Control on standardisation of, e.g. fat content of raw milk for production of various products;
  • Quality and activity of dairy cultures (lactic acid starters), rennet etc.
  • Hygienic quality of packaging materials;
  • Quality and strength of cleaning and sanitation solutions;
  • Hygienic status of processing line, milk tanks, pipes line, packaging machines, etc.;
  • Overall cleanliness and hygiene during reception, processing, storage and distribution;
  • Organoleptic, hygienic, chemical and physical qualities of the end products including packages;
  • Keeping quality properties (shelf life) of end products;
  • Well organised recording of test results and findings as well as regular and timely reporting.

3.2.1 Sampling

The above described quality control activities involve quite extensive sampling and samples of different nature. Major part of quality control activities and sampling should be carried out regularly. Some of them can be made time to time. The leading principle in sampling is that the samples taken will represent the whole as well as possible. Therefore, e.g. when taking samples from a bulk milk or storage tank the contents of a tank is to be agitated thoroughly prior to sampling. The sampling should be made clean and hygienic way using samplers and sample bottles of appropriate quality as described earlier.

Samples are also taken at various stages of process in order to follow up the adequacy of heat treatment (phosphatase test), the development of lactic acid in fermentation processes etc. These samples can be taken, e.g. from milk and cream after pasteurisation, from milk during fermentation process, cheese whey, cream for butter making etc.

Activity of dairy cultures, rennets etc. can be tested in the laboratory by using small quantities of milk, e.g. in test tubes.

It is also important to test a small sample of milk to be used for fermented milk products as above by adding lactic acid starter. If acidity does not increase during incubation, the milk probably contains antibiotics and is not fit for processing.

For controlling the quality of end products, packaging materials etc., either systematic (one sample per certain number of units produced) or random sampling can be applied. In case of batch production (e.g. cheese) representative samples from each batch produced should be taken.

It is important to know what will happen to the quality of a product during storage, transport and distribution and for how long time the consumer can keep it, e.g. in the refrigerator without affecting the quality. Therefore samples of products are kept under similar conditions in the laboratory and tested for various quality parameters over sufficient period of time in order to find out the keeping quality properties (shelf life) of the product.

It is essential for the quality of end products that the equipment used for processes are clean and sterile which means that cleaning and sanitation has been made correctly. Those milk processing plants having microbiological testing methods are regularly checking the cleanliness and sterility of equipment after cleaning, before starting the processing. The samples from the surfaces of equipment which will be in contact with milk during process like, tanks, pipe line, milk cans etc. are collected into sterile solution by using Swab test and Rinse test.

In Swab test a surface area of known size of equipment is wiped with sterile cotton bad and collected in sterile solution. In Rinse tests the surface of, e.g. a milk can is rinsed carefully with sterile solution which is collected in a sterile bottle. The subsequent analyses of Total Plate Counts of Bacteria and Coliform Plate Count will give the results in micro-organism count per square cm or, e.g. per milk can. Coliform bacteria should be absent.

At small milk processing plants, which do not have microbiological methods available, this type of control is not possible. Anyhow, the level of hygiene should be observed regularly and proper methods for cleaning and sanitation should be used.

 

4. TESTING METHODS

4.1 Organoleptic test

The organoleptic testing of raw milk and milk products uses normal senses of sight, smell and taste in order to observe and record the overall quality. The result of this test is obtained immediately on the spot where and when it is carried out. This method is of minimum cost but when correctly used it is very useful and, e.g. permits rapid screening out of poorest quality milk at reception. It is applicable on farms, during milk collection, at milk reception and at the milk processing plant.

The organoleptic testing applied for judgement of qualities of various milk products is the first and basic testing method.

Nevertheless, an organoleptic test is a subjective test and depends on skills, experience and senses of the person carrying out the test. Therefore, these tests should be complemented by further laboratory tests.

4.1.1 Application to raw/liquid milk

- The organoleptic test should be carried out immediately after opening the lid of the milk can/container.

- Observe the colour, appearance, and cleanliness of milk - Smell from milk can be sensed just above the milk surface immediately after removal the lid. - Taste of milk is more permanent and easy to define than smell. Taste raw milk only after making sure that it is from healthy animals.

- After emptying the can/container inspect the insides of the lid and the can/container for visible dirt and impurities.

The appearance, colour, smell and taste of milk should be normal and typical to, e.g. those of fresh goat's milk.

The following abnormal colours, appearances, smells and flavours can be, however, detected:

A. Abnormal colour/consistency

  • Pink colour: Polluted with blood;
  • Yellowish creamy colour: Colostrum or late milk;
  • Blue thin colour: Adulterated by adding water;
  • Large clots or flakes: Sour milk or mastitis milk;
  • Small white clots or grains: Mastitis milk or adulterated with flour and skim milk powder;
  • Visible dirt and impurities: Produced under unhygienic conditions.

B. Abnormal smell and/or taste

  • Souring: Lactose fermenting, acid producing bacteria
  • Malt: Streptococcus lactis var. maltigenes Bitter: Peptonising of milk by Streptococcus liquefraciens
  • Blue souring: Unpleasant sweet and sour smell, thin and waterish appearance caused by bacterial activity and storage in a closed container without ventilation
  • Fruit aroma: Pseudomonas fragi producing esters
  • Slimy milk: Capsule forming bacteria, e.g. Aerobacter aerogenes and Alcaligenes viscosus
  • Bubbles, coagulation and whey separation: Fermentation by yeast

C. Chemical changes

  • Salt: Increase in chlorine content and decrease in lactose content. Mastitis milk, colostrums
  • Boiled: Release of volatile sulphides
  • Rancidity: Lipolysis of fat
  • Tallow: Oxidation of fat

D. Off-flavours from feeds

Garlic, onion, beets, bad silage, certain plants and pastures can cause off-flavours to milk

E. Absorption of off-flavours from air, milk containers etc

  • It is well known that milk and cream can absorb smelling compounds from the air. This is caused by the ability of butter fat to absorb, specially after milking when the milk is warm, strong smells like paint, phenol, cresol, lysol, petroleum, etc. Strongly smelling paints, disinfectants and other chemicals should be not handled and stored in places where the dairy animals are kept and milked.
  • Storage of milk together with fruits and fish also causes off-flavours to milk

4.1.2 Application to m ilk products

The organoleptic evaluation of quality parameters and standards set for milk products, e.g. for cheese is of outmost importance because there are no laboratory methods which can replace human senses in judging the parameters like appearance, colour, aromas, taste, quality of a package etc. Anyhow, sensory testing being the most important and first testing method applied to finished products these should be complemented by laboratory tests. Especially important is to analyse hygienic quality and composition of the product.

The organoleptic testing of a milk product includes:

- Flavour and taste

- Body and texture

- Colour and appearance

- Packing qualities

4.2 Lactometer test (Lactodensimeter test, Specific gravity)

This test is used to determine whether the milk is likely adulterated: A. Contains added water B. Sub-standards in total solids content C. Fat removed from original milk D. Contains added skim milk or skim milk powder, dissolved in water or as powder

Apparatus:

- Lactometer

- Measuring cylinder 250 ml

- Thermometer

Procedure:

Agitate the milk carefully with a plunger before sampling. Take care not to introduce air bubbles into the milk. Air bubbles would interfere the test reading the result.

Place the milk sample in a cylinder the size of which is determined by the dimensions of lactometer.

Place the cylinder on a table or other level surface. Place and lower down the lactometer slowly into the milk till it is floating freely.

Read the result at eye level at the point of the scale where the surface of milk cuts across the stem of lactometer. The reading result is recorded together with the milk temperature. The lactometer can give correct results only when the milk temperature corresponds with the calibration temperature indicated on the lactometer stem. For other temperatures temperature correction is to be applied for correct results.

The correction is 0.2 lactometer divisions for each degree centigrade; to be added when milk temperature is above the calibration temperature and to be subtracted if below it.

4.3 Alcohol test

This test is used for immediate detection of increased acidity level in milk.

Apparatus:

- Alcohol gun, or test tube and pipette

Reagent:

- 68% ethanol

Procedure: Equal amounts (e.g. 2 ml each) of milk and 68% alcohol are mixed in a test tube. The content is agitated by gentle movement. The result can be observed immediately. Look carefully the inner surface of the test tube. If there is no reaction by alcohol test, the acidity of milk is likely at normal levels and the milk can be used for pasteurisation. If there, is any trace of; coagulation, thickening, lumping, flaking, graining, precipitation, dense cloud formations the milk is likely sour and not fit for pasteurisation. However, a sample from this milk should be taken for the laboratory for a further acidity test.

4.4 Clot-on-boiling test

Like the alcohol test also this test is used during milk collection for detection of instability of milk proteins for heating because either lactic acid or rennin enzymes produced by bacteria.

Apparatus:

  • Burner or boiling water bath
  • Test tube
  • Pipette 2-5 ml

Procedure:

A small amount of milk is boiled over a flame or immersed in constantly boiling water in a test tube. The result will show up instantly. The content of a test tube is examined by running the milk along and around the inner walls of the test tube. No coagulation indicates that milk is fit for heating operations at the time of testing. If coagulation, thickening, lumping, flaking, graining or precipitation is observed milk is likely not fit for pasteurisation.

4.5 Titratable acidity

The titratable acidity test is used for quantifying the acidity in milk. This test should be used for samples from milks, which are suspected to be sour according to results of alcohol and/or clot-on boiling tests.

Apparatus:

  • Burette reading to 0,1 ml
  • Porcelain dishes
  • Pipette 10 ml reading 1-10 ml

Reagents: - Phenolphthalein indicator solution (in ethanol) 0.5% 0.1 N Sodium hydroxide solution (water), (NaOH-solution) Procedure:

  • After agitation well 9 ml of milk is pipetted into the porcelain dish.
  • 3 drops of phenolphthalein indicator is added to the milk in the dish
  • Add carefully drop by drop NaOH solution from the burette to the milk in the dish and agitate carefully. When a pink colour appears and remains stop adding NaOH solution and read off from the column of the burette the volume of 0.1 N NaOH solution consumed.
  • Add one more drop of NaOH solution to the dish. If pink colour is produced and remains the previous reading was correct.
  • Divide the reading by 10, which gives the result in % of lactic acid.

If the reading was 1.6 ml (amount of 0.1 N NaOH used) this is recorded as 0.16% lactic acid.

If a lactic acid level 0.16% is detected in a raw milk sample, it is usually considered to be too high and it is doubtful to accept this milk for pasteurisation.

4.5.1 Preparation of reagents

  • 0.1 N NaOH solution: Dissolve 4 g NaOH (an ampoule) in 0.5 litre of distilled water in a measuring flask and fill up to 1 000 ml.
  • 0.5/a phenolphthalein indicator in 50% ethanol: Mix 100,5 ml 99,5% ethanol with 99,5 ml distilled water and thereafter dissolve 1,00 g phenolphthalein in the mixture.
  • 68% ethanol (alcohol): To make up app. 1 litre of 68% ethanol (by volume) 1000 x 68/a ml a % ethanol and 1000 x (a - 68)/a ml distilled water is to be mixed.

a = alcohol % in absolute ethanol

4.6 Phosphatase test (IDF Method)

The phosphatase test is used to detect inadequate pasteurisation and possible raw milk contamination of milk and milk products.

The phosphatase enzyme, which is present and active in fresh milk, is completely destroyed by the heat treatments applied both for low temperature pasteurisation and for the hightemperature shorttime pasteurisation (HTST) processes.

This IDF-method specifies the determination of the phosphatase activity in milk and can be applied for the control of adequate pasteurisation. The phosphatase activity is a measure of the quality of active alkaline phosphatase present in milk and milk products. The liquid sample is diluted with a buffer solution at pH 10.2 and incubated at temperature of 37 C for two hours. Any active alkaline phosphatase present in the sample will liberate p.nitrophenol from the added p.nitrophenyl phosphate. The p.nitrophenol liberated is measured by direct comparison with standard colour glasses in a comparator.

Duration:

  • Preparation: 5 minutes
  • Incubation: 2 hours
  • Reading: 2 minutes

Apparatus: Volumetric Flask Test Tubes Lovibond Comparator Pipettes 5 ml, 1 ml Reagents:

  • Dissolve 3.5 g of anhydrous sodium carbonate and 1.5 g of sodium bicarbonate in distilled water and dilute to 1 000 ml in a volumetric flask.
  • Dissolve 0.15 g of p.nitrophenyl phosphate in 50 ml of sodium carbonate-bicarbonate buffer solution and dilute with the buffer to 100 ml in a volumetric flask.

The solution keeps stable for one month if stored in refrigerator (below 5 C). Any instability is denoted by the formation of a yellow colour. Procedure:

  • Analysis should take place preferably shortly after sampling. Otherwise keep the sample in a refrigerator (maximum two days). Mix the sample carefully for the analysis.
  • Pipette 5 ml of buffer solution into a clean and dry test tube followed by 1 ml of milk sample. Close the test tube with a test tube stopper. Mix by inversion and place in the 37 C water bath.
  • At the same time place in the water bath a control sample test tube containing 5 ml of the buffer solution and 1 ml of boiled milk of same kind as under test. 4. After two hours, remove the tubes from the water bath, invert each and read the colour developed using the Lovibond comparator and special disc, the test tube containing the boiled control milk being placed on the left of the stand and the tube containing the milk sample under testing on the right. The direct reading is recorded as ppm nitro-phenol in milk. Results:
    • Results of duplicate determination should not differ by more than 2 ppm of p.nitrophenol over the critical range of 0-14 ppm of p.nitrophenol. Any reading of excess of 10 ppm is recorded as unsatisfactory.

Precautions:

a) After use, test tubes must be emptied, rinsed in water, washed in hot water containing an alkaline detergent, following by rinsing in clean hot water. Finally the tubes must be rinsed in distilled water and dried.

b) Pipettes must be rinsed in clean cold water immediately after use, washed as above followed by rinsing in distilled water and dried before use.

c) The test tube stoppers must be rinsed in hot water immediately after use followed by boiling for two minutes in distilled water. d) Use a separate pipette for each sample and avoid contaminating the pipette with saliva. e) The test must not be exposed to direct sunlight at any time.

f) The use of plastic utensils should be avoided since they may contain phenol compounds that may give false positive results of the test.

 

5. MAINTENANCE OF LABORATORY EQUIPMENT

The laboratory equipment should all times kept clean and free from traces of milk residues and chemicals, which might interfere the testing. At the end of each working day the glassware and equipment are rinsed first with cold water and then washed in a detergent, rinsed with distilled water and dried.

The glass ware and equipment used for microbiological samples, dairy cultures, agitating, sampling and other equipment in contact with milk should be not only kept constantly clean but must also be sterilised before use.

5.1 Routine sanitising of all equipment

  • Rinse with cold water;
  • Wash and brush in hot water containing detergent in 1% solution, e.g. washing soda;
  • Rinse in hot distilled water and examine for cleanliness;
  • Allow to get dry upsidedown in dust-free surrounding;
  • Pipettes should after use be placed vertically in a cylinder containing a mild solution of hypochlorite in concentration of 200 ppm. This eases cleaning and minimises the risk of contamination.

5.2 Simple sterilisation methods of equipment

  • Immersion in boiling water for not less than five minutes Ensure that the water is kept on the boil all the time;
  • Heating in a hot air oven 160 C for two hours;
  • Steam under pressure in an autoclave 120 C for 20 minutes;
  • Immersion in 70% ethanol and flaming just before use.

 

6. SAFETY IN LABORATORY

  • Work in a quality control laboratory involves handling of highly corrosive and irritating chemicals, open flame and boiling water.
  • The most efficient prevention against accidents is to have a proper working routine including the layout of working area, use of protective glove, eyewear and clothing.
  • The most common accidents are corrosion and burning of skin and eyes caused by alkalis, acids, and hot water and hot surfaces.
  • The first aid in these cases is immediate application of COLD WATER.
  • The damaged area should be rinsed in plenty of water for 2-30 minutes depending on degree of damage.
  • Do not apply grease or bandages as first aid Do not try to neutralise alkalis or acids as this may increase the damage.
  • In case of damages to eyes, apply cold water until medical assistance can be obtained
  • One or two trained operators should only operate the autoclave
  • The manufacturer's instructions should be strictly followed, as autoclaves produce lethal pressures
  • All containers and flasks with chemicals and solutions should be clearly labelled with details of contents, date of preparation or opening
  • When diluting acid or alkaline with water, the following rule is imperative: POUR THE ACID OR ALKALINE SLOWLY INTO THE WATER.

 

7. SCOPE OF QUALITY CONTROL

The quality control activity in the dairy industry must cover all the quality aspects of milk and milk products and all the way from the dairy cow to the consumer. The list of subjects of quality control activity below gives an idea about the scope of the tasks of the quality control in the dairy industry.

7.1 Dairy farm

  • dairy animal
  • personnel
  • cow shed and milking parlour
  • milk handling and storage rooms
  • cold and hot water
  • milking including milking machines, utensils and materials
  • milk handling including utensils and materials
  • quality of cleaning and washing
  • milk cooling and storage
  • delivery for collection
  • possible direct sales of milk on farm

7.2 Transport of milk from farm to Milk Collecting Point/Centre or to Milk Plant

  • cleanliness and condition of transport vehicle
  • temperature during transport
  • duration of transport
  • unloading and reception methods

7.3 Milk Collection Point/Centre, Milk Processing Plant

  • overall cleanliness and condition
  • personnel
  • cold and hot water
  • milk reception timetable
  • milk cooling and storage
  • storage time
  • heat treatments
  • milk handling and processing
  • packaging and labelling (production and best before dates)
  • storage
  • loading and transport for distribution

7.4 Transport of milk products

  • cleanliness and condition of transport vehicle
  • loading and transport system
  • temperature during transport
  • duration of transport
  • unloading system and place

7.5 Distribution and resale

  • general cleanliness and condition
  • personnel
  • cold and hot water
  • quality of cleaning
  • storage time of milk products
  • storage temperature
  • methods of handling, distribution and sale