Quality of Dairy Products
Judging Grading and Standardizing
Booklet No. 278
Dairy Management & Milk Products: DMMPS - 8
Contents
Preface
I. Introduction
II. Milk Grades
III. Fundamental Rules for Judging Milk
IV. Detection of Adulteration in Milk
V. Detection of Water in Milk
VI. Detection of Adulteration in Ghee
VII. Off-Flavours in Milk
VIII. Quality Control Tests
IX. Bacteriological Tests
X. Standards of Dairy Products
XI. PFA Standards for Milk Products
Preface
Milk is a complete food which is liked by almost all the people. Despite 'Operation Flood' there is still scarcity of milk the market. A common man is still deprived of milk. The milk & milk products in the market is mostly adulterated. People follow different methods to test the purity of the milk. But, best is to test the milk & milk products in the laboratory by scientific methods. This booklet describes in detail different lab methods to test the adulteration of milk & milk products.
Dr. K. T. Chandy, Agricultural & Environmental Education
I. Introduction
Judging of milk refers to the evaluation of its "eating quality" on the basis of various attributes; Grading refers to the classification of milk into different grades. The 'eating quality' of any dairy product is determined by organoleptic/sensory tests, smell taste, touch and sound. Of these, taste and smell are the most important in judging and grading. Based on the quality as determined by the evaluation of the milk, samples could be broadly classified into two categories: acceptable and rejected.
The samples which are grouped under "acceptable category" can further be differentiated in grades. This classification is based on physical characteristics such as colour, smell, taste and presence of visible foreign material in milk. The evaluator makes use of the most sensitive organs viz. eyes, nose, tongue and hands.
II. Milk Grades
The quality of milk of various grades is described here.
Grade I
This includes the fresh milk completely free from off-flavours, abnormal colours and visible foreign materials, such milk when tested should give over five and half Methyl Blue Reduction (MBR) time.
Grade II
Milk having off-flavours such as cowy/barny, flat, foreign, metallic, or rancid to a moderate degree and MBR time between two and half hours to five and half hours. Presence of small amounts of foreign material placed under this category. The off- flavours present in grade II milk could be easily removed by suitable processing techniques.
Grade III
Milk having off-flavours such as acidic, bitter, weedy, oxidized, metallic and extraneous material distinctly visible in great amount is classified as grade III milk. This milk has less than two and half hours MBR time. It is difficult to remove completely the off-flavours by easy processing techniques. Such milk is not considered suitable for market milk operations. It is generally separated and cream is used for making ghee and skim milk for casein manufacture.
III. Fundamental Rules for Judging Milk
While judging 'the quality of milk, the evaluator must keep following points in mind. He should,
-be in physically and mentally in sound condition before scoring.
-know the score card or ideal set up for each product.
-learn the grades of each product and defect intensities allowed in each grade.
-have the samples at proper temperature ego ice-cream at 20.6 to 23.3°C, butter, cheese and milk at l5o Centigrade.
-observe the aroma immediately after removal of the sample and introduce into the mouth a sufficiently large volume for tasting.
-observe the sequence of flavours and make a mental picture of the taste and smell reactions and concentrate upon the sample being examined.
IV. Detection of Adulteration in Milk
The following tests may be done for the detection of adulteration in the milk sample.
1. Detection of sucrose
Add 0.1 g of resorcinol powder in 10 ml of milk. If sucrose is mixed, the milk will give red colour.
2. Detection of starch
Add little iodine solution in milk. Milk containing starch will show violet colour.
3. Detection of skim milk powder
If colour of milk by addition of nitric acid turns light violet it indicates mixing of skim milk powder otherwise normal milk shows yellow colour.
4. Detection of gelatin
Add picric acid and shake well and observe the precipitation which in case is of yellow colour, indicates mixing good amount of gelatin and if is shows light brown colour it indicates mixing in very little quantity.
5. Detection of total solids
This adulteration can be determined by following Richmond's formula:
T.S. =G/4 + 1.2F + 0.14
Where,
T.S. = total solids percentage in milk
G = correct lactometer rca4ing of milk at 6()0 Fahrenheit
F = fat percentage in milk
V. Detection of Water in Milk
The water adulteration in milk can be detected by following tests.
1. Detection by vieth ratio
Vieth ratio in milk is 13:9:2 for the presence of lactose : protein : mineral matter and if it gets altered indicates the adulteration.
2. Nitrate test
Water from ponds, river, wells and hand pump contains nitrate, therefore, if milk is adulterated with such water this test is positive. Treated tap water normally does not contain nitrate. This test is not useful in case milk is adulterated with tap water.
3. Freezing point test
The freezing point of normal milk is -0.544o C. In case milk is adulterated with water the fat and total solids in milk are reduced causing a rise in freezing point. This can be determined by use of standardized apparatus Hortvet cryoscope.
l00(T -T 1)
Percent of water added in milk = ------
T
Where,
T and T 1 is freezing point of pure and adulterated milk respectively.
Here, some examples of percent of water added in milk depending on the freezing point of milk is given.
Freezing point of milk Water added in milk
(OC) (%)
-0.5381.1
-0.536 1.5
-0.520 4.4
-0.510 6.3
-0.500 8.1
VI. Detection of Adulteration in Ghee
The following tests may be used for the detection of adulterants in ghee sample.
A. Test for vegetable fat
The following tests help to detect the presence of vegetable fat or the hydrogenated fat in ghee.
1. Bomer's phyto-sterol- Test
Fifty grams of ghee is weighed and saponified with 100ml of alcoholic potassium hydroxide and 300 ml of distilled water is added. This solution is passed through ether to dissolve unsaponified cholestrine and phytosterine. Ether is evaporated and residue (Unsaponified matter) is treated with 2 ml acetic anhydride for 15 minutes and is filtered. When crystals of acetate are formed filtrate is evaporated. These acetate crystals are dissolved in alcohol and recrystallized. If melting point of these determined is above 17°C it indicates presence of phystosterol acetate which means presence of vegetable fat or oil.
2. Valenta test
Take 3ml of ghee and 3ml of glacial acetic acid and warm the content in a test tube. Agitate it frequently and note the melting point. If it is above 39oC it indicates presence of animal fat, but if abnormally low (i.e.below 29oC), then there is presence of coconut oil.
3. Baudouin test
Melted sample of ghee is shaken with cane sugar solution in hydrochloric acid. If sesame oil is present in ghee it shows red colour.
4. Halphens test
Take 5 ml of amyl alcohol and add 1 ml of 1% of sulphur solution in carbon-disulphide, and mix with 10 ml of sample ghee. This is heated at 125°C for one hour. If red colour appears it indicates presence of cotton seed oil in ghee.
5. Nitric acid test
A sample of 3 ml of ghee is melted in a tube and 3 drops of concentrated colourles nitric acid is added and kept in hot water bath. Observations are made as given here.
Colour Ghee
No colour Pure ghee
Yellow Presence of vegetable fat
Orange Presence of tallow or lard
Reddish yellow Presence of wax.
6. Soda ash test
Ghee sample is melted and equal amount of 25% sodium carbonate solution is added. It is shaked well and kept in boiling water for few minutes. Turbid liquid is formed. When it is kept for few more minutes two layers formed are on standing. It indicates presence of vegetable fat/tallow or lard in the lower layer, while the top layer is of the pure ghee.
B. Test for animal fat
Ghee adulterated by animal fat can be detected by the tests given here.
1. Valenta test
This test is already described in the text.
2. Nitric acid test
This test is also described in the text.
3. Carbolic acid test
Take one part of ghee sample and add 2.5 parts dilute carbolic acid. If it is pure ghee sample will get dissolved in this, otherwise, in case of adulteration by animal fat, pure ghee will form a top layer on the animal fat.
VII. Off-Flavours in Milk
Some times milk has been found to possess some abnormal or off-flavours which could be due to the following reasons.
1. Those related to the animal itself.
2. Those related to production, processing and storage. The common off-flavours, their causes, origin and control are given in table 1.
Table 1: Off-f1avours in Milk
Sl.No / Off-flavours / Causes / Control (prevention)1 / Salty / High content of chloride, mastitis, late lactation, breed and individuality of animals / Such milk should be detected and discarded. Mastitis treatment
2 / Barny/cowy / Unclean milking procedures, unclean milking utensils poor ventilation, overcrowding of animals in the cowshed / Avoid dirt and manure contamination by proper cleaning of cowshed and animals, arrange proper cooling of milk and general sanitation
3 / Feed/weed / Milk tainting fodder or weeds, feeding animals silage during milking or just before. Milking, feed silage after / Use feed that cause little or no food flavour allow a time interval of 5 hrs between feeding & never before or during milking , eliminate weeds from crops to be used as fodder
4 / Bitter / Weeds and the enzyme lipase in milk / Proper cooling of raw vaccum Pasteurisation of milk
5 / Stale and flat / Feeding of concentrate especially in the winter low temperature long time storage / Proper cooling and disposal of old stocks
6 / Rancid / Lipase (Butyric acid) fermentation, late lactation / Avoid excessive agitation of raw milk, provide adequate refrigeration system. Select protective feed and proper pasteurization of milk
7 / Oxidized/oily/metallic / Direct contact of milk with traces of metals especially copper, Vitamin C, sunlight storage, late lactation, dry feeds seasonal variation etc. / Elimination of metal contamination, proper sanitation use of coloured bottle and paper contaminers or use aluminium alloys/stainless steel as milk contact surfaces
8 / Cooked / Overheating of milk / Avoid overheating of milk
9 / Malty / Grpwth of str. Lactis var.maltigenes microorganisms in milk / Store milk at 5C or below to check bacterial growth . proper cooling and heat treatment
10 / Musty / Moisture left in utensils uses of cloth during transportation and storage / Use of clean and dry containers, elimination of cloth covering milk cans and leakage control
11 / High acid sourness / Excessive lactic acid development (due to considerable growth of lactic acid microorganism) / Store milk at 5C (40F) or below to check bacterial growth and producing proper pasteurization of milk
VIII. Quality Control Tests
Quality control is vital in the production and handling of milk and milk products. Milk is not only the ideal food for man, it is also a very good food for many species of bacteria which rapidly bring about its spoilage. The bacteria are present everywhere and unless specific precautions are taken at each step, they are likely to get into the milk or milk products in large numbers. The presence of bacteria is difficult to detect unless elaborate tests are carried out, with the result that without having this information in advance and if the activities of bacteria are not checked, there may be very heavy financial losses to the manufacturer because of the spoilage of the product.
For these reasons, it is extremely important that some tests which will help in following up the quality of milk are carried out as a matter of routine. There are some tests which are called ‘platform tests' which can be used at the receiving platform of the dairy to judge the quality of the milk received from each supplier. Some platform tests are described here.
1. Organolaptic tests
Soon after milk is received on the platform of dairy the lid of can is opened and milk is stirred up with a plunger to test for smell whether pleasant or unpleasant. Then it is tested for appearance (colour e.tc.), taste and flavour.
2. Acidity test
The acidity developed in milk as a result of bacterial activity is measured by titration against standard alkali solutions. The normal acidity of milk ranges from 0.12 to 0.16 % lactic acid. Acidity in excess of about 0.20 % indicates poor quality of milk.
3. Indicator test
The acidity developed in milk due to bacterial activity is measured in terms of pH value as indicated by special indicator dyes, e.g. brom thymol blue and brom-cresol purple.
4. Sediment test
Milk is passed through a funnel containing a filter disc and the amount of dirt and dust collected is compared visually or by weight. The test indicates the gross impurities and dirt in milk as a result of unhygienic conditions of production.
5. Clot on boiling (C.O.B) test
This test is used to determine the shelf life of milk samples and its ability to withstand pasteurization. A small amount of milk is boiled in a test tube over a flame for two minutes. When acidity exceeds beyond 0.2% the milk clots and hence it is COB positive which means unsuitable to heat treatment.
6. Alcohol test
This test determines the stability of proteins when subjected to heat treatment and is, therefore, useful in milk powder, sterilized milk factories etc., where milk is subjected to high temperatures. Ten ml milk is taken in a tube and mixed with 10 ml of 75% ethyl alcohol. It is shaken to observe the clots. If acidity is beyond 0.2% the particles of curd will be noted along the wall of test tube-rendering it unsuitable to heat treatment.
7. Fat test
Pour 10 cc of sulphuric acid (specific gravity 1.82) into the butyrometer and add 11cc of milk gently along the side of butyrometer so that it floats on acid. Add 1ml of amyl alcohol (specific gravity 0.81). Rubber stopper is inserted to close it. Mix the fluids in the tube by gently shaking. The normal specific gravity of pure fresh cow and buffalo milk varies from 1.028 to 1.030 and 1.030 to 1.032 respectively. It is affected by the nature of adulteration. Specific gravity of milk can also be determined by Wesphal balance.
8. Resazurin test
Take 10 ml of milk sample in a sterile test tube and add one ml resazurin dye solution (0.005%). Mix the content and incubate at 370 C for 10 minutes. After this the colour is read against a control milk through resazurin disc comparator to judge the quality of milk, which is given in Table 2. This method at the most takes 10 minutes to test.
Table 2: Ten minutes resazurin test
Sl.No / Disc reading / Category / Action taken1 / 4 or higher / A / Accepted
2 / 1-3.5 / B / Salvaged if possible or rejected
3 / 0-1 / C / Rejected
9. Specific gravity test
It is the density of milk showing the ratio of heaviness/mass of milk to the water. It depends upon the amounts and nature of solid substances present in milk. The solids not fat (SNF) increases the specific gravity (sp.gr.) of milk, where as fat decreases it. To determine specific gravity mix the sample of milk (standard temperature 60o F) in a jar and lower the lactometer gently into milk to f1oat. Leave it for few seconds. When it is stationary and docs not touch the side of the jar, then read the scale of lactometer. If the temperature is above or below the standard, then add 0.1 to the lactometer reading. The lactometer reading after correction is known as corrected lactometer reading.
1 + CLR
Sp. gr. of milk = ------
1000
Where,
CLR = Corrected Lactomeler Rcading
10. Bromocresol purple test
This tests alkalinity in milk due to mastitis or colostral milk. A little milk is placed on the strip of bromocresol paper. If the milk turns bright purple, it is an indication of alkalinity in milk. This is quick and effective test.
11. Hansa test
It is meant for testing adulteration of cow milk with buffalo milk. Skim milk is injected into rabbits and the antibodies produced are collected and purified to make specific anti-scrum. When anti-scrum is added to buffalo milk, tiny particles can easily be seen.
A drop of milk is placed on a clean glass slide and one drop of scrum is added and mixed. If sample contains buffalo milk the mixture on slide shows clear flocculation of casein particles within a minute. However, if there is no change in case then it is pure cow milk.
12. Test for heated milk
Take 10cc of milk in a test tube and add 3ml of starch solution, 1 ml of potassiurn iodide solution and 2 drops of hydrogcn peroxide in it. Shake the contents well. If the milk is already heated, a pale yellow colour appears, otherwise unheated i (raw) milk shows a blue colour. The reagents take 4 to 5 minutes to react.
13. Guaical test for heated milk
Take 5 ml (J milk in a test tube and add 5 m11% solution of crystallized guaical and 1 drop of hydrogen peroxide. If the milk is heated, show no colour but unheated raw milk gives a pomegranate colour.
IX. Bacteriological Tests
The following tests are done to detect the presence of micro-organisms in the milk.
1. Standard plate bacterial count
Collect representative sample and pipette out one ml of milk and mix it in a 9 ml priorly sterilized normal saline blank in a tube to prepare 1:10 dilution. Similarly subsequent dilution are prepared (1:100, 1:1000, 1:10,0000 etc.). Then one ml of the suitable dilution mixture is pipette out to transfer into a sterilized petriplate and is mixed with melted sterilized nutrient agar medium (15g Agar-agar + 5g peptone + 3g beef extract + 5g sodium chloride + 1000 ml distilled water) and after setting, it is incubated at 37o C for 48 hours. The bacterial colonies developed due to proper temperature and nutrition present in medium are counted with the help of colony counter and only such dilution which show number of colonies between 30 to 300 per plate is considered ideal for greater accuracy of results. The number of colonies is multiplied by dilution presented as number Standard plate bacterial count (Spe/ml). In table 3 bacterial standards of raw milk based on Spe/ml is given.