Quality tests for milk need a concerted effort to ensure that the whole dairy value chain is involved. This is because dairy products are very susceptible to bacterial contamination.
To ascertain the quality of the raw material and monitor the quality throughout the value chain, the following tests will be very instrumental.
1. Organoleptic Quality Tests for Milk
This involves all the preliminary quality tests conducted at the farm level, at the processing plant’s reception platform or at the collection center.
It is a simple, quick and cost effective method of checking quality of milk and allows for segregation of good quality and poor quality milk.
The grader must have good senses of sight, smell, and touch because the results of this test is obtained instantly.
Start testing by shaking the milk can a little bit then opening the lid and check for any aroma/flavor that emanates from the can by smelling it.
Check for visual appearance on the milk such as colour, any other foreign objects on the surface of the milk and the sanitary condition of the milk can. The sanitation of the can indicates how the milk has been handled.
Feel the milk can to check the milk temperature. The temperature of the milk can give more information about when the milking was done.
2. Clot on Boiling Quality Tests for Milk
This is a cheap, quick and easy to perform quality test that is mainly used to give information on the acidity of the milk. A small amount of milk is boiled in a test tube and checked for any signs of coagulation.
If the milk coagulates, it is an indication that the milk will not be able to withstand processing conditions due to high acidity.
This test is however not quantitative and does not detect any acidity level below 0.2% of lactic acid concentration. Any milk that fails this test is considered too sour for processing.
3. Alcohol Quality Tests for Milk
This is a simple and quick method used to test the acidity of milk. Mix equal amounts of alcohol (72% ethanol concentration) and milk sample and check for signs of coagulation.
The mechanism behind alcohol test.
Milk proteins remain stable due to two factors:
- Hydration forces
- Negative charges on the protein particles
Hydration forces keep the proteins suspended in milk owing to the hydrophilic property of ϰ-casein. Negative charges keep the proteins repellant from each other hence they do not agglomerate into lumps. Destabilizing these forces will lead to flocculation of the proteins.
Alcohol is a dehydrator which removes water from the system thereby affecting the hydration forces that are responsible for keeping the proteins suspended.
If such milk has elevated acidity due to microbial activity, the proteins will flocculate due to the combinative effect of hydration force destabilization and charge neutralization.
When combined with alizarin, it gives alcohol alizarin test, which is more informative than just the alcohol test. Alizarin is a dye that changes colour depending on the acidity level.
|pH||6.6 – 6.7||6.4 – 6.6||≤6.3||≥6.8|
|Normal milk||Slightly acidic||Acidic (coagulates)||Alkaline|
Confirmatory Quality Tests for Milk
1. Acidity Test (Titration/pH method)
Acidity test is performed to determine the level of natural or developed acidity in milk. The level of acidity in milk can be expressed in two ways.
Titratable acidity: – shows the total acidity but does not show the strength of the acid
pH: = shows the strength of acid
Reasons for doing acidity test:
- To find out the quality of milk
- To monitor storage conditions because when dairy products are stored in favourable conditions for the growth of microorganisms, they produce acidity.
- To control manufacturing e.g. for yoghurt, fermentation is done until a desired level of acidity is achieved and then stored in a cold room.
- Cheese processing also depends on acidity
Natural acidity of freshly drawn milk is due to its constituents e.g. CO2, albumins and globulins, casein and phosphates. These substances react in the milk and produce acidity.
|Constituent||Lactic acid concentration (%)|
|Albumins and globulins||0.01|
|Titratable acidity equivalent||0.13|
Titratable acidity is based on the principle that an acid of a known volume and strength will completely neutralize an alkali with equal volume and strength. The alkali that is commonly used in titration of acidity in milk is 0.1N NaOH.
Titratable acidity is the measure of the amount of alkali required to shift the pH of milk of 6.6 to a determined pH based on the indicator used.
When measuring the acidity of the milk, total acidity is usually used because it encompasses both natural and developed acidity.
Expression of titratable acidity:
The lactic acid concentration is measured in terms of:
Soxhlet Henkel degree (°SH): – the volume (ml) of 0.25N NaOH used per 100ml of sample
Thorner degree (°Th): – the volume (ml) of 0.1N NaOH used per 100ml of sample
Domic degree (°D): – the volume (ml) of 0.1N NaOH used per 100ml of the sample
Acid determination should not be used as the sole basis of accepting or rejecting milk because milk can have acceptable acidity but suffers from flavor defects and other bacterial problems.
Relationship of acidity in different products
The acidity of milk and milk products is usually on the plasma portion of the milk.
Therefore, when you want to compare two dairy products (milk and cream), which have different plasma and fat contents, the calculation of the acidity should be based on the plasma portion of the milk.
Milk with 4% fat content and 0.16% lactic acid and cream with 40% fat content, what will be the acidity of the cream?
Plasma: milk – 96%, cream – 60%.
Comparable acidity of the cream to milk
X=0.1% lactic acid
The same method can be used for cream and butter.
Factors affecting titratable acidity in milk:
- Changes from calcium di-calcium phosphate to tri-calcium phosphate and then to phosphoric acid.
- Action of calcium phosphate
- The stage of lactation e.g. colostrum milk has high proteins hence high acidity
- Mastitis or condition of the udder. Mastitis lowers the acidity because blood (which is alkaline) leaks into the milk.
- Enzyme activity e.g. lipolysis of fats into fatty acids
- The effect of the feeds
2. pH Determination
pH is the concentration of hydrogen ions in milk. It is determined using two methods
a. Calorimetric method
Different indicators that cover various pH ranges are used. The unknown substance with indicator added to it will match the colour of solutions with known pH.
The colour solutions should contain the same indicators as the unknown. The accuracy of this method can be increased by using a comparator which will compensate for the turbidity of the milk.
b. Electrometric method
Uses pH meter/ potentiometer. This method involves measuring the difference in two different voltages of two electrodes in a sample. It gives direct measure of pH with direct values on the display unit.
3. Inhibitor Quality Tests
Performed to check for antibiotics and drug/antibiotic residue in milk. When the antibiotics are present in significant amounts, they inhibit fermentation process. This makes it an important test to consider when dealing with fermented milk products.
Drug residues and the antibiotics are also a health hazard as some people are sensitive to antibiotics. The antibiotic test can be done through various methods, the most common of which are:
Delvo test: – This is the easiest and the fastest method to use but it is very expensive to acquire and operate.
Fermentation test: – bacteria in milk produce acid as they metabolize the milk, antibiotics and drugs inhibit growth of micro-organisms. Milk with antibiotics will take too long to ferment or fail to ferment completely.
4. Specific gravity (Lactometer test)
Lactometer is a hydrometer used to measure specific gravity of milk and scale is limited to the specific gravity of milk.
Milk fat has a specific gravity of 0.93, milk has a specific gravity of 1.032 and the milk solids have a specific gravity of 1.036.
Lactometer works on the principle of fluid displacement. The displaced fluid has equal weight as the floating body. Liquids with lower specific gravity have larger displacements than liquids with higher specific gravity.
Lactometers have temperature sensitive and are designed to be used at specific temperatures. It is used to check for adulteration of milk, either by skimming or addition of water or both. When water is added, the specific gravity is lowered towards one.
The design of the lactometer allows for direct reading on the stem whose calibration ranges between 26 to 38. This is because the figure obtained from the reading are on the second and third decimal points of the specific gravity.
The first figures of the specific gravity are 1.0, which are then succeeded by the figures obtained from the lactometer reading.
Any deviation in temperature from the recommended reading temperature is adjusted by either adding or subtracting accordingly.
Common methods of adulterating milk:
Skimming is the removal of fat from the milk. Given that fat is the lightest portion of the milk, its removal will increase the lactometer reading.
A sample of milk that has low lactometer reading has a high fat content compared to the one with a low fat content.
|Fat content||Lactometer reading||SNF content|
|Calculate % fat content removed||Calculate % lowering SNF|
When adulteration is watering lactometer reading will go low and SNF will reduce as well as the fat content.
3. Skimming and watering
The percentage of fat removed and the percentage reduction of SNF are not the same when watering and skimming are done. Such samples have low fat content, low lactometer reading and low SNF than normal milk.
5. Freezing point quality tests for milk
Milk freezes at -0.55°C while pure water freezes at 0°C. A small adulteration with water will cause detectable change in freezing point of milk. This makes freezing point test the most accurate test for milk adulteration by water addition.
Freezing point depends on the soluble constituents of milk, especially lactose and chlorides. Freezing point remains the same even though these are differences in the composition of the two components.The freezing point of milk never changes (-0.55°C ): testing for #adulteration Click To Tweet
The freezing point does not depend on composition of the sample but on the concentration of dissolved salts (lactose and chloride).
As milk freezes, its composition gets unbalanced; solids get concentrated on the remaining liquid part of the milk.
How to determine the amount of water in adulterated milk
Amount of added water = 100(Freezing temp of milk – Freezing temp of suspect milk)
6. Dye Reduction Quality Tests for Milk
These quality tests show comparative activity of the micro-organism in milk hence used as a rough indication of the microbial load in milk. It is based on the observation of the blue colour imparted in the milk, which will disappear with time.
The length of time depends on the number of microorganisms present in the milk holding all other factors (such as nutrient content, moisture content, and temperature) constant.
The colour change is assumed to be due to two reasons:
- Consumption of oxygen in milk by the microorganism
- Enzyme reductase produced by the microorganisms
The time reduction indicates the possible number of microorganism. This test has low correlation with other tests especially those that show presence of bacteria.
To perform this test, take a definite amount of methylene blue and mix with 10 ml of milk. Mix uniformly and incubate at 37°C in a water bath and wait for colour change.
The longer it will take for the colour to change, the lower the microbial load. All the glassware used must be sterile. The interpretation of the results is as follows:
Table 1: Dye reduction test result intepretation of pasteurized milk
|Length of time (hours)||Quality of milk||Grade of milk|
|5 or more||Excellent||1|
|3 – 5||Good||2|
|2 – 3||Fair||3|
|Less than 2 hours||Poor||4|
Raw milk test has a much rapid decolouration.
7. Resazurin Quality Tests for Milk
Resazurin test can take 10 minutes for fresh milk and up to one hour for pasteurized milk. This test works on the principle of a dye indicator, which changes colour when oxidized.
Using sterile equipment; add 1 ml of the dye into 9 ml of milk sample, mix uniformly and incubate at 37°C in a water bath for 10 minutes for fresh milk and up to one hour for pasteurized milk.
After the time has elapsed, observe the colour change using a lovibond comparator. The results are interpreted according to the table below
|Reading||Colour||Quality of milk||Verdict|
|5||Light blue||Very good||Accept|
The 10 minutes resazurin test is used at the reception to accept or reject milk due to its rapidity.
8. Butterfat Quality Tests for Milk
The most common and accurate method used for butterfat testing is the Gerber method. The method is used for determining the butterfat content in milk and milk products.
Butterfat determination is important because it can be used to value the milk for payment purposes and to determine how to standardize the milk.
You will require the following chemicals and ingredients
- Amyl alcohol
- Sulphuric acid (S.G 1.820 – 1.825 or 90 -91% concentration)
Sulphuric acid dissolves all the non-fat content of the milk. The fat content remains in a liquid form due to high heat produced by the acid-protein reaction.
Amyl alcohol provides distinct layer between the two phases, which are separated due to their density differences.
Stronger/weaker concentration should be available. Stronger concentration attacks the amyl alcohol while weaker concentration reduces oxidative effect of the acid.
Amyl alcohol used should be colourless – iso-amyl alcohol – an isomer of two alcohols; methyl and butanol.
Different Gerber butyrometers are used for different products depending on their different fat contents.
Problems encountered during butterfat quality tests:
i) Curd test
There is a formation of a slightly coloured solution after addition of the acid or a curdy fat column. This usually results from using a weak acid, which fails to break the fat globule membrane. It may also result from putting insufficient acid or insufficient mixing of milk and acid.
ii) Charred test
This happens when there is a darkened fat column with dark specs at the base of the fat column. This may be because the acid is too strong, or the milk and acid were mixed slowly, or the milk was added/dropped into the acid. There might be too much acid causing the milk to burn.
When testing low fat content sample, centrifugation should be done more times (three times) because the milk is homogenized.
iii) Babcock method of testing butterfat
This method uses the same principle as the Gerber method, the only difference is the design of the butyrometers used. Babcock method uses butyrometers with cylindrical stems.