Just as people like milk for nutritive value, bacteria that cause milk spoilage also do. Milk has nutrients that make it suitable for the rapid multiplication of bacteria that cause spoilage.
Milk from the udder of a healthy cow contains very few bacteria.
To ensure that milk remains fresh for a longer time it is necessary to practice good hygiene. Below is some advice that farmers and milk traders should follow to ensure good quality:
Before re-using the milk container:
To ensure that the milk you receive from the farmers or milk traders is of good quality by carrying out the following simple basic tests.
Organoleptic test (Using your senses of sight and smell)
This test should be performed first.
It simply requires the use of the senses of sight and smell to test the milk. Milk that cannot be adequately judged this way should be subjected to other more sensitive and objective tests.
Procedure:
Abnormal appearance and smell that may cause milk to be rejected could be due to:
Clot on Boiling Test
This test is quick and simple. It allows you to detect milk that has been kept for too long without cooling and has developed high acidity, or colostrum milk that has a very high percentage of protein. Such milk does not withstand heat treatment hence clot on boiling test could be positive at a much lower acidity.
Procedure:
Boil a small amount of milk in a spoon or other suitable container. If there is clotting, coagulation or precipitation, the milk has failed the test and should be rejected.
Alcohol Test
The test is quick and simple. The specific type of alcohol used is known as 'ethanol'. It enables you to detect bad milk that may have passed the previous two tests because it is more sensitive to lower levels of acidity. It also detects milk that has been kept for long without cooling, colostrum or milk from a cow with mastitis.
Procedure:
Use a syringe to draw equal amounts of milk and 70% alcohol solutions in a small tube or glass cup (such as those used to administer medicine to children). Mix 2 ml milk with 2 ml 70% alcohol.
If the tested milk sample coagulates clots or precipitates, it will have failed the test and the milk should be rejected. Because this test is quite sensitive, milk that passes this test can keep for some hours (at least two hours) before it goes bad.
Lactometer Test
Some dishonest milk suppliers adulterate milk with added water to increase the volume or added solids such as flour to make it look thicker. Addition of anything to milk can introduce bacteria that will make it spoil quickly.
Adulteration of milk is also illegal.
To test the milk to check if it has been adulterated. The test is based on the fact that milk has a heavier weight or density (1.026 - 1.032 g/ml) compared to water (1.000 g/ml). When milk is adulterated with water or other solids are added, the density either decreases (if water is added) or increases (if solids are added).
If milk fat (cream) is added to milk, the density becomes lower.
The equipment used to determine milk density is called a lactometer. Most lactometers are usually marked from "0" (representing density of 1.000 g/ml) to "40" (representing density of 1.040 g/ml).
Procedure:
Ensure that milk has been left to cool at room temperature for at least 30 minutes and its temperature is about 20°C. Stir the milk sample and pour it gently into a 200 ml measuring cylinder (or any container deeper than the length of lactometer).
Let the lactometer sink slowly into the milk. Take the lactometer reading just above the surface of the milk.
If the temperature of the milk is different from the lactometer calibration temperature (20°C), then use the below correction factor:
For each °C above the calibration temperature add 0.2° lactometer "degrees" (°L) to the observed reading, and for each °C below calibration temperature subtract 0.2° lactometer "degrees" (°L) from the observed lactometer reading. These calculations are done on the lactometer readings that is, 29 instead of true density of 1.029 g/ml.
Judgement:
Normal milk has a density of 1.026 - 1.032 g/ml (or 26 - 32 0n the lactometer reading). If water has been added, the lactometer reading will be below 26. If any solid such as flour has been added, the reading will be above 32.
Milk from the udder of a healthy cow contains very few bacteria.
To ensure that milk remains fresh for a longer time it is necessary to practice good hygiene. Below is some advice that farmers and milk traders should follow to ensure good quality:
- Maintain clean and healthy cows
- Keep a clean milking environment
- Wash hands with soap and clean water before milking
- Wash the udder with a clean cloth and warm water
- Dry the udder with a clean dry cloth
- Make the first draw into a strip cup to check for mastitis and throw away from the milking area even it appears clean
- Use clean containers for milking
- Cows with mastitis should be milked last and their milk discarded
- Milk from cows under antibiotic treatment should not be sold until 3 days after last treatment or advised by the veterinarian
- After every milking, dip the teats into an "antiseptic"
- During milking, the milker should not have long nails, sneeze or cough and smoke
- Release the cow from the milking area as soon as milking is finished
- After milking, cover the milk to avoid contamination
- Move the milk to a clean and cool area
Hygienic milk handling
Always use metal such as aluminium containers and not plastic containers. When transferring milk between containers, pour, do not scoop. Scooping may introduce spoilage bacteria.Before re-using the milk container:
- Pre-rinse the container soon after use
- Thoroughly scrub the container with warm water and detergent or soap (using a stiff bristled hand brush or scouring pad such as super brite)
- Rinse the container in running water
- Dip-rinse the container in boiling water for at least one minute
- Air-dry the milk container in the open in inverted position
How to test if milk is acceptable
During testing only a small amount of milk is used. For the result of the test to give a true picture of the state of milk,it is important to mix milk before obtaining the sample or testing.To ensure that the milk you receive from the farmers or milk traders is of good quality by carrying out the following simple basic tests.
Organoleptic test (Using your senses of sight and smell)
This test should be performed first.
It simply requires the use of the senses of sight and smell to test the milk. Milk that cannot be adequately judged this way should be subjected to other more sensitive and objective tests.
Procedure:
- Open a can of milk
- Immediately smell the milk and establish the nature and intensity of smell, if any.The milk may smell non-fresh or foreign odours may be detected.
- Observe the appearance of the milk.Look at the colour of milk, any marked separations of fat, colour and physical state of the fat, foreign bodies or physical dirt
- Touch the milk container to feel if it is warm or cold. This may indicate to you how long milk has taken since milking (if not chilled thereafter) and will influence the lactometer test for adulteration (see below)
Abnormal appearance and smell that may cause milk to be rejected could be due to:
- Type of feed or atmospheric taint
- Cows in late lactation
- Chemicals taints or discolouring
- Advanced acidification or scouring
- Milk previously chilled and subjected to disturbances during transportation
- Adulterations with other solids (may also show as sediments or particles)
- Boiling, if milk fat is hardened
Clot on Boiling Test
This test is quick and simple. It allows you to detect milk that has been kept for too long without cooling and has developed high acidity, or colostrum milk that has a very high percentage of protein. Such milk does not withstand heat treatment hence clot on boiling test could be positive at a much lower acidity.
Procedure:
Boil a small amount of milk in a spoon or other suitable container. If there is clotting, coagulation or precipitation, the milk has failed the test and should be rejected.
Alcohol Test
The test is quick and simple. The specific type of alcohol used is known as 'ethanol'. It enables you to detect bad milk that may have passed the previous two tests because it is more sensitive to lower levels of acidity. It also detects milk that has been kept for long without cooling, colostrum or milk from a cow with mastitis.
Procedure:
Use a syringe to draw equal amounts of milk and 70% alcohol solutions in a small tube or glass cup (such as those used to administer medicine to children). Mix 2 ml milk with 2 ml 70% alcohol.
If the tested milk sample coagulates clots or precipitates, it will have failed the test and the milk should be rejected. Because this test is quite sensitive, milk that passes this test can keep for some hours (at least two hours) before it goes bad.
Lactometer Test
Some dishonest milk suppliers adulterate milk with added water to increase the volume or added solids such as flour to make it look thicker. Addition of anything to milk can introduce bacteria that will make it spoil quickly.
Adulteration of milk is also illegal.
To test the milk to check if it has been adulterated. The test is based on the fact that milk has a heavier weight or density (1.026 - 1.032 g/ml) compared to water (1.000 g/ml). When milk is adulterated with water or other solids are added, the density either decreases (if water is added) or increases (if solids are added).
If milk fat (cream) is added to milk, the density becomes lower.
The equipment used to determine milk density is called a lactometer. Most lactometers are usually marked from "0" (representing density of 1.000 g/ml) to "40" (representing density of 1.040 g/ml).
Procedure:
Ensure that milk has been left to cool at room temperature for at least 30 minutes and its temperature is about 20°C. Stir the milk sample and pour it gently into a 200 ml measuring cylinder (or any container deeper than the length of lactometer).
Let the lactometer sink slowly into the milk. Take the lactometer reading just above the surface of the milk.
If the temperature of the milk is different from the lactometer calibration temperature (20°C), then use the below correction factor:
For each °C above the calibration temperature add 0.2° lactometer "degrees" (°L) to the observed reading, and for each °C below calibration temperature subtract 0.2° lactometer "degrees" (°L) from the observed lactometer reading. These calculations are done on the lactometer readings that is, 29 instead of true density of 1.029 g/ml.
Judgement:
Normal milk has a density of 1.026 - 1.032 g/ml (or 26 - 32 0n the lactometer reading). If water has been added, the lactometer reading will be below 26. If any solid such as flour has been added, the reading will be above 32.
How to preserve milk to reduce spoilage
In order to ensure that you further increase the shelf life of your milk after receiving it, you need to keep it in a cool place. Lowering of milk temperature reduces "speed" (rate) of bacterial growth. If you do not have a refrigerator, you may keep the milk in cold water or in a hole under shade.
Hey very useful information about milk and its teasing......
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