Milk contains approximately 86% water, 4.7% sugar (lactose), 4.1% fat, 4.2% protein and 1% minerals. It supports the growth of micro-organisms and thus is prone to contamination.
The purpose of milking a dairy cow is to obtain milk that is fit for human consumption.
Milk from the udder of a healthy cow contains very few bacteria and to ensure that it remains fresh for long it should be handled under conditions of good hygiene.
Unclean milk can be a source of disease to the consumer, rejected at the market and so is a loss to the farmer, does not keep for long and is not good for processing.
The milking procedure is the first step in obtaining clean milk. At the farm this starts with ensuring the cow to be milked is healthy.
The cow should be well fed with a diet well balanced with forage and concentrates to ensure high production of good quality milk.Feeding very high amounts of concentrates and low amounts of forages results in milk with low butter fat. On the other hand feeding too little concentrates leads to low milk yield.
An unhealthy cow will feed less and produce less milk. Cows should always be kept healthy and clean as sick animals can transmit diseases like tuberculosis and brucellosis to milk consumers. If a cow is suspected to be sick, a qualified veterinary practitioner should be contacted immediately. Milk from a cow that is being treated with antibiotics should not be consumed or sold until the withdrawal period is over.
Farmers are encouraged to vaccinate their animals against brucellosis. Animals should also be checked periodically for all types of contagious diseases and treated promptly in case they are infected.
Mastitis is an inflammation of the mammary glands in the udder caused by infection with disease-causing bacteria which can be controlled by observing general hygiene and proper milking procedure.
- be healthy and clean
- Maintain short nails and hair (for ladies, cover the head when milking)
- Never smoke during milking time
- Milk quickly and completely without interruptions
The do’s and don’ts of hand milking a dairy cow
- A milking shed (parlor) which can be permanent or movable should be constructed. It should be located away from any smells.
- The floor of shed should be clean and dry and if possible have a cement floor for ease of cleaning.
- The shed should be cleaned after every milking and animals kept off outside milking time.
- Use seamless aluminum or stainless steel cans for milking and storing milk. Plastic container is difficult to clean.
- Clean utensils immediately after milking or after emptying milk: rinse with cold water, scrub with a brush using hot water with detergent then rinse with cold water.
- Place upside down on a rack and dry in the sun.
- Store utensils in a safe, clean and well ventilated room.
Milking is the most important activity in a dairy farm. Milk can be extracted either by hand or by machine. Hand milking is an art, which is improved with practice.
Alveolar cells synthesize milk, which is stored in the gland cistern. The sphincter muscle at the tip of the teat (teat sphincter) control milk let down. For efficient milking, teat should be of moderate size, symmetrical and enough tension of the sphincter muscle.
Practical aspects of milking:
Milk synthesis and secretion is continuous unless interfered with by pressure from the filling of the gland cistern (this explains why more milk is extracted by frequent emptying (milking) to ensure pressure does not built up). The ejection of milk from alveolar lumen is under influence of oxytocin (hormone).
The cow is brought to the milking parlor as calmly as possible. Frightening the animal at this stage has a negative effect on milk let down due to release of adrenaline (hormone) which has a negative effect on milk letdown.
- Feed the cow its production ration (this is optional depending on the feeding system) this calms the animal and stimulates milk letdown.
- Restrain animal – tie hind legs above hock joint in the form of a figure 8. A loose knot should be used to safeguard both animal and man (applicable only for hand milking)
- Wash hands with soap and clean water before milking. Dry hands with towel.
- Test for mastitis using a strip cup – strip first few rays of milk into strip cup from each quarter and observe for any abnormalities. If mastitis is detected, the cow should be milked last.
Using a strip cup
- Wash udder with warm clean water with disinfectant using a clean towel.
Warm water also stimulates milk let down. Dry udder using a dry towel.
Area of udder to be cleaned
- Apply milking jelly – prevents cracking of teats and eases milking (for hand milking only)
- Milk quickly and completely by squeezing the teat, do not pull. Milking each cow should take 7–10 minutes at most.
- Use clean containers for milking.
- After milking: Strip the animal – getting last drops of milk from udder to avoid incomplete milking (can lead to mastitis).
- After milking dip the teats in a teat dip (disinfectant to ensure that bacteria do not gain entry through the teat sphincter which is loose immediately after milking).
Step by step method of hand milking
Dipping cow teats in disinfectant
It is recommended that the animal remain in a standing position for at least one hour to ensure the teat does not come into contact with the ground while the sphincter is still loose.
- Routine milking procedures stimulate milk letdown and should therefore not be changed
- After cow has been maximally stimulated for milk let down, it should be milked immediately since the stimulus reduces over time. Oxytocin effects are maximum between 5-10 minutes, thus milking should be completed during this time.
- Don’t harass animal since adrenaline (hormone produced due to fright) has opposite effect of oxytocin (milk let down hormone).
Handling the milk
The following guidelines should be followed to avoid milk spoilage:
- Filter milk immediately after milking: Use a white filter cloth or strainer. Disinfect, wash and dry the cloth/strainer after use
- Always handle milk in clean, preferably metal, containers.
- When transferring milk between containers, pour the milk instead of scooping since scooping may introduce spoilage bacteria.
- Do not store milk at high temperatures.
- Do not handle milk if you are sick. Seek medical treatment and resume work only when the doctor says you are fit to do so
- Store milk in a cool clean place preferably lockable room set aside for milk only. If storing overnight, keep the milk in cold/ chilled water.
- Deliver milk to the market as soon as possible preferably in the cool morning or evening.
Store the milk without chemicals in a lockable cool and clean place. Do not mix warm (morning) milk with cool (evening) milk; deliver to the collection centre separately or cool the warm milk before mixing.
Milk is highly perishable hence it should be preserved to ensure it is safe for human consumption at the home and that it reaches the processor and/or final consumer in good condition. The success of any preservation method is highly dependent on hygiene conditions under which the milk was produced. Hence milk produced from a healthy cow, milked by a healthy milker using clean equipment will be clean and more likely to keep long. Milk can be preserved using the following simple methods:
Cooling milk slows down the growth and activity of germs and hence prevents spoilage.
Milk can be cooled through:
- Keeping under a shade
- Dipping the containers with milk in a cold water bath, flowing stream of cooling tank
- Keeping the milk in a refrigerator
- Using a charcoal cooler
- Using cooling rings: if cool (10°C or less) running water is available, you can pass it through a perforated ring so that it flows over the cans
- Using an electrical cooling tank.
When cooling milk, loosen the lids of the cans to allow the air to escape, and make sure no water gets into the milk. Cover the cooling tank with a lid to protect the milk from insects and dust.
Heating kills many bacteria and heated milk will keep longer. It also gets rid of harmful micro-organisms that could transfer diseases from the cow to humans. The best method of heating milk (to retain the taste and avoiding off-flavours) is to immerse the milk can in boiling water for at least 30 minutes.
Milk to be consumed at home should be boiled, using a large pan or other cooking container. Milk can be heated to a certain temperature and kept at that temperature for some time to kill germs, then cooled. This is called pasteurization. A thermometer is required for monitoring the temperatures. Milk can also be subjected to low heat treatment. Heat the milk to 65°C then cool.
Chemicals can be used to preserve milk but only on advice from the collecting centre because it is important to use the correct types and amounts. Use of chemicals is illegal in some countries and only milk delivered to a dairy plant should be preserved with chemicals. Nevertheless, chemicals allow un-cooled milk to keep longer even in high temperatures and, if used correctly, chemicals have little effect on the physical quality of the milk.
Proper cleaning of milk equipment
Immediately cans are emptied of milk they should be cleaned as follows:
- Rinse with cold water.
- Scrubbing with brush and warm detergent (any un-perfumed liquid soap will do).
- Rinse with cold water.
- Sterilize (sanitize) with boiling water or steam if available or use dairy sanitizing solution such hypochlorite or commercial brand preparations in accordance with manufacturer’s instructions.
- Dry cans on a drying rack. Exposure to sunlight will enhance killing off bacteria during drip drying of cans.
Milking machines should be cleaned according to recommended practice:
- Rinse with cold water.
- Use the “cleaning-in-place” (CIP) method where detergent in hot water is circulated in the system.
- Rinse with hot water.
Timely replacement of worn out rubber parts should be undertaken regularly.
Cleaning and sanitation of milk transportation equipment
Transport of larger quantities of milk requires insulated bulk tankers. These are very expensive and require special additional equipment like pumps which should also be thoroughly cleaned by the “cleaning-in-place” (CIP) method.
Milk transportation equipment should be properly cleaned and sanitized because milk provides an ideal medium for growth of bacteria. Select detergents and sanitizers that will not corrode the material from which the equipment is made. Cleaning and sanitizing are complementary processes.
Maintenance of milk handling and cooling equipment
Maintenance of milk coolers
For best use of milk cooling equipment, it is important to adhere to the following:
- Avoid opening the milk cooler unnecessarily to prevent warm air from entering
- Ensure that the evaporator is well ventilated for proper function of cooler
- Ensure that the cooler always has enough refrigerant in the system.
- Connect the cooler to a voltage stabilizer to provide for a constant supply of electricity.
- Set up schedules for cleaning and preventive maintenance and ensure that they are followed. Any mechanical repairs should be carried out by a trained technician.
- Have a standby generator in case of power failure.
Characteristics and maintenance of bulk tanks
Milk cans should be maintained by proper handling and adherence to regular cleaning and sanitation schedules. Cleaning, sanitizing and rinsing of bulk tankers and accessories like pumps should be done immediately after emptying the milk. The valves, hose connections and lid of the tanker should be covered to prevent the milk from being contaminated with dirt.
Importance of carrier maintenance
Milk transport vehicles often get dented during loading and offloading. Milk cans are designed with rims at the bottom to resist deformation during rough handling. Milk should be transported as quickly as possible to the milk cooling centre or processing factory to avoid spoilage. But milk that already has many bacteria will not keep for long, even when cooled.
For small scale dairy farmers, setting up a milk cooling centre centrally may be the ideal solution. Where farmers bring their milk to a cooling centre through a cooperative, they should do so as soon as milking is completed.
A Milk cooling centre with a capacity of 1000 – 3000 litres will serve up to 300 small holder farmers ensuring that the quality of their milk when produced under hygienic conditions is well preserved and accepted at the processing plant.
It is important to remember that under a hot environment milk will spoil within 3-4 hours.
So any means of cooling that will lower the temperature of milk from 380C at milking will help to prevent multiplication of bacteria. There are several options available.
In highland areas where the water temperature can be as low as 10oC, the milk may be cooled down to 20oC using water temperature through immersing milk cans in a water trough connected to a water tap or water spring.
Surface milk cooler
In hot areas like in the coast, Western Province, North Eastern, Nyanza, cooling of milk to 3-50C below ambient temperature may be achieved through use of charcoal lined evaporative cooling cabinet.
Milk transport to processing factory
Bulk milk transport
Milk cooled on the farm or cooling centre may be transported in milk cans or in bulk tankers. Bulk tankers are insulated, so the milk will remain cold until it reaches the plant. This is dependent on transport being fast, i.e. short distance or good roads enabling milk to be delivered before the temperature of milk rises above 100C.
Alternatively, such milk may be filled in cans and transported in milk cans. This has advantageous in case of a farmer’s delivering a can of POOR quality milk; it does not get mixed with other farmers’ GOOD quality milk and spoil the lot. Since the cans are not insulated, the transport to the factory must be efficient enough to enable milk reach the factory in acceptable condition.
In the case of farmers delivering milk to a collection point it is advisable that the milk cans are placed in a shaded area while awaiting pick-up by a milk transport vehicle.
Provision of shade at pick up points is important
Bad milk will be rejected at the dairy plant resulting in the farmer losing money. The milk transporter may lose money if the spoilage was due their fault. The nation will suffer because its people will not have the high quality food. To avoid all these bad things happening, hygienic milk handling is essential at each stage; at the farm, cooling centre and during transport.
BASIC MILK QUALITY TESTS
There are four simple milk quality tests that may be carried out routinely both at the farm and milk collection centre:
- Sight-and-smell (organoleptic) test
- Clot-on-boiling test
- Alcohol test
- Lactometer test
These tests ensure that only milk of acceptable quality is received and require only a small amount (sample) of milk from each container. If the sample of milk doesn’t pass the test, the milk from that container will be rejected and in most cases, the farmer bears the loss. Thus, it is important that milk is handled in accordance with good hygienic practice particularly at the farm. The procedures of these milk quality tests are described below:
This should be the first test to be performed and it involves assessing the milk with regard to its smell, appearance and color. This test is quick and cheap to carry out, allowing for segregation of poor quality milk.
No equipment is required, but the tester should have a good sense of sight and smell. Milk that cannot be adequately judged in this way is subjected to tests that are more objective.
- Open a can of milk.
- Immediately smell the milk and establish the nature and intensity of smell, if any.
- The milk will not be accepted if it smells slightly sour or has foreign odors like paint or paraffin.
- Observe the color of milk. Deviation from the normal yellowish-white color indicates damage to the udder (reddish—blood, or yellow—pus).
- Check for any foreign bodies or physical dirt, which may indicate that the milking and handling were not done hygienically.
- Touch the milk container to feel whether it is warm or cold. This indicates how long the 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 (e.g. feeding silage or brewer’s waste too close to milking time)
- Cows in late lactation or in some cows when on heat or soon after conception (due to hormonal changes)
- Bacterial taints (from cows with mastitis)
- Chemical taints or discoloring (may be due to equipment not rinsed properly)
- Advanced acidification or souring (milk that is fermenting)
- Marked separation of fat may be caused by:
- Milk previously chilled and subjected to excessive shaking during transportation
- Adulteration with other solids (may also show as sediments or particles)
- Boiling, if milk fat is hardened
This test is quick and simple. It allows for detection of milk that has been kept for too long without cooling and has developed high acidity, or milk that has a very high percentage of colostrums and hence protein. Such milk does not withstand heat treatment hence this test could be positive at a much lower acidity.
- Boil a small amount of milk for a few seconds in a spoon or other suitable container.
- Observe immediately for clotting.
- The milk will be rejected if there is visible clotting, coagulation or precipitation.
The test is quick and simple. The specific type of alcohol used is known as “ethanol”.
This test is more sensitive to lower levels of acidity and can therefore detect bad milk that may have passed the previous two tests.
It also detects milk that has kept for long cooling, colostrum or milk from a cow with mastitis. Because this test is quite sensitive, milk that passes this test can keep for some hours (at least two hours) before it goes bad.
- Use a syringe to draw equal amounts of milk and 70% alcohol solution into
- A small tube or glass cup (such as those used to administer medicine to children).
- Mix 2 ml milk with 2 ml 70% alcohol and observe for clotting or coagulation.
- If the tested milk sample coagulates, clots or precipitates, the milk will be rejected.
This test is used to determine if the milk has been adulterated with water or solids.
Addition of anything to milk can introduce bacteria that will make it spoil quickly, is dishonest and is therefore illegal. The lactometer 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 water or other solids are added to milk, the density either decreases (if water is added) or increases (if solids are added). If milk fat (cream) is added to milk, the density decreases. The equipment used to measure 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).
- Leave the milk to cool at room temperature for at least 30 minutes and ensure 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 the 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 this correction factor:
- For each °C above the calibration temperature, add 0.2 lactometer “degrees” (°L) to the observed lactometer reading.
- For each °C below calibration temperature, subtract 0.2 lactometer “degrees” (°L) from the observed lactometer reading.
- Note: These calculations are done on the lactometer readings (e.g. 29 instead of the true density of 1.029 g/ml).
Examples of how to calculate the true lactometer readings when the milk temperature differs from the calibration temperature of 20°C
Milk Observed lactometer Correction°L True lactometer True density
Temperature °C reading °L reading °L G/ml
17 30.6 – 0.6 30.0 1.030
20 30.0 nil 30.0 1.030
23 29.4 + 0.6 30.0 1.030
If the milk is normal, its lactometer reading will be between 26 and 32. If the lactometer reading is below 26 or above 32, the milk will be rejected because it means that it has been adulterated with added water or solids.
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