Dairy Council

Turning grass into milk 

Cows are ruminants which means they have four stomachs, all with different functions. This complex digestive system allows cows to turn grass and other plants which are inedible to humans into milk.

During the grazing season (usually from April through to October in Northern Ireland, although it can be longer) grass is a cow’s main source of food. In the colder winter months cows are housed indoors, and are usually fed on grass which has been preserved by the farmer during the summer – this often takes the form of silage or hay. The cow may also be given additional foods (for example grain) - a world-leading feed assurance programme in Northern Ireland means only the highest quality feed is allowed onto our dairy farms.

When a cow eats, she swallows her food quickly with very little chewing. The food then, passes into the first stomach. 

First Stomach

This stomach (called the Rumen) contains special bacteria which can break down grass and release nutrients.

Second Stomach

In this stomach (called the Reticulum) grass is softened further and formed into a ball called “the cud”. The cud is then brought back up into the cow’s mouth where it is chewed again until it is very soft (this is known as “chewing the cud”) and then it is re–swallowed.

Third Stomach

In this stomach (called the Omasum) the cud is broken down even further.

Fourth Stomach

This stomach (the Abomasum) is the most like a human stomach, and is where the grass or other food undergoes its final breakdown.

From here, it will pass into the small intestine, where nutrients will be absorbed into the bloodstream and be carried around the body and to the udder where the milk is produced.

How are cows milked? 

When a cow enters the milking parlour her teats are washed and the dairy farmer fits four teat cups to them. The milking machine then copies the natural sucking of a calf, gently sucking the milk from the cow’s udder. 

When the milk leaves the cow’s udder (at around 38°C) it flows into a glass jar which measures the amount of milk given.

The milk is then pumped through pipes into a large refrigerated tank called a vat, where it is cooled (to around 4°C) and stored until it is collected and brought to the dairy.

When milking is complete the cow’s teats are washed again. The milking parlour is then thoroughly cleaned for the next milking session…most cows will be milked twice a day. 

Although this is the traditional method used to milk cows, a growing number of dairy farms in Northern Ireland have robotic milking parlours. These parlours use automated machines to milk the cows – the computerised system allows each cow to decide when she’s ready to be milked rather than having a scheduled milking time. 

Once the milk has been collected from the dairy herd, a tanker collects the milk from the farm. The tanker driver tests the temperature of the milk, and checks that it smells and looks fresh. The driver will also take a sample so that the milk’s quality can be tested and also to ensure that it has been produced in clean conditions and that it is free from harmful bacteria.

Once this has been done, the driver can transfer the milk from the vat to the tanker. The milk is sucked into the tanker by a vacuum pump, and a special meter which is fitted to the tanker, called a ‘flowmeter’, measures the amount of milk collected from the farm.

When the farm’s vat is empty it is washed to make sure that it is clean and ready for the next milking. Once the milk has been collected the driver takes the tanker to the dairy for processing. The tanker is well insulated to make sure the milk stays cool on its journey to the dairy.

When the milk tanker arrives at the dairy it is weighed. The milk is then tested to make sure it still looks and smells fresh, and also to ensure it is at the correct temperature. Once the milk has been checked it is pumped out of the tanker into storage tanks. The empty tanker is then weighed again to calculate how much milk has been delivered to the dairy. The tanker is then carefully washed before it is used for the next milk collection.

The next step is to heat–treat the milk. Heat–treatment ensures the milk is safe to drink (by killing any harmful bacteria) and also helps to keep it fresh for longer. The most common form of heat treatment used on milk in Northern Ireland is pasteurisation, which involves heating it to 71.7°C for at least 15 seconds, and then cooling it very quickly to less than 3°C. 

More on pasteurisation (

Once the milk has been heat treated it is ready to be packaged and sold to consumers.

Some milk is sent for cheese making or to a creamery to make cream, butter or yogurt. 

Find out how milk is used to make cheese, yogurt and butter

Did you know?

From leaving the cow to becoming a dairy product in your fridge, milk

is tested at least ten times to ensure the optimum quality.