What happens to milk when it is pasteurized?

When you go to the market to buy packaged milk, be it in a pouch or in a bottle or any other form of packaging, it is highly likely it would be labelled as ‘Pasteurized Milk’. This could be toned milk, double toned milk, full fat or full cream milk, or standardized milk.

Why is milk pasteurized?

What happens when milk is pasteurized?

Is pasteurized milk dangerous?

Does pasteurization trigger or worsen lactose intolerance?

In this article, I will try to address these questions and present the facts to you to help clear all the fear, anxiety, and rumors that surround pasteurization as a process of milk processing.

If you are looking for a TL/DR, here you go:

Raw milk potentially DOES contain dangerous pathogens

Raw milk DOES NOT kill dangerous pathogens by itself

Pasteurizing milk DOES NOT cause lactose intolerance or milk allergies

Both raw milk and pasteurized milk are equally likely trigger an allergic or intolerant reaction if you have an existing condition, but the pasteurization process is not causing it

Raw milk consumption is dangerous, and in some severe cases very highly dangerous

Raw milk consumption DOES NOT treat allergies or cancer or lactose intolerance

Pasteurization DOES NOT destroy or even significantly reduce the milk's nutritional value

Pasteurization DOES NOT make milk safe to leave out at ambient temperature or eliminate the need for refrigeration post-pasteurization

Pasteurization DOES kill harmful bacteria

Pasteurization ACTUALLY DOES save lives

Let us begin by addressing a basic question –

What is pasteurization?

Pasteurization is a process of applying heat to a food product followed by immediate cooling to destroy the pathogens in the food and make it safe for consumption. The process is not limited to milk and lots of food products apart from milk are pasteurized every day to ensure food safety and better shelf life. Different food products would be pasteurized at different temperatures for different time spans.

In the context of milk pasteurization, here are the temperature to time combinations that are commonly used:

Type of Pasteurization TemperatureTime
Vat pasteurization63ºC (145ºF)*30 minutes
High Temperature Short Time Pasteurization (HTST)72ºC (161ºF)*15 seconds
Higher Heat Shorter Time Pasteurization (HHST)89ºC (191ºF)1 second
Higher Heat Shorter Time Pasteurization (HHST)90ºC (194ºF)0.5 seconds
Higher Heat Shorter Time Pasteurization (HHST)94ºC (201ºF)0.1 seconds
Higher Heat Shorter Time Pasteurization (HHST)96ºC (204ºF)0.05 seconds
Higher Heat Shorter Time Pasteurization (HHST)100ºC (212ºF)0.01 seconds
Ultra Pasteurization (UT)138ºC (280ºF)2 seconds
General indicator of time-temperature combination for different types of pasteurization for Milk

* When the fat content in milk is more than 10% or sweeteners have been added or it is a concentrated or condensed milk, the specific temperature should be increased by 3ºC (5ºF).

So, you can see that milk is exposed to a higher temperature for a very short span of time and then immediately cooled to 4ºC. This temperature change from low to high to low kills most pathogens and makes the milk safe for human consumption while also increasing its shelf life from a few hours (for raw untreated milk) to approximately 2 days (for pasteurized standardized milk maintained under refrigeration conditions).

The most commonly deployed time-temperature combination for pasteurizing milk in the dairies is the HTST or the High Temperature Short Time Pasteurization. Here, the milk is heated to 72ºC (161ºF) for 15 seconds and then immediately cooled to below 4ºC.

How is milk usually pasteurized?

This is quite a simple process preceded and succeeded by other steps, though it can still sound very complicated sometimes. I am going to try to simplify it as much as possible.

In earlier times, vat pasteurization was the most commonly used method. This involved heating milk to 63ºC (145ºF)* and holding it at that temperature for 30 minutes, followed by cooling it. Milk was constantly stirred (agitated) throughout the process to ensure uniform heat transfer. The vats were double-walled, so hot water (for heating) and cold water (for cooling) were circulated while milk inside the vat, with the lid of the vat having an agitator that kept the milk moving.

However, since the early 90s, plate heat exchangers have been increasingly common for milk pasteurization. They save space, are more energy efficient, are quicker, more compatible with the Cleaning-in-Place process, use regenerative heat transfer, cost-effective, and safer from a food safety standpoint. Here’s what a plate pasteurizer looks like:

A plate milk pasteurization apparatus

Here, as you see in the picture, a lot of food grade stainless steel plates are lined up together, sealed with the best gaskets to prevent leakage and kept tightly packed together. There are two fluids that enter and exit this apparatus:

  1. Milk
  2. Water

Hot water and cold water are circulated in alternate plates. Now raw milk goes in from one side of the plate pasteurizer, it gets circulated in thin pipes (to increase surface area and maximize heat transfer) in the plates that are circulating the hot water. Milk, coming in contact with the hot water, will heat up. Temperature is constantly monitored. The apparatus is calibrated in a way that the milk reaches the required temperature for the desired time, then the milk exits from the other side of the pasteurizer, and goes back into the apparatus, this time circulating through similar thin pipes through the plates circulating the chilled water to cool the milk.

This is what I mean when I say hot water and cold water are circulated in alternate plates:

Plate heat exchanger: Red indicates hot water, Blue indicates cold water

This is what each of these plates look like on both sides:

Fishbone design on the plates of a plate heat exchanger

They are called fishbone plates because the design of the pipes on the plate resembles a fishbone. There are four holes in each plate. The gaskets seal the flow and the design of the pipes in the plate determine as well as their placement decide what goes in and out and what just bypasses that particular plate.

Once the whole cycle of heating and cooling is complete, samples of the milk are drawn and the quality assurance team will perform the mandatory tests on it. This plate heat exchanger process is a continuous process, so multiple samples will be drawn through the course of operation every day.

How is milk confirmed to be pasteurized? How is the efficacy of pasteurization checked?

The efficacy of pasteurization is confirmed by the existence, or rather, the non-existence of an indicator organism. The indicator organism is a microbe whose existence should be effectively destroyed in regular pasteurization conditions. Microbes also evolve like humans do, they can become temperature resistant or more dangerous or resistant microbes get discovered, so the indicator organism for pasteurization is also updated from time-to-time.

Currently, the indicator organism for milk pasteurization is Coxiella burnetii. If a pasteurized milk samples is not found to contain the indicator organism, the pasteurization is considered to be effective.

What is the effect of pasteurization on the nutritional value of milk?

The benefits of milk pasteurization are well-established. I have also mentioned above that pasteurization kills dangerous pathogens in milk. But what about its effect on the nutritional value? Let us go component-by-component to make it easy to understand.

Effect of milk pasteurization on Vitamins

Milk, including pasteurized milk, is an important source of Vitamin B1 and B12. Pasteurization, like any other cooking or processing method, will slightly decrease the concentration of Vitamin B2, Vitamin C, B12, and Folate. Despite the decrease post-pasteurization, milk is still an important source of Vitamin B2 and B12. The Vitamin E content also decreases slightly.

However, there are two very important things to consider here. Milk, naturally, in its raw state is anyways NOT a significant source of these nutrients. Secondly, no food can give you all nutrients in the quantities your body needs them, which is why eating a balanced diet is important.

Also worth mentioning, Vitamin A content in milk has been found to increase a bit on pasteurization.

Vitamin C is acidic and very, very highly heat sensitive. The slightest heat, including the heat of the subzi or dal it gets added to, will cause the breakdown and subsequent loss of Vitamin C.

Effect of milk pasteurization on milk proteins & enzymes

Milk contains two types of proteins – casein and whey proteins. Whey proteins, especially beta-lactoglobulin get partially denatured on heating. Instead, denaturation opens up the proteins, making them more accessible for the hydrolytic enzymes in digestion, thereby improving the protein digestibility. However, the denaturing of whey proteins DOES NOT lower their nutritional value. Also, only 10% of these proteins are denatured during the pasteurization process.

It is a myth and a misconception that raw milk contains ‘active enzymes’ which help digest milk and get destroyed on pasteurization. The enzymes present in milk naturally are not required for digesting the milk, our digestive systems are perfectly equipped to do it by themselves without external aid. Unless, you are unwell, in which case please talk to a doctor.

Pasteurization has little to no impact on the casein proteins. Pasteurization also works in the exact same way for A1 and A2 milk, no miracles happening there either.

Effect of milk pasteurization on lactose

Pasteurization has no effect on the lactose content of milk. UHT and UT method can trigger a little Maillard reaction, but regular market milk pasteurization has no effect on lactose in milk. Pasteurization will also not induce or cause you to have an allergic reaction or lactose intolerance. If you have a pre-existing condition, both raw milk and pasteurized milk are equally likely to make you sick.


Pasteurization of milk makes it safe to consume by killing the dangerous pathogens that may be present in raw milk. It also makes milk shelf-stable to be transported to some distance under refrigerated conditions and increases shelf life for up to 2 days under refrigeration.

Pasteurization of milk could potentially destroy a very teeny tiny insignificant amount of nutritive value, but milk is anyways not a significant source of most of those nutrients even in its raw form, so that makes no difference. Also, no food gives you all the nutrients you need in the volumes you need, so have a balanced diet, please.

If you are buying raw milk from local vendors, please boil the milk prior to consumption. Consuming milk straight from a cow’s udder looks exotic only in Tiktok and Instagram videos, it could be fatally dangerous in real life, so don’t try it.

Food safety and stability is highly important, do not take it lightly.

Here are my references:

Cover image source: Pexels.com

What do you think of this article? What milk do you consume? Drop a line in the comments or find me on Instagram @banjaranfoodie.

Penny for your thoughts!