I recently rewrote much of the information about the cheeses on our website. It’s been a challenge, as there are a lot of them, but it’s kept things up to date. You may have noticed that I’ve included a ‘new’ milk treatment, thermised. So I thought I should explain what the various heat treatment options mean (plus I get to do some cheese nerdy research, so it’s a win-win!).

Here in the UK, milk goes through a heat treatment before it ends up in your fridge or on your cheeseboard: pasteurised, thermised, or raw. Each uses a different amount of heat, which affects the product’s safety, microbes, and flavour. There are strict regulations in place to ensure these processes are properly followed.

Most milk in the UK, for drinking or major cheesemaking, is pasteurised, i.e., heated to a temperature high enough to kill harmful bacteria while preserving most of the nutrients. This is a legal term, and the milk must be heated to 72°C for 15 seconds, then rapidly cooled. This method, discovered by Louis Pasteur, prevents spoilage and illness.

Pasteurised milk is a safe and reliable choice for cheesemaking. Heating the milk kills bacteria like E. Coli and Salmonella, which is definitely a good thing. The downside is that it also removes many of the natural microbes that give cheese its unique flavour.

Unpasteurised milk includes raw milk and milk warmed below pasteurisation temperatures. In cheesemaking, “unpasteurised” usually means raw milk cheese; this means all the natural bacteria from the farm stay in the milk and help develop flavour as the cheese ages. The flavour of raw milk cheeses can be more interesting and unique than that of pasteurised milk cheeses. But using raw milk also brings some safety concerns.

Since raw milk can sometimes carry harmful bugs, there are strict rules about using it in the UK. Farmers must maintain high standards, and buying raw milk to drink is usually limited to the farm gate or specialist suppliers.

Thermised milk has been gently heated to 57°C-68°C for about 15 seconds. It is not a legally recognised process, and, as such, milk treated in this way is still considered unpasteurised, but not raw. It is something of a halfway house, falling between raw and pasteurised. Not only does thermising reduce spoilage bacteria, but it also preserves more natural microbes than pasteurisation. Since not all bacteria are removed, thermised milk still requires careful handling.

In cheesemaking, thermisation is useful when milk quality varies or when makers seek a balance between safety and flavour. The gentle heat reduces harmful bacteria, making the milk more stable and consistent for production, while leaving more of the natural microbes. This means the cheese can still develop interesting flavours.

Heat treatment does impact the cheese. Pasteurised milk allows flavours to be controlled and consistent. Raw milk cheeses have more complex flavours from natural microbes. Thermised milk cheeses offer a middle ground, keeping some natural microbes while reducing risk. The type of milk a cheesemaker uses is what makes cheese so varied and exciting.