Pasteurisation of milk is a process used to heat raw milk to make it safe for human consumption. It is aimed at destroying all of the pathogenic or disease-causing bacteria that are in raw milk. It follows on that cheese made from pasteurised milk should also be free from these same pathogenic or disease-causing bacteria. That is providing the milk was pasteurised correctly and there is no recontamination of the milk or cheese with pathogens post the pasteurisation process.

There are three commonly used heat treatments that can be applied to milk to pasteurise it to make cheese:

• continuous or High-Temperature Short Time (HTST) heating, a continuous process and is the domain of medium to larger processors. A typical time temperature is 72°C for 15 seconds;
• batch heating which is a less severe heat treatment but is used for smaller quantities of milk and is commonly used by smaller milk and cheese processors. A typical time temperature is 65°C for 30 minutes
• many home cheesemakers and some small commercial processors use 68°C and hold it for two minutes

As a cheesemaker you may want to know that the pasteurisation process can affect milk that is used to make cheese. The main areas are:

1. The indigenous lactic acid bacteria cultures that we want to make use of when we make the cheese are destroyed by the heating. So lactic starter cultures are added to the milk to replace those indigenous cultures that previously existed. The indigenous lactic acid bacteria will normally be more varied, versatile, and better acclimatized in the milk than the added lactic starter cultures. However, research into starter cultures has significantly closed the gap between indigenous cultures and added lactic cultures. The starter culture companies look at raw milk and raw milk cheese and select the best of best cultures that they can find, isolate them, and grow them, do cheesemaking trials to see how well they work. They are then grown up in bulk and sold to cheesemakers. The lactic starter cultures available are now very complex and there are numerous varieties of very good flavour producing cultures available to the cheesemaker. This allows cheesemakers using pasteurised milk to attain significantly improved flavours than was attainable previously.
2. Pasteurisation provides more regular starter performance and more uniform cheese quality. But is this uniformity and quality beneficial or not? It’s a very controversial point.  A large part of that answer really depends on the skill of the cheesemaker to make a good cheese. Simply put, very good quality cheese can be made using both pasteurised or raw milk if the cheesemaker applies good cheesemaking practices.
3. If raw milk is left for several days it will turn sour, caused by the indigenous lactic acid bacteria cultures in the milk. If pasteurised milk is left for several days, even if kept in the fridge, it will go off or be spoilt. Pasteurisation does not kill all the microbes in the milk, some still survive the pasteurisation process and some also make their way into the milk as we make the cheese. These are called NSLAB (non-starter lactic acid bacteria). As milk ages the NSLAB increase in both variety and number. As a cheesemaker use the freshest pasteurised milk that you can get. Practice good hygiene. Your milk will have a lower number of NSLAB in it.
4. Some of the soluble calcium salts in the milk are precipitated by the heat of the pasteurisation process. The soluble calcium salts are essential for the firm coagulation of milk with rennet. This precipitation of these soluble calcium salts is greatly increased by unnecessary extra heating during pasteurisation. Large milk processors and even home cheesemakers may overheat milk with a view to extending the shelf life of milk, or by trying to destroy more NSLAB, so don’t think that extra heating is better, it is not. This extra hating is detrimental to the quality of the milk for cheesemaking. With many pasteurised milks, it is good practice to add calcium chloride back into the milk. It is also better to add this calcium at least 40 minutes before adding the rennet. Lack of soluble calcium salts leads to poor curd formation and difficulty in moisture expulsion (in the form of whey) in cheesemaking. This may lead to broken up curds, lower yields, soft and over acid cheese.
5. Pasteurisation of milk can lead to denaturation of the whey proteins. The interaction of the whey proteins with the Casein leads to impairment of the rennet set. The effect on your cheese will also be poor curd formation and whey drainage.
6. Pasteurisation inactivates many of the indigenous enzymes in the milk such as Lipase.  Lipase enzymes are important in developing flavours in cheese. Cheeses made from raw milk have higher lipolysis flavours than the same cheese made with pasteurised milk.  Lipase can be added back into milk, but this process is used mainly for cheeses that require a stronger flavour eg Romano, Greek Feta