This Wiki Article discusses the different forms of milk processing. Milk straight from the cow, goat, or ewe is often referred to as “raw” milk. there are five processes commonly applied to raw milk, depending on if home dairy or mass manufactured milk that is bought in stores:
- Filtration to remove any foreign bodies.
- Pasteurization to kill any harmful bacteria and increase the milk’s shelf life.
- Cooling either after pasteurization or after milking to increase the milk’s shelf life.
- Ultrafiltration, where butterfat is removed from the raw milk.
- Homogenization to provide a more stable uniform product.
Filtration of milk is to remove any foreign bodies that may have entered the milk during the milking and collection process.
For a home dairy this may be as simple as a very fine mesh kitchen strainer. For larger quantities, a purpose made milk filtration unit is often used.
If the milk is to be pasteurized or used in cheese a making immediately, then this step is omitted. If not then the milk must be cooled immediately to slow natural bacteria from multiplying and reducing the milk’s like.
For small home dairies, the easiest way is to place the milk in bottles of stainless steel containers with lids and stand the containers in a sink of running cold water. For larger quantities, an in-churn milk cooler is often used.
Once cooled the milk should be placed in a refrigerator until at 4C / 40F or below.
Note, even if making cheese immediately, the milk temperature after milking may still be too high and thus cooling required.
Pasteurization is named after its creator, French chemist and microbiologist Louis Pasteuris and it involves the heating of milk for short time periods to kill bacteria and thereby reduce health risks to its consumers. A secondary benefit of this process is to increase its shelf life. This process kills both wanted and unwanted bacteria.
Pasteurization can be at different temperature and time, with generally the higher the temperature, the less time. There is no global standard for pasteurization, but in general the temperature and time are grouped into 3 levels:
- Pasteurized: Milk is heated to ~66C / 150F and held for ~30 seconds before cooling. This is the most common and kills the least bacteria. Milk normally requires refrigeration to prolong its life.
- Ultra-Pasteurized: Milk is heated to ~72C / 162F and held for ~15 seconds before cooling. Higher temperature and shorter time, the second most common, kills more bacteria than Pasteurized. Thus enables longer refrigerated shelf life and is common on products that are consumed or sold slower such as creams and organic milks.
- Ultra Heat Treated or UHT: Milk is heated to ~82C / 180F and cooled immediately. Highest temperature and shortest time, the least common method, kills the most bacteria. Results in shelf life of years even without refrigeration. Commonly used where milk is not common or logistics of getting Pasteurized milk or other products is problematic.
After heating cooling is performed rapidly, normally via cold water and on larger equipment through heat exchangers.
Note, often manufactured milk (and products derived from it) are labeled Pasteurized or Ultra-Pasteurized or UHT, this may not describe the level of pasteurization.
In general, good bacteria in the milk makes a better cheese. Thus, hygiene issues aside, raw milk is the best for making cheese, followed by Pasteurized, then Ultra-Pasteurized, and the UHT (almost completely dead) milk. Unless you know the source and the hygiene conditions of the source of your milk, Pasteurized is always best. Check local laws at your location as to the use of unpasteurized milk in cheese making.
Note, many people new to cheese making if using store-bought milk assume organic milk is the best, this is normally not true as it is often Ultra-Pasteurized to provide longer shelf life but also making it poorer for cheese-making than cheaper Pasteurized milk.
There are considerable arguments about the merits or advantages of pasteurization versus the disadvantages of also killing off the good bacteria in milk. For more information, see the International Dairy Foods Association’s website on pasteurization, go to Resource Center > Industry Facts > Milk > Pasteurization pdf.
For counter arguments to pasteurization, see the US-based Campaign for Real Milk’s website.
To reduce fat content of cow’s milk, butterfat is removed from the raw milk to make lower % fat milks and the high fat milk used for making butter, heavy creams, and other high fat milk products including cheese.
For small home dairies this is normally done via skimming the cream off the top of cow’s milk after it has separated or by using a hand crank or electric centrifuge which separates the lighter cream. Industrial large dairies use a process known as Ultrafiltration where milk under high pressure is pumped through a semi-permeable membrane to remove the fat and other high molecular weight components. UF milk is low-cost to manufacture as i) the permeate (leftovers) can be sold for whey protein, ii) labor is reduced, and iii) if using UF to make cheese you use less milk to make the same volume as you are binding more of the lost fat and protein up front rather than losing it in the whey and lower shipping cost to transport the concentrate.
Ultrafiltration was applied to industrial cheese making in France in the late 70’s to significantly lower costs in making Camembert and Brie cheeses. Since then it has been applied to many industrially made high moisture cheeses around the world such as Cream Cheese, Chevre, and Feta where you simply filter the milk to give you your final moisture, fat and protein content. Most industrial milk processing plants use UF and industrial cheese making dairies order their milk to a specified concentration which is used to make almost instant cheese by spraying rennet into it as the molds are filled. If the cheese is retailed in plastic containers then often that is the mold. As the material is already de-wheyed, there is no draining or drying. UF cheeses generally have very-creamy texture but none of the flavour of traditional artisan made cheese and the high moisture leads to poor shelf life. To add flavour, often manufactured artificial cheese flavours are added which depending on the country and labeling requirements, may not explicitly be disclosed in the list of ingredients.
Homogenization is the process where milk is (in general) pumped at high pressure through small orifices which breaks up the fat globules in milk into smaller sizes so that they have significantly less tendency to coagulate. Before mass homogenization of milk, the fat in milk, or butterfat as it is called in the industry, would coagulate and as lighter than the water part of milk, float to the top of the milk and was known as the cream. Homogenization thus prevents or delays this natural separation.
Homogenization is common with processing of cow’s milk but not with goat’s or ewe’s milk as the fat globules are already small.