DAIRY TECHNOLOGY: MILK PROCESSING
Diagram
Raw milk and pasteurized milk side by side
Raw milk
Milk as it comes from the animal: no heat treatment, all its microbes still alive.
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Pasteurized milk
The same milk heated just enough to kill the harmful microbes, then cooled fast.
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The core difference
Heat. Pasteurization kills the pathogens that cause milk-borne illness, and raw milk carries them.
Figure 1: Raw milk compared with pasteurized milk. Drawn in our own style from Anne Mendelson, Milk: The Surprising Story of Milk Through the Ages, Knopf, 2008, chapter RAW VS PASTEURIZED ORGANIC VS CONVENTIONAL, p. 54.
Introduction
Raw milk and pasteurized milk look identical in the bottle, and they taste almost the same. The difference is invisible: one has been heated to kill harmful microbes, and the other has not.
The core difference is safety. Pasteurization was built to destroy the pathogens that milk can carry, such as the organisms that cause tuberculosis, brucellosis and salmonellosis. Raw milk keeps its natural enzymes and its full microbial load, including any pathogens present.
Quick comparison
Table 1: Raw Milk compared with Pasteurized Milk
| Aspect | Raw Milk | Pasteurized Milk |
|---|---|---|
| Heat treatment | None, milk is used as drawn | Heated, then cooled quickly |
| Common methods | No heating step at all | HTST: 72 degrees C for 15 seconds; LTLT: 63 degrees C for 30 minutes |
| Pathogens | Any pathogens present stay alive | Disease-causing microbes are destroyed |
| Enzymes | Natural enzymes stay active | Most enzymes are destroyed or reduced |
| Shelf life | Very short, spoils in a day or two even when chilled | About 1 to 2 weeks when kept cold |
| Flavour | Full fresh flavour, varies with the herd and feed | Almost the same, with a very slight cooked note |
| Nutrition | Same nutrients in principle | Almost the same; small losses of some heat-sensitive vitamins |
| Cream line | Cream rises naturally | Cream still rises unless the milk is also homogenized |
| Legal sale | Restricted or banned in many places | The legal standard for milk sold in shops in most countries |
| Risk | Can carry tuberculosis, brucellosis, Salmonella, E. coli and Listeria | Safe when pasteurized correctly and kept cold |
Scientific explanation
Definition
Raw milk is milk that has not been heated above the temperature of the animal. It is used as drawn, after straining and cooling.
Pasteurized milk is milk heated to a set temperature for a set time to destroy pathogenic microbes, then cooled quickly to below 10 degrees C. Pasteurization is not sterilization: it kills pathogens and most spoilage organisms, but not all microbes.
Working principle
Pasteurization works because heat destroys microbes at a rate that rises steeply with temperature. A short time at a higher temperature kills the same number of microbes as a longer time at a lower temperature, which is why HTST and LTLT are equivalent in effect.
The target organisms are the most heat-resistant pathogens that milk can carry. Once they are destroyed, the milk is safe, and fast cooling stops the survivors from growing.
Apparatus
A pasteurizer: a plate heat exchanger or a vat with heating and cooling sections, a holding tube that sets the holding time, and a temperature recorder.
Raw milk needs no heating apparatus, only clean milking equipment, a strainer and a cooler. A thermometer and a clock are the control tools for pasteurization.
Measurement procedure
For HTST pasteurization, pump the milk through the heating section to 72 degrees C, hold it at that temperature for 15 seconds in the holding tube, then cool it at once to below 10 degrees C.
For LTLT pasteurization, heat the milk in a vat to 63 degrees C, hold it for 30 minutes with stirring, then cool it quickly. Check the result with the phosphatase test: the enzyme alkaline phosphatase is destroyed by correct pasteurization, so a negative test shows the milk was heated enough.
Factors affecting readings
The fat content and the solids of the milk protect microbes slightly, so richer milk needs the full time and temperature.
The starting microbial load matters: dirty milk with a high count is harder to make safe, which is why clean milking comes first. A faulty thermometer, a short holding time or slow cooling can all leave the milk unsafe.
Formulas and calculations
The books give no formula that calculates pasteurization from first principles for this pair. The time and temperature pairs are set by law and by experiment, not by a formula.
The phosphatase test is a pass or fail check, not a calculation: a negative result means the milk reached the pasteurization temperature.
Industrial applications
- Raw milk: farmhouse use and traditional products in places where its sale is allowed.
- Raw milk: some traditional cheeses, where the natural microbes shape the flavour.
- Pasteurized milk: drinking milk sold in shops, the standard fluid milk in most countries.
- Pasteurized milk: tea, coffee and cooking, and the base milk for yoghurt, cheese and other products.
- Pasteurized milk: school and hospital milk supply, where safety is the first requirement.
Advantages and limitations
- Raw milk advantage: full fresh flavour and active natural enzymes.
- Raw milk limitation: it can carry dangerous pathogens, and it spoils fast.
- Pasteurized milk advantage: safe to drink, with a shelf life of 1 to 2 weeks when kept cold.
- Pasteurized milk limitation: a slight cooked flavour and small losses of heat-sensitive vitamins.
FAQ
Does pasteurization change the taste of milk?
Barely. Most people cannot tell pasteurized milk from raw milk in a blind taste, though a careful taster may notice a very slight cooked note.
Is raw milk more nutritious than pasteurized milk?
Not in any way that matters. The protein, fat, lactose, calcium and most vitamins are the same; only small amounts of heat-sensitive vitamins are lost.
Why is raw milk risky if it looks and smells fine?
Pathogens do not change the look, smell or taste of milk. Milk can carry tuberculosis, brucellosis, Salmonella and E. coli while looking perfectly fresh.
Does pasteurization kill all microbes?
No. It kills pathogens and most spoilage organisms, but some heat-resistant microbes survive, which is why pasteurized milk still spoils and must be kept cold.
What is the phosphatase test?
A test for the enzyme alkaline phosphatase, which is destroyed by correct pasteurization. A negative test shows the milk was heated enough; a positive test shows it was not.
References and further reading
- Anne Mendelson, Milk: The Surprising Story of Milk Through the Ages, Knopf, 2008, chapter RAW VS PASTEURIZED ORGANIC VS CONVENTIONAL, p. 54 (Google Books page read)
- FTN books on disk: Market Milk, Milk Processing and Quality Management, Dairy Processing Handbook by Bylund (agrimoon dairy series).
