What causes milk to spoil?

Updated May 31, 2017

Although milk is sterile inside of the cow's udder, contaminants in the teat, hairs, milking equipment and storage facilities introduce bacteria and fungi. Most of these are killed during pasteurisation, but some survive and reproduce, even when the milk is refrigerated. As the microorganisms grow, they break down the milk's carbohydrates, proteins and fats, causing spoilage. Depending upon the type of organisms responsible, the milk may sour, curdle, develop slimy strings, red or grey rot, or mould.

Spoilage Microorganisms

The microorganisms that cause milk to spoil are present in small numbers from the moment you open the lid. In these concentrations, they are harmless. However, as they multiply, they begin to change the milk's composition. Common microorganisms include lactic acid bacteria, or Lactobacillus, which cause fermentation; Bacillus, Microoccus and coliform bacteria, which cause curdling; Clostridium and Serratia bacteria, which cause rot; and Penicilium and Geotrichum fungi, which create mould.

Indications of Spoilage

Spoiled milk often has a strong smell and an acidic taste. It also may form semi-solid clumps called curds, change colour or consistency, or contain mould. Spoiled milk is not the same as soured or cultured milk, which also is acidic and forms curds. These properties of cultured milk are created by beneficial bacteria, which are present in raw milk but are killed by pasteurisation. When pasteurised milk sours or curdles, it is due to harmful bacteria, which are unfit for human consumption.


Pasteurisation is the process of heating milk to destroy harmful bacteria. There are several processes, including batch pasteurisation and the continuous, high temperature short time (HTST) process. Batch pasteurisation heats the milk to 62.8 degrees Cor 30 minutes to kill pathogenic and spoilage microorganisms. HTST pasteurisation heats the milk to 71.7 degrees Cor 15 seconds. Both of these processes destroy most microorganisms, but a few hardy bacteria often survive.

Preventing Food Poisoning

Never drink milk that smells or tastes sour. If it has any trace of clumps, strings, red or grey colouration or mould, pour it out. Also observe the expiration date. This reflects the amount of time you can expect the bacteria to remain at a benign level. Store milk and milk products in the refrigerator at 0.556 to 4.44 degrees Celsius. The lower the temperature, the slower the bacteria will propagate.

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About the Author

Kylene Arnold is a freelance writer who has written for a variety of print and online publications. She has acted as a copywriter and screenplay consultant for Advent Film Group and as a promotional writer for Cinnamom Bakery. She holds a Bachelor of Science in cinema and video production from Bob Jones University.