Difference Between Fermented Milk & Spoiled Milk

Cheese is the product of fermented milk.
Cheese is the product of fermented milk. (Image: Creatas Images/Creatas/Getty Images)

Manufacturers go through a lot to ensure that milk does not spoil, or go bad, before the expiration date. Pasteurization, refrigerated transportation and even special temperature controls in the store's dairy case are all measures taken to avoid spoiled milk. That's why the idea of spoiling milk on purpose seems silly at first. However, there's a difference between fermented milk and spoiled milk. The difference also determines when old milk or curdled milk is OK to drink.

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The Rundown on Fermented Milk

Milk that is "spoiled" in a controlled setting by adding organisms is called fermented milk. It is made from whole milk and is produced by lactic acid-making bacteria that are mixed into the milk. The bacteria eat the lactose sugars in the milk, making lactic acid. This causes the casein proteins in the milk to bind together, making the milk curdle. A pure culture containing a specific bacteria is grown, measured and added to the milk.

The Story on Spoiled Milk

Spoiled milk is milk that has been out in temperatures above 45 degrees Fahrenheit or in the refrigerator long enough to become a hospitable environment for bacteria. These bacteria work the same as the bacteria added to fermented milk. The difference is that the bacteria infiltrating spoiled milk are a variety of unknown organisms. They contaminate the milk in varied concentrations. There is no control in this environment.

More Difference

The bacteria used in fermented milk are safe and healthy to eat, while the bacteria in spoiled milk are harmful and will make you sick. Fermented milk takes 18 to 24 hours to ripen. Spoiled milk can harm you after a few hours of bacteria infiltration. Fermented milk is made using the strictest sanitary conditions, and spoiled milk is made in unsanitary conditions and your common refrigerator.

Ferment Milk Products

The fermented milk is used to make some of the most common foods in the human diet. Yogurt, cheese, sour cream and a dairy product known as kefir are all made from fermented milk. The different foods are made from various bacteria and are aged for specific amounts of time. The choice of bacteria used to ferment the milk also affects the flavor of the finished product. The type of milk product that starts the creation of the food also matters. For example, light cream fermented with a specific bacteria makes sour cream. Fermenting skim milk with Lactobacillus bulgaria bacteria creates yogurt.


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