When milk sours is it a chemical change? 

In this brief article, we are going to answer the question “When milk sours is it a chemical change?”. We will also discuss the nutritional benefits of milk and talk about how we can ensure that the milk stays fresh.

When milk sours is it a chemical change? 

Souring of milk is considered as a chemical reaction which results in the production of lactic acid, which has a pungent odor. The taste and smell of spoiled milk are both unpleasant. It has a sour taste.

The fermentation process is responsible for the sour taste in milk.  It is also possible for it to curdle and therefore become lumpy. 

The process of pasteurization kills the vast majority of the germs that are found in milk. However, very few resistant microorganisms remain in the milk. Lactic acid is produced when lactose is digested by bacteria; the transformation of these sugar molecules into lactic acid.

The production of lactic acid is what gives the milk sour flavor and aroma. Lactic acid also causes the proteins in milk to change their structure, which results in the milk being curdled and clumped together. 

This process, like other chemical reactions, cannot be reversed once it has begun.

The primary effects that are caused by sour milk include the conversion of some of the milk sugar into lactic acid, which causes casein to precipitate and makes the other insoluble ingredients more soluble. This process takes place when the milk is soured.

The fermentation process caused by lactic acid bacteria results in a reduction in the amount of sugar that is present in milk.

There is a loss equal to 22 %, or 1.27 gram, of sugar for every 100 cc of milk that is consumed by bacteria. The amount of sugar that is degraded is 1.124 gram, and 88.5 percent of that sugar is converted into lactic acid. The quantity of lactic acid that is produced is 1.124 gram.

Due to the action of Bacteria Lactis aerogenes, the citric acid that was present in milk is completely eliminated. In its place, acetic acid and carbon dioxide are produced as a result of this transformation.

The acid that is formed as a by-product of the activity of bacteria is responsible for making the inorganic components of ordinary milk that are insoluble.

The structure of the albumin protein in soured milk changes because of the acidification process that occurs in the milk.

Calcium caseinate derived from regular milk transformed into the free protein, which subsequently precipitates, and the generation of calcium lactate takes place, which is soluble in the serum.

The action of Bacteria Lactis aerogenes is responsible for the transformation of citric acid into acetic acid and carbon dioxide as part of the process of sour fermentation.

How can we ensure that the milk stays fresh?

  1. Milk should only ever be poured into containers that are completely dry, spotless, and, if possible, made of glass or ceramic.
  1. When cleaning milk containers, you should NEVER use hot water since doing so speeds up the fermentation process. Instead, the use of cold water is always recommended.
  1. It is important to keep milk as far away from other odors and air as possible.
  1. Avoid prolonged exposure to intense sunshine, since this might cause vitamins to become less effective over time.

If you make it a priority to ensure that everyone in your home follows these few fundamental recommendations, milk will continue to provide complete nutrition because of its rich content of macronutrients.

People often say, “Consume milk, and you’ll be healthy,” and there’s a solid reason: “Milk is very beneficial to human health.”

What are the advantages of milk in terms of nutrition?

The mammary glands are responsible for the production of milk, which is a nutrient-rich and multi-component fluid that is used for the purpose of feeding infants.

The following nutrients contribute to its composition:

  • Macronutrients (calcium, magnesium, fluorine, sodium, phosphorus, iodine, chlorine, sulfur etc.)
  • Micronutrients (iron, copper, zinc, iodine, fluorine, silicon, chromium etc.)
  • Fats
  • Proteins (albumin, globulin)
  • Vitamins (A, D, E, K, B-complex)
  • Enzymes (lactose)
  • Bacteria

The amount of the milk’s many different components is strongly influenced by its source, which in turn has a direct effect on how those components are distributed throughout the milk.


In this brief article, we answered the question “When milk sours is it a chemical change?”. We also discussed the nutritional benefits of milk and talked about how we can ensure that the milk stays fresh.




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Hi, I am Charlotte, I love cooking and in my previous life, I was a chef. I bring some of my experience to the recipes on this hub and answer your food questions.