Understanding Why Milk Becomes Sour in Summer

Milk becomes sour if left outside the refrigerator, especially in summer. This happens due to microbial activity. Bacteria naturally present in milk or introduced from the environment multiply rapidly at room temperature, and even faster in the warmth of summer.

These bacteria, primarily Lactic Acid Bacteria (LAB), feed on the sugar naturally found in milk. This sugar is called lactose.

The Role of Lactose to Lactic Acid Conversion

The main reason milk sours is a chemical change where lactose sugar is converted into lactic acid. This process is called fermentation and is carried out by the bacteria.

• Milk contains lactose, a disaccharide sugar.
• Lactic Acid Bacteria (like Lactobacillus species) consume lactose.
• Through metabolic processes, they convert lactose into lactic acid.
• Lactic acid is an organic acid that increases the acidity of the milk.
• The increased acidity lowers the pH of the milk, causing the milk proteins (caseins) to coagulate or curdle.
• Lactic acid also has a distinct sour taste.

In summer, the higher temperature provides an optimal environment for these bacteria to grow and carry out the conversion of lactose to lactic acid at a much faster rate compared to cooler temperatures or when milk is refrigerated.

Analyzing the Given Options

Let's look at why the other options are not the primary cause of milk souring:

• Its colour: The colour of milk (white) is due to the presence of casein proteins and dispersed fat globules, which scatter light. The colour itself has no direct impact on whether milk will sour.
• The breakdown of proteins: While bacterial action can eventually lead to the breakdown of proteins (proteolysis), the initial and primary cause of the sour taste and curdling associated with souring is the production of lactic acid from lactose fermentation. Protein breakdown contributes more to changes in texture and flavour in later stages of spoilage, but the sour taste is mainly from lactic acid.
• The calcium present in it: Calcium is an essential mineral in milk and plays roles in nutrition and milk structure (associated with casein micelles). However, calcium does not cause milk to become sour when left outside the refrigerator. The souring is a biological process driven by bacterial metabolism of sugar.

Therefore, the correct explanation for milk becoming sour, especially rapidly in summer, is the conversion of lactose sugar to lactic acid by bacteria.

Revision Table: Milk Souring Factors

Additional Information on Milk Preservation and Souring

Keeping milk in the refrigerator slows down the activity of Lactic Acid Bacteria significantly, which is why milk lasts longer when refrigerated. Pasteurization is a process where milk is heated to a specific temperature for a set time to kill most of the harmful bacteria and reduce the number of spoilage-causing microorganisms, extending its shelf life before souring occurs.

Some dairy products, like yogurt, are made by intentionally adding specific Lactic Acid Bacteria cultures to milk under controlled conditions to convert lactose into lactic acid. This controlled souring process creates the desired texture, flavour, and acidity of yogurt.