Understanding Curd Formation and Bacteria

Curd, also known as yogurt in many parts of the world, is a popular dairy product formed through a process called fermentation. This process relies heavily on the action of specific types of bacteria that convert the lactose (milk sugar) present in milk into lactic acid.

The Role of Lactic Acid Bacteria (LAB)

A group of bacteria collectively known as Lactic Acid Bacteria (LAB) are primarily responsible for the fermentation that creates curd. These bacteria produce lactic acid as a major metabolic end product from the fermentation of carbohydrates like lactose.

The accumulation of lactic acid causes the pH of the milk to drop. This acidic environment leads to the coagulation or thickening of milk proteins (casein), resulting in the characteristic texture of curd.

Key Bacteria in Curd Formation

While many different species of LAB can contribute to fermentation, certain species are commonly used in the production of curd and yogurt. Prominent among these are species belonging to the genus Lactobacillus and Streptococcus.

• Lactobacillus: Various species like Lactobacillus bulgaricus and Lactobacillus acidophilus are widely used. They are thermophilic, meaning they grow well at higher temperatures typically used for fermentation (around 40-45°C).
• Streptococcus: Streptococcus thermophilus is another common bacterium used in combination with Lactobacillus species. It is also thermophilic and grows alongside Lactobacillus.

The combination of these bacteria contributes to both the texture and the distinct flavour profile of the curd.

Analyzing the Options

Let's look at the given options in the context of curd formation bacteria:

• Bifidobacteria: These are beneficial probiotic bacteria often found in the gut and added to some fermented dairy products, but they are not the primary bacteria used for the fundamental fermentation that turns milk into curd.
• Lactic acid: Lactic acid is the *product* of the fermentation process, not the bacterium itself. Bacteria *produce* lactic acid.
• Lactobacillus acidophilus: This is a well-known species of Lactobacillus bacteria frequently used in the production of curd and other fermented milk products. It contributes to the fermentation process and the resulting texture and acidity.
• Bacillus coagulans: This is a spore-forming bacterium that is also a lactic acid producer and is often used as a probiotic. While it produces lactic acid, it is less commonly associated with traditional curd or yogurt fermentation compared to species like Lactobacillus bulgaricus or Streptococcus thermophilus, or even Lactobacillus acidophilus which is a direct option.

Conclusion

Based on the function and common usage in curd production, Lactobacillus acidophilus is a type of bacteria directly involved in forming curd through lactic acid fermentation.

Revision Table: Bacteria and Curd Formation

Additional Information on Lactic Acid Fermentation

Lactic acid fermentation is an anaerobic metabolic process where glucose and other six-carbon sugars are converted into cellular energy and lactate (the ionized form of lactic acid).

In the context of milk, the bacteria consume lactose, convert it to glucose and galactose, and then ferment these sugars to produce primarily lactic acid. This increase in acidity preserves the milk, inhibits the growth of spoilage bacteria, and changes the physical properties of the milk, leading to curd formation.

Different bacterial cultures and fermentation conditions can result in different types of fermented milk products, each with unique textures, flavours, and health benefits.