Chapter 2: Problem 12

A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?

Short Answer

Expert verified

(a) To prevent milk from souring quickly. (b) Alkaline conditions inhibit lactic acid bacteria, slowing curd setting.

Step by step solution

Understanding pH Levels

pH measures the acidity or basicity of a solution. Fresh milk has a pH of around 6, which is slightly acidic.

Effect of Baking Soda on pH

Baking soda is a basic substance. When added to milk, it neutralizes some of the acids present, thus increasing the pH level and making the milk slightly alkaline.

Reason for Shifting pH

By increasing the pH to slightly alkaline, the milkman prevents the milk from turning sour quickly. Bacteria that cause milk to sour thrive in acidic conditions. Alkaline conditions slow down their growth.

Setting Curd - The Role of Acidity

Curd formation occurs due to the action of lactic acid bacteria, which convert lactose in milk to lactic acid, making the milk acidic. This increased acidity causes proteins (casein) to coagulate, forming curd.

Delayed Curd Formation

In slightly alkaline conditions, the growth of lactic acid bacteria is inhibited, leading to a slower acidification process. Because acid formation is slowed down, the proteins take longer to coagulate, delaying the setting of curd.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

pH Levels

pH is a scale used to measure how acidic or basic (alkaline) a substance is. This scale ranges from 0 to 14: where 0 is very acidic, 14 is very alkaline, and 7 is neutral. Fresh milk naturally has a pH of around 6, making it slightly acidic. Understanding the pH levels in substances like milk is crucial because it affects various chemical and biological processes. When the pH of milk changes, it can influence how certain bacteria grow and how proteins behave. This is why even a small shift in pH can have significant effects on milk and dairy products.

Baking Soda Effect

Baking soda, or sodium bicarbonate, is a basic (alkaline) substance. When added to milk, it reacts with the acids in the milk, neutralizing them. This reaction shifts the pH of the milk from slightly acidic (around 6) to slightly alkaline. The change in pH due to baking soda has practical applications. In the context of the exercise, the milkman uses baking soda to make fresh milk last longer by slowing down the activity of bacteria that thrive in acidic environments. By making the milk slightly alkaline, baking soda helps to maintain the milk's freshness for a longer time.

Curd Formation Process

Curd formation is a process that converts milk into a thicker, more solid state, often enjoyed as yogurt or soft cheese. The key player in this transformation is lactic acid bacteria. These bacteria consume lactose (milk sugar) and produce lactic acid as a byproduct. The increase in lactic acid lowers the pH of the milk, making it more acidic. As the milk becomes more acidic, the proteins (mainly casein) start to coagulate, or clump together, forming curd. This process is what turns liquid milk into a solid-like curd, creating a tangy taste and thick texture.

Lactic Acid Bacteria

Lactic acid bacteria are the microorganisms responsible for souring milk and forming curd. They thrive in warm, acidic environments, converting lactose into lactic acid. The lactic acid they produce creates the acidic conditions needed for curd formation. However, these bacteria do not grow well in alkaline environments. When the pH of milk is shifted to be more alkaline, the activity and growth of lactic acid bacteria are inhibited. This slower bacterial activity delays the production of lactic acid, which in turn delays the curd formation process.

Milk Acidity

Milk is naturally slightly acidic with a pH around 6, due to its various components, including lactic acid. When milk becomes more acidic, it can lead to souring, as the environment becomes ideal for lactic acid bacteria to thrive. These bacteria accelerate the production of more lactic acid, further lowering the pH and encouraging processes like curd formation. Maintaining the acidity of milk is essential for controlling its freshness and the rate at which it turns into curd. By controlling the pH, such as through the addition of baking soda, the souring and curd formation processes can be slowed down, extending the milk's shelf life.

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A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?

Short Answer

Step by step solution

Understanding pH Levels

Effect of Baking Soda on pH

Reason for Shifting pH

Setting Curd - The Role of Acidity

Delayed Curd Formation

Key Concepts

pH Levels

Baking Soda Effect

Curd Formation Process

Lactic Acid Bacteria

Milk Acidity

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