Isoelectric Precipitation of Proteins - Casein from Milk - Amrita University

Summary

This video explains the isoelectric precipitation of casein from milk, a process used to isolate milk proteins. Casein, the dominant protein in milk, is extracted by adjusting the pH to its isoelectric point (around 4.6-4.8) using hydrochloric acid. The video outlines the materials and apparatus required, including milk, distilled water, 0.2N HCl, a magnetic stirrer, and a pH meter. Step-by-step instructions guide viewers through centrifuging the milk, adding HCl, precipitating casein, and filtering it. The casein is then washed, dried, and weighed, with a final expected yield of 3.5 grams.

Takeaways

• 🥛 Milk contains various proteins, with casein being the dominant type.
• 🧪 Casein gives milk its white, opaque appearance by forming micelles with calcium and phosphorus.
• ⚖️ The isoelectric point (I.E.P.) of a protein is the pH at which the protein is electrically neutral.
• 🌡️ At its isoelectric point, casein becomes insoluble and can be precipitated from milk.
• 🧫 To isolate casein, the pH of milk is adjusted to 4.5-4.8 using dilute hydrochloric acid.
• 🔬 Centrifugation at 4000 rpm for 20-25 minutes is used to skim the milk, separating the cream/fat.
• 💧 Hydrochloric acid is added drop by drop while stirring, and casein starts to precipitate at a pH of 4.8 to 4.6.
• 📉 The precipitated casein is filtered, washed with distilled water, ethanol, and diethyl ether, then dried.
• ⚖️ The final weight of the dried casein should be 3.5 grams.
• 🔬 Equipment used includes a pH meter, burette, magnetic stirrer, and centrifuge.

Q & A

What are the main types of proteins found in milk?

Milk contains various types of proteins, including caseins, whey proteins, and minor proteins.

What role does casein play in milk?

Casein is the dominant class of proteins in milk, responsible for its white, opaque appearance by forming clusters with calcium and phosphorus called micelles.

What is the isoelectric point (I.E.P.) of a protein?

The isoelectric point (I.E.P.) is the pH at which a protein molecule has no net electrical charge, meaning it is electrically neutral.

Why is casein isolated by adjusting the pH of milk?

Casein is isolated by adjusting the pH of milk to its isoelectric point (4.5-4.8) because most proteins show minimum solubility at this point, causing the casein to precipitate.

What is the purpose of adding hydrochloric acid (HCl) to milk during the casein precipitation process?

Hydrochloric acid (HCl) is added drop by drop to reduce the pH of milk until the isoelectric point of casein is reached, causing it to precipitate.

What are the key steps in preparing skimmed milk for the casein isolation procedure?

The milk is first centrifuged at 4000 rpm for 20-25 minutes at room temperature to separate the cream/fat, which is then removed to obtain skimmed milk.

What is the initial pH of milk before the precipitation process begins?

The initial pH of milk is around 6.6 before the addition of hydrochloric acid.

At what pH does casein start precipitating during the process?

Casein starts precipitating at a pH between 4.8 and 4.6.

What is done after casein precipitates and settles in the beaker?

Once casein precipitates, it is filtered out using a funnel and filter paper, then washed with distilled water, ethanol, and diethyl ether.

What is the expected weight of the dried casein after filtration and drying?

The expected weight of the dried casein is approximately 3.5 grams.