grade EXPERIMENT 40 Pre-Lab Questions 1. When milk gets sour, it curdles. What a

Question

grade EXPERIMENT 40 Pre-Lab Questions 1. When milk gets sour, it curdles. What are the "curdles?"What brings about the curdling of the milk? 2. Fat is removed by both an ethanol wash and a diethyl ether-ethanol wash. Why are these solvents effective in removing the fat from the casein? 3. The skin on your fingers can get stained in two of the tests carried out in this experiment. Why will ninhydrin cause blue-purple stains? a. b. Why will concentrated nitric acid cause yellow stains? 4. Why do heavy metals, like Hg and Pb?, cause poisoning? How does an antidote, like milk or raw egg white, work? 483

Explanation / Answer

Ans. #1. Milk gets sour due to bacterial fermentation in it. During the process, the pH of milks is reduced (becomes acidic). The increase in [H+] (from fermentation) induces denaturation of proteins in milk. The denaturation of protein further leads to “curding” of milk.

#2. Fat are soluble in ethanol, diethyl ether and other organic solvents. When casein protein is mixed well with ethanol (diethyl ether) in a separatory funnel, the fat is solvated in ethanol whereas casein proteins remains un-solvated. The fat-free casein can be obtained by decanting off the liquid ethanol-fat solution.

#3. A. Ninhydrin reacts with amino acids to form blue-purple colored complex. Out skin is made up of cells, which contain abundance of keratin protein besides other protein. Protein are polymer of amino acids.

So, the amino acids in the proteins in skin also react with ninhydrid and stain it blue-purple.

#3. B. The aromatic amino acids in protein react with concentrated HNO3 to form yellow-colored nitrate derivative of the aromatic rings.

Since skin also contains protein (including aromatic amino acids in them), it is stained yellow with concentrated nitric acid.

#4. He heavy metals like Hg2+ and Pb2+ exhibits wide mechanisms of toxicities including protein denaturation, adverse effect on serval enzymes and, in turn, the metabolic pathways associated with those enzymes.

Milk and raw egg white are rich in protein. The proteins act as chelating agent that bind to Hg2+ and Pb2+. Chelated Hg2+ and Pb2+ are unable to exert their toxicity because they are not free to react with any other molecule in the body.

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