Qualitative tests for food Flashcards

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1
Q

Explain the steps of the experiment to test for reducing sugars (6)

A
  1. dissolve glucose in water in test tube.
  2. Add an equal amount of benedicts solution.
  3. In a 2nd test rube mix equal volumes of water and benedicts solution. This is the control
  4. Heat both test tubes in a boiling water bath
  5. If reducing sugar is present, the solution turns brick red
  6. If it isn’t present the solution remains blue
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2
Q

What colour is bendict’s solution and what colour does it turn
In the presence of what does this colour change happen?

A

Blue to brick red when reducing sugar is present

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3
Q

Explain the test for starch (4)

A
  1. Add a few drops of iodine solution to some starch dissolved in water.
  2. Add a few drops of iodine solution to some water. This acts as the control.
  3. If starch is present the colour turns from red-yellow to blue-black or purple.
  4. If it is absent the solution stays red-yellow
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4
Q

What colour does iodine change from and to?

In the presence of what does this happen?

A

red-yellow to blue-black or purple in the presence of starch

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5
Q

Explain the test for lipids (6)

A
  1. label a piece of brown paper as lipid.
  2. Rub a small piece of butter or cooking oil on the paper.
  3. Repeat process using a few drops of water on a piece of paper labeled water. This acts as a control.
  4. Leave the 2 pieces of paper over a radiator to dry
  5. Lipid produces a permanent stain or translucent spot on the paper
  6. The water stain dries out
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6
Q

Explain the test for protein (6)

A
  1. Dissolve a smaple of soluble protein (egg white or milk) in water.
  2. Add sodium hydroxide (colourless) until the solution clears.
  3. Then add a few drops of dilute copper sulfate (blue).
  4. As a control, add sodium hydroxide and copper sulfate ( or biuret solution) to water.
  5. The appearance of a purple-violet colour shows that proteins are present.
  6. If protein isn’t present the colour remains blue.
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7
Q

What is a negative result and positive result of a protein test using biuret solution?

A

A negative result is blue and a positive result is purple-violet

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