Food tests

• What are carbohydrates, lipids and proteins all examples of?

  They are all biological molecules.
• How do you test for sugars?


  Benedict's test.
• What are the steps to the Benedict test and their results?

  Add Benedict's regent (blue) to a sample and heat it in a water bath at 75C.

  Positive result: coloured precipitate (green/orange/ brick red)

  Negative result: stays the same (blue)
• How do you test for starch?

  Iodine test.
• What are the steps to the iodine test and their results?

  Add iodine solution to the sample.

  Positive result: dark blue/black colour.

  Negative result: browny-orange.
• How do you test for lipids?

  Emulsion test.
• What are the steps to the emulsion test and their results?

  Shake the test substance with ethanol for about a minute until it dissolved, then pour the solution into water.

  Positive result: Milky/cloudy emulsion

  Negative result: Clear
• How do you test for proteins?

  Biuret test.
• What are the steps to the biuret test and their results?

  Add a few drops of potassium hydroxide solution to make the solution alkaline. Then add some copper sulfate solution (blue).

  Positive result: Purple

  Negative result: Blue
• What is calorimetry?

  Food can be burnt to see how much energy it contains.
• What is the experiment for calorimetry?

  1)You need a food that'll burn easily e.g dried beans or pasta.

  2)Weigh small amount of food and skewer it onto a mounted needle.

  3)Add set volume of water to a boiling tube which will measure the amount of energy when food is burnt.

  4)Measure temperature of water, then set fire to food.

  5)Hold the burning food under boiling tube until it goes out and repeat this until food won't catch fire. Then measure temperature of water again
• How do you calculate the amount of energy in food?

  Energy in food (J) = Mass of water (g) x temperature change in water (C) x 4.2
• How do you work out how many joules are in each gram of the food?

  Energy per gram of food (J/g) = Energy in food (J) divided by mass of food (g)