Biochemical Tests

• What is the test for starch?

  Crush/grind the seed/sample (if required) then add iodine solution to the sample.

  • Positive = Blue/black
  • Negative = No colour change
• What is the test for lipids?

  Crush/grind the seed/sample (if required), add ethanol then water to the sample then mix.

  • Positive = Milky white emulsion
  • Negative = Colourless
• What is the test for proteins?

  Crush/grind the seed/sample (if required), add Biuret solution and mix.

  • Positive = Purple
  • Negative = Blue
• All monosaccharides are reducing sugars
• What is the test for reducing sugars?

  Heat the solution with benedict's solution.

  • Positive = Brick red precipitate
  • Negative = No colour change
• The test for sugars is semi-quantitative, meaning that it can be used to estimate the approximate amount of sugar in a sample.

  • High in sugar = brick red
  • Medium levels of sugar = orange
  • Low levels of sugar = yellow
  • Very low levels of sugar = green
• What is a disadvantage of the test for sugars?

  It's a qualitative method.
• What is the test for non-reducing sugars?

  Heat the solution with benedict's, after achieving a negative result boil the sugar solution with an acid, neutralise the solution with an alkali then heat it again with benedict's.

  • Positive = Brick red precipitate
  • Negative = No colour change
• When testing for non-reducing sugars why do we boil the solution with an acid?

  This will hydrolyse any disaccharides that are present, hydrolysing them into monosaccharides.
• Why do we have to neutralise the solution boiled with an acid in the test for non-reducing sugars?

  Benedict's won't work in acidic conditions.
• To make the test for sugars quantitative we can do one of two things:

  • Measure and compare the mass
  • Use a colorimeter
• In the test for sugars how can we make the test more quantitative by measuring and comparing the mass?

  Filter the precipitate produced and then measure the mass and compare it. The greater the mass, the more sugar that's present.
• In the test for sugars how can we make the test more quantitative using colorimetry?

  Place the samples in a colorimeter and measure the light passing through the sample. The less light that passes through the more sugar.
• What is an advantage of quantitating the test for sugars?

  This standardises the method