Food tests

• proteins

  biuret added to sample in a solution

  positive -> purple
• reducing sugars

  heat benedicts reagent with a solution of the sample

  forms a red/orange precipitate
• non-reducing sugars

  sucrose

  heat benedicts with a solution of the sample

  if remains colourless, heat with hcl to reduce the sugar

  add NaCO3 -> neutralize the acid

  re-add benedicts reagent and heat

  red/orange precipitate should form
• why is sucrose a non-reducing sugar

  lacks a free ketone or aldehyde group that can reduce copper (2) sulfate to copper (1) oxide
• starch

  add iodine

  positive: blue/black
• emulsion test

  dissolve in ethanol

  add water

  shake

  milky white solution formed
• why do you dissolve in ethanol before water

  lipids are non-polar and so they readily dissolve in ethanol and not water