1.5 — carbohydrates: tests Flashcards
what molecules are reducing sugars?
all monosaccharides and some disaccharides (such as maltose and lactose)
what molecules are non-reducing sugars?
some disaccharides (such as sucrose) and all polysaccharides
what are the steps to find out whether a sample contains a reducing sugar?
- add 2cm3 of food sample into a test tube
- add equal volume of benedict’s reagent
- heat in boiling water for 5 mins
- if positive, green —> brick red precipitate will form
what does a blue mixture after conducting the benedict’s test mean?
no reducing sugar present
what does a green mixture after conducting the benedict’s test mean?
low concentration of reducing sugar present
what does a orange mixture after conducting the benedict’s test mean?
medium concentration of reducing sugar present
what does a brick red mixture after conducting the benedict’s test mean?
a high concentration of reducing sugar present
what are the methods of determining the concentration of reducing sugars?
(after conducting the benedict’s test)
- use a colorimeter to measure absorbanvce of each solution
- filter the solution and weight the precipitate
what must you do to test for non-reducing sugars?
hydrolyse them into their monosaccharide components
what are the steps to find out whether a sample contains a non-reducing sugar?
- first conduct the test for reducing sugars
- add equal parts food sample to dilute hydrochloric acid (HCl)
- heat mixture in a boiling water bath for 5 mins (the HCl hydrolyses the disaccharides into monosaccharides)
- re conduct the test for reducing sugars
- positive results are the same as for reducing sugars
what are the steps to find out whether a sample contains starch?
- place 2cm3 of food sample into a test tube
- add a few drops of iodine and shake
- if starch is present, the solution will turn from orange to blue-black
