biochemical tests

Question 1

What is a reducing and non reducing sugar
Examples

Answer 1

reducing sugars - They can donate electrons and becomes oxidised and become the reducing agent
examples : glucose, fructose and galactose

non- reducing sugars - Cannot donate electrons
example: sucrose

Question 2

How do you test for starch

Answer 2

Add a few drops of potassium iodide solution to the sample (orange/brown)
If starch is present iodine ions will react with the centre of the starch creating a blue/black colour.

Question 3

what is a serial dilution and how to you do it

Answer 3

Taking a series of solutions of a stock solution
- concentration decreases by the same every time
- can be doubling dilutions where the concentration is halved between each test tube.

Question 4

How do you test for lipids in solids

Answer 4

• Solid = grounded up food in a test tube and add ethanol (2cm^3) then shake to mix then add distilled water (2cm^3) then shake again to mix. If becomes milky/emulsion it is a positive test
• Liquid = add ethanol (2cm^3) along with distilled water (2cm^3) then shake to mix. If the solution turns milky/white emulsion it is positive

Question 5

What is the test for reducing sugars and non reducing sugars

Answer 5

Reducing sugars - Add benedict’s reagent
- heat in a water bath above 80 degrees
Non reducing sugars - Add dilute hydrochloric acid and heat in a water bath (break bonds and produce monosaccharides)
- Add an alkali
- Add excess benedict’s solution
- heat in a water bath above 80 degrees

if positive test the solution will turn from pale blue to brick red

Question 6

how can the test for reducing sugars be quantitive?

Answer 6

• Filter the solution and weight the participate
• Remove the precipitate and use a colourimeter to measure the absorbance of the remaining benedict’s solution

Question 7

How do you test for proteins

Answer 7

• Add biurets reagent and shake
• positive test will turn from light blue to lilac/purple

you make biurets by adding dilute copper sulphate and a alkali such as sodium hydroxide

Question 8

what is the test for lipids

Answer 8

Add 2cm^3 of ethanol and distilled water
- shake

if lipids are present they will become milky and form a white emulsion

Question 9

What are the positive test results for each test

Answer 9

• lipids = white emulsion / milky
• protein = light blue to lilac
• starch = brown to blue/black
• sugars = blue to brick red