Food tests & Colorimeter

Question 1

What is the benedict`s test used for

Answer 1

To test whether a substance is a reducing sugar or non-reducing sugar

Question 2

Describe how to carry out a test for a student that wants to test whether a substance is a reducing-sugar or non-reducing sugar

Answer 2

1. ) Place the samples to be tested in a boiling tube - if it is not in liquid form grind it up or blend it in water
2. ) Add an equal amount of Benedict’s reagent
3. ) Heat the mixture gently in a water bath for five minutes
4. ) If the solution remains blue then the sugar is non reducing, if the solution turns brick red there is a high concentration of reducing sugar - note that this is a qualitative test so you can get a colour of yellow which would be moderately positive for example

Question 3

What is the colour change for benedict’s test

Answer 3

• From Blue to Brick red, varying colours in this range

Question 4

Why is there a colour change in benedict`s test when reducing sugars are present and what is this

Answer 4

• When benedicts solution is exposed to reducing sugar its blue Cu2+ ions are reduced to brick red Cu+ ions
• The more reduced sugar the more blue Cu2+ ions change colour which is why there is a gradient according to the concentration of copper ions

Question 5

What happens when non-reducing sugars are boiled with hydrochloric acid and then the benedict`s test is completed on them

Answer 5

They will become reduced into their reducing sugar counterparts since they have been hydrolised by the acid and will therefore give a positive result

Question 6

Why might you employ an iodine test

Answer 6

• To check for the presence of Starch

Question 7

How is an iodine test carried out

Answer 7

• Drops of iodine are dissolved into potassium iodide solution
• This solution is then mixed with the solution that is to be tested
• If the solution changes colour from yellow/brown to purple/black, starch is present in the sample

Question 8

What is the test and colour change when looking for the presence of starch in a sample

Answer 8

• Iodine test

- Yellow//brown to purple/black

Question 9

What are reagent test strips for reducing sugars and how can they be beneficial

Answer 9

Reagent test strips are test strips for reducing sugars (most commonly glucose)
They come with a colour coded chart which allows you to determine the concentration of the solution

Question 10

What is a lipid emulsion test and how do you carry it out

Answer 10

• Used to test for the presence of lipids
• Mix the test sample with ethanol and then mix the resulting solution with water and shake it
• If a white emulsion forms as a white layer on top of the solution - lipid is present
• If the solution remains clear - there is no lipid

Question 11

How do you know if lipid is present in a substance

Answer 11

• Carry out lipid emulsion test

- A white emulsion layer should form on top of the solution

Question 12

How can you quantitively determine the glucose concentration

Answer 12

• Carry out a benedict`s test on the selected solutions
• Prepare the precipitate of the solutions from the solutions reacted with the benedict`s reagent
• Calibrate the colorimeter using distilled water
• Place the correct coloured filter (Mcalonan will accept just that)
• Place the solution in the colorimeter and use it
• Read the transmission % of each of the solutions
• The lower the transmission % of the solution the higher the concentration of the precipitate
• Using this information you can plot a calibration curve

Question 13

What is a biuret test used for

Answer 13

• To test for the presence of protein

Question 14

How do you carry out a biuret test

Answer 14

• Take 2ml of the solution to be tested in a test tube
• Mix the sample with 2ml of 5% sodium hydroxide solution
• Add two drops of 1% copper sulphate solution
• If the solution turns blue, violet or purple, proteins are present, if not then there are no proteins