mod 2, food tests

Question 1

how do you test for the presence of carbohydrates (starch)- what’s a positive result

Answer 1

add iodine to food sample

positive result= will turn yellow-brown to blue black

Question 2

what type of sugars are all monosaccharides and some disaccharides

Answer 2

reducing sugars

Question 3

why are all monosaccharides known as reducing sugars

Answer 3

because they can reduce or give electrons to other molecules

Question 4

explain how you test for reducing sugars

Answer 4

• if you heat a reducing sugar with benedict’s solution there is a colour change from blue to green, yellow or brick red.
• benedicts solution contains Cu2+ ions which are reduced to Cu+ ions, forming an orange red copper oxide. This is called a precipitate as it comes out f solution and forms a solid suspended in the reaction mixture.

Question 5

benedict’s: what colour will the reaction mix appear if little precipitate is formed

Answer 5

green

Question 6

example of a non reducing sugar

Answer 6

sucrose

Question 7

how to test for a non reducing sugar

Answer 7

-first test for a reducing sugar
-take a separate sample and boil it with hydrochloric acid to hydrolyse the sucrose into glucose and fructose.
-cool the solution and use sodium hydrogen carbonate to neutralise it
test for reducing sugars::
a positive result= (green, yellow, orange-red) indicates a non reducing sugar was present in the test

Question 8

how do you test for lipids

Answer 8

the emulsion test:
-take a sample and mix it thoroughly with ethanol
-any lipid will go into solution in the ethanol
-filter
-pour the solution into water in a clean test tube
- a cloudy white emulsion indicates the presence of lipids
^^ This is made up of tiny lipid droplets that come out of solution when mixed with water

Question 9

testing for proteins

Answer 9

biurets test::
colour change from light blue to lilac
the test detects the presence of peptide bonds
-place a sample of food in spotting tile and add biurets solution

Question 10

what is the quantitative test for reducing sugars

Answer 10

The reducing sugar is first verified that it is a reducing sugar, it is placed in a beaker where benedicts solution is added and it is heated to 80(degrees/celsius) in a water bath. If a reducing sugar is present the solution will change colour from blue to orange-red, this solution is then placed in a curvette the colorimeter shines a beam of light and a photoelectric cell picks up the percentage transmission, this is then measure on a calibration curve with a set of known concentrations, the value can then be read off the curve for its percentage transmission.

Question 11

how do you quantitively test for reducing sugars

Answer 11

using a colorimeter

Question 12

what does a high reading of absorbance mean

Answer 12

• High reading for absorbance means lower conc. of reducing sugar.

Question 13

colorimeter, the more concentrated the colour is, the higher the…

Answer 13

absorbance

Question 14

what is paper chromatography used to identify?

Answer 14

unknown amino acids

Question 15

how do you calculate the Rf value:

Answer 15

distance travelled by solute/ distance travelled by solvent

Question 16

how do you use paper chromatography to find out the unknown amino acid?

Answer 16

• draw a pencil line near the bottom of chromatography paper, put a conc spot of amino acids.
• add an alcohol or water as the solvent-put in a fume cupboard
• different amino acids will spread out a different rates
• when solvent is nearly at the top take it out and mark the solvent front
• use rf values to identify separated molecules

Question 17

what can biosensors detect

Answer 17

• uses a biological molecule to detect a chemical.
• the biological molecule produces a signal which is converted to an electrical signal by a transducer (part of the biosensor)
• the electrical signal is processed to work out other info

Question 18

whats the mobile phase

Answer 18

-where molecules can move. e.g the liquid solvent

Question 19

whats the stationary phase

Answer 19

-where molecules cant move e.g the paper