food tests Flashcards
what is the test for reducing sugars?
benedicts test
describe how to do the benedict’s test
- add blue benedicts reagent to a sample solution
- heat in a water bath
- brick red precipitate forms if reducing sugar is present
explain the reason for the colour change in the benedicts test
cooper sulphate has been reduced copper oxide (Cu2+ –> Cu+)
what is the method for testing non-reducing sugars?
- boil with dilute HCl
- sucrose is hydrolysed to fructose and glucose which will give a positive result in the benedicts test
what is the test for starch?
iodine test
describe the method for the iodine test
- add a few drops of iodine to the sample
- iodide ions react with the centre of starch molecules
- turns blue black in presence of starch
explain the colour change in the iodine test
starch has been digested by enzymes
what is the test for lipids
emulsion test
describe the emulsion test
- add ethanol to sample
- shake
- add cold water
- milky emulsion forms in presence of lipids
explain the precipitation formation in the emulsion test
lipids dissolve in non-polar substances but not polar substances (water)
what is the test for proteins
biuret test
describe the method for the biuret test
- add sodium or potassium to make the sample alkaline
- add a few drops to the sample
- turns lilac
describe how a biosensor is used
- blind taste test of a controlled volume of juice
- rank sweetness
- biosensor is used to find concentration
- match the concentrations and ranks
describe how a serial dilution is carried out
- 5 test tubes
- each contain a known concentration of solution
- equal volumes
- each tube has a concentration decrease of the same amount each time
- e.g. 1 = 10cm3 solution
2 = 8cm3 solution, 2cm3 water
3 = 6cm3 solution, 4cm3 water
describe how colorimetry is carried out
- create a serial dilution including a water control
- benedicts test
- put into cuvettes
- red filter on colorimeter
- measure absorbance
- draw a calibration curve
