food tests and calibration curves

no calibration curve detail - watch primrose kitten video because i can't explain it to save my life

how can you test for reducing sugars?

(typical) Benedict’s test:

• place 2 cm³ of liquid testing substance in a boiling tube (if not liquid, first crush w/ pestle and mortar and add distilled water, then use filter paper and funnel to create a filtrate)

• add 10 drops of Benedict’s

• place in a boiling water bath for 3-5 mins

• if positive - blue → green/yellow/orange/brick red precipitate (depending on conc of reducing sugar)

how can you test for non reducing sugars?

(altered) Benedict’s test:

• boil in diute HCl (to hydrolyse the non-reducing sugar)

• add sodium hydrogen carbonate (to neutralise solution)

• place 2 cm³ of liquid testing substance in a boiling tube (if not liquid, first crush w/ pestle and mortar and add distilled water, then use filter paper and funnel to create a filtrate)

• add 10 drops of Benedict’s

• place in a boiling water bath for 3-5 mins

• if positive - blue → green/yellow/orange/brick red precipitate (depending on conc of non reducing sugar)

name the reducing sugars:

• (all monosaccharides)

  • glucose

  • galactose

  • fructose

• maltose

• lactose

name a non reducing sugar:

lactose

what is the difference between a reducing and a non reducing sugar?

reducing sugars act as reducing agents in chemical reactions, whereas non reduicng sugars do not

how can you test for starch?

iodine test:

• place 3 cm³ of liquid testing substance in a boiling tube (if not liquid, first crush w/ pestle and mortar and add distilled water, then use filter paper and funnel to create a filtrate)

• add 1 cm³ of iodine solution (containing iodine and potassium iodide)

• if positive - orange → blue black

how can you test for lipids?

emulsion test:

• place 3 cm³ of liquid testing substance in a boiling tube (if not liquid, first crush w/ pestle and mortar and add distilled water, then leave mixture for a while to allow particles to settle)

• add 3 cm³ of ethanol

• add 3cm³ of water

• shake solution

• if positive - clear → milky white emulsion

why do we not filter the lipid solution when testing for lipids?

lipids may stick to filter paper

how can you test for proteins?

biuret test:

• place 3 cm³ of liquid testing substance in a boiling tube (if not liquid, first crush w/ pestle and mortar and add distilled water, then use filter paper and funnel to create a filtrate)

• add 3 cm³ of dilute NaOH solution and mix

• add 10 drops of dilute copper (II) sulfate solution and mix

• (this is biuret solution - may be premixed)

• if positive - blue → violet

why would you get a negative result in the biuret test for a solution w/ just amino acids (and not proteins)?

tests for peptide bonds (which would not be present)

what is a calibration curve used for?

to demonstrate the conc of a substance in an unknown sample

creatinine detecting solution reacts w/ creatinine to produce an orange colour - how could you produce a calibration curve for creatinine?

• use distilled water and creatinine solutio to produce a serial dilution

• add creatinine detecting solution to each soltuion

• use a known/specified/constant vol of a soltuion (e.g. diluted creatinine solution)

• record absorbance/transmission of solution(s)using a colourimeter

• plot dilution/conc of creatinine solution against absorbance/transmission

summarise how a colourimeter works

• colourimeter detects how much light is absorbed by solution (how much of the light that is travelling through the solution is being stopped by the solution)

• the more concentrated the solution, the more light it will absorb

• we can use this to determine to conc of a solution