food tests Flashcards
glucose test
on a spot tile
• spot A: glucose solution
• spot B: distilled water
using Diastix test paper
colour change from blue to brown in the presence of glucose
starch test
on a spot tile:
• spot C: starch solution
• spot D: distilled water
using iodine solution (take 1 drop of iodine solution and add it to the solution)
colour change from reddish-brown to blue-black in the presence of starch
protein test
on a spot tile
• spot E: protein solution
• spot F: distilled water
using Abustix test paper
colour change from yellow to green in the presence of protein
lipids test
using filter paper
procedure:
add one drop of water and another drop of lipids/oil onto the filter paper
allow the paper to dry for a period of time
observe under a light source (bench lamp)
immerse in organic solvent (eg. ether)
observe under light source again
2 observations:
• translucent spot of oil/lipids is permanent, while the translucent spot of water disappears over time
• when the paper is immersed in organic solvent the translucent oil spot disappears
what should we do before carrying out the protein and starch tests and why
for both tests the solution should be shaken in the reagent bottle before use (as both are insoluble in water => they tend to settle at the bottom of the bottle if left on the table for a long amount of time; shaking ensures the even distribution of starch and protein in the solution to get a valid result
vitamin c test
in a test tube with DCPIP solution
procedure: add the vitamin C solution drop by drop
gently shake the tube after the addition of each drop of vitamin C solution until DCPIP solution changes from blue to colourless (decolourises)
to measure the concentration of vitamin C you measure the amount of drops of vitamin C solution needed to decolourise the DCPIP solution
the more the amount of drops needed for decolorisation of the same volume of DCPIP solution => the lower the relative vitamin concentration is in the food extract
how to deal with solid food samples
cut the food sample into smaller pieces
grind the sample with a mortar and pestle with a fixed amount of distilled water added
squeeze the ground materials through several layers of pre-moistened muslin cloths and collect the extract in a beaker
iv + dv of DCPIP exp
iv: type of food sample
dv: relative vitamin C concentration (arbitrary unit)
potential errors of fruit DCPIP exp
potential errors:
• varying sizes of each drop
• judgement of complete decolorisation of DCPIP is subjective
• DCPIP solution turns pink due to the acids in the fruit
what may the potential errors in the fruit DCPIP exp lead to
relative vitamin C concentration will be over/underestimated —> reduce reliability + validity of results
DCPIP solution turns pink in acids (ie. does not completely decolorise), vitamin C concentration cannot be fairly compared —> reduce validity of results DCPIP solution
how to improve the fruit DCPIP exp?
can use syringe / pipette instead of dropper to transfer the fruit extract to the test tube w/ the DCPIP solution
experiment can be repeated using same extracts / extracts of different fruits of the same type (reduces effect of individual differences among fruits // reduces random errors)
cvs of DCPIP
volume of DCPIP solution, concentration of DCPIP solution, temperature of food samples, volume of distilled water used to prepare the food extracts and therefore the mass of the food sample used to make the extract
