During my work in the laboratory, I noticed that fructose, starch and dextrin did not give positive results with their statements such as iodine and sulwanof when prepared 3 to 5 days ago. Why?
Kindly check https://alevelbiology.co.uk/notes/tests-for-carbohydrates/
Also check http://www.chem.boun.edu.tr/wp-content/uploads/2014/04/Chem-415-Experiment-1.pdf
Hello Asawer A.mhammed Alzayd
I think the reason starch gives a negative result after 3 days could not be far from the possibility that it hydrolyses into simple sugars, possibly due to the presence of activated amylase enzymes or other hydrolytic conditions. The amylose in starch is responsible for the formation of a deep blue colour in the presence of iodine. What the amylase enzyme (or any starch hydrolysis conditions) does is that it chops down the amylose polymer chain into its monomeric units (glucose). We know that iodine reacts with complex carbohydrates but does NOT react with glucose and other simple sugars, so if for some reason your starch breaks down over time into simple sugars, iodine will give you a negative result.
Kavnen Tseke how release the amylase enzyme from starch solution if the condition saved in dark
Hi Asawer A.mhammed Alzayd if you've got the amylase enzyme in the solution (which could come by chance from droplets of saliva) before saving it in the dark, it will still hydrolyse the starch. You may wish to inspect every stage of your preparation and check the 3 day starch solution if it tests positive for simple sugars. One thing to also consider is that the pH of the water used in preparing the starch solution may also facilitate hydrolysis even in the absence of amylase enzyme so you may wish to check that also. The mechanism for acid hydrolysis is that the O in the glycosidic linkage picks up a proton, then the bond breaks and water is added to the sugar. This could happen in the presence of water alone especially if the pH is somewhat acidic.
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