Explain the different principles of testing sugar with the Phenol method.
The method is based on the interaction of the decomposition products of sugars in a strongly acidic medium with phenol, as a result of which condensation products are formed, colored in a yellow-orange color. The intensity of the color within certain concentration limits is proportional to the amount of sugars in the solution. Under the influence of an acidic environment, when heated, sucrose and other oligosaccharides undergo hydrolysis with the formation of monosaccharides, which are then converted into furfural or oxymethyl furfural as a result of dehydration (splitting off water molecules).
Furfural and oxymethylfurfural then enter into condensation reactions with phenol, forming condensation products colored yellow-orange. The optical density of the resulting colored solution is determined on a photoelectric colorimeter or spectrophotometer.
For the reaction with phenol, a sugar solution is used, purified from amino acids, which can also interact with furfural and oxymethylfurfural to form melanoidins. Therefore, when separating sugars from plant samples, they are selectively extracted with ethyl alcohol, and then, after removing the alcohol by evaporation, an aqueous solution of sugars is obtained, which is then taken for colorimetric analysis. The calculation of the amount of sugars is based on comparing the optical density of the analyzed solution and a set of standard solutions with a known sugar concentration. This method is highly sensitive and allows you to determine the amount of sugars in the analyzed sample up to 10 μg.