Chemical Test
To test for the presence of monosaccharides and reducing disaccharide sugars in food, the food sample is dissolved in water, and a small amount of Benedict's reagent is added. During a water bath, which is usually 4–10 minutes, the solution should progress in the colors of blue (with no glucose present), green, yellow, orange, red, and then brick red or brown (with high glucose present).A colour change would signify the presence of glucose. The common disaccharides lactose and maltose are directly detected by Benedict's reagent, because each contains a glucose with a free reducing aldehyde moiety, after isomerization.
Sucrose (table sugar) contains two sugars (fructose and glucose) joined by their glycosidic bond in such a way as to prevent the glucose isomerizing to aldehyde, or the fructose to alpha-hydroxy-ketone form. Sucrose is thus a non-reducing sugar which does not react with Benedict's reagent. Sucrose indirectly produces a positive result with Benedict's reagent if heated with dilute hydrochloric acid prior to the test, although after this treatment it is no longer sucrose. The acidic conditions and heat break the glycosidic bond in sucrose through hydrolysis. The products of sucrose decomposition are glucose and fructose, both of which can be detected by Benedict's reagent, as described above.
Starches do not react or react very poorly with Benedict's reagent, due to the relatively small number of reducing sugar moieties, which occur only at the ends of carbohydrate chains. Inositol (myo-inositol) is another carbohydrate which produces a negative test.
Benedict's reagent can be used to test for the presence of glucose in urine. Glucose found to be present in urine is an indication of diabetes mellitus. Once a reducing sugar is detected in urine, further tests have to be undergone in order to ascertain which sugar is present. Only glucose is indicative of diabetes.
| Experiment | Observation | Inference |
|---|---|---|
| Substance in water + 3ml Benedict's solution, then boil for few minutes and allow to cool. | Red ppt or green ppt or yellow ppt obtained | Reducing sugar e.g. Glucose is present |
| Substance in water + 3ml Benedict's solution, then boil for few minutes and allow to cool. | Solution remains clear or is a little blue | Reducing sugar is not present |
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