<P> 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 reducing sugar present), green, yellow, orange, red, and then brick red precipitate or brown (with high reducing sugar present). A color change would signify the presence of a reducing sugar . 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 . </P> <P> 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 . </P> <P> 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 (myoinositol) is another carbohydrate which produces a negative test . </P> <P> Benedict's reagent can be used to test for the presence of glucose in urine . Glucose in urine is called glucosuria and can be indicative of diabetes mellitus, but the test is not recommended or used for the diagnosis of diabetes mellitus . A false positive reaction can be due to the presence of other reducing substances in urine such as ascorbic acid (during intake of vitamin supplements), drugs (levodopa, contrast used in radiological procedures) and homogentisic acid (alkaptonuria). </P>

Why do you not see false positives due to starch
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