<P> The principle of Benedict's test is that when reducing sugars are heated in the presence of an alkali they are converted to powerful reducing species known as enediols . Enediols reduce the cupric compounds (Cu) present in the Benedict's reagent to cuprous compounds (Cu) which are precipitated as insoluble red copper (I) oxide (Cu O). </P> <P> The color of the obtained precipitate gives an idea about the quantity of sugar present in the solution, hence the test is semi-quantitative . A greenish precipitate indicates about 0.5 g% concentration; yellow precipitate indicates 1 g% concentration; orange indicates 1.5 g% and red indicates 2 g% or higher concentration . </P> <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), orange, yellow, green, 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>

What color is a negative benedict's test