<P> Generally, Benedict's test detects the presence of aldehydes and alpha - hydroxy - ketones, also by hemiacetal, including those that occur in certain ketoses . Thus, although the ketose fructose is not strictly a reducing sugar, it is an alpha - hydroxy - ketone, and gives a positive test because it is converted to the aldoses glucose and mannose by the base in the reagent . </P> <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>

Why benedict test is called semi quantitative test
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