Question

1. You are a food scientist trying to determine the nutritional content of certain food samples. Foor food sample D you preform a Benedict test to see if it contains any monosaccharides. For this test, you take 3 droplets of food sample D and mix it with 2 droplets of the Benedict test solution. The solution is heated for 5 minutes and you observe the color of the solution changing from blue to green to orange. A color change from blue to orange means that sample D does in fact contain monosaccharides as the solution color changed from blue to green to orange (an orange color = presence of monosaccharides). Disaccharides such as sucrose (table sugar) and lactose (milk sugar) are more complex than monosaccharides. The Benedict test does not detect disaccharides directly - answer the questions below:

a. Circle the name of the functional group involved in the Benedict test is it a(n): hydroxyl/carbonyl/carboxyl. (Circle and explain)

b. Does the Benedict test specifically indicates the presence of an aldose or ketose sugar. (Circle which one and explain)

c. If a sample tested with the Benedict test turns green but not orange, what can be concluded?

d. The Benedict test heats the sample, but only for 5 minutes. Suggest a reason why heating for longer could result in a false-positive test.

Answer 1

a. Benedicts test is for reducing sugars. the reducing sugars contain free functional group in their terminal carbon that can easily get involved in the reaction. The fuctional group involved in benedicts test are the carbonyl group. the sugar containing free carbonyl group in their terminal carbon will give benedicts positive.

b. the benedicts test specifically indicates the aldose sugar, as in aldose sugar, the terminal carbon contain the carbonyl group(aldehyde group).

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c. any color change from the original color( blue) of the reagent signifies that the test is positive. according to the concentration of the sugar present in the test sample, the color will come.

blue(no reducing sugar)----- green(traces of reducing sugar)------ orange ( moderate amount of reducin sugar)------ brick red(high amount of reducing sugar in the sample).

d. Gentle heating with time allows the ether linkages between the rings to undergo a hydration reaction, unlock, open and then give a positive test. Also many ketose sugars tautomerize into aldose sugar, that will give false positive benedicts test.