1. Practical Biochemistry - Food Tests
Qualitative tests for Carbohydrates
Starch
Add potassium iodide (iodine) to the sample. If starch is present, the colour turns form yellow to blue-black.
Reducing Sugars – all Monosaccharides e.g. glucose
Add Benedict’s solution (alkaline copper sulphate) and heat to 80 degrees for 3 minutes. The colour changes from blue to orange-red. The orange red is a precipitate.
Non reducing sugars – sucrose
Perform the Benedict’s test above and obtain a negative result. Boil the sample with hydrochloric acid which hydrolyses the sucrose, breaking the glycosidic bond and splitting it into two Monosaccharides. Cool the solution and add sodium hydrogencarbonate. Perform the Benedict’s test again, and an orange-red precipitate will form if there was sucrose in the original sample.
Quantitative Tests using a colorimeter
The Benedict’s test reveals the presence of reducing sugars. The more reducing sugar there is present, the more precipitate will be formed and the more Benedict’s solution will be used up. As the amount of Benedict’s solution used up rises, more light will be transmitted; the readings therefore give a measure of the Benedict’s reaction. The sample is placed in a cuvette between a light source and a photoelectric cell. The display then shows the % transmission.
In order to quantify the amount of sugar present, a calibration curve must be made. This is done by carrying out the Benedict’s test on a number of known concentration solutions. Plot a graph of transmission against sugar concentration. Do the Benedict’s test on the unknown sample, and place it in the colorimeter. Read off the graph at the % transmission to find the concentration of the unknown solution.
Proteins - Add biuret reagent (sodium hydroxide and copper sulphate). The chemicals react with the peptide bonds and cause a colour change from blue to lilac.
Lipids – mix the sample with ethanol which dissolves any lipid present. Pour this into water and if lipid is present, a cloudy white emulsion will form at the top of the water as the lipid comes out of solution.