Potatoes are underground organs made by the potato plant, Solanum tuberosum. Fig. 1.1 shows a potato plant. [Figure 1.1] Potatoes contain high quantities of starch and are a popular food. After harvesting, potatoes are stored. During storage, the starch content of potatoes gradually decreases due to the breakdown of starch molecules into glucose. A glucose assay using a Benedict's test and a colorimeter can be used to determine the concentration of glucose in potatoes. The glucose assay uses these steps: • The outer skin is removed from a potato. • The potato is cut into small pieces that are then ground into a pulp using a mortar and pestle. • The potato pulp is filtered to remove most of the solids and obtain potato juice. • Excess Benedict's solution is added to the potato juice. • The mixture is heated in a water-bath at 90°C for five minutes. During this five-minute period, copper ions (Cu2+) in the Benedict's solution react with glucose in the potato juice to form an insoluble precipitate. • The mixture is filtered to remove the precipitate. The filtrate, which is blue, contains copper ions that did not react with the glucose in the potato juice while the mixture was being heated. • A sample of the blue filtrate is transferred to a colorimeter tube (cuvette). • A colorimeter is used to measure the absorbance of the blue filtrate. • A calibration curve is used to determine the glucose concentration of the potato juice from the measurement of absorbance. Fig. 1.2 shows one type of colorimeter. [Figure 1.2] A clean colorimeter tube containing the sample to be tested is placed in the colorimeter. Light is passed through the sample and the absorbance of light by the sample is measured.