In this lab you will isolate starch from potatoes and investigate if different food samples contain starch. This is done with the help of Lugol’s solution (iodine/ potassium iodide solution).
Starch is an organic compounds that belongs to the carbohydrates. Carbohydrates are an energy storage for both plants and animals. Starch molecules are very long and the building blocks repeat themselves. They form long chains and belong to the so-called poly saccharides. The two building blocks of starch are amylose which forms spiral chains and amylopectin which forms branched chains. Both are built up from glucose rests which is why the chemical formula can generally be written as (C6H10O5)n. Starch can be found, e.g. in root crops and grains.
The presence of starch can be detected with with the help of Lugol’s solution which is a mixture of iodine and potassium iodide dissolve din water. Potassium iodide is added to increase iodine’s solubility in water. Iodine molecules (I2) are stored in the spiral chains of amylose when Lugol’s solution (brown solution due to the iodine) is added to the starch. This storage compound (”iodine starch”) causes a blue-black colour. A schematic of the compound is shown to the right in the figure above.
First, the potatoes need to be cleaned and peeled. Thereafter, they are grated and put into a bowl. 500 ml of water are added and the mixture is stirred thoroughly with a spoon for at least five minutes. A linen cloth is placed in a funnel and the mixture is pressed through into a large beaker (800 ml or 1000 ml). A part of the grated potatoes, for example the cellulose, will stay behind in the linen cloth. The liquid in the large beaker needs to stand and rest for approximately ve minutes. Then more water is added, approximately 100 – 200 ml. The finely dispersed, solid starch particles will slowly settle at the bottom of the beaker. Afterwards, the
water is decanted (= poured off). Then 100-200 ml of water are added again to
the large beaker and decanted when the solid starch particles have settled on the
bottom a second time. This cleaning step is repeated until the starch particles have
a completely white colour. Afterwards, the starch is dried in a at bowl in air and
at room temperature.
1. What is observed macroscopically when iodine is built into starch molecules? What happens when no starch is present?
2. Which function does starch have?
3. In which foods do you expect to detect starch? In which foods should there be no starch?
4. Do the results in the table match your expectations? If not, why could they be different?
