The Isolation and Detection of Starch – A Practical for Science Lessons

1 Goal

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).

2 Introduction

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.

3 Materials and Chemicals

  • Two Potatoes
  • Different other foods: Flour, bacon, cheese, apples, pasta and rice are recommended
  • Lugol’s solution (= iodine/potassium iodide solution)
    – Preparation: Dissolve 10 g potassium iodide in 100 ml of distilled water. Then slowly add 5 g iodine crystals, while shaking. Finally, lter and store in a tightly stoppered brown glass bottle.
  • Knife and Spoon
  • Several Test Tubes with Gummy Plugs
  • Mortar
  • One 250 ml-Beaker and one 800 ml- or 1000 ml-Beaker
  • Heating plate
  • Linen cloth
  • Funnel
  • Two Bowls
  • Grater

    4 Implementation

    4.1 The isolation of starch

    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.

    4.2 The detection of starch

    The foods are crushed in a mortar and and small amounts of each are put into their respective test tube. The test tubes are filled up to a third with water and shaken vigorously. In case not all the food particles are suspended, the test tubes are heated in a water bath (water bath = a 250 ml-beaker filled with water and the test tube inside is heated carefully on a heating plate, the test tube is afterwards cooled under owing, cold water). Then one drop of Lugol’s solution is added to each test tube and the test tubes are shaken with a gummy plug on top.

5 Questions for Discussion

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?

References

Image retrieved from: Petra Mischnick, Skolan för kemivetenskap , Kungliga Tekniska Högskolan, Stockholm, 11 January 2013 (https://www.kth.se/che/archive/arkiv/molnov-1.272910, 30 August 2017).

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