How to Measure Sugar in Potatoes

Updated March 23, 2017

Most people think of sugar as the sweet, white, granulated ingredient added to desserts and cookies. Glucose, another form of sugar, is naturally present in fruits and vegetables. Glucose is an important source of energy to the body for muscles and other tissues and acts as the fuel for our brain cells. There are several methods used to test and measure the sugar content of food. The Benedict's reagent (or solution) method is a good way to measure the sugar content in potatoes and other foods.

Put on your protective safety goggles. Pour 4ml of the potato juice into the test tube. Potato juice can be collected from raw grated potatoes. Put the grated potatoes into a small sieve and press the potato into the sieve to release the juice.

Add 1ml of Benedict's reagent to the raw potato juice. This reagent is typically available at local pharmacies. The solution is a mixture of sodium citrate, copper sulphate and sodium carbonate.

Mix Benedict's reagent and the raw potato juice together. Thoroughly blend the two by whirling the tube or stirring with a glass rod.

Place the test tube in a container with boiling water high enough to cover the level of the solution in the test tube. A canning jar or Pyrex container can be used to hold the boiling water safely . Baby food jars are also good for 50 to 100ml.

Leave the test tube in the boiling water for 2 to 3 minutes. Return the hot tube to the test-tube rack using the test-tube holder.

Look for colour changes in the solution. The substance in your test tube will change colour if it contains glucose. Using a white paper as a background, hold the test tube to determine the colour.

A blue colour indicates no sugar.

A blue-green colour indicates a trace of sugar.

A green colour indicates a little sugar.

A yellow colour indicates some sugar.

An orange-red colour indicates a large amount of sugar.


Test tubes and beakers are typically quite inexpensive. They should be made of glass capable of being heated. Pyrex is a good choice. Test tubes and beakers can be purchased online, at college bookstores or stores that carry children's chemistry kits. After you complete your measurement, empty your test tube, clean the tube thoroughly and return it to its rack. For reference, 1ml equals 20 drops of liquid, and 480ml equals 1 pint.


Benedict's solution is poisonous. Keep it away from the eyes and mouth. You will be using boiling water. Be alert and cautious. Wear protective safety goggles. This is always an important step when working with chemicals of any kind. Wear appropriate clothing when conducting an experiment.

Things You'll Need

  • Benedict's reagent
  • Medicine dropper
  • Beaker, size 1000ml
  • Test tubes
  • Test tube holder and rack
  • Raw potato juice
  • Graduated cylinder, size 10ml or 100ml
  • Protective goggles
  • Lab apron or coat
  • Plastic gloves
  • Boiling water
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