���˿���

There are different tests which can be used to detect carbohydrates, proteins and lipids in foods.

Aims

To investigate the use of chemical reagents to identify starch, reducing sugars, proteins and fats.

Iodine test for starch

Method:

1. Place one spatula of the food sample on a dish or 1 cm³ if the sample is liquid.
2. Using a dropper, place a few drops of iodine solution onto the food.
3. Record any change in the colour of the solution.

Starch is detected using iodine solution. This turns blue-black in the presence of starch.

Benedict's test for reducing sugars

1. Place two spatulas of the food sample into a test tube or 1 cm³ if the sample is liquid. Add about 1 cm³ depth of water to the tube and stir to mix.
2. Add an equal volume of Benedict's solution and mix.
3. Place the tube in a water bath at about 95°C for a few minutes.
4. Record the colour of the solution.

Reducing sugars are detected using Benedict's solution.

Reducing sugars include:

• monosaccharides – such as glucose and fructose
• disaccharides – such as maltose.

Benedict's solution gradually turns from blue to cloudy orange or brick red when heated with a reducing sugar. Sucrose is a non-reducing sugar and does not react with Benedict's solution.

Biuret test for proteins

1. Place one-two spatulas of the food sample into a test tube or 1 cm³ if the sample is liquid. Add about 1 cm³ depth of water to the tube and stir to mix.
2. Add an equal volume of potassium hydroxide solution to the tube and stir.
3. Add two drops of copper sulfate solution and stir for two minutes.
4. Record the colour of the solution.

Proteins are detected using Biuret reagent . This turns a mauve or purple colour when mixed with protein.

Emulsion test for lipids

1. Place two spatulas of the food sample into a test tube or 1 cm³ if the sample is liquid.
2. Add 2 cm³ of ethanol to the tube. Cover the end of the tube and shake the tube vigorously.
3. Allow the contents to settle.
4. Pour the liquid from the top of the mixture into a test tube half-filled with water.
5. Record the level of the food and whether the water is cloudy or clear.

Lipids are detected using the emulsion test. This is what happens:

1. The test substance is mixed with 2 cm³ of ethanol.
2. An equal volume of distilled water is added.
3. A milky-whiteemulsion forms if the test substance contains lipids.

Risks

• Potassium hydroxide can be harmful to skin and eyes.
• Hot water can scald.

Control measures

• Wear eye protection.
• Do not taste any of the foods.
• Take care with hot water.
• Wash off any splashes immediately.

Results

Colour observed at end of the test:

Conclusions

The colour changes show that beef is the only food tested that contains protein. Beef, vegetable oil and breakfast cereal contain lipids. Both rice and breakfast cereal contain starch. Breakfast cereal is the only food tested that contains reducing sugars.