3a: Nutrients (Food test)

Summary video of all 4 food tests

Note: If it is solid, need to chop into small pieces and add a few drops of water.

If it is powder, need to dissolve the powder in water before conducting the food test.

Lighting of bunsen burner

a. Benedict's test (reducing sugar)

- Positive test: The blue solution forms a yellow / orange / brick-red precipitate.

- Negative test: The solution remains blue.

Range of colours for Benedict's test

Colour changes with different amounts of reducing sugar

Blue solution to green precipitate: Traces of reducing sugar

Blue solution to orange precipitate: Moderate amount of reducing sugar

Blue solution to brick-red precipitate: large amount of reducing sugar

b. Biuret test (proteins)

1. Add 2cm3 of biuret reagent (pre-mix of sodium hydroxide + copper (II) sulfate solution)  to 2cm3 of sample solution in a test-tube.

2. Shake thoroughly.

- Positive test: The blue solution turns violet.

- Negative test: The solution remains blue.

c. Iodine test (starch)

- Positive test: The brown solution turns blue-black.

- Negative test: The solution remains brown.

d. Ethanol emulsion test (fats)

- Positive test: The clear solution forms a cloudy white emulsion.

- Negative test: The solution remains clear.