RP4: Required practical activity 4: Food tests
Not started yet — this one needs some love.
Aim: use qualitative reagents to test a range of foods for carbohydrates, lipids and proteins.
This is a qualitative identification practical — you are observing colour changes to determine which biological molecules are present, not measuring a rate. There is no single independent variable.
What is being changed: the food sample and/or the reagent used.
What is being observed: whether a colour change occurs (positive or negative result).
Controls: same volume of food extract; same volume of reagent; same heating time and temperature (where heating is required); distilled water tested alongside each reagent as a negative control.
Prepare the food extract first (for tests 1–3)
Grind a small sample of food using a pestle and mortar.
Transfer to a small beaker, add a little distilled water and stir to dissolve.
Filter through a funnel with filter paper into a conical flask to obtain as clear a solution as possible.
Test 1 — Starch (iodine test)
Place 2–3 cm³ of food extract in a test tube.
Add a few drops of iodine solution.
Positive result: orange-brown changes to blue-black (starch present). Negative: remains orange-brown.
Test 2 — Reducing sugar (Benedict's test)
Place 2–3 cm³ of food extract in a test tube.
Add 10 drops of Benedict's solution.
Place the test tube in a beaker of hot water (approximately 80 °C) for 5 minutes.
Positive result: blue changes to green, yellow, orange or brick-red depending on sugar concentration. Negative: remains blue.
Test 3 — Protein (Biuret test)
Place 2–3 cm³ of food extract in a test tube.
Add 2 cm³ of Biuret reagent. Shake gently.
Positive result: blue changes to lilac or purple (protein present). Negative: remains blue.
Test 4 — Lipids (emulsion test)
Place a small amount of food in a test tube and add 2 cm³ of ethanol. Shake to mix.
Pour the ethanol extract into a fresh test tube containing cold water.
Positive result: a white cloudy emulsion forms (lipid present). Negative: solution remains clear.
Safety
Iodine solution is an irritant — wear safety goggles; wash any spills off skin immediately.
Hot water bath (Benedict's test) — risk of burns and scalding; use a beaker of hot (not boiling) water; hold test tubes with a test tube holder; do not point the tube at anyone.
Biuret reagent contains sodium hydroxide (corrosive) and copper sulfate (harmful) — wear safety goggles at all times; wash any spill off skin immediately with plenty of water.
Ethanol (emulsion test) is highly flammable — keep all ethanol away from naked flames and heat sources; no Bunsen burners should be lit while ethanol is in use.
No eating or drinking in the laboratory during food tests.
Common exam mistakes
Benedict's test must be heated in a water bath; adding Benedict's reagent at room temperature is not a complete reducing-sugar test.
Say iodine solution, not solid iodine, for the starch test; the positive result is orange-brown to blue-black.
Do not mix up the food-test colours: Biuret gives lilac/purple for protein, and the ethanol emulsion test gives a white cloudy layer for lipids.
Safety answers need a linked risk and control, such as ethanol is flammable so keep it away from flames; generic "wear goggles" alone is weak.