Food tests detect nutrients in a sample. Crush solid food with distilled water to prepare a food solution or suspension. Use separate samples and clean equipment for each test. Wear eye protection; follow your teacher’s instructions for reagents and hot water baths.
Add a few drops of iodine solution. Orange-brown → blue-black is a positive result. No heating is needed.
Add Benedict’s solution and warm in a hot water bath. A positive result changes blue to green, yellow, orange or a brick-red precipitate, depending on the amount of reducing sugar. Blue is negative.
Add Biuret reagent. Blue → lilac/purple indicates protein. No heating is needed.
Add Sudan III reagent and shake carefully as instructed. A red-stained oil layer indicates lipid. The reagent stains the lipid; do not confuse this with the protein or starch colours.
A distilled-water sample is a negative control. Known nutrient samples provide positive controls. Keep sample and reagent amounts consistent when comparing results. These tests mainly show presence or absence, not an exact nutrient mass.
Iodine: starch. Benedict’s + heat: reducing sugar. Biuret: protein. Sudan III: lipid.
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