Find out a qualitative test for proteins, fats and oils, amino acids and test any fruit juice, saliva, sweat and urine for them.

Qualitative tests

• Biuret test

  • Principle: copper(II) ions in alkaline solution form a violet complex with peptide bonds in proteins

  • Reagent: mixture of NaOH and CuSO₄ (Biuret reagent)

  • Positive result: appearance of violet color indicates proteins

• Sudan III test

  • Principle: Sudan III is a fat-soluble dye that selectively stains lipids red by partitioning into nonpolar oil droplets

  • Reagent: alcoholic or aqueous Sudan III solution

  • Positive result: red-stained oil layer or droplets indicate presence of fats and oils

• Ninhydrin test

  • Principle: ninhydrin reacts with free amino (–NH₂) groups of amino acids to yield a purple (Ruhemann’s purple) complex

  • Reagent: 0.2–0.5% ninhydrin in ethanol or acetone

  • Positive result: blue-purple color upon heating indicates amino acids

Testing procedure for each sample

• Prepare fresh samples of fruit juice, saliva, sweat and urine in clean test tubes

• Add 1 mL of Biuret reagent to each sample, mix and stand 3–5 min, observe color

• Add a few drops of Sudan III to separate aliquots of each sample, shake and let stand, observe oil layer under light

• Add 1 mL of ninhydrin reagent to separate aliquots of each sample, heat in boiling water bath for 3–5 min, observe color

Expected results and interpretation

• Fruit juice

  • Biuret test: no violet color, proteins absent or below detection limit

  • Sudan III test: no red layer, fats and oils absent

  • Ninhydrin test: faint to moderate purple indicating small amounts of free amino acids

• Saliva

  • Biuret test: violet color, proteins (e.g., amylase) present

  • Sudan III test: no red layer, fats and oils absent

  • Ninhydrin test: light purple, free amino acids and small peptides presentSweat

  • Biuret test: no violet color, proteins generally absent or very low

  • Sudan III test: no red layer, fats and oils absent

  • Ninhydrin test: no color change or very faint purple, minimal free amino acids

• Urine

  • Biuret test: no violet color in healthy urine, proteins absent

  • Sudan III test: no red layer, fats and oils absent

  • Ninhydrin test: no color change under normal conditions, free amino acids absent or below detection limit; positive only in pathological aminoaciduria

Notes for accurate interpretation

• Include positive controls (known protein, oil, amino acid solutions) and negative controls (distilled water)

• Ensure reagents are fresh and at proper concentration

• Use the same sample volumes and observation times for consistency

• Record intensity of color changes semi-quantitatively (e.g., none, light, moderate, strong) for comparative analysis