Food Chemistry and Food Analysis
This domain tests the chemistry behind what a laboratory reports: proximate composition, the behaviour of proteins, fats and carbohydrates, and the classical and instrumental methods a Food Analyst uses under the FSS Act.
Proximate analysis - method to value
- Moisture: hot-air oven at 105 °C; vacuum oven or Karl Fischer titration for heat-labile or high-sugar matrices.
- Ash: muffle furnace at 550 °C - total mineral matter.
- Crude fat: Soxhlet extraction with petroleum ether; milk fat by the Gerber method (sulphuric acid and amyl alcohol, read in the butyrometer).
- Crude protein: Kjeldahl digestion, distillation and titration; nitrogen x 6.25 (x 6.38 for milk and milk products).
- Carbohydrate by difference; crude fibre by acid-alkali digestion, dietary fibre by enzymatic-gravimetric assay.
Adulteration chemistry (high-yield pairs)
- Starch in milk → iodine, blue-black colour.
- Metanil yellow in turmeric or dal → hydrochloric acid gives a magenta colour.
- Argemone oil in mustard oil → nitric acid test, red colour from sanguinarine.
- Vanaspati in ghee → Baudouin test (sesame-oil marker); ghee purity is also read from the Reichert–Meissl value.
- Rancidity → peroxide value (meq O2/kg) and acid value; TBA test for malondialdehyde.
Worked example
A 2.000 g wheat-flour sample gives 0.0320 g nitrogen on Kjeldahl. % protein = (0.0320 ÷ 2.000) x 100 x 5.70 = 9.12%. Using 6.25 would give 10.0%, which is wrong because the conversion factor is commodity-specific - always state the factor with the result.
Traps
- Quoting 6.25 for dairy (use 6.38).
- Mixing up acid value (free fatty acids) with peroxide value (oxidation).
- Reporting fat on a dry-matter basis when the standard is as-is.
- Treating crude fibre as identical to total dietary fibre.