Basic operations in the food lab
Every analysis starts the same way. First take a representative sample: grind or mix the food so that a small piece looks like the whole. Then weigh it on an analytical balance (it reads to 0.0001 g). Use clean, dry glassware and write every weight down. Heating is done in an oven or muffle furnace, and liquids are measured with a pipette or burette. Repeat each test at least twice. If the two results are close, you can trust them.
Quantitative analysis: how much, not just what
Qualitative analysis tells you what is present (for example, "this has starch"). Quantitative analysis tells you how much (for example, "20 g of starch in 100 g"). Three tools are used. Gravimetric methods weigh something (dry weight, ash, fat). Titration adds a liquid of known strength until a colour changes (protein by nitrogen, acidity, vitamin C). Instrument methods use light or other signals (colour meter, spectrophotometer).
Percent = (mass of the part ÷ mass of the sample) × 100.
Water content
Water is found by drying. Weigh a sample, dry it in an oven at about 105 °C until the weight stops falling, cool it in a desiccator, and weigh again.
Water % = (loss in weight ÷ starting weight) × 100
Water is important because it decides how fast a food spoils, how it tastes and how much it should cost. Very sugary or oily foods are dried under vacuum at a lower temperature so they do not burn.
Proteins, fats and carbohydrates
Protein. Protein is the main source of nitrogen in food. In the Kjeldahl method the sample is digested in hot acid, then the nitrogen is collected and measured by titration. Protein = nitrogen × 6.25 (because protein is about 16% nitrogen).
Fat. Dry the sample, then wash it with a solvent such as petroleum ether in a Soxhlet apparatus. Evaporate the solvent and weigh the fat.
Carbohydrate. Usually found by difference: 100 − (water + ash + fat + protein). Sugars and starch can also be measured directly with special tests.
Minerals and vitamins
Ash and minerals. Burn the sample at about 550 °C in a muffle furnace. Carbon, hydrogen and nitrogen leave as gases. The grey-white ash is the total mineral content. To find single minerals such as calcium or iron, the ash is dissolved and measured by titration or by an instrument.
Vitamins. Vitamins are present in very small amounts (milligrams or micrograms) and many break down in light or heat, so they are measured with gentle, special methods. Vitamin C can be found by titrating with a blue dye that turns colourless. Other vitamins are separated and measured by an instrument called HPLC.
Key formulas and definitions
- Water % = (loss in weight ÷ sample weight) × 100
- Ash % = (weight of ash ÷ sample weight) × 100
- Protein % = nitrogen % × 6.25
- Carbohydrate % = 100 − (water + ash + fat + protein)
- Key terms: representative sample, gravimetric, titration, Soxhlet, Kjeldahl, desiccator, muffle furnace
Worked examples
1. A 5.00 g sample weighs 3.40 g after drying. Find the water percent.
Loss = 5.00 − 3.40 = 1.60 g. Water % = 1.60 ÷ 5.00 × 100 = 32%.
2. 2.0 g of flour gives 0.0196 g of nitrogen. Find the protein percent.
Nitrogen % = 0.0196 ÷ 2.0 × 100 = 0.98%. Protein % = 0.98 × 6.25 ≈ 6.1%.
3. A food has water 12%, ash 1%, fat 2% and protein 9%. Find the carbohydrate.
Sum = 12 + 1 + 2 + 9 = 24. Carbohydrate = 100 − 24 = 76%.
4. 10 g of nuts give 4.5 g of fat after Soxhlet extraction. What is the fat percent?
4.5 ÷ 10 × 100 = 45% fat.
Common mistakes
- Weighing a hot sample. Hot air rises and the balance reads too low. Cool it in a desiccator first.
- Forgetting to multiply nitrogen by 6.25 and reporting nitrogen as protein.
- Dividing by the dried weight instead of the starting weight for water percent.
- Taking one reading only. Always repeat and compare.