Practice of component analysis
A full analysis follows the same path every time.
- Plan. Decide the food, the tests (water, ash, fat, protein, carbohydrate, and any minerals or vitamins) and the order. Always do the water test first, because the other results are often quoted on a dry or wet basis.
- Prepare. Take a representative sample. Grind, mix, label, and keep it in a closed container.
- Test in duplicate. Do each test twice. If the two results differ by more than about 2% of their value, repeat.
- Calculate. Write the formula, the numbers and the units for each line. Check that water + ash + fat + protein + carbohydrate is close to 100.
- Report. Give the food name, the method, the results in a table (g per 100 g), the average, and any problem you noticed. A good report lets another person repeat your work.
Wet basis vs dry basis. Wet basis is per 100 g of the food as it is. Dry basis is per 100 g of dry matter, so it is not changed by how much water the food holds: dry basis % = wet basis % ÷ (100 − water %) × 100.
Food hygiene testing
Hygiene tests ask: is this food safe? Common tests:
- pH and acidity. Below pH 4.6 most harmful bacteria cannot grow. Use pH paper or a pH meter. Acidity is measured by titrating with alkali.
- Total plate count. Mix a little food in sterile water, make a dilution (for example 1 in 1000), spread a small amount on agar, incubate, and count the colonies. CFU per gram = colonies × dilution factor. CFU means colony forming unit: one colony came from one living cell or clump.
- Indicator germs. Coliform bacteria or E. coli show there may be dirt or faecal contamination. Fewer is better; many foods must have none in 25 g.
- Added or harmful substances. Simple tests find starch in milk (iodine turns blue-black), or an unlawful dye; labs also check preservatives, pesticide and metal traces.
- Surface swabs. Swab a table or knife and plate it, to see if cleaning works.
Limits differ by food and country, so always read the rule for your food. Never taste a sample in the lab.
Key formulas and definitions
- Dry basis % = wet basis % ÷ (100 − water %) × 100
- CFU per g = colonies counted × dilution factor
- Dilution factor = 1 ÷ dilution (10⁻³ → 1000)
- Total of five components ≈ 100 g per 100 g
- Key terms: duplicate, wet basis, dry basis, dilution, CFU, indicator organism
Worked examples
1. A plate from a 10⁻³ dilution shows 42 colonies. Find CFU per gram.
CFU/g = 42 × 1000 = 42,000.
2. Rice has 8% protein on a wet basis and 12% water. Find protein on a dry basis.
Dry basis = 8 ÷ (100 − 12) × 100 = 8 ÷ 88 × 100 ≈ 9.1%.
3. Two water tests of the same flour give 11.8% and 12.2%. Report the result.
The two are close (0.4 apart), so the average is (11.8 + 12.2) ÷ 2 = 12.0% water.
4. A plate from a 10⁻² dilution has 320 colonies. Is this a good count?
Plates are best counted when they have about 25 to 250 colonies. 320 is too many to count well, so repeat with a 10⁻³ dilution.
Common mistakes
- Running a test only once and reporting it as sure.
- Counting colonies from a plate with hundreds of colonies (too crowded). Use a plate with about 25 to 250.
- Forgetting the dilution factor and reporting colonies as CFU per gram.
- Mixing wet-basis and dry-basis numbers in one table.