Fat content by the Soxhlet method (lab visit)
Fat is washed out of a dried sample by petroleum ether that boils, condenses and siphons back again and again; the flask's gain in weight is the fat.
Aim and principle
Lab visit: observed at a food analysis laboratory, not performed by you (but the principle, diagram and calculation are asked). Aim: To find the fat content of a food by the Soxhlet method. Principle: Fats dissolve in non-polar solvents such as petroleum ether or hexane. A dried, ground sample in a porous thimble is washed again and again by fresh, warm solvent: the solvent boils in the flask, its vapour is condensed, the liquid drips onto the sample and dissolves fat, and when the chamber fills to the top of the siphon tube the whole solution drains back into the flask. The fat stays in the flask while the solvent boils again, so every cycle uses clean solvent. After several hours the solvent is removed and the fat left in the pre-weighed flask is weighed.
Requirements
| Item | What it does |
|---|---|
| Soxhlet apparatus: round-bottom flask, extractor with siphon tube, reflux condenser | The continuous extraction system |
| Extraction thimble (cellulose or filter paper) and fat-free cotton | Holds the sample and lets solvent pass through |
| Petroleum ether (40-60 °C) or hexane | Dissolves the fat |
| Heating mantle or water bath (no open flame) | Boils the solvent safely |
| Hot air oven, desiccator, analytical balance | Drying the sample and flask, cooling and weighing |
| Dried, ground food sample (2-5 g) | Material whose fat is measured |
Procedure
Observed at the laboratory. Typical quantities are given; follow the exact method in your lab manual.
- 1Dry the empty flask at about 105 °C, cool it in a desiccator and weigh it (W₁).
- 2Weigh 2-5 g of dried, ground sample (W) into the thimble; plug with fat-free cotton and place it in the extractor.
- 3Pour petroleum ether into the flask (about one and a half times the extractor's siphon volume) and assemble flask, extractor and condenser.
- 4Turn on the condenser water first, then heat on a water bath or mantle so that the solvent siphons regularly.
- 5★Continue the extraction for about 6-8 hours.many siphon cycles
- 6Remove the thimble, recover most of the solvent, and evaporate the rest on the water bath in a fume hood.
- 7Dry the flask with the fat at about 105 °C (about 30 minutes), cool in a desiccator and weigh (W₂); repeat to constant weight.
★ one-way, controls the pace
The Soxhlet extraction cycle
The last step leads back to the first: the cycle repeats for about 6-8 hours, each time with freshly distilled, fat-free solvent.
Worked example: fat in soybean flour
Soybean flour (moisture 10.0%) was dried, and 5.000 g of the dried sample was extracted. Empty flask W₁ = 92.350 g; flask + fat W₂ = 93.330 g. Find % fat on dry basis and on fresh (as-purchased) basis.
- Weight of fat = W₂ − W₁ = 93.330 − 92.350 = 0.980 g.
- % fat (dry basis) = 0.980 ÷ 5.000 × 100 = 19.6%.
- Convert to fresh basis: % fat (dry) × (100 − % moisture) ÷ 100.
- = 19.6 × (100 − 10.0) ÷ 100 = 19.6 × 0.90 = 17.64%.
- Check: 5.000 g × 19.6 ÷ 100 = 0.980 g of fat.
Fat = 19.6% on dry basis, about 17.6% in the flour as purchased (fresh basis).
Notes and alternatives
- The extract is called crude fat (ether extract) because the solvent also takes up phospholipids, sterols, pigments, waxes and fat-soluble vitamins.
- Petroleum ether is often chosen over diethyl ether (which some official methods use): it extracts fewer non-fat substances, takes up less water and does not form explosive peroxides.
- Alternative: weigh the thimble before and after extraction (loss-in-weight method).
- For milk and milk products, fat is found by the Gerber or Röse-Gottlieb method; foods with bound fat may need acid hydrolysis before extraction.
Precautions and result
- Petroleum ether is highly flammable: no open flames or sparks, heat only on a water bath or mantle, work in a fume hood, and check the joints for leaks.
- Turn on the condenser water before heating, never let the flask boil dry, and do not leave the set-up unattended.
- Use a dried, finely ground sample; water keeps the solvent out and carries sugars into the extract.
- Cool the flask in a desiccator before weighing and dry to constant weight. Wear gloves and goggles.
- Result: The fat content of the given sample is ___ g per 100 g (dry basis), ___ g per 100 g (fresh basis).
Fat content is needed for nutrition labels (total fat per 100 g), for energy calculation (9 kcal/g) and for food composition tables. It is used to check products against food standards and 'low-fat' claims, to plan low-fat and therapeutic diets, and as part of proximate analysis, where 'carbohydrate by difference' = 100 − (moisture + protein + fat + ash + fibre).
Short answer: “Why is the sample dried before Soxhlet extraction?” Petroleum ether does not mix with water, so water stops the solvent from reaching the fat; water also carries sugars into the extract. A dry, ground sample is extracted fully.
Short answer: “Why is the Soxhlet extract called crude fat?” Besides true fat it contains other ether-soluble matter such as phospholipids, sterols, pigments, waxes and fat-soluble vitamins.
Structured question: “Explain the working of a Soxhlet extractor with a neat diagram.” Diagram: flask, extractor with thimble and siphon tube, condenser. Cycle: boil, vapour rises, condenses, drips on sample, chamber fills, siphons back; fat stays in the flask. About 6-8 h. % fat = fat ÷ sample × 100. Safety with petroleum ether.
Boil → Climb → Cool → Drip → Fill → Siphon: six words for one Soxhlet cycle.