🧈 Fat — Soxhlet extraction
Determination of crude fat by multiple extraction with organic solvent (petroleum ether) in Soxhlet apparatus. Classical reference method.
Overview
Soxhlet extraction is the classical reference method for determining crude fat in food, recognized by AOAC, ISO, and Polish standards. The method developed by Franz von Soxhlet in 1879 involves multiple, cyclic extraction of fat from a sample using a volatile organic solvent (most commonly petroleum ether, b.p. 40–60°C).
The food sample, placed in a cellulose thimble in the extraction chamber, is repeatedly flooded with condensed solvent, which dissolves fats and other lipophilic substances. When the solvent level in the chamber reaches the siphon arm, the solution automatically flows back to the collection flask. The cycle is repeated 15–30 times over 4–8 hours of extraction. After completion, the solvent is evaporated and the fat residue is dried and weighed.
The method is required in quality control of meat and meat products (PN-ISO 1443), dairy products, cereals, feed, and many other food matrices. It is the basis for nutritional value declaration on labels (EU Reg. 1169/2011). Modern laboratories use automated systems (BÜCHI E-500, FOSS Soxtec, VELP SER 148) that shorten extraction time to 1–2 hours.
The method extracts so-called crude fat — in addition to triacylglycerols, free fatty acids, phospholipids, sterols, carotenoids, and waxes are also extracted. For samples requiring acid hydrolysis (e.g., dairy products, chocolate), preliminary HCl digestion is used before extraction.
Method principle
The dried sample is placed in a cellulose thimble in the extraction chamber of the Soxhlet apparatus. Solvent (petroleum ether) in a round-bottom flask is heated, evaporates and condenses in the reflux condenser, dripping onto the sample. The solvent dissolves fats, and when the level in the chamber reaches the siphon arm, the solution automatically returns to the flask. The evaporation–condensation–extraction–siphoning cycle is repeated multiple times (4–8 h). After extraction, the solvent is evaporated (rotary evaporator or distillation), the flask is dried at 103°C and weighed. Fat content is calculated gravimetrically: fat [%] = (flask mass with fat − empty flask mass) × 100 / sample mass.
Applications
- Fat determination in meat and meat products (PN-ISO 1443)
- Quality control of dairy products (butter, cheese, milk powder)
- Fat analysis in cereal products (flour, bread, pasta)
- Testing of feed and feed mixtures
- Nutritional value declaration on labels (EU Reg. 1169/2011)
- Quality control of edible oils, fats, and margarines
- Analysis of special-purpose food (dietary, infant)
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.5–100% fat |
| Solvent | Petroleum ether 40–60°C |
| Extraction time | 4–8 h (classical), 1–2 h (automated) |
| Repeatability | ±0.2–0.5% fat (depending on matrix) |
| Sample size | 2–5 g (dried) |
| Number of cycles | 15–30 siphoning cycles |
Standard
- Standard number
- PN-ISO 1443:2000
- Title (PL)
- Mięso i przetwory mięsne — Oznaczanie zawartości tłuszczu całkowitego
- Title (EN)
- Meat and meat products — Determination of total fat content
Step-by-step procedure
1. Sample preparation
Homogenize sample. Dry 3–5 g at 103°C to constant mass (or use sample after moisture determination). Weigh with 0.001 g accuracy.
2. Acid hydrolysis (optional)
For dairy/chocolate samples: boil with 50 mL 25% HCl for 1 h. Filter, wash with hot water, dry.
3. Thimble preparation
Place dried sample mixed with sand in cellulose thimble. Close with cotton wool. Place thimble in extraction chamber.
4. Flask preparation
Dry collection flask at 103°C, cool in desiccator, weigh with 0.001 g accuracy. Add boiling beads.
5. Filling with solvent
Pour petroleum ether into flask (150–200 mL). Assemble Soxhlet apparatus. Turn on cooling water.
6. Extraction
Turn on heating. Ether boils, evaporates, condenses, and floods sample. After chamber fills, siphoning occurs. Repeat 15–30 cycles.
7. Solvent evaporation
After extraction completion, evaporate ether on rotary evaporator or distill to near dryness.
8. Flask drying
Dry flask with fat at 103°C for 1 h. Cool in desiccator for 30 min. Weigh.
9. Drying to constant mass
Repeat drying (30 min) and weighing until difference between consecutive weighings < 0.001 g.
10. Calculations
Fat [%] = (flask mass with fat − empty flask mass) × 100 / sample mass. Report result to 0.1% accuracy.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Automated fat extractor | BÜCHI FatExtractor E-500, FOSS Soxtec 8000, VELP SER 148/6 | 45 000–90 000 PLN |
| Classical Soxhlet apparatus (glass) | Soxhlet set 250 mL (flask + chamber + condenser), Simax/DURAN | 800–1 500 PLN |
| Heating mantle / water bath | Electrothermal EME, Lauda Ecoline | 2 000–5 000 PLN |
| Analytical balance 0.1 mg | Mettler Toledo ME204, Sartorius Quintix 224 | 8 000–20 000 PLN |
| Laboratory drying oven | Binder FD 56, POL-EKO SLW 32 | 6 000–15 000 PLN |
| Cellulose extraction thimbles | Whatman 603 (33 × 80 mm), pack of 25 pcs. | 80–150 PLN |
| Fume hood / fume cupboard | Required due to solvent vapors | 15 000–40 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Petroleum ether 40–60°C p.a. | 8032-32-4 | Main extraction solvent, 2.5 L for ~20 determinations |
| Hydrochloric acid 25% (for hydrolysis) | 7647-01-0 | For acid hydrolysis of dairy/chocolate samples before extraction |
| Purified sea sand | 14808-60-7 | For mixing with sample, facilitates extraction (flowability) |
| Glass boiling beads | — | For collection flask, prevents bumping |
| Defatted cotton wool | — | For closing extraction thimbles from top |
Health and safety (OHS)
- Petroleum ether — highly flammable (H225), vapors heavier than air. Work under fume hood, away from ignition sources.
- HCl (hydrolysis) — corrosive (H314), use gloves and goggles. Work under fume hood.
- Hot flasks and heating mantles — risk of burns. Use holders.
- Solvent vapors — fume hood ventilation mandatory. Explosion risk at concentration > LEL.
- Solvent waste — collect separately as hazardous waste.