🧈 Fat — Soxhlet extraction

Food Chemistry PN-ISO 1443

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

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.

   ⏱ Time: 2–4 h • 🌡 Temperature: 103°C

2. Acid hydrolysis (optional)

   For dairy/chocolate samples: boil with 50 mL 25% HCl for 1 h. Filter, wash with hot water, dry.

   ⏱ Time: 1–2 h

3. Thimble preparation

   Place dried sample mixed with sand in cellulose thimble. Close with cotton wool. Place thimble in extraction chamber.

   ⏱ Time: 5 min

4. Flask preparation

   Dry collection flask at 103°C, cool in desiccator, weigh with 0.001 g accuracy. Add boiling beads.

   ⏱ Time: 30 min • 🌡 Temperature: 103°C

5. Filling with solvent

   Pour petroleum ether into flask (150–200 mL). Assemble Soxhlet apparatus. Turn on cooling water.

   ⏱ Time: 5 min

6. Extraction

   Turn on heating. Ether boils, evaporates, condenses, and floods sample. After chamber fills, siphoning occurs. Repeat 15–30 cycles.

   ⏱ Time: 4–8 h • 🌡 Temperature: 60–70°C

7. Solvent evaporation

   After extraction completion, evaporate ether on rotary evaporator or distill to near dryness.

   ⏱ Time: 15–30 min

8. Flask drying

   Dry flask with fat at 103°C for 1 h. Cool in desiccator for 30 min. Weigh.

   ⏱ Time: 1.5 h • 🌡 Temperature: 103°C

9. Drying to constant mass

   Repeat drying (30 min) and weighing until difference between consecutive weighings < 0.001 g.

   ⏱ Time: 30–60 min • 🌡 Temperature: 103°C

10. Calculations

    Fat [%] = (flask mass with fat − empty flask mass) × 100 / sample mass. Report result to 0.1% accuracy.

Required equipment and apparatus

Reagents, media and consumables

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.

Frequently asked questions