The mainstream extraction processes for walnut oil fall into three categories: cold pressing, hot pressing, and solvent extraction. Among these, cold pressing (≤60°C) best preserves nutritional integrity and is the preferred method for producing premium-grade walnut oil.

I. Cold Pressing Process (≤60°C; Optimal Nutrient Retention)

1. Raw Material Pre-treatment

Selection: Select plump, mold-free, and insect-free walnuts; prioritize thin-shelled varieties with high oil content.

Cleaning: Use air classification and sieving to remove stones, twigs, leaves, and shriveled nuts; use high-pressure water washing to remove soil, sand, and pesticide residues.

Shelling: Mechanical shelling (shelling rate ≥95%); use air classification to separate kernels from shells; keep kernel breakage rate ≤8%.

Peeling (Optional): Briefly soak in a dilute alkaline solution (1–2% NaOH) for 5–10 minutes, then rinse thoroughly with clean water to remove the seed coat and reduce bitterness.

Low-Temperature Drying: Dry using hot air at 40–60°C to reduce moisture content to 5–8% (prevents mold growth and maximizes oil yield).

Crushing/Flaking: Crush the nuts into particles of 2–4 mm, then roll them into thin flakes (0.3–0.5 mm) to increase the surface area for oil extraction.

2. Low-Temperature Pressing

Equipment: Low-temperature screw oil press or hydraulic oil press.

Parameters: Temperature ≤60°C; Pressure 20–40 MPa; Oil yield 28–32%; Residual oil in press cake/meal 8–12%.

Crude Oil: Light yellow and transparent with a mild flavor; retention rates for Vitamin E, polyphenols, and Omega-3 fatty acids exceed 90%.

3. Crude Oil Refining

Filtration: Plate-and-frame filtration or centrifugal filtration to remove solid residues.

Degumming (Optional): Add water (2–3% by weight) at 50–60°C, stir for 30 minutes, and then use centrifugation to remove phospholipid gums and impurities.

Dehydration: Vacuum drying at 60°C (-0.08 to -0.1 MPa) to reduce moisture content to ≤0.1%.

Nitrogen Flushing for Preservation: Store away from light, flush with nitrogen gas, and bottle in dark-colored glass containers; can be stored under refrigeration for 12–18 months. II. Hot Pressing Process (100–120°C; High Oil Yield, Rich Aroma)

Pre-treatment: Cleaning → De-shelling → Drying → High-temperature Steaming and Roasting (100–120°C, 5–10 min). This step releases the walnut's characteristic aroma and facilitates the easy extraction of oil.

Pressing: Temperature: 80–100°C; Oil Yield: 42–46%; Residual Oil in Cake/Meal: 6–8%.

Characteristics: Deep yellow color, rich aroma; high temperatures result in the loss of some Vitamin E and polyphenols, and render Omega-3 fatty acids susceptible to oxidation.

III. Solvent Extraction Process (Industrial Scale)

Process Flow: Walnut Cake (post-cold/hot pressing) → Crushing → n-Hexane Extraction (60°C) → Miscella (Oil-Solvent Mixture) → Low-temperature Desolventization (≤60°C) → Crude Extracted Oil → Refining (Degumming / Deacidification / Decolorization / Deodorization).

Characteristics: Oil yield of 95%+; low production costs; risks associated with residual solvents; significant loss of nutrients; primarily used for lower-grade blended oils.

IV. Process Comparison

Cold Pressing: Temperature ≤60°C; Oil Yield: 28–32%; Extremely high nutrient retention; Mild flavor; Medium-to-high cost.

Hot Pressing: Temperature 100–120°C; Oil Yield: 42–46%; Moderate nutrient retention; Rich, aromatic flavor; Medium cost.

Solvent Extraction: Temperature 60°C (during desolventization); Oil Yield: 95%+; Low nutrient retention; Mild flavor; Low cost.