Notes on Seed Oil Extraction Methods

Introduction

• Global context: food waste is a major issue driving sustainable processing; roughly 1.3\times10^9\ tons wasted annually, about $30\%$ of total food production, worth around $7.5\times10^8\$ USD$. Oilseed production is large (roughly $632\text{ million tons}$) and expected to be worth about $162.5\text{ billion USD}$ by 2025. Demand for non-soybean oils and valorization of seed by-products are growing.
• Focus: balance oil and phytochemical yields with environmental and economic impacts; green extraction options (SCO2, enzyme-assisted) are pursued, but practicality and cost remain concerns.

Extraction Methods Overview

• Main methods: expeller pressing, organic solvent extraction (e.g., hexane), and green alternatives (supercritical CO2 SCO2 and enzyme-assisted aqueous extraction).
• Key consideration: maximize oil and valuable phytochemicals while minimizing environmental footprint and cost; co-product valorization drives profitability.

Mechanical Pressing

• Two main equipment types: expeller press and extruder.
• Expeller pressing: simple, solvent-free, can keep temps under $50^{\circ}\text{C}$ (cold pressing) but often with lower oil recovery; 5–20% of oil can remain in the cake if barrel spacing is too tight or seeds are highly compacted.
• Process optimization examples:
  • Screw rotation: increasing from $1.2\,\text{rpm}$ to $18\,\text{rpm}$ raises capacity (e.g., from $2.2\text{ kg seed/h}$ to $29.4\text{ seed/h}$) but may reduce oil yield (
  • Linseed oil: two passes increased yield from $19\%$ to $32\%$; a third pass offered no significant gain and could degrade phenolics and flavonoids due to heat/pressure.
• Extrusion (twin- vs single-screw): pretreatment via extrusion can boost oil recovery and enable co-treatment with solvent; twin-screw systems allow thermomechanical treatment with fewer pre-treatments; examples show up to $50\%$ oil recovery for coriander seeds without pretreatment and $~$ for sunflower seeds under specific conditions; lab vs industrial scale differences exist.

Solvent Extraction

• Hexane is the dominant solvent due to high efficiency, low cost, recyclability, and favorable physicochemical properties; widely used for soybean oil.
• Drawbacks: hexane is flammable and poses health/environmental risks; residual hexane found in some products; regulatory limits differ by region (EU strict limits; FDA varies by product).
• Alternatives/co-solvents discussed: ethanol, isopropanol, acetone, ethyl acetate, etc.; ethanol is allowed in organic production and can extract more polar compounds (e.g., polyphenols, pigments).
• Comparative yields: hexane often yields high oil quantities; alternative solvents can achieve similar oil yields in many seeds, with different selectivity for co-extracted components (e.g., tocopherols, phospholipids, sterols).

Supercritical CO2 (SCO2) Extraction

• SCO2 is a greener option with no solvent residues and good selectivity for non-polar compounds; applicable to many seeds (canola, soybean, sunflower, grape, peach, etc.).
• Key advantages: high diffusivity, low viscosity, recyclable CO2, no solvent residues, high purity.
• Limitations: non-polar CO2 struggles with polar phytochemicals; high capital and operating costs; need for high-pressure equipment and scalable continuous systems.
• Enhancements: use polar co-solvents (e.g., ethanol) to improve solubility of polar lipids and phenolics; sequential SCO2 with ethanol can increase phenol yields from defatted seeds.
• Process parametrics: yields depend on seed type, particle size, pressure, temperature, moisture, and flow rates; examples show specific trade-offs (e.g., hemp seeds faster extraction with higher pressure; grape seeds yield lycopene with SCO2).

Aqueous Extraction Processing (AEP) and Enzyme-Assisted AEP (EAEP)

• AEP uses water as solvent; oil partitions into fractions (solid residue, skim, cream, free oil); demulsification is needed to recover free oil.
• Pros/cons: environmentally friendlier than organic solvents; yields are often lower than solvent-based methods, but some studies report competitive yields up to $96\%$ for free oil under optimized pretreatments.
• Pretreatments to boost yields: roasting, flaking, extrusion, moisture conditioning, acidic pretreatments (e.g., flaxseed treated with citric acid) can destabilize emulsions and disrupt matrix to release oil.
• Enzyme-assisted AEP (EAEP): adding carbohydrases and proteases breaks cell walls and membranes, increasing oil release; can reduce environmental burden and energy use; optimal enzyme activity depends on pH (away from seed protein pI) and temperature (commonly $45-55^{\circ}\text{C}$).
• Examples of enzyme benefits:
  • Moringa seeds: enzyme additions raised oil yields from $8\%$ (control) to $17-23\%$.
  • Almond cake: enzyme addition raised oil yield to $50\%$ (vs $42\%$ control) under certain conditions; scale can affect outcomes.
  • Cocktail enzyme blends (cellulase, pectinase, hemicellulase) improved oil yields (e.g., mustard flour: $76\%$ oil, $75\%$ protein) vs lower yields with AEP alone.
• Enzyme-assisted demulsification can significantly boost free oil yields (e.g., almond cake demulsification with protease yielded $60-63\%$ free oil vs up to $39\%$ control).

Life Cycle and Environmental Impact Analyses (LCA/EIA)

• LCA vs EIA: LCA focuses on cradle-to-grave impacts; EIA adds social and time/location factors.
• Environmental performance varies by method and defnition of system boundaries:
  • Mustard seed hexane: higher environmental impact overall; hexane emissions contribute to specific categories.
  • Hexane vs ethanol (as hexane replacement): ethanol can reduce some pollutants but may increase energy use depending on process boundaries.
  • Expelling (mechanical pressing) can have low chemical inputs but may incur higher GHGs due to energy use; EAEP can reduce some impacts but pretreatment energy can raise CO2e in some cases.
• Comparative TEA/LCAs (example trends):
  • Hexane: high oil yield but highest solvent-related environmental impact in several studies; energy efficiency can be favorable.
  • Expelling: lowest chemical inputs, but energy intensity can drive higher GHGs; often seen as cleaner in solvent-use terms but not necessarily overall.
  • EAEP: often lower environmental impacts than hexane, but total CO2 emissions depend on pretreatment energy; may still be viable with co-product valorization.
• Tables in literature summarize techno-economic and environmental analyses, highlighting co-products as critical to environmental and economic viability.

Techno-Economic Analysis (TEA)

• TEA breaks down capital/operating costs and potential profits; co-products often drive profitability, offsetting higher processing costs.
• Examples: soybean oil pathways illustrate the value of co-products (e.g., soybean meal, ethanol co-products) in achieving profitability.
• Economic thresholds:
  • Hexane extraction profitability may require large annual oil production (e.g., around $3.46\times10^7\text{ kg/year}$) for break-even in some models.
  • EAEP profitability for soybean oil may require annual production above $8.5\times10^6\text{ kg}$.
• SCO2 case studies (grape marc, grapeseed oil): co-product revenues (e.g., dried skins, exhausted seed powder) can help meet break-even; grapeseed oil market values can be substantial (up to a few tens of euros per kg).
• Important caveats: TEA models often assume lab-scale data; scale-up validation is critical; energy and equipment costs dominate SCO2 and EAEP economics; integration with existing facilities and co-product markets strongly influences viability.

Conclusions and Practical Implications

• A circular, green economy approach is increasingly adopted, prioritizing sustainability and waste reduction.
• Ethanol extraction and SCO2 are viable alternatives to hexane in many cases, especially with attention to energy use and safety.
• Enzyme-assisted aqueous extraction offers environmental advantages and can achieve competitive yields with proper optimization, particularly when co-products are valorized.
• The best method depends on the seed matrix, target products (oil vs. phytochemicals), and boundaries for environmental and economic assessment.
• A combination of techno-economic analysis and life cycle assessment is essential for industrial adoption, along with validation at pilot/industrial scales.
• Co-product valorization is critical to economic viability across all extraction pathways.