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In modern oilseed processing, pressing and expansion represent two core unit operations that can function independently or be combined to form multiple technological routes. Their synergy and divergence constitute an important dimension in the design of oilseed processing systems.

I. Technical Foundation and Operational Flexibility of the Pressing Process

The Screw Press is the core equipment in any pressing plant, yet its performance is highly dependent on the quality of seed preconditioning. The fundamental goal of preconditioning is to disrupt the cellular structure of oilseeds to release intracellular oil droplets. However, excessive cell wall disruption results in a powdery material lacking sufficient structural integrity—unable to sustain effective pressure within the press cage or facilitate smooth oil phase separation. Roll conditioning thus represents an inherent compromise between “opening oil pathways” and “maintaining structural strength.” Similarly, the degree of dehulling must be carefully controlled: for seeds such as sunflower or rapeseed, a moderate retention of hulls actually enhances cake strength, which can improve the economic performance of pressing operations.

Given the critical importance of flaking quality to press performance, modern screw geometries—incorporating high-pressure zones, high-shear sections, and shaped cage bars—can partially compensate for upstream preparation deficiencies, reducing the stringency of flaking requirements. Nevertheless, it must be emphasized that any poor-quality flakes will degrade overall system performance. Good flaking remains the key to successful pressing, particularly in prepress-solvent extraction processes.

Thermal-moisture conditioning (cooking and tempering) is equally critical. While temperature is not the sole determinant, it directly affects drying efficiency; the moisture content of the feed significantly influences frictional behavior within the press cage, thereby regulating pressure build-up and oil expression. Although the industry has accumulated considerable empirical knowledge regarding the moisture-pressure relationship, the underlying mechanisms are not yet fully understood. Notably, the rheological behavior of the feed depends not only on final moisture content but also on the drying history itself. Protein denaturation is further influenced by the temperature, humidity, and residence time profile within the conditioning equipment. Different Conditioner types—such as vertical stack cookers (characterized by long residence times and relatively humid environments) versus rotary or tube-type dryers (rapid moisture release, short residence times)—dictate different optimal feed moisture levels, which also vary with seed type.

The pressing process can be further categorized into two application scenarios:

• Prepress (Pre-pressing):As a preliminary step before solvent extraction, prepress extracts only a moderate fraction of the oil. It is typically applied to high-oil “soft seeds” such as rapeseed and sunflower. Modern prepress systems can achieve capacities exceeding 3,000 tonnes per day, with Oil Extraction rates of approximately 65%–75%, corresponding to a prepressed cake oil content of about 18%–20%. Excessive pressing not only increases power consumption and wear dramatically while reducing throughput, but also produces a brittle cake that disintegrates into fines in the extractor, impairing solvent percolation and drainage. Conversely, insufficient oil removal yields a soft cake that similarly compromises extraction performance.

• Full-Pressing (Direct Pressing):This approach relies solely on mechanical means for Oil Extraction, without subsequent solvent extraction. Constrained by the physical limits imposed by intermolecular and capillary forces, full-pressing can achieve Oil Extraction rates above 90%, with residual cake oil contents of 5%–8%. To attain this level, the oilseed must be subjected to higher pressures for extended residence times, typically achieved through lower feed moisture to increase internal friction. However, high-pressure operation is limited by the mechanical strength of the press cage, which constrains the maximum barrel diameter; the slower screw speeds required for longer retention further restrict throughput. Consequently, full-press press capacities are generally about one-tenth those of modern prepress units.

The two-stage full-pressing process developed by Harburg-Freudenberger employs a small number of prepresses operating under relatively mild conditions to remove the bulk of the oil, followed by finishing presses to achieve final oil yield. Each stage is preceded by thermal conditioning to adjust moisture and temperature for optimal processing. Modern screw designs, featuring multiple high-shear sections, can even eliminate the need for conventional cracking and flaking. Advanced two-stage systems incorporate cold prepressing at ambient temperatures; the resulting cold-pressed oil can be separately filtered for enhanced product value.

The economic choice between full-pressing and prepress-solvent extraction routes must be evaluated on a project-by-project basis, taking into account local permitting requirements, environmental impacts, energy costs, local meal markets, safety considerations, and numerous other factors—no universal preference exists.

II.Extended Applications of Screw Pressing Technology

The screw press, as a continuous solid-liquid separation device, has found applications well beyond vegetable Oil Extraction. The fundamental prerequisite is that the feed material must possess sufficient structural strength to sustain pressure build-up and permit low-viscosity liquid flow, making the system unsuitable for slurries. For dewatering applications, twin-screw presses—featuring two counter-rotating shafts for forced feeding and conveying—are typically employed, characterized by simple mechanical construction and low system pressures. Heavy-duty single-shaft screw presses are used in the rendering industry for fat separation from animal tissues, as well as in wood chip dewatering, spoilt grain processing for brewing, and synthetic rubber dewatering.

III.Combined Routes of Pressing and Expansion

Pressing and expansion share certain mechanical similarities. Since expansion is fundamentally a thermal process that complements the cooking/tempering step required for effective pressing, their combination is technically logical.

Expansion → Pressing Route: This combination initially emerged in the treatment of full-fat soybeans or cottonseed meal for animal feed, where extrusion cooking reduces antinutritional factors. Given the commercial value of these oil-bearing products, subsequent oil removal via screw pressing became a natural extension. However, as noted earlier, direct expansion of high-oil materials is relatively inefficient, often requiring special screw geometries and high power consumption. Once these initial challenges are addressed, this route offers advantages including simple system configuration, good Oil Extraction efficiency, high press throughput, and low energy consumption. A critical design consideration is the flash evaporation of moisture upon discharge from the expander, which demands effective exhaust systems to prevent excessive condensation in downstream conveyors or presses.

Pressing → Expansion Route: The motivation for this reverse sequence lies in the expander’s ability to convert high-oil cake into granular material with stable structural characteristics, thereby enhancing solvent extraction performance. When prepress cake exhibits excessive fines that impair solvent percolation and result in high residual oil, post-pressing expansion can strengthen cake structure. This approach was tested in multi-oilseed plants where expanders were already available for soybean processing. However, its overall adoption has been limited due to: (i) additional power requirements, (ii) extra heat generation necessitating cake drying and cooling, and (iii) increased dust and odor emissions requiring air treatment. Without expansion, investment in upstream preparation and pressing equipment must be sufficient to produce a prepress cake suitable for direct solvent extraction without additional post-treatment.

IV.Future Outlook

Expanders and Screw Presses will continue to occupy designated and significant positions within the oilseed processing industry in the foreseeable future. Single-unit press capacities will continue to increase, bounded only by mechanical constraints. Prepress-extraction and full-pressing routes will coexist, with ongoing engineering efforts to narrow the oil yield gap between them, thereby enhancing the economic viability of full-pressing. Combinations with expanders—as dual-function thermal and mechanical preparation units—are likely to become more common. Meanwhile, continued research into alternative innovations, such as supercritical carbon dioxide injection during pressing, may one day give rise to a new paradigm in oil extraction—one that optimizes not only oil yield but also the quality of both oil and protein fractions.

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