Mechanical Oil Expulsion Methods: Ghanis, Presses, and Expellers

Every bottle of cooking oil you use – whether mustard, sesame, sunflower, or groundnut – starts its journey under pressure. Before modern refineries or chemical solvents entered the picture, physical force was the only way to get oil out of seeds. That approach, known as mechanical oil expulsion, remains widely relevant today, spanning everything from traditional village-level ghanis to high-capacity industrial screw expellers. Understanding how each method works, and where it fits best, is fundamental to oilseed processing.

What is mechanical oil expulsion? 🔗

Mechanical seed oil extraction is one of the oldest and simplest methods for separating oil from oil-bearing materials. It works on a straightforward principle: apply sufficient physical pressure to oilseeds so that cell walls rupture and oil is forced out. No chemical solvents are involved, making the process cleaner, safer, and more environmentally friendly than solvent-based alternatives. The two main outputs are crude oil and an oil cake (also called press cake or meal), which is protein-rich and valuable as animal feed. According to research published on ScienceDirect, in India alone, nearly 90% of the total 24 million tonnes of produced oilseeds are crushed using the mechanical pressing method, reflecting just how central this approach is to global oil production.

Mechanical expulsion methods can be broadly grouped into three categories: traditional ghanis, press-type oil expellers (including hydraulic presses), and mechanical screw expellers. Each operates on the same foundational principle – pressure – but differs substantially in design, throughput, efficiency, and the quality of oil produced.

The ghani: traditional oil extraction at the village level 🔗

The ghani is among the oldest mechanical oil extraction tools still in active use. As documented by Appropedia’s resource on small-scale vegetable oil extraction, a ghani (also known as a chekku or kol) is essentially a mortar and pestle device that grinds oilseeds into fine particles and extracts oil from them. The mortar is fixed to the floor – traditionally made of wood – while the pestle, made of wood or stone, is driven by a bullock harnessed to a long lever that rotates it inside the mortar.

How the ghani works 🔗

A batch of oilseed is loaded into the mortar. As the pestle rotates, it grinds the seeds. After sufficient grinding, a small quantity of water is added. The water combines with the ground material, and the kneading action of the pestle pushes the oil out through a hole at the base of the mortar, where it is collected in a container below. Modern motorised versions of the ghani replace the bullock with an electric motor, but the operating principle remains the same. Ghanis are used extensively in the Indian subcontinent for processing mustard seed, sesame, copra, and groundnuts.

Efficiency and suitability 🔗

The ghani delivers an oil yield of approximately 60%, which is considered reasonable for a device of its simplicity. It requires no pre-grinding equipment for smaller oilseeds, can be fabricated locally, and has low running costs. The oil produced is often valued for its distinctive flavour and aroma – qualities that come precisely because of the low-temperature, slow grinding process. However, the ghani is a batch system: only a fixed quantity of seed can be processed at one time, which limits throughput. CSIR-Central Mechanical Engineering Research Institute notes that traditional wooden ghanis have low crushing capacity and leave a high residual oil in the cake, which is why upgraded mechanical expellers have been developed to improve yield while retaining oil flavour and pungency. The ghani is best suited for small community-level operations where quality of oil matters more than volume.

Press-type oil expellers: hydraulic and cage presses 🔗

Press-type oil expellers operate on a batch system, applying direct compressive force to oilseeds held in a chamber or cage. The most commonly used type in this category today is the hydraulic press.

Hydraulic presses 🔗

Hydraulic press-based expellers use the principle of liquid pressure transmission to compress oilseeds and extract oil. A typical system consists of a hydraulic cylinder, a piston within a pressing chamber, a hydraulic pump station, a control valve, and a motor. The oilseeds are placed inside the press chamber, and hydraulic pressure is applied – often up to 30-50 MPa – to crush the material and expel the oil. The expelled oil drains through a filter system, while the remaining solid forms a press cake.

One key advantage of hydraulic pressing is that it operates at relatively low temperatures, which prevents oxidation and preserves the oil’s natural nutrients, antioxidants, and flavour compounds. Research published in ScienceDirect confirms that cold press extraction is a hydraulic, energy-efficient method that produces high-quality oils at low temperatures, with oils rich in lipophilic phytochemicals like antioxidants. This makes hydraulic pressing particularly well suited for premium or specialty oils – sesame, walnut, almond, olive – where oil quality is the primary concern.

The main limitation of hydraulic presses is their batch operation mode: the chamber must be loaded, pressed, and then unloaded before the next cycle begins. This reduces throughput compared to continuous screw expellers, making hydraulic presses more appropriate for small to medium-scale specialty oil production rather than high-volume industrial operations.

Cage and bridge presses 🔗

As outlined on the Wikipedia entry for oilseed presses, manual cage-style and ram presses form another class of press-type extractors. These presses use a cage structure to contain the oilseed material while a ram or screw applies compressive force. Like ghanis and hydraulic presses, cage presses operate on a batch basis, meaning they must be cleared and reloaded between pressing cycles. They are generally suited to small-scale or artisanal operations, particularly where electricity supply is unreliable and manual effort is available.

Mechanical screw expellers: continuous, high-throughput oil extraction 🔗

The mechanical screw expeller, often simply called a screw press or oil expeller, represents the most efficient and widely used mechanical oil extraction technology for medium to large-scale operations. According to Wikipedia’s article on expeller pressing, an expeller press is a screw-type machine that presses oilseeds through a caged barrel-like cavity. The term “expeller” was trademarked by Anderson International Corporation in 1900 after Valerius Anderson invented the interrupted screw design, and it has since become the standard machine in the modern oil milling industry worldwide.

Operating principle 🔗

At the heart of a screw expeller is a rotating wormshaft (also called a screw shaft) that turns inside a closely fitting cylindrical cage or barrel. Oilseeds are fed continuously into a hopper at one end of the machine. As the wormshaft rotates, it pushes the seeds forward through progressively narrowing spaces within the barrel, building up intense pressure. Research published in ScienceDirect explains that the oilseed is compressed under high pressure – typically 4 to 35 MPa – which ruptures cell walls so that oil globules escape and are forced out through slots or perforations in the barrel wall. The oil drains out of the cage, while the remaining solid material (the press cake or oil cake) is discharged from the far end of the barrel through a choke mechanism that controls the degree of compression.

One important characteristic of screw expellers is that friction between the seeds and the screw generates heat. As noted by Wikipedia, the pressure involved typically creates temperatures in the range of 60-99°C during pressing. This heat helps reduce oil viscosity and improves yield, but it also means the oil is subject to some thermal degradation. For cold-pressed oil production, manufacturers run the press at lower speeds and sometimes add cooling apparatus to keep extraction temperatures below 50°C.

Hot pressing vs. cold pressing in screw expellers 🔗

According to Desmet, a leading oilseed processing technology provider, hot pressing involves feeding seeds into the press at temperatures of 105-120°C after conditioning them with steam, which achieves higher oil extraction rates and leaves the cake with around 7-10% residual oil. Cold pressing, by contrast, uses ambient-temperature feed material and lower screw speeds, prioritising oil quality and protein preservation in the cake over maximum oil yield. A peer-reviewed study in the journal Foods (MDPI) found that screw-press cold pressing of sesame seeds achieved the highest oil yield among tested oilseeds at around 30.6%, with material losses across all seeds remaining between 1.5% and 3.3%, indicating strong overall efficiency of the press.

Pre-pressing and full pressing 🔗

Screw expellers can be used in two processing modes. In full pressing, the expeller is set to extract as much oil as possible in a single pass, leaving a press cake with roughly 4-9% residual oil depending on the seed type. French Oil Mill Machinery Company, one of the oldest manufacturers of screw presses, notes that full pressing is generally suited to smaller operations processing under 200 tons of oilseed per day. In pre-pressing, the expeller performs only a partial extraction, and the resulting cake – still containing 18-20% oil – is then sent to a solvent extraction plant for further processing. This combined approach is commonly used for high-oil-content seeds like canola, sunflower, and palm kernels, and is standard in large industrial oil mills.

Efficiency and throughput 🔗

Research on screw press design published in ScienceDirect shows that conventional screw expellers leave about 8-14% of expressible oil in the de-oiled cake, and typically require multiple passes to achieve around 80% oil recovery. Screw presses have capacities ranging from as little as 9 kg per hour to over 16,000 kg per hour, making them scalable across a wide range of production environments – from small rural processing units to large commercial oil mills. Their key advantages over batch systems are continuous operation, higher throughput, and reduced labour costs per unit of oil produced.

Comparing the three methods: which suits which scale? 🔗

Each mechanical oil expulsion method has a distinct operational niche. The table below summarises the key differences:

Ghani: Suitable for village or community-level production. Batch operation, low throughput (up to 100 kg seed per day), approximately 60% oil yield, low capital cost, produces traditionally flavoured oil. Best for mustard, sesame, groundnut, and copra.

Hydraulic press: Suited to small and medium specialty oil producers. Batch operation, low to medium throughput (processing cycles of 5-8 minutes per batch), very high oil quality due to low-temperature pressing, higher capital cost than a ghani but lower than large expellers. Best for sesame, walnut, almond, olive, and other premium seed oils.

Mechanical screw expeller: Suited to medium and large-scale operations. Continuous operation, high throughput (9 to 16,000+ kg/h), oil yield of 80-92% depending on seed and settings, higher capital and energy cost, produces oil that may require further refining. Applicable to virtually all commercially processed oilseeds including soybean, canola, sunflower, cotton, and palm kernel.

As noted by French Oil Mill Machinery, mechanical extraction is generally more economical for smaller-capacity operations under 500 tons per day, while solvent extraction becomes more cost-effective at larger scales. Residual oil in the press cake from mechanical methods – typically 4-8% – is higher than what solvent extraction achieves, but the process is solvent-free and produces a protein-rich cake that commands higher value as livestock feed.

The oil cake: a valuable by-product 🔗

Regardless of which mechanical method is used, the press cake left behind after oil extraction is not waste – it is a valuable co-product. Oil cake produced by the mechanical pressing method retains approximately 5-10% or more of the original oil content of the seeds and is widely used as a protein supplement for cattle, poultry, and other livestock. Cakes from different oilseeds carry different protein levels and nutritional profiles, which influence their market value. Soybean cake and canola meal, for instance, are among the most commercially important protein feeds globally. Some cakes – such as mustard seed meal – contain compounds that limit their direct use in animal feed, and require further processing before use.

Pre-treatment of oilseeds before mechanical pressing 🔗

Mechanical oil expulsion does not begin with simply feeding seeds into a machine. As explained on the Wikipedia page for oilseed presses, effective pressing requires clean, dry seed to begin with – foreign material such as rocks, soil, chaff, and dust must be removed, as they reduce efficiency and cause wear. Seeds with hard coats, such as sunflowers and groundnuts, need dehusking or decortication before pressing. Moisture content of the seed is also critical: Pennsylvania State University Extension notes that seeds at the typical storage moisture of around 10% do not press well, as excess moisture ties up the oil and prevents clean separation from the meal. For most oilseeds, the optimal pressing moisture is in the 7-8% range. For hot-pressing operations, seeds may also be heated or conditioned with steam before entering the screw press to break down cell walls and reduce oil viscosity, improving extraction efficiency.

What do you think? Given that ghanis produce traditionally valued oil but with lower efficiency, and screw expellers offer higher yields but at greater energy cost – how should small-scale oilseed processors in developing countries weigh oil quality against extraction efficiency when choosing their processing method? And as consumer demand for cold-pressed, chemical-free oils continues to grow, do you think hydraulic pressing will become a more prominent method in commercial oil production?

References

1. https://frenchoil.com/articles-events/oilseed-extraction-using-mechanical-extraction-and-beyond/
2. https://www.sciencedirect.com/topics/engineering/pressing-extraction
3. https://www.appropedia.org/Small_Scale_Vegetable_Oil_Extraction/Oil_extraction
4. https://www.cmeri.res.in/technology/oil-expellers
5. https://kumarmetal.com/expellers-edible-oil-production-everything-you-need-to-know/
6. https://www.sciencedirect.com/science/article/abs/pii/S2949824425002022
7. https://en.wikipedia.org/wiki/Oilseed_press
8. https://en.wikipedia.org/wiki/Expeller_pressing
9. https://www.sciencedirect.com/science/article/abs/pii/S026087749900134X
10. https://www.desmet.com/en/feed/full-pressing
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC10573014/
12. https://frenchoil.com/products/oilseed-equipment/
13. https://www.kmecomp.com/oil-mill-plant/oil-pressing/
14. https://extension.psu.edu/oilseed-presses