Borgmann Aquaponics Hydroponics
Cold Pressing
Cold pressing sounds like a uniform process. In reality, this term conceals two fundamentally different procedures that, aside from the absence of external heat application, have little in common: mechanical pressing of oil-rich seeds and fruits to obtain fixed vegetable oils on the one hand – and abrasion of citrus peels to obtain essential oils on the other. Both methods follow purely physical-mechanical principles; solvents, external heat, and chemical auxiliaries play no role. This is precisely what makes their products so valuable – and their correct execution so demanding.
Two Methods Under One Name – A Necessary Distinction
The term "cold-pressed" appears on cooking oil bottles as well as on flacons of bergamot oil. Behind this apparent uniformity lie two processes that must be considered separately, as they involve different raw materials, different physical principles, and different end products.
Method A: Pressing Fixed Oils
Raw materials: Oil-rich seeds, kernels, and fruits – rapeseed, flax, hemp, sunflower, walnut, olive, pumpkin seed, black cumin, almond.
Principle: Mechanical pressure ruptures cell walls and expels the fixed oil stored in fat cells (lipid droplets/oleosomes). The oil is the plant's storage lipid – not a volatile aromatic oil.
Product: Fixed, non-volatile vegetable oil (triglycerides). Leaves a permanent grease stain on paper.
Method B: Abrasion of Citrus Peels
Raw materials: Exclusively the peels of citrus fruits – lemon, orange, bergamot, grapefruit, mandarin, lime.
Principle: Mechanical rupturing of the oil glands (schizolysigenous passages/otricoles) in the outermost peel layer, the flavedo. The released oil is washed away with water and centrifuged.
Product: Essential citrus oil (volatile, terpene-rich). Evaporates without residue from paper.
This distinction is not academic but practically relevant: anyone wanting to cold-press flaxseed oil needs a screw press and oil-rich seeds. Anyone wanting to obtain lemon oil needs a grater and lemon peels – and no pressure in the technical sense. Confusing these two processes is the most common source of misunderstanding regarding cold pressing.
Physics and Technology of Seed Pressing – Fixed Oils
Cellular Mechanics Under Pressure
Oil-rich seeds store their energy reserves in the form of triglycerides in membrane-enclosed lipid vacuoles, the so-called oleosomes (also lipid bodies or spherosomes). These are embedded within the cell interior and protected by the surrounding cell wall. Cold pressing uses mechanical pressure to first rupture the cell wall and then the oleosome membranes, releasing the oil and allowing it to escape through fine sieve gaps from the press cake. This is a purely physical process without any solvent content – no substance is dissolved, only mechanically liberated.
Critical for yield is the preparation of the seed material. Raw, intact seed material releases only a fraction of its oil content under pressure because elastic cell walls initially yield to pressure without breaking. Rolling mills or crushers pre-comminute the material and pre-break the cell structure – only then is efficient pressing possible. For hard-shelled seeds such as rapeseed or sunflower kernels, this step is mandatory; for soft oil fruits like olives, it can be omitted.
The Screw Press – Standard Equipment for Cold Pressing
Modern oil mills and also the electric oil presses available for home use operate on the principle of the screw or expeller press: a rotating auger moves the press material through a gradually narrowing channel in which pressure continuously increases. The oil emerges through fine slots in the surrounding sieve cylinder (screen), while the press cake is expelled at the end of the channel. The most important physical side effect of this process is unavoidable: frictional heat. Depending on the setting, throughput, and oil content of the material, the temperature of the emerging oil in home devices can reach 40–70 °C – even though no external heat is applied.
This makes the term "cold-pressed" in the home context a matter of interpretation. EU Regulation No. 29/2012 defines the designation "first cold pressing" for olive oil precisely: the oil must be obtained at a maximum temperature of 27 °C. This threshold is difficult to reliably achieve with poorly regulated home presses. For sensitive oils such as flaxseed oil or hemp oil, where α-linolenic acid dominates, temperatures above 40 °C represent a genuine loss of quality: polyunsaturated fatty acids oxidize more quickly, and the formation of lipid peroxides begins.
Press Cake – The Underestimated Byproduct
The oil cake remaining after pressing still contains 8–15 % residual oil that cannot be recovered through simple pressing. Industrially, this residue is fully extracted using solvent extraction (hexane) – a process that fundamentally distinguishes cold-pressed quality oil from refined products. The oil cake is protein-rich and used as animal feed or fertilizer; with nuts and almonds, it is commonly used as a flour substitute in cooking.
Active Ingredient Profile of Fixed Oils
In contrast to distillation, where only volatile substances are captured, cold pressing captures the entire lipophilic active ingredient spectrum: triglycerides, phospholipids, sterols (e.g., β-sitosterol), fat-soluble vitamins (particularly vitamin E as a natural antioxidant), carotenoids, and plant-specific accompanying substances that give the oil its aroma and color. This density of active ingredients is the central quality advantage of cold-pressed oils over refined products, where degumming, bleaching, and deodorization systematically remove these accompanying substances.
The Anatomy of Citrus Fruit and the Principle of Abrasion
Where the Oil Resides – The Flavedo
Citrus fruits do not store their essential oils evenly distributed throughout the tissue, but in highly specialized structures visible to the naked eye: the oil glands (botanically: schizolysigenous passages, also called otricoles), which are located exclusively in the outermost peel layer, the flavedo (the colored peel, also exocarp). The underlying white, spongy tissue – the albedo – contains no essential oil but is rich in pectin and bitter substances that transfer into the oil-water mixture if abraded too deeply, impairing quality.
The art of citrus oil production therefore lies in mechanically rupturing only the flavedo without damaging the albedo. Abrading too deeply introduces pectin and bitter substances into the oil and negatively alters its profile. This principle applies to hand methods as well as industrial machines.
Traditional Hand Methods – Sfumatura and Écuelle à piquer
Before mechanization in the 20th century, citrus oils were obtained through laborious manual work. Two methods have historical significance:
- Sfumatura (from Calabria and Sicily): Halved fruits were rubbed by hand over a sponge covered with natural bristles. The oil-water mixture saturated the sponge, which was regularly pressed out into a collection vessel. An experienced worker could produce about 3 kg of bergamot oil per working day – an extraordinary quantity for the time. The method delivered exceptionally high-quality oil through minimal mechanical stress and complete avoidance of heat. Genuine sfumatura bergamot oil is now a rarity on the world market.
- Écuelle à piquer (from Grasse/Provence): A bowl-shaped vessel with metal spikes mounted inside. The citrus fruit was moved rotatingly over the spikes, which punctured the oil glands in the flavedo point by point. The emerging oil collected in the basin. This method was also artisanal and is no longer industrially relevant, though it can still be found in specialized workshops.
Industrial Processes – Sfumatrice, Pelatrice, and Centrifuge
Today, the industrial production of citrus oils is carried out exclusively by machine. The dominant method is the Sfumatrice process: whole fruits or just the peels are abraded in drum-like chambers with rasping elements. Water is continuously supplied to transport the emerging oil away as an oil-water emulsion. This mixture is separated in centrifuges: since essential citrus oils are lighter than water (density typically 0.84–0.86 g/ml), they float to the top and are continuously drawn off.
In the Pelatrice, the fruit peels are guided against rotating grinding discs. It is preferred when fruit juice and peel oil are to be obtained simultaneously – for example in orange processing, where both products are equally valuable commodities.
Plant-Specific Characteristics
Both for fixed oils and citrus oils, the botany of the raw material crucially determines how pressing must be carried out and what product quality is realistically achievable.
Olive Oil (Olea europaea)
Olive oil is the only common case where an oil-rich fruit (rather than a seed) is pressed. The oil resides in the fruit pulp (mesocarp), not the pit. The fruits are crushed by millstones or metal hammers into a paste, which is then pressed or centrifuged. EU Regulation No. 29/2012 stipulates for the designation "first cold pressing" a maximum temperature of 27 °C and a traditional pressing system. Quality-determining factors are harvest time (early = bitter, polyphenol-rich oil; late = mild, but fewer antioxidants), harvest damage (mechanical injury increases free fatty acids through enzymatic processes), and processing time (within 24 hours of harvest is standard for quality producers).
Flaxseed Oil (Linum usitatissimum)
Flaxseed oil is the most sensitive of all common pressed oils. With a content of up to 60 % α-linolenic acid (omega-3), it is exceptionally oxidation-sensitive. Quality can irreversibly decline within days with improper handling. For home production: press only small quantities, fill immediately into light-protective bottles, minimize headspace, and store in the refrigerator. Pressing temperature should not exceed 30 °C – achievable with home devices only through slow operation with pauses. Unpressed seeds keep for years when stored cool and dry; the finished oil should be consumed within 4–8 weeks.
Bergamot (Citrus bergamia)
Bergamot oil from Calabria is considered one of the finest citrus oils, central to classic Eau de Cologne compositions. The oil is cold-pressed exclusively from the flavedo; the fruit itself is unsuitable as food due to its bitterness. Bergamot oil contains bergapten (5-methoxypsoralen), a furanocoumarin with strong photosensitizing effects: direct skin contact followed by UV exposure can cause intense, persistent pigmentation. Bergapten-free bergamot oil (produced by vacuum distillation or activated carbon adsorption) is the safer alternative for cosmetic applications. This safety aspect must be carefully considered for home production.
Black Cumin Oil (Nigella sativa)
Black cumin oil has attracted considerable medical interest, particularly due to its content of thymoquinone, a benzoquinone derivative with well-documented antioxidant and immunomodulatory properties. The oil content is about 28–35 %, allowing good pressing yields. The small, hard seeds must be pre-cracked for efficient extraction; whole seeds produce very poor yields in home presses. Thymoquinone is relatively heat-sensitive, making temperature control during pressing more important than for more robust oils. The deep black to dark green color of the cold-pressed oil is plant-typical and not a quality defect.
Home Application – A Realistic Picture
Fixed Oils: Possible, with Limitations
Electric screw presses for household use are available from about 100–350 EUR from specialty retailers and are well-suited for a range of oilseeds: pumpkin seeds, sunflower seeds, walnuts, hazelnuts, almonds, sesame. For sensitive oils like flaxseed or hemp, quality losses due to frictional heat are realistic unless the press is actively cooled. For very small, hard seeds like rapeseed, home application is technically demanding and yields are low.
Quality control without a laboratory: An infrared thermometer measures the temperature of the emerging oil. Above 40 °C: pause and allow the device to cool. The oil should be clear to slightly cloudy immediately after pressing; pronounced cloudiness indicates press setting or raw material moisture issues.
Citrus Oils: Home Version Following the Sfumatura Principle
Industrial centrifugation is not feasible at home. However, a practical home version is possible:
- Grate only the colored outer peel (flavedo) with a fine grater, working over a collection vessel. Do not grate into the white layer.
- Place the grated peel in a fine cloth and squeeze firmly.
- Allow the cloudy oil-water mixture to stand in a narrow glass for at least 24 hours – the oil rises to the top and can be pipetted off.
- Yield: from 5–6 organic lemons approximately 0.5–1 ml of oil. Small, but of exceptional freshness and authenticity.
Use only certified organic fruit – fat-soluble pesticides from conventional cultivation accumulate in the peel oil.
Critical Parameters & Safety Information
Pesticides in Citrus Oils
Since cold pressing only processes the peel structure, all lipophilic pesticides on or in the peel transfer directly into the oil. Pyrethroids, organophosphates, and many fungicides are fat-soluble and become concentrated through cold pressing, not diluted – unlike fruit juice, which comes primarily from the fruit pulp and contains little peel pesticide. Use only certified organic fruit; the term "untreated" is not legally sufficiently defined.
Furanocoumarins – Photosensitization
Bergamot, grapefruit, and lime contain relevant amounts of photosensitizing furanocoumarins (bergapten, psoralen). Cold-pressed oils of these fruits applied to the skin followed by UV exposure: risk of phototoxic reactions (redness, blistering, persistent hyperpigmentation). Safe alternative: distilled citrus oils, where furanocoumarins remain in the residue.
Shelf Life of Cold-Pressed Oils
Citrus oils: high monoterpene content (> 90 % limonene for orange and lemon oil) makes them particularly oxidation-sensitive. Consume within 6–12 months after opening; always store in dark glass bottles in a cool place.
Fixed oils: depending on fatty acid profile, 4 weeks (flaxseed oil, refrigerated) to several years (jojoba oil, coconut oil). Indicator of rancidity: typical tallowy, soapy odor – then discard, do not continue using.
Yields and Key Figures at a Glance
Home yields are typically 20–40 % below the stated values due to equipment losses and incomplete cell preparation.
| Raw Material | Method | Oil Content | Home Yield (approx.) | Special Features |
|---|---|---|---|---|
| Olive Oil Olea europaea | Paste → Press/Centrifuge | 12–25 % (fruit pulp) | 80–150 ml / kg fruit | EU regulation: Temp. ≤ 27 °C for designation "first cold pressing" |
| Pumpkin Seed Oil Cucurbita pepo | Screw press | 35–50 % | 200–300 ml / kg seeds | Styrian: seeds pre-roasted (then not cold-pressed oil in the strict sense) |
| Flaxseed Oil Linum usitatissimum | Screw press | 38–45 % | 200–280 ml / kg seeds | Extremely oxidation-sensitive; pressing temperature ≤ 30 °C mandatory |
| Black Cumin Oil Nigella sativa | Screw press (pre-crack seeds) | 28–35 % | 150–220 ml / kg seeds | Thymoquinone heat-sensitive; dark oil typical |
| Lemon Oil Citrus limon | Abrasion (Sfumatrice / home grater) | 0.2–0.4 % (fresh peel) | 0.5–1 ml / 5–6 fruits | Only organic fruit; very high limonene content, rapidly oxidizing |
| Bergamot Oil Citrus bergamia | Abrasion (Sfumatrice) | 0.3–0.6 % (fresh peel) | 1–2 ml / 6–8 fruits | Bergapten → photosensitization; use only organic fruit |
Further Reading and References
All works listed below have been verified for existence (SpringerLink, German National Library, VLB book directory):
- Matthäus B., Brühl L.: Handbuch Native Pflanzenöle. Agrimedia GmbH & Co. KG, 2008. ISBN 978-3-86037-304-2. — Standard work on cold pressing of fixed oils with detailed chapters on raw material quality, pressing technology, yield, and quality assurance.
- Belitz H.-D., Grosch W., Schieberle P.: Food Chemistry. 4th edition, Springer-Verlag, 2009. DOI 10.1007/978-3-540-69934-1. — Chapter "Lipids": fatty acids, oxidation mechanisms, and production methods for vegetable oils.
- Carle R. (ed.): Ätherische Öle – Anspruch und Wirklichkeit. Wissenschaftliche Verlagsgesellschaft Stuttgart, 1993. — Contains chapter on expression of citrus oils including traditional and industrial production methods.
- European Union: Commission Implementing Regulation (EU) No. 29/2012 of 13 January 2012, Official Journal L 12/14. — Legally binding definitions of the terms "first cold pressing" (Art. 5a) and "cold extraction" (Art. 5b) for olive oils.
- Blaschek W. (ed.): Wichtl – Teedrogen und Phytopharmaka. 6th edition, Wissenschaftliche Verlagsgesellschaft Stuttgart, 2016. ISBN 978-3-8047-3068-7. — Monographs on black cumin and flaxseed oil with information on ingredients and pharmaceutical quality.
The yield guidelines are based on published literature values and have been conservatively adjusted for home application conditions. Industrial yields are systematically higher. Information on historical production methods (Sfumatura, Écuelle à piquer) is documented in secondary literature on perfume history and citrus oil production in Calabria.
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