OIL AND FAT: OIL EXTRACTION
Diagram
Two ways to get oil out of a seed
Cold-pressed oil
Oil squeezed out by mechanical pressure only, with no added heat and no solvent. Keeps flavour and colour.
↓
Solvent-extracted oil
Oil pulled out with a solvent, usually hexane, then the solvent is removed and the oil is refined. Higher yield, neutral taste.
Figure 1: Cold pressing squeezes oil out with pressure only, while solvent extraction pulls it out with a chemical solvent. Drawn in our own style from the extraction discussion in Gunstone, Rapeseed and Canola Oil, and in Akoh and Min, Food Lipids.
Introduction
Cold-pressed oil and solvent-extracted oil are both vegetable oils, but they are taken out of the seed in different ways. Cold pressing squeezes the oil out with mechanical pressure only, with no added heat and no chemical solvent, so the oil keeps the flavour, colour and aroma of the seed or fruit. Solvent extraction soaks the crushed seed in a solvent, usually hexane, which dissolves the oil out, then the solvent is removed and the oil is refined. Solvent extraction pulls out far more oil than pressing alone, but the oil is then refined and comes out neutral.
Quick comparison
Table 1: Cold-Pressed Oil compared with Solvent-Extracted Oil
| Aspect | Cold-Pressed Oil | Solvent-Extracted Oil |
|---|---|---|
| Method | Mechanical pressing only | Soaking in a solvent, usually hexane |
| Heat | No added heat | Heat is used to remove the solvent and to refine the oil |
| Chemicals | None | A solvent is used, then removed |
| Yield | Lower, some oil stays in the press cake | Higher, almost all the oil is recovered |
| Flavour | Keeps the flavour and aroma of the seed or fruit | Neutral after refining |
| Colour | Keeps its natural colour | Pale after refining |
| Refining | Usually none, or only settling and filtering | Refining is needed: neutralising, bleaching and deodorising |
| Cost | Higher, because the yield is lower | Lower per litre, because the yield is higher |
| Use | Dressings, dipping and finishing, where flavour matters | General cooking, frying and food manufacture |
| Press cake | Still holds oil, used as feed or pressed again | Holds little oil after extraction |
Scientific explanation
Definition
Cold-pressed oil is oil obtained from seeds or fruits by mechanical pressure only, without added heat and without a chemical solvent. Solvent-extracted oil is oil obtained by treating crushed seed with a solvent that dissolves the oil, then removing the solvent and refining the oil.
Working principle
In cold pressing, cleaned seed is crushed and pressed in a screw press or a hydraulic press, and the oil runs out. Because no heat and no solvent are used, the flavour compounds, pigments and aroma stay in the oil, but a good part of the oil stays behind in the press cake. In solvent extraction, the crushed seed is washed with hexane, which dissolves the oil out of the seed. The solvent is then boiled off and recovered, and the crude oil is refined to remove free fatty acids, colour and odour. The yield is much higher, which is why most commodity oils are solvent extracted.
Apparatus
The books used here name no laboratory apparatus for measuring or comparing either term, so no apparatus list is given.
Measurement procedure
The books used here give no standard laboratory procedure with volumes and steps for either term, so no procedure is invented here.
Factors affecting readings
For cold pressing, the seed quality, the moisture and the press pressure decide the yield and the flavour: poor seed gives poor oil that cannot be fixed by refining. For solvent extraction, the solvent, the temperature, the time and the refining steps decide the yield and the neutrality of the oil. In both cases the seed decides the fatty acid profile, because pressing and extraction do not change the fatty acids themselves.
Formulas and calculations
The books used here give no formula for either term, so no formula is given here.
Industrial applications
- Cold-pressed oil: salad dressings and dipping, where the flavour of the seed or fruit is wanted.
- Cold-pressed oil: finishing oils, such as extra virgin olive oil, where aroma matters.
- Solvent-extracted oil: general cooking and frying, where a neutral, low cost oil is wanted.
- Solvent-extracted oil: food manufacture, where large volumes of neutral oil are needed.
- Solvent-extracted oil: the base for margarines and shortenings, after refining and modification.
Advantages and limitations
- Cold-pressed oil, advantage: it keeps the natural flavour, colour and aroma, with no chemicals used.
- Cold-pressed oil, limitation: the yield is lower, so it costs more, and it needs good seed because defects cannot be refined away.
- Solvent-extracted oil, advantage: the yield is much higher, so the oil costs less and suits large scale cooking and manufacture.
- Solvent-extracted oil, limitation: refining strips out flavour and colour, so the oil is neutral and needs the solvent to be fully removed.
FAQ
What is the difference between cold-pressed and solvent-extracted oil?
Cold pressing squeezes oil out with pressure only, while solvent extraction dissolves the oil out with a solvent such as hexane, then removes the solvent and refines the oil.
Which one tastes better?
Cold-pressed oil keeps the flavour of the seed or fruit. Solvent-extracted oil is neutral after refining.
Which one gives more oil?
Solvent extraction gives a much higher yield, because it pulls out oil that pressing leaves behind.
Is solvent left in the oil?
The solvent is boiled off and recovered, and the oil is refined before sale, so the finished oil is not a solvent mixture.
Is cold-pressed oil always unrefined?
Usually it is only settled and filtered, not refined, so it keeps its flavour and colour.
References and further reading
- FTN books on disk: Technology of Fats and Oils.
- Gunstone, Frank. Rapeseed and Canola Oil. 2004.
- Akoh, Casimir C. and David B. Min (eds). Food Lipids: Chemistry, Nutrition, and Biotechnology. 2nd ed., CRC Press, 2002 (Google Books page read)
- Google Books pages read for this article:
- Akoh, Casimir C. and David B. Min (eds). Food Lipids: Chemistry, Nutrition, and Biotechnology. 2nd ed., CRC Press, 2002 (Google Books page read)
