Edible oils are nutrition rich organic vegetable oils that are hygienic and contain no toxic substance. Such oils are healthy for human consumption either directly or as food ingredient. Primary sources of edible oils are oilseeds, nuts and fruits of various plants such as groundnut, soybean, sunflower, rapeseed, linseed, safflower, peanut, palm, coconut, olive, etc. There are certain other oilseeds which usually do not provide edible oil; but non-edible oils extracted from such vegetable sources can be used commercially for several industrial purposes. Examples include oil-bearing parts of rubber, neem, castor, drumstick, almond, avocado, bean, date, rice, milkweed, salmon, jojoba, etc. Whether it is edible or not, oil is first required to extract from such vegetable sources and subsequently refine the extracted oil prior to use.
Different vegetable sources are required to process in different ways to harness maximum yield or better quality oil. Accordingly various oil extraction methods have emerged over the years, each of them provides certain benefits over others. Such processes can be broadly grouped as mechanical extraction, solvent extraction and hybrid extraction (pre-processing and solvent method). In mechanical extraction, oil-bearing part of vegetable sources (like dry seeds, fruits, nuts, etc.) are mechanically compressed by exerting high pressure, which results in separation of oils from solid matter (that ultimately becomes oil cake). Pressing can be carried out in hydraulic press or in modern expeller (screw press). Prior to pressing, sometimes oilseeds are heated to an elevated temperature in order to get maximum yield with minimum effort. Accordingly, mechanical extraction can be classified as cold extraction and hot extraction.
There exist one temperature boundary to scientifically differentiate this two methods, which is 120°C. Cold extraction indicates temperature of the oilseeds, oil or oil cake has never exceeded 120°C prior or during extraction; while, if temperature exceeds 120°C, then the process is termed as hot extraction. With the divergence of oilseed cultivation in last few decades, a number of vegetable sources are now available and they require different ways of processing. In some cases, two or three stage processing is employed where in first stage oilseeds are cold pressed to extract 60 – 80% of total oil and then the oleaginous oil cake is again heated up at certain temperature before pressing second time and third time (rarely) to extract rest of the oil. Although it gives maximum yielding, oil quality and purity degrade. Various differences between cold extraction and hot extraction are given below in table format.
Table: Differences between cold extraction and hot extraction
| Cold Extraction | Hot Extraction |
|---|---|
| In cold extraction process temperature is always maintained below 120C. | In hot extraction oilseeds or oil cake is heated up above 120C during or prior to squeezing. |
| No heat source is required in this method. | Heat source like electrical heater, gas flame, or hot water is desired. |
| Cold pressed oil color varies from light to moderate yellow. | Hot pressed oil becomes dark in color (usually dark brown). |
| Cold pressed oils can be consumed by human just after filtering the oil after extraction. | Hot pressed oil mandatorily require refining before it is consumed. |
| Cold pressed oils are used mainly for human consumption as food ingredient. | Hot pressed oils are mainly used in the varnish, paint, detergent, soap, etc. production industries. |
| Cold pressed oils are pure, nutrition rich and hygienic. | Hot pressed oils lacks purity and nutrition quality due to heat treatment. They may even contain toxic substances. |
| Oil yielding is less but oil quality is high. | High oil yielding but oil quality hampers. |
