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

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Vegetable oils are all those oils that are derived from plant products like seeds, nuts and fruits. Its popular sources include groundnut, soybean, sunflower, rapeseed, linseed, safflower, peanut, coconut, plum, olive, etc. Such oils usually don’t contain any toxic substance like lectin. With the diversification of crop production over last couple of decades, now-a-days many other sources have been emerged that include neem, tobacco, castor, drumstick, almond, avocado, bean, rice, salmon, etc. Oil extracted from such non-conventional plant sources are not commonly used as food; rather they fulfil growing industrial demands for producing various items. Therefore, vegetable oils may not necessarily be edible; however, they are organic, bio-degradable and reusable.

Mineral oils are petroleum based hydrocarbons that are derived by distillation of crude oil. These are usually by-products of crude oil refining process while extracting gasoline and other petroleum products. These are usually colorless and odourless but not suitable as food ingredient for human consumption as they content various toxic compounds. However, they are overwhelmingly used for several industrial applications such as production of baby oil, cosmetics, laxative, paint, varnish, lubricant, coolant, engine oil, brake fluid, cutting oil, etc. Apart from crude oil, mineral oils can also be extracted from oil shale, especially cannel coal; however, extraction is very costly and are not conventionally done. Various differences between vegetable oils and mineral oils are given below in table format.

Table: Differences between vegetable oil and mineral oil

Sources: Mineral oil and vegetable oil are obtained from different sources and thus have different properties. Vegetable oils have predominantly plant based sources like oilseeds, nuts and fruits. Typical sources from where vegetable oil is extracted include seeds or fruits of groundnut, soybean, sunflower, rapeseed, safflower, peanut, neem, tobacco, castor, drumstick, almond, avocado, palm, coconut, olive, etc. Such seeds are first dried and then compressed without or without heating (cold pressing and hot pressing). On the other hand, mineral oils are extracted from crude oil (unprocessed petroleum). These are by-products of crude oil refining process obtained during extracting gasoline and other petroleum products. Mineral oil can also be extracted from candle coal (terrestrial type oil shale) but is not commonly done due to high extraction cost and low output.

Mineral oil is not actually oil: Biologically, fat is considered as oil. So oil must contain fatty acid. Vegetable oils do contain fatty acid and thus are fat; however, mineral oil is not a fat. Outside the boundary of biology, every liquid (at normal temperature) derived from petroleum can be considered as oil.

Bio-degradability and spoilage: Biodegradation is defined as the bio-chemical process by which compound organic substances are gradually broken down (reduced) into simpler compounds by living microbial organisms including bacteria and fungi. Biodegradability is the measure of possibility or rate of biodegradation of specific substance under defined conditions. Organic substances are commonly biodegradable but not all substances depict same level of biodegradability. Vegetable oil, being the simplest form of hydrocarbon, shows high biodegradability as microorganisms can easily digest to reduce it to carbon di-oxide. Their spoilage also has little concern towards environment pollution as these are degraded naturally. Mineral oils, being complex hydrocarbon, show very low bioremediation rate in natural conditions and thus its spilling (either accidentally or due to human activities) poses great hazard for terrestrial and marine ecosystems. Most bacteria fail to establish a favourable condition to enhance degradation rate due to toxicity of mineral oils and thus yeast and fungi became primary decomposer.

Edible or non-edible oil: An edible oil is one that is nutritious and digestible for human when consumed directly or indirectly as food ingredient. They must not contain toxic element that can pose health risk. Majority of vegetable oils are edible; however, certain oils (like oil extracted from groundnut, soybean, sunflower, rapeseed, safflower, peanut, etc.) are nutrition rich and thus are consumed commonly. Other vegetable oils are commercially used for various industrial applications. Mineral oils are non-edible; they are produced for industrial or household usage. Their typical applications include production of animal laxative, lubricant, coolant, engine oil, brake fluid, grease, etc. Some of them also have pesticidal, medicinal and cosmetic usages.

Reproduction: Vegetable oils are extracted from plant derivatives and thus these are renewable and inexhaustible. Mineral oils are based on petroleum or crude oil which has limited storage. Global crude oil reserve is depleting day by day. Crude oil forms naturally by geothermal process when large scale dead zooplankton and algae are buried under sedimentary rock and subjected to intense heat and pressure. Naturally occurring petroleum takes millions of years for formation, which is far longer than can be deemed renewable. So mineral oil cannot be replenished, at least within the time scale of human. This global concern has paved the way for extensive research to find out alternative. Many vegetable oils have showed potential capability in several fields most prominently bio-fuel.

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Apart from vegetable oil, there are many petroleum based oils which are non-edible. Such petroleum oils are extensively used for various industrial applications like fuels. Rapid depletion of petroleum oil reserve along with the steep increase in energy demand throughout the world paved the way for finding out alternative sources for wide industrial requirements. As a result of extensive researches, now-a-days it is possible to utilize certain vegetable oils as bio-diesel and others. However, large scale commercialization is still pending owing to its high cost and low rate of supply. Therefore, all vegetable and petroleum oils can be broadly classified into another two groups—edible oil and non-edible oil.

As the name suggests, edible oils are beneficial for human consumption and are popularly used for such purpose. They have high nutritional value and less industrial demand. Such oils also require less processing to make it safe and hygienic for health. Majority of vegetable oils come under edible oil; however not all. There are few vegetable oils and all petroleum oils which are not suitable for eating and have industrial demand. Such oils are grouped under the umbrella of non-edible oils. Even animal fats also come under this group. Various differences between edible oil and non-edible oil are given below in table format.

Table: Differences between edible oil and non-edible oil

Sources: Edible oils are directly extracted from oilseed grains, nuts and tree fruits. Thus their sources are predominantly organic; for example seeds of groundnut, soybean, sunflower, rapeseed, linseed, safflower, peanut, etc. Oil extracted from tree fruits or seeds like palm oil, coconut oil, olive oil, etc. are also edible. All vegetable oils are not edible; some vegetable oils have low demand as food source. There are more than 50 crops whose oil is primarily used for industrial purposes, some popular examples are rubber, neem, tobacco, castor, drumstick, almond, avocado, tomato, bean, date, rice, milkweed, sal, salmon, kusum, jojoba, etc. Apart from these non-edible vegetable oils, petroleum based oils and animal fat are predominantly used for non-edible purposes. So non-edible oils consist of non-edible vegetable oils, petroleum oils, and animal fats and therefore, it can be organic or inorganic. However, there exist no clear boundary between culinary use and industrial use, so certain oils can be used for both purposes based on market demand and production.

Applications: Edible oils are consumed, directly or indirectly, as food ingredients. They are mainly used for cooking including sautéing, frying and baking. They can also be consumed directly (like flavouring) without any culinary processing; however, in limited amount. Non-edible oils are not used as food intake, rather they are utilized in various forms for several industrial applications. Few major consumers of such oils are transport industries, candle, soap, synthetic detergent, resin, ink, wax, dye, paint and varnish factories. Non-edible vegetable oils, which are also known as second generation feedstock, can be used for sustainable production of biodiesel and thus have emerged as promising substitute for rapidly depleting petroleum oil reserves. Apart from fuel, they can also be used for the production of lubricant, coolant, engine oil, grease, etc. Some of them also have pesticidal, medicinal and cosmetic usages.

Nutritional value: Edible oils must be rich in nutrition; ideally such oils should be digestible, rich in caloric value and free from unsaturated fat. Most edible oils are actually healthy fat and don’t contain any toxic component like lectin that can pose a risk on human health. Oils help absorbing fat soluble vitamins (A, D, E, and K) and also provide other benefits. Non-edible oils are either not safe for human consumption as food or not rich in nutrition.

Processing requirement: Usually edible oils do not require any chemical processing for its extraction from oilseeds, nuts or fruits. Cold or hot pressing are considered as sufficient for extraction. First pressed oil are of high quality in terms of nutrition and purity and the same has extra value towards culinary usage. However, animal fat must be processed in various physical and chemical methods before its consumption. Non-edible organic or inorganic oil usually requires several chemical treatments to make it suitable for specific purpose.

Oil cost: Edible oil extraction process must be hygienic with highest level of purity. Any deviation may cause altercation with toxic elements or bacterial infection that can make the oil unsafe for human consumption. Tight quality control requirement along with higher demands and limited production made edible oil costlier. Non-edible vegetable oil plants can be grown in marginal lands having low fertility. They can be grown in arid zones (regions with low rainfall) with minimum requirement of irrigation facility. Apart from oil, such plants also generate by-products that have sufficient commercial value. So non-edible vegetable oils are cheaper as compared to edible one. Other forms of non-edible oils are also cheaper but their resources are limited and are influenced by geo-political factors.

A through comparison among edible oil and non-edible oil is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Atabani et al. (2013); Non-edible vegetable oils: A critical evaluation of oil extraction, fatty acid compositions, biodiesel production, characteristics, engine performance and emissions production; Renewable and Sustainable Energy Reviews; Vol.18; pp. 211-245.
• Oilseeds: Properties, Processing, Products and Procedures by G. Nagaraj (2009, NIPA).

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