Oils of plant origin have been predominantly used for food-based applications. Plant oils not only represent a non-polluting renewable resource but also provide a wide diversity in fatty acids (FAs) composition with diverse applications. Besides being edible, they are now increasingly being used in industrial applications such as paints, lubricants, soaps, biofuels etc. In addition, plants can be engineered to produce fatty acids which are nutritionally beneficial to human health.
Vegetable edible oil refers to oil obtained from oilseeds and nuts through an extraction process. It can be extracted from various oil seeds such as mustard, coconut, soybean, peanut, rapeseed, cottonseed, sesame, etc. via pressing process. Plant based extracted oils are generally used for salad dressing, deep fat frying and pan frying.
Until the industrial revolution in the 19th century, rapeseed, linseed, olives, and nuts were the primary sources of vegetable oils. Today, the world market is dominated by palm and soybean oil, followed by rapeseed and sunflower oil. This has led to a change in the extraction and purification/modification processes. Originally, the oil extraction process consisted of cleaning, crushing, heating, and pressing only. But from 1900 onwards, solvent extraction was applied to recover the residual oil from the pressed cake or to replace the pressing process completely (e.g., for soybean oil). At the same time, the oil purification process changed from a simple decanting and filtration to a combination of neutralization with caustic, bleaching with active clay and deodorization at high temperature under vacuum with steam.
Commercial Process of Manufacturing Edible Vegetable Oil
In a typical edible oil processing plant oil is extracted from the seed first using mechanical extraction (expeller press) process followed by chemical extraction (hexane extraction) process. By using both methods less than 1% of the oil is left in the meal. Then the left out residual meal is sold as an animal feed raw material. Following are the steps involved for the processing of vegetable oil:
Step 1: Harvesting of Oil Seeds
Seed is planted and harvested as with any other crop. This is followed by the cleaning process, which removes unwanted materials such as soil and other seeds from the harvest. In some cases, it is preferable to shell the seed, removing hulls for a better-quality final product.
Step 2: Processing
At this point, if the seed is large, the seed is crushed or broken up into smaller pieces. These uniform pieces are then conditioned by heating before being pressed for oil. The two products of this process are the raw pressed oil and the press cake, which is the compressed dry material of the seed. The raw oil is filtered before moving on to the final steps.
Step 3: Solvent Extraction
Pressed cake is flaked and broken down for additional oil extraction. The flakes are ground up and mixed with hexane to produce a slurry, which is heated. During heating, the hexane evaporates, and is collected for further use. While being heated, the meal releases the remaining oil, which is mixed with a small amount of hexane that did not evaporate.
Step 4: Refining
It is a combination of the following process steps for producing an edible oil with characteristics that consumers desire such as bland flavor and odor, clear appearance, light color, stability to oxidation and suitability for frying:
- Degumming: A pretreatment process applied to seed oils to reduce the phosphorus content. It is a two-step process with addition of water and/or acid to hydrate phospholipids. The phospholipids are subsequently removed by centrifugation.
- Neutralization: The purpose of neutralization is to reduce the concentration of free fatty acids to a maximum of 0.10% with the use of a diluted alkali solution, typically sodium hydroxide. This process can be applied batch-wise in stirred vessels and continuously by means of centrifuges. After alkali treatment, the oil is washed with hot water or treated with silica to reduce the residual soap level in the neutralized oil.
- Bleaching: The main purpose is to remove residual soap, pigments, and oxidized components. In this process, bleaching earth (activated clay and/or silica) is added to the oil as absorbent. The earth and absorbed impurities are subsequently removed by filtration. Addition of activated carbon in the bleaching process will also reduce the polycyclic aromatic hydrocarbon level. An acid pretreatment before bleaching earth addition will improve the removal of phosphorous (max 30ppm) and/or metals during the bleaching process.
- Deodorization: Under high vacuum the oil is heated to 180–240°C and brought in contact with stripping steam to remove volatile components and to create an odorless oil with a bland taste and increased storage stability. Also, free fatty acids can be removed during deodorization, at increased temperatures (220–270°C).
The process sequence of combined degumming/neutralization followed by bleaching and deodorization is called chemical refining, referring to the chemical removal of free fatty acids. The process sequence of degumming followed by bleaching with acid pretreatment and deodorization at high temperature is called physical refining, referring to the physical removal of free fatty acids (stripping). The physical refining process is generally preferred for low phosphorous oils (acid degummed seed oils and tropical oils) due to lower oil losses and less liquid effluent production.
Industry can design almost any fat or oil for a specific application by the use of various modification processes, such as hydrogenation, interesterification, fractionation or blending. Hydrogenation typically reduces essential fatty acid content and creates various fatty acid isomers, both cis and trans. The wide flexibility available to industry through the selection of raw materials and different modification processes allows for the production of oils at the lowest cost possible, an important aspect of food production.