In the domain of food science, there is a so-called “smoke point” of oil, which refers to the temperature at which it stops shimmering and starts smoking. Compared with refined oil, crude oil contains more moisture, impurities, free fatty acids and other components, and its smoke point is generally lower, sometimes even lower than the usual cooking temperature (120°C-180°C). If use directly this kind of oil, there is a lot of oily smoke formation during the cooking process.
The formation of oil fume is bad for health. Thermal decomposition of oil may form polycyclic aromatic hydrocarbons (PAHs), small molecular aldehydes and other substances, which are currently considered to have a higher possibility of carcinogenesis. Lampblack itself is also one of the sources of PM2.5 air pollution. There is currently evidence that frequent exposure to oil fume increases the risk of lung cancer. From this point of view, it is necessary to refine the crude oil.
Many unrefined oils are packed with minerals, enzymes, and other compounds that don’t play well with heat and tend to be especially susceptible to rancidity. To produce an oil with a high smoke point, manufacturers use industrial-level refinement processes like bleaching, filtering, and high-temperature heating to extract and eliminate those extraneous compounds. What you’re left with is a neutral-flavored oil with a longer shelf life and a higher smoke point. The following table shows the different smoke point between crude and refined oils, which are the most common in our daily lives.
|Crude oil||Refined oil|
|Coconut Oil||350°F / 177°C||450°F / 232°C|
|Corn germe oil||320°F / 160°C|
|Peanut oil||320°F / 160°C|
|Sunflower oil||320°F / 160°C|
|Sesame oil||350°F / 177°C|
|Soybean oil||320°F / 160°C|
|Canola oil||464°F / 240°C||470°F / 240°C|
Ref: The Vegetarian Helth Institute
In addition to improving the smoke point, refining has the following benefits :
1) Refined oil looks better.
Refining includes the steps of decolorization and deodorization, which can remove the unpleasant colors and odors in the crude oil, and make it become clear and translucent, and there is no strange taste.
2) Refined oil can meet the needs of more end products.
That’s to say, as an industrial product, need to be consider to meet various needs in the later period. At this time, the “refined oil” with higher purity and suitable for various processing needs is more in line with the needs of industrialization than the “crude oil” with limited use, and it is easier to be standardize.
3) The oil refining process can produce some by-products incidentally. For exemple, the process of degumming crude oil can produce phospholipids. Phospholipids are highly value-added products and can be used as emulsifiers and stabilizers in many foods. Some vegetable oil deodorization processes can be used to produce tocopherols (vitamin E). Some oil refineries also separate the oils according to the melting points of fatty acids. In other words, palm oil production companies often extract palm oil into “palm stearin” with a relatively high melting point and “palm liquid oil” with a relatively low melting point.
4) Food safety requires a refining process to remove impurities and toxins harmful to the human body in the raw materials, and the refined oil is generally safer. Cottonseed oil is a special example: it contains gossypol, which is toxic for reproduction, so it must be refined for edible purposes.
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