For refining of edible oil, there are three basic processes in the refinery as follows,
  • Neutralizing
  • Bleaching
  • Dewaxing
  • Deodorizing
First process is neutralising the oil is neutralised to remove free fatty acids (FFA) by a addition of caustic soda. Then oil is heated upto about 60˚c by thermic fluid coils or close loop steam circulation and oil is stirred by stirrer. Then soap stock is formed due to chemical reaction of acid with alkali. The formed soap is allowed to settle at the bottom of the neutraliser from where it is taken out into soap pan otherwise it send to centrifuge separator for spliting it instantly then the process is followed by refining and washing.

Second process is the bleaching process, here neutralised oil is drawn into the second vessel called bleacher where colour of oil is removed by bleaching process with aid of chemicals such as carbon black and bleaching earth. Oil is generaly is heated upto 110˚c by thermic fluid coils or by close loop steam circulation. Stirring is also continued. And this process i.e BLEACHING process is done under vacuum. Bleaching oil then goes to the pressure leaf filter where bleaching earth and chemicals are separated.

After De-gumming, Neutralization, water washing, vacuum drying and bleaching the Dewaxing is done. Dewaxing oil like sunflower, corn and rice bran contain waxes (which are esters of long chain fatty acid and alcohol) in the oil. These waxes impart haziness to the oil at lower temperature. To remove this haziness, the oil is subjected to the de-waxing process. In this process oil is cooled to de-waxing temperature and proper retention time is provided in the crystallizer to form crystals. Filtration of the oil removes the waxes.

Fourth process is the Deodorizing here clean bleached oil is then drawn to deodourizer where oil is heated above 110 ˚c through thermic fluid coils or by steam and then live steam is given to the oil from the bottom steam nozzles and temperature of oil is raised upto 200 to 220˚c through thermic fluid coils. Entire process is done under high vacuum. Thus smell is removed from the oil in the deodouriser. Then it goes to cooler where water circulating coils take away heat and oil is cooled. Again it goes to second polishing filter where completely refined and transparent golden yellow colour oil is obtained.
The miscella refining is the advance process in cottonseed oil refining. This process refines the crude oil as early as the extraction stage and was developed specifically for the neutralization of cottonseed oil to remove gossypol (a particularly stubborn pigment, toxic in nature) effectively.

The oil is dissolved in hexane in miscella refining, which is why the separators which separate the soap stock from the neutralization process are blanketed with inert gas (preferably nitrogen). The hexane is removed from the oil phase by evaporation and the cottonseed oil is then bleached and deodorized.

Therefore, the caustic treat selected for the crude oil will vary with the FFA content and the level of caustic excess over the theoretical quantity determined for each oil. The theoretical quantity of caustic is based on the ratio of molecular weights of NaOH to oleic fatty acid.

The refining conditions for cottonseed oil are chosen more for the improvement of colour because of the presence of gossypol. This pigment is sensitive to heat and oxidation, forming colour compounds that are difficult to remove from the oil other than by reaction with caustic. Therefore, a larger excess of a more concentrated NaOH solution treat is used for cottonseed oil than most other vegetable oils; good quality cottonseed oils generally require 0.2% excess, while darker colour oils may require up to 0.4% excess treat. The pretreated crude miscella then enters the caustic soda addition and mixing section where the caustic is intimately mixed and dispersed. The crude miscella caustic mixture is then contacted for a period of time to allow for proper refining to occur. The reacted mixture then passes through a trim heat exchanger to ensure the proper temperature for centrifugation in the MACFUGE-710 NYTROGEN BLANKETING SELF CLEANING CENTRIFUGE. The light colored, refined miscella is easily separated from the dark brown gelatinous soap stock in the specially designed centrifuge.

The refined miscella then reenters the extraction plant stripping system for removal of remaining of hexane. So we get at the end pure oil i.e. without solvent oil, & this oil is send forward to further operation.

The neutral oil is subjected to the bleaching process. In miscella refining, the Neutral oil contains very less % of yellow, green, and red pigments. For removing of these unwanted coloring pigments we heat the oil to bring the temperature of oil at required temperature and then bleaching earth is added which adsorb the coloring bodies. This oil is passes through pressure leaf filters and then the polishing filter.

Edible fats and oils retain undesirable flavours and odours after refining, and they develop other organoleptic undesirables during bleaching, hydrogenation, fractionation, and interesterification processing. Deodorization is basically a vacuum-steam distillation process operated at elevated temperatures to remove FFA and other volatile odoriferous components that cause the undesirable flavours and odours. Additional deodorization benefits include heat bleaching to destroy carotenoid pigments, pesticide removal, and cyclopropenoid fatty acid reduction to negligible level, all of which ensure oil purity. Deodorization is the last major processing step where flavour, odour, and many of the other qualities of an edible fat and oil product can be controlled. From this point forward, all efforts are directed toward retaining the quality of the deodorized product. Experience has shown that edible fats and oils flavour and odour removal correlates well with the reduction of FFA. The odour and flavour of oil with a 0.1% FFA will be eliminated when the FFA is reduced to 0.01% to 0.03%, assuming a zero peroxide value. Therefore, all commercial deodorization consists of steam stripping the oil for FFA removal.

NOTE: The major advantage of miscella refining over a conventional refining is in the immediate refining of crude miscella directly after the oil has been before the solvent stripping has set the color.