Refining (refinement) is generally a process of refining the raw material into a cleaner form. In particular, refining of vegetable oils is common in the food industry. Raw oils are obtained from seeds using the following methods:
- cold pressing (approx. 35 °C – extra virgin: preserves the most nutritional ingredients, antioxidants, vitamins etc.)
- pressing at higher temperatures (approx. 90 °C, mostly by pressing already cold pressed residues – lower quality)
- extraction (a solvent dissolves pressing waste, and oil is separated from the solution by distillation – poorest quality)
Although the raw pressed oils are the healthiest, they have a distinctive smell, they are cloudy (often even after mechanical filtration) and dense. The customer is more satisfied with a processed oil, which is clear, thin, does not grow turbid (similarly to homogenization) and has no distinctive scent or taste. In addition, the raw oil is much more prone to oxidation (less durable) and in case the extraction process was used, it also contains toxic substances to be disposed of. All this will solve the refining process.
The refinement consists of the following steps:
- stripping of the dirt (the residues of protein, lecithin or other substances are removed, that would make the oil cloudy. The process is carried out at high temperature, with the aid of a centrifuge and salts)
- neutralization or alkaline refining (removal of free fatty acids, ie those not bound to triglycerides – to improve odor and taste). The process is done with the aid of lye (NaOH), a sodium soap is formed, which must then be washed with hot water.
- acidic refining (sulfuric acid (H2SO4) is added at high temperature to remove (mineralize) other impurities, subsequently the oil is washed with hot water)
- bleaching (using bleaching clay – pigments removal)
- deodorization (application of overheated water vapor under negative pressure at a temperature of about 200 °C – removal of volatile substances that’ll attach to the steam particles)
- dewaxing (cooling and filtration)
These steps also result in:
- removing some of the beneficial substances (vitamins, antioxidants – tocopherols, carotenoids, phytosterols) – although manufacturers sometimes add them back when bottling, efficiency is questionable
- deterioration (partial degradation) by multiple heating to high temperature
- trans fats (up to 1%)
- there may be a risk that spoiling oil will also be used (refining will mask the difference)
Since refined oil has better oxidative stability, it can be filled into cheaper transparent plastic bottles. Oil prone to oxidation is usually in dark glass bottles or opaque tin containers.