Fats and oils are fractionated in order to enhance their working value and/or to increase their commercial value, or to make special products. Fractionation can be applied to crude oils, to deacidified and bleached oils or to oils that have only been bleached.
Fats and oils are not homogeneous substances, but mixtures of various triglycerides with differing melting points. This affects their physical properties. Thus, fats and oils do not have a definite melting point, but a melting range. This characteristic is exploited for separation purposes.
Fractionation is a purely physical process, in which both fat and triglyceride components remain unchanged. Separation is governed exclusively by the melting point or, in the case of wet fractionation, by solubility in a suitable solvent. The liquid components of a fat are present as droplets in the crystal lattice of components with higher melting-points and are squeezed out. Despite the use of modern equipment, the process takes several hours and can be broken down into the following stages:
- Melting or dissolving the fat / oil
The three methods currently used for fractionation are:
1. Dry fractionation (without adjuvants)
Dry fractionation is based upon the principle of separation according to the differing melting points for the various components, e.g. olein and stearin. The process has the advantage of being purely physical, involving no additives of any kind and, thus, dispensing with the need for subsequent product purification. One disadvantage is relatively poor separation, compared with the two other processes. Dry fractionation is performed by heating the mixture to a temperature above the melting point and then leaving it to cool and separate, usually into two fractions.
2. Lanza fractionation
The principle is based on a patent granted to the Lanza brothers in 1905. Separation is not performed mechanically, but by wetting the crystal surface with aqueous detergents, into which salts may also be mixed.
This hinders emulsification. Wetting causes higher melting-point components in the oil to become hydrophilic and to migrate into the aqueous phase. This produces large crystal-free oil droplets that then coalesce. In the next step in processing, the two phases are separated by centrifugation.
3. Wet fractionation
The principle is based upon differences in solvent solubility at a given temperature. This results in sharp separation of the different phases, which can be controlled by adjusting the temperature and the amount of solvent. If particularly pure fractions are required, the product can be re-washed with a solvent and the latter is subsequently removed.
Winterising is a very simple form of fractionation, in which it is not two fractions with similar particle sizes that are separated, but small portions that are separated from the bulk of the oil. This process is not used to modify an oil, but to alter its behaviour, so that it becomes easier to handle at low temperatures. At refrigerator temperatures, waxes, along with high melting-point glycerides, tend to curdle. This is sometimes mistaken for spoiling.
De-acidified and bleached oil is cooled down to 5-15°C. Unwanted waxes, mucilages and high melting-point glycerides precipitate out and are filtered off. In most cases, this filtering process is problematic, due either to the small amounts of waxes and glycerides involved or to the properties of the substances that need to be separated. This problem can be tackled by adding small amounts of fuller's earth.
Winterising can be performed in batches or continuously and takes somewhere in the region of 4-8 hours, depending on the type of oil and on the process used (batch or continuous).
There is also the option of solvent winterising. The oil is mixed with the solvent (to avoid losses) and cooled. The high melting-point crystals in the triglycerides are filtered off and the solvent is then removed.