The main component of oil is fatty acid triglyceride. The carbon chain length of fatty acid in triglyceride is different, and the degree of unsaturation is also different. This makes the oil contains a considerable amount of triglyceride with low melting point and high melting point , low melting point triglyceride in liquid form is referred to as the "oils", while the high melting point triglyceride in solid or semi-solid form, usually called "fats." The fractionation process is oil and fat processing method that divides the triglyceride into low-melting liquid phase (olein) and high melting point solid fat (stearin) by controlling the cooling and crystallization process, and separates "olein" from "stearin".

Ⅰ.Early history of the fractionation process

The oil fractionation process has been used for nearly 150 years. The invention of oil fractionation technology is attributed to the French Hippolyte Mège-Mouriès, Hippolyte is his mother's maiden name. Mège-Mouriès(1817-1880), born in Draguignan, southern France. Mège-Mouriès was busy doing dairy research on the imperial farm near Paris in 1867,he found that even though thin cows reduced milk production, they still produced fat in milk.Therefore, he created a process to produce butter by artificial methods, and thus became a pioneer in biotechnology, which gave him lasting reputation, namely the invention of margarine.

On July 15, 1869, the French Ministry of Agriculture and Trade granted Mecca Molière with 15-year patent for processing and producing some animal fats,the patent was also registered in the United Kingdom in 1873. In 1873, Mecca Molière received U.S. patent (146012; 1-1-1873) entitled "Modification of Animal Fat". Soon after this, his US patent was obtained by the American Dairy Company in New York City for the production of margarine.

The fat in the patented method came from animal fat, tallow and mutton fat, after moderate cooling, the liquid components are separated from ordinary tallow. However, only the temperature difference is the driving force, and the step-by-step crystallization of fat is a completely natural phenomenon. Therefore, it was also observed that palm (kernel) oil harvested in tropical regions was transported to cold Western Europe in wooden barrels, and the crystals formed after cooling were suspended in the wooden barrels. These small amounts of dense solids eventually settled, and such components can effectively replace the hardened fat in margarine.Therefore, we can consider these wooden oil barrels as the first oil fractionation crystallizer, and the waves provided the necessary stirring for mixed suspension. In addition, palm oil naturally crystallizes in winter, which is an economical way of oil fractionation.

II. The development of fractionation technology

The technology of fractionation did not take off until the 1960s, when palm oil production in Southeast Asia surged and export taxes on processed palm oil were reduced. However, the technical boundary at that time was mainly determined by the phase separation.In the early stages of the fractionation technology, oil and fat rely solely on gravitational forces to cause the separation between the heavier solid phase and the lighter liquid phase, which contains a large amount of liquid oil, almost over 75%. In the past few decades, the continuous development of separation technology from vacuum filtration to centrifuge and membrane filter press makes fractionation become a general economic modification technology. Although some specific technologies rely on surfactants for very specific production, actually there are only two kinds of main fractionation technology used in the 21st century edible oil industry: dry fractionation and solvent fractionation.

III Factors affecting fractionation

The factors affecting the palm oil crystallization process include the oil grease composition, homogeneous polycrystals and cooling conditions.

Grease composition

Palm oil contains 4% to 8% triglycerides, which will form eutectic mixture with triglycerides, thereby reducing the solid content and slowing down the crystallization rate of oils. The content of monoglycerides in palm oil is less than 1%, so it has no obvious effect on the crystallization process of palm oil.

Homogeneous polycrystals

The triglycerides of palm oil have homogeneity and polymorphism, so they have different forms of crystals, namely α-type, β'-type and β-type crystals. The stability and melting point of these crystal forms gradually increase. When palm oil is cooled, α-type crystals are obtained first, and then they are converted into β'-type and β-type crystals in turn. In order to obtain a good separation effect, it is desirable to obtain β'and β-type crystals during the crystallization process, because β'-type crystals have characteristics of large and hard crystals, beneficial to subsequent filtration and separation.

Cooling rate

The cooling rate will affect the formation of crystal nucleus and crystal growth. When the temperature is low enough (32~36℃), saturated glycerides will crystallize. These crystals become crystal nucleus for further crystallization of low-melting glycerides, and then form large crystals. Slow cooling rate and proper stirring rate are necessary conditions for obtaining ideal crystals.

IV The application and development of oil fractionation technology

The first step of dry fractionation of palm oil is crystallization below 10°C. The liquid component is used in frying, cooking and as a substitute of salad oils, or is being further separated. With the further development of single-stage palm oil fractionation technology, there is a tendency to increase, perform double or triple components of palm oil to produce components with specific characteristics, such as high iodine number super oleic acid (iv> 65) and super hard palm stearin and intermediate components.

Intermediate components can be used as raw materials for the production of typical cocoa butter (CBE), which are similar in physical and chemical properties to cocoa butter and are non-laurel butter.

Another technical field is the use of plug flow reactor (Plug Flow Reactor, PFR), which allows continuous step crystallization, thereby greatly reducing operating costs (such as steam usage and cooling power). In fact, just like other edible oil processing technologies, we constantly pursue economic and process optimization. The heat recovery system, feed crystal device, optimized mixing procedure and elegant plant layout help to maximize the capacity and cost of the dry fractionation device.

The fractionation process produces two products, so the sum of two components should always exceed the processing cost and raw material cost. This is why the feasibility of multi-stage fractionation is not only a question of technical know-how, but also a question there is market for all "by-products".