There are two technological methods for extracting edible oil from plants, differentiation oil extraction method and physical oil extraction method, that is, what we usually call leaching method (chemical) and pressing method (physical):
Press method: press method has a long history, and its process is relatively simple: after steaming and frying the oil, squeeze the oil out of the oil by mechanical method. The ancient pressing method requires operators to pay heavy physical labor. The residual oil content in the extruded oil residue (oil cake) is quite high, which wastes extremely valuable oil resources. Modern pressing method has been industrialized and automatic operation, but the problem of high residual oil content in oil residue can not be solved.
From the pretreatment of pressed raw materials, there are cold pressing method and hot pressing method, also known as mature pressing method. Cold pressing means that the raw materials are directly put into the oil press to extrude the oil without baking or steaming. This method has relatively light oil color and brighter color, but the oil yield is low, and the oil taste is not strong and mellow. The purpose of ripe pressing is to dry the oil crops before pressing, so as to reduce the moisture of raw materials and increase the activity and fluidity of oil molecules, so as to improve the oil yield and ensure the flavor of oil quality. But it also destroys the chemical composition of the oil, resulting in a darker and darker color of the oil.
The basic process of oil pressing process is as follows:
1. Conventional production process: raw meal – steaming frying – pressing – machine pressing crude oil
2. Special oil production process: oilseeds – fried seeds – pressing – filtering – scented oil
3. Special oil production process: oilseed – whole seed cold pressing – Filtration – cold pressing oil
Process of oil being squeezed out: when the oil enters the press chamber of the oil press, the pressure increases with the rotation of the press chamber. The seed material is squeezed tightly with the extrusion of oil, and the pressure is generated between the directly contacted pressed particles, resulting in the plastic deformation of the seed material, especially at the oil film rupture and the bonding. In this way, after being extruded and blasted, the pressed material is no longer loose, but forms a plastic body, called oil cake. During pressing, due to the dual action of temperature and pressure, the protein will continue to deteriorate, and then affect the plasticity of the pressing material. In short, the best oil pressing effect can be guaranteed only if the degree of protein denaturation is appropriate.
Leaching method: leaching method is an oil production process. Its theoretical basis is the extraction principle. It originated in France in 1843. It is a safe, hygienic, scientific and advanced oil making process. At present, more than 90% of the total output of oil esters produced by leaching in industrial developed countries. The advantages of oil extraction method are less residual oil, high oil yield, low processing cost, high economic benefit, high quality of meal and good feeding effect in feed industry.
The special solvent oil specially arranged and produced by the state for oil processing can be completely separated from oil and grease with its own composition characteristics. The leaching method is to use the nature of mutual dissolution of oil and organic solvent to crush and press the oil into embryo or expand it, and then contact the oil embryo with organic solvent (generally n-Hexane) in the equipment called extractor to extract and dissolve the oil in the oil. Then, the solvent in the oil is removed by heating and stripping. By this method, the residual oil in the oil residue can be reduced to less than 1%. Taking soybean as an example, the oil yield of leaching method is 50% higher than that of pressing method.
Oil extraction process
(1) According to the operation mode, the leaching process can be divided into intermittent leaching and continuous leaching:
① Batch leaching process operations, such as the embryo entering the extractor, the meal unloading from the extractor, the injection of fresh solvent and the extraction of concentrated mixed oil, are batch, intermittent and periodic. The leaching process belongs to this type of process.
② The process operations of continuous leaching, such as material embryo entering the extractor, meal unloading from the extractor, injection of fresh solvent and extraction of concentrated mixed oil, are all continuous leaching processes, which belong to this type of process.
According to the contact mode, the leaching process can be divided into immersion leaching, spray leaching and mixed leaching:
③ Immersion leaching is called immersion leaching when the material embryo is immersed in solvent to complete the leaching process. The leaching equipment belonging to immersion type includes tank group type, bow type, U-type and Y-type leaching devices, etc.
④ Spray leaching is called spray leaching when the leaching solvent contacts the material embryo in a spray state to complete the leaching process. The leaching equipment of spray type includes crawler extractor, etc.
⑤ Mixed leaching is a leaching method combining spraying and soaking. It belongs to mixed leaching equipment, including horizontal rotating extractor and annular extractor.
(2) The oil extraction process can be divided into direct leaching and pre pressing leaching according to the production method:
① Direct leaching direct leaching is also called “one-time leaching”. It is a process of leaching oil directly after pretreatment. This process is suitable for processing oil with low oil content.
② Pre press leaching the process of extracting some oil from pre press oil, and then leaching the cake with high oil content. This process is suitable for oil with high oil content.
(3) The selection basis and basic process flow of leaching process are closely related to the selected process flow. It directly affects many aspects such as product quality, production cost, production capacity and operating conditions after the oil plant is put into operation. Therefore, both advanced and reasonable process flow should be adopted. The selection of process flow is based on:
① According to the varieties and properties of raw materials, different technological processes are adopted according to different varieties of raw materials, such as processing cottonseed. The technological process is: Cottonseed → cleaning → depilation → shelling → kernel and shell separation → softening → rolling embryo → steaming and frying → pre pressing → leaching;
If rapeseed is processed, the process flow is: Rapeseed → cleaning → rolling embryo → steaming and frying → pre pressing → leaching;
According to the different oil content of raw materials, determine whether to adopt one-time leaching or pre press leaching. As mentioned above, rapeseed and Cottonseed Kernel are high oil-bearing raw materials, so the pre press leaching process should be adopted. The oil content of soybean is low, so the one-time leaching process should be adopted.
Soybean → cleaning → crushing → softening → embryo rolling → drying → leaching;
② According to the requirements for products and by-products, the process conditions should be changed accordingly. For example, when processing soybean, soybean meal is used to extract protein powder, soybean peeling is required to reduce the content of crude fiber and relatively improve the protein content. The process flow is as follows:
Soybean → cleaning → drying → temperature regulation → crushing → peeling → softening → embryo rolling → leaching → leached meal → baking → cooling → crushing → high protein soybean powder
③ Select oil plants with large production capacity according to the production capacity, and select more complex processes and advanced equipment if possible; For oil plants with small production capacity, relatively simple processes and equipment can be selected. For example, for the leaching workshop with a daily treatment capacity of more than 50 tons, paraffin oil tail gas absorption unit and frozen tail gas recovery solvent unit can be considered.
Food science and engineering professionals pointed out that for most oil plants, whether pressed or leached, the crude oil obtained cannot be eaten directly because it contains phospholipids, free fatty acids and pesticide residues. Only after refining processes such as degumming, deacidification, decolorization and distillation deodorization can we get oil for our consumption. Therefore, whether the edible oil is safe or not depends not on whether the pressing or leaching process is adopted for the preparation of the front section, but mainly on the subsequent refining process.
The n-Hexane used in the leaching oil plant must be food grade. After heavy metal removal treatment, the residue of harmful metals such as lead and arsenic is less than 10 ppb (1 / 100 million). Such a low residue will not be harmful to the produced oil.
China’s national standard stipulates that the preparation process must be marked on the outer package of edible oil, which is to give consumers the right to know, which does not mean that the two preparation processes are different in food safety.