At present, extracting oil by leaching is the method adopted by most edible oils, while squeezing edible oils is only concentrated on a few medium and high-grade small packaging oils.
Generally speaking, squeezing method is greener and healthier than leaching method, but now many edible oil production enterprises still need to use leaching process to complete the necessary links such as phosphorus removal and Aspergillus flavus removal. It seems that Lu Hua is the only person in China who is completely physical. Hu Jihua is not sure.
Of course, there is no problem for consumers to consume any kind of edible oil as long as it meets the national standards.
After finishing, introduce two ways:
Leaching type
Extraction of oil by leaching is a method of extracting oil from oil by selecting some organic solvents that can dissolve oil, spraying and soaking the oil.
The basic process of extracting oil by leaching is to soak the oil-bearing material blank, pre-pressed cake or puffed material blank in the selected solvent to dissolve the oil in the solvent to form mixed oil, and then separate the mixed oil from the leached solid meal. Evaporating and stripping the mixed oil to evaporate the solvent and separate the oil to obtain the leached crude oil. The leached solid powder contains a certain amount of solvent, and the finished powder is obtained after evaporation. Solvent vapor discharged from wet meal steaming, mixed oil evaporation, stripping and other equipment needs to be recovered by condensation, mineral oil absorption and other methods, and the recovered solvent can be recycled.
The advantages of oil extraction by leaching method are: high oil extraction efficiency, low residual oil (below 65438 0%) and good quality of meal; The production process can be controlled at a lower temperature to obtain meal with a small degree of protein denaturation, which is beneficial to the extraction and utilization of oil protein; Easy to realize large-scale and automatic production; Because of its closed production and low temperature, the production environment is better than that of pressing method; Power consumption is less than that of squeezing method. However, most of the solvents used in oil leaching are flammable and explosive, and the solvent vapor has certain toxicity, which is dangerous in the production process. In addition, due to the residual solvent (generally less than 500ppm) in the extracted crude oil, its quality is worse than that of the pressed crude oil, so it cannot be eaten directly. However, these shortcomings can be overcome by improving the process, developing suitable solvents and improving production management. Therefore, oil extraction has been widely used at home and abroad.
Introduction of physical suppression methods
The history of squeezing oil can be traced back to China, Egypt, India and other ancient countries more than 5,000 years ago. The original extruders include lever extruders, wedge extruders and human screw extruders. With the invention of Bramacher hydraulic press in 1795, 18 18 ~ 1824 began to extract oil from oil with hydraulic oil press. 1895, China built the first hydraulic press oil mill in Yingkou, Liaoning. 1900, Valerius D.Anderson invented the continuous screw oil press (trade name is Expeller). Since then, the continuous screw oil press has become the main equipment for squeezing oil. At present, Anderson International Co., Ltd. and French Oil Equipment Co., Ltd. are the two major manufacturers of continuous screw oil presses in the United States, while Krupp in Germany and Rothstein in Desmet, England are the major manufacturers in Europe. The earliest equipment manufacturer of screw oil press in China is Hubei Anlu Grain Machinery Factory (formerly Dongfanghong Grain Machinery Factory).
First, the basic principle of oil extraction by pressing method
(1) pressing process
The process of squeezing oil is the process of squeezing oil out of the squeezed material with the help of mechanical external force. In the pressing process, physical changes mainly occur, such as material deformation, oil separation, friction heating, water evaporation and so on. However, due to the influence of temperature, moisture and microorganisms, some biochemical changes will also occur, such as denaturation of protein, inactivation and destruction of enzymes, and combination of certain substances. In the process of pressing, the inner and outer surfaces of the pressed material particles are pressed against each other under pressure, which leads to two different processes in the liquid part and the gel part, that is, the oil is squeezed out from the gap of the pressed material, and the pressed material particles are deformed to form hard oil cakes. The extrusion process of grease is shown in Figure 2-4.
1, the process of partial separation of oil and gel
In the main stage of pressing, the oil under pressure can be regarded as following the hydrodynamics principle of viscous fluid, that is, the extrusion of oil can be regarded as the movement of incompressible liquid in deformed porous media. Therefore, the average speed of oil flow mainly depends on the friction (viscosity) and driving force (pressure) inside the liquid layer in the pores. At the same time, the thickness of liquid layer (pore size and number) and the length of oil path are also important factors affecting the oil discharge speed at this stage. Generally speaking, the smaller the viscosity, the greater the pressure, and the faster the oil flows out of the pores. At the same time, the longer the oil flow distance, the smaller the pores and the slower the pressing speed.
Under the strong extrusion, the surface of the extruded material particles will be extruded to the final stage, and there will inevitably be such a limit situation that the extruded surface will eventually leave a monomolecular oil layer or a monomolecular oil layer. Because of the huge molecular force field on the surface, this oil layer is completely combined between the surfaces, and it no longer follows the general laws of fluid mechanics and cannot be squeezed out from the cracks on the surfaces. At this time, the oil molecules can be an extremely thin adsorption film in an oriented state. Of course, these oil films will also break in some places, so that the parts can directly contact and combine with each other. It can be seen that the oil content is very low when the pressed particles are pressed into oil-film dense bodies at the end of pressing. In fact, the residual oil in the filter cake is much higher than the monomolecular oil layer retained on the particle surface. This is because the inner and outer surfaces of the particles are not all compacted, and the surfaces of a single compacted particle are in direct contact, so some grease will remain in the closed oil circuit.
2, the formation process of oil cake
Under the action of pressure, the compressed particles are constantly squeezed with the discharge of oil, and the compressed particles in direct contact generate pressure between each other, resulting in plastic deformation of the compressed materials, especially at the oil film rupture, which will merge with each other. In this way, at the end of pressing, the pressed material is no longer loose but begins to form a complete plastic body, which is called oil cake. It should be noted that the oil cake is not all particles combined, but an incompletely combined gel porous body with a large number of pores. That is to say, there are still many pores between particles or between groups of particles, except that some particles combine to form a continuous gel skeleton of cakes. It is likely that some of these pores are not connected with each other to close the oil path, while the other parts are connected with each other to form a channel, and it is still possible to continue to squeeze oil. It can be seen that the residual oil in the filter cake is composed of oil sealed in pores, oil combined with the inner and outer surfaces of particles and oil remaining in undamaged oil cells. It must be pointed out that in the actual press process, the distribution of residual oil in the press cake is inevitably inconsistent due to factors such as uneven pressure distribution and inconsistent oil flow speed. At the same time, it can not be ignored that in the process of pressing, especially in the final stage, due to friction heating or other factors, the discharged oil will contain a certain amount of gas mixture, mainly water vapor. Therefore, the actual oil recovery process should include two situations: extrusion of liquid oil in deformed porous media and extrusion of a mixture of water vapor and liquid oil.
(B) the main factors affecting the effect of oil extraction by pressing
The effect of oil extraction by pressing depends on many factors, mainly including the structure of pressed materials and pressing conditions. In addition, the structure and selection of oil press equipment will also affect the oil production effect to some extent.
1, the influence of publishing house structure
Press structure refers to the mechanical structure and internal and external structure of the press. The structural properties of pressed products mainly depend on the quality of pretreatment (mainly steaming and frying) and the composition of oil itself.
(1) General requirements for the structure of pressed materials. It is required that the particle size of the press material should be appropriate and consistent, the consistency of the internal and external structures of the press material should be better, and the number of intact cells in the press material should be as small as possible. On the premise of not affecting the internal and external structures, the packing density of the press material should be large. It is required that the viscosity and surface tension of oil in the binder should be as low as possible, and the binder particles should have enough plasticity.
(2) Factors affecting the structural properties of pressed materials. Among the structural properties of pressed materials, the mechanical properties of pressed materials, especially plasticity, have the greatest influence on oil recovery. When the oil content and shell content are almost the same, the plasticity of extrudate is mainly affected by water content, temperature and protein denaturation.
Generally speaking, with the increase of water content of pressed materials, their plasticity also increases gradually. When the moisture reaches a certain point, the press oil is the best, and the moisture content at this time is called "optimal moisture" or critical moisture. For a certain pressing material, there is a narrow optimum moisture range under some conditions. Of course, the optimum moisture range is closely related to other factors, first of all, temperature and protein denaturation.
Generally speaking, the plasticity of pressed materials increases when heated, but decreases when cooled. Extrusion temperature not only affects its plasticity and oil production effect, but also affects the quality of oil and cake. Therefore, the temperature also has an "optimal range".
Excessive deformation of protein will reduce the plasticity of the press, thus increasing the necessary working pressure of the press. Excessive steaming and frying will make the green body develop in the direction of hardening, which will have an impact on the pressure, oil output and cake formation in the press chamber. However, protein denaturation is necessary to extract oil by pressing, because whether the protein denaturation in the pressed material is sufficient or not measures the degree of colloid structure damage in the oil and also affects the effect of pressing oil. Figure 2-5 shows the relationship between protein denaturation degree and cake residual oil in pressed cake. It should be noted, however, that due to the combined effect of temperature and pressure, protein will be constantly changing. For example, the denaturation degree of protein before pressing is about 74.4 ~ 77.03%, and it can reach 9 1.75 ~ 93% after pressing. In a word, a proper degree of protein denaturation can ensure a good oil extraction effect.
In fact, the properties of pressed materials are reflected by the synergistic effect of moisture, temperature, oil content, protein denaturation and other factors. But in normal production, we often only pay attention to the influence of moisture and temperature. The combination of water content and temperature is that the lower the water content, the higher the required temperature. Under the condition of low residual oil rate, it is necessary to press the material with reasonable low moisture and high temperature. However, the pressing temperature is not allowed to exceed a certain limit (such as 130℃). In addition, different pretreatment processes may get the same water content and temperature, but the degree of protein denaturation is quite different.
2. Influence of pressing conditions
In addition to the structural conditions of the pressed material itself, the pressing conditions such as pressure, time, temperature, material layer thickness and oil discharge resistance are the decisive factors to improve the oil output effect.
(1) Pressure during pressing. The essence of oil extraction by pressing method is to exert pressure on the pressed material to remove grease. However, pressure, pressing state, pressing speed and changing law have different effects on pressing effect.
(1) The relationship between the pressure and the compression of the compressed material. The compression of the pressed material in the process of pressing is mainly caused by the extrusion of the inner and outer surfaces of the solid after the pressed material is pressed out of oil. At the same time, the evaporation of water, the taking away of cake crumbs in the discharged liquid, the coagulation of gel after pressurization and the density change caused by some chemical changes also caused the volume shrinkage of the pressed material. The higher the pressure, the greater the plastic deformation of the particles and the more thorough the oil production. However, under a certain pressure, there is always a limit to the compression of a certain compressed substance. At this time, even if the pressure increases to the maximum, its compression is very small, so it is called an incompressible body. The pressure at this incompressible starting point is called "limit pressure" (or critical pressure).