We are Benteng Machine, specialize in producing oil press and oil press line for various oilseeds, if you have need for the machine, please contact us any time. Today we will talk mainly about the cotton seed oil.

The main reason for the deep color of cottonseed oil is the presence of gossypol and its derivatives in the oil. Gossypol mainly exists in the pigment glands of cotton kernels and is the most important pigment in cottonseed, accounting for 20%-40% of the total pigment. Gossypol is easily denatured by heat to form a very stable pigment, which is difficult to remove whether it is alkali refining, adsorbent adsorption or high temperature decolorization.

For cottonseed oil, the less gossypol in cottonseed oil, the lower the degree of gossypol in the oil, the easier it is to refine and decolorize, and the alkali consumption and refining consumption will decrease accordingly. The content of gossypol in cottonseed oil and the degree of denaturation mainly depends on the control of the process parameters in the oil pressing section and the leaching section. If the preliminary oil pressing section and the leaching section are not handled well, the quality of crude oil is poor, and the gossypol in the oil is severely denatured, even if a large amount of excess alkali or white clay is added to the subsequent section, it will not help.

In order to systematically and comprehensively solve the problem of decolorization and produce high-quality cottonseed oil, it is necessary to consider many aspects such as cottonseed processing technology and cottonseed oil refining technology. The following focuses on the process that affects the color of cottonseed oil, analyzes the causes of pigment formation, and discusses how to reduce the color and other issues.

1、Cottonseed Oil pressing process

1.1 Safe storage of cottonseed

Because of the protection of cotton husks, cotton seeds are easier to store. However, cottonseed has poor thermal conductivity, strong hygroscopicity, damp heat is not easy to spread, and easy to heat and mildew. In the process of mildew, cottonseed not only becomes rancid, but also due to the presence of moisture and temperature, it will cause the oxidation of gossypol and other pigments, making it difficult to decolorize cottonseed oil. Therefore, in order to avoid the darkening of cottonseed oil color, the storage time of cottonseed should be shortened as much as possible, the moisture content should be strictly controlled, and the temperature of cottonseed should be checked in time. Particularly, light cottonseeds must be stacked reasonably, paving the bottom of the pile and preventing rain and snow. Ensure the quality of processed raw materials.

Generally speaking, the safe storage moisture of cottonseed should be less than 10%. If the cottonseed temperature is higher than 30°C, and the temperature rises quickly, even with a musty smell, measures should be taken as soon as possible to deal with it.

                                                           1.2 Reduce the shell content in the kernel

Cotton seed contains about 40% of the hull, and the hull generally contains only 0.5% to 1.0% oil, and contains brown pigment, pesticide residues, and high wax content. In addition, it contains about 4% of the lint. If it is not removed during the oil production process, it will not only reduce the production capacity of the oil production equipment, resulting in a decrease in the oil yield, but also affect the quality and crude protein content of the meal, which will affect the color and luster of the crude oil. The flavor has a great influence.

In order to pursue the quantity of meal, some manufacturers ignore the shell content in the kernels, resulting in high crude oil pigment and wax content, resulting in difficulty in alkali refining and increased refining consumption. Therefore, the cottonseed processing must increase the peeling rate, improve the quality of peeling, and avoid the pigment in the cotton husk from being dissolved in the oil to deepen the oil color and cause difficulty in decolorization. Generally, the hull content in the kernel (10 mesh/inch sieve inspection) should not be more than 10%, and the kernel content in the hull should not be more than 0.5%.

                                                          1.3 High-moisture steaming and pre-pressing

The effect of the steaming process on the refining of cottonseed oil is very critical. The cotton blank should be steamed and fried with high moisture content, and its functions are as follows:

(1) While controlling the moisture content, the cell wall is broken, the protein is coagulated and denatured, and the viscosity of the oil is reduced. However, only higher moisture content is not enough, and higher temperature and longer time of steaming and frying are required.

(2) The important point of high-moisture steaming for the refining of cottonseed oil lies in: ①Promoting phospholipids to absorb water and expand. Phospholipids in cottonseed exist in two forms: bound state and free state. Bound state phospholipids refer to a complex formed by combining phospholipids with proteins and carbohydrates. There is very little free phospholipid in cottonseed. During the steaming and frying process, due to protein denaturation, the phospholipids in the complex are released. Under the action of water, the phospholipids will swell in a condensed state and will no longer dissolve in the oil. Therefore, the condensed phospholipids remain in the meal during oil production. ②Promote the combination of gossypol and protein. Free gossypol is easily soluble in oils and solvents, but insoluble in water, and can generate denatured gossypol under the action of light, water and heat. Denatured gossypol is non-toxic, but it has a very deep color and is difficult to be destroyed by alkali. This is an extremely undesirable situation in the oil production process. But free gossypol can combine with the phospholipids and proteins in cotton kernels to form bound gossypol. The combined gossypol is also non-toxic, but it has a greater impact on the color of the crude oil than denatured gossypol, especially when free gossypol is combined with phospholipids. The combined gossypol can be dissolved in the oil and affect the quality of the crude oil. Therefore, steaming and frying can be used to combine free gossypol with protein and stay in the meal, thereby improving the quality of crude oil and reducing refining consumption. On the contrary, after low moisture, high temperature, long time steaming and frying, the pressed crude oil has a deep sauce color, which is very difficult to remove.

Regardless of the puffing-leaching process or the traditional pressing-leaching process, the steaming and frying operation cannot be omitted or ignored. In order to improve the quality and yield of crude oil, and reduce the loss of alkali refining, the preform must be steamed and fried with high moisture.

The first step of high-moisture steaming requires the billet to be heated with water to moisten it to a moisture content of 16% to 20%, and at the same time, direct steam to steam the billet. The height of the material layer should be more than 1m. In the second step, indirect steam or heat transfer oil is used to heat and dry to adjust the moisture. Generally, the steaming and frying pan is designed with 4 or 5 layers, the height of each layer is about 300mm, and the processing time of each layer is controlled within 20~30min. The time of the whole steaming and frying process should be controlled within 1.5~2h. Even if the puffing-leaching process does not have strict requirements on the temperature and moisture of the blank, the full steaming and frying process time should not be less than 1.5h. The temperature of the discharged blank should reach 100~110℃, and the moisture content should be reduced to 5%~7%.

The crude oil produced by pressing should be filtered and cooled in time to prevent the gossypol contained in the oil from generating gossypol and gossypol derivatives at high temperatures, which will deepen the color of the crude oil and bring difficulties to decolorization.

It is also worth mentioning that in recent years, many cottonseed protein processing enterprises have adopted the one-time extraction method to prepare cottonseed oil. Compared with the pre-press extraction method, this process lacks the high-moisture, high-temperature steaming and frying process. It only undergoes low-temperature softening or instantaneous high-temperature puffing. Gossypol, pigments, phospholipids, and proteins have little or no denaturation, so they are more dissolved in oil, causing. The quality of the crude oil leached at one time is poor. Moreover, the crude oil prepared by one-step leaching method is generally high in acid value, dark in color, high in phospholipid content, difficult to grasp the refining parameters, and the alkali consumption and refining consumption are relatively large.

2 、The leaching process

In order to improve the concentration effect, reduce the temperature of the gas phase, improve the quality of crude oil, and prevent oxidative deterioration, negative pressure evaporation technology has been widely used in the production of cottonseed oil. Cottonseed oil is prone to oxidation, polymerization, and decomposition when it is higher than 130°C, producing peroxides and many thermally unstable products. Especially under the combined action of high temperature and water vapor, the gossypol, phospholipids and protein mucilage in the crude oil are easily oxidized and denatured, leading to the increase of the acid value of the crude oil, forming melanin that dissolves in the oil and darkening the oil color, making it difficult to decolorize.Therefore, the gas phase pressure of the evaporation system should be reduced as much as possible, and the operating temperature should be lowered to ensure the quality of the leached crude oil. Generally, the residual pressure of the negative pressure evaporation system is 27~40kPa, and the temperature of the gas phase is controlled at 85~95℃.

Like the crude oil produced by pressing, the leached crude oil also needs to be cooled in time and should not be stored for a long time to prevent the gossypol contained in the oil from oxidative denaturation and make the crude oil darker in color and difficult to refine.

3、The refining process of cottonseed oil

There are two common refining methods for cottonseed crude oil: the traditional chemical alkali refining process and the mixed oil refining process. At this stage, the former method is mostly used in China. The chemical alkali refining process route and parameters will be discussed below.

3.1 Medium temperature and long mixed alkali refining process

Generally speaking, cottonseed oil contains 2% to 4% gossypol, phospholipids and pigments. These components must be removed during the oil refining process, otherwise the flavor and sensory indicators of cottonseed oil will be affected. Alkali refining and decolorization are the two processes for removing the pigment of crude oil. If the gossypol cannot be removed in the alkali refining section, the remaining gossypol and pigment will be denatured during the decolorization and deodorization heating process to make the oil color fixed. Resulting in irremovable color. Therefore, the alkali refining section is the key to the removal of gossypol.

Many manufacturers ignore the acid reaction process in the actual production process. Sufficient and reasonable acid reaction can not only remove non-hydratable phospholipids in cottonseed oil, but the presence of phosphoric acid can also convert chlorophyll into light-colored pheophytin, and it is also effective in reducing redness. In addition, phosphoric acid can also complex metal ions, passivate the catalytic effect of iron, copper and other trace metals on the oxidation of fat, increase the oxidation stability of fat, and improve the flavor of fat.

Generally, cottonseed oil with an acid value (KOH) of 5-10mg/g uses a lye concentration of 16%-18%. The amount of alkali added should be determined by the acid value of the cottonseed oil and soap stock after washing. The acid value (KOH) of the cottonseed oil after alkali centrifugation should be below 1.0mg/g, and the soap stock should be neutral with a smooth and soft texture. Too much excess alkali not only reduces the refining rate, but also has a high soap content in the refined oil and poor transparency, which may even cause salt precipitation in the centrifuge.

Alkaline refining of high acid value cottonseed crude oil can also use two high-temperature short-mixing alkali refining processes. The first pass controls the acid value (KOH) at about 1.5mg/g, and the second pass controls the acid value (KOH) below 0.6mg/g. However, the use of two alkali refining will increase equipment investment and refining consumption will increase.

3.2 Conventional decolorization and deodorization

The decolorization and deodorization of cottonseed oil is basically similar to other oil processing. The pre-decolorization-re-decolorization process can be adopted in the decolorization section to establish two adsorption balances, and the decolorization effect will be ideal. Under the premise of ensuring a high degree of vacuum, the decolorization temperature can be appropriately increased to ensure the decolorization effect.

Compared with soybean oil and rapeseed oil, the thermal decolorization effect of cottonseed oil is relatively poor. The red value of ordinary soybean oil after decolorization is 4.0 (Lovibond colorimetric groove 133.4mm). After high temperature deodorization, the color can be easily reduced to the standard requirements. But the color of cottonseed oil will not change significantly after deodorization. If conditions permit, the deodorization time can be appropriately extended (not less than 90min) and the deodorization temperature (above 250°C) can be increased to enhance the thermal decolorization effect.

4、Conclusion

The color of cottonseed oil not only depends on the processing technology, refining technology and degree of refining, but also is closely related to raw material storage, cleaning, shelling and separation of kernels, steaming and frying, oil extraction, and leaching evaporation. Improper storage conditions of cottonseed, unreasonable oil extraction processing technology, and improper selection of refining routes will lead to high gossypol content in cottonseed oil, gossypol denaturation and even color solidification, which will have a great impact on the color of cottonseed oil. Therefore, by analyzing the specific reasons in production, the problem of cottonseed oil decolorization can be effectively solved by prescribing the right medicine.