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Color Control Measures In Soybean Oil Processing Ⅰ

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(Last Updated On: September 3, 2021)

Some soybean oils contain special impurities and pigments, and the amount of clay used in the bleaching process is too high, resulting in poor oxidation stability of the finished oil.In order to prevent and control the darkening of soybean oil during the process of making and refining, and to reduce the amount of clay, different measures can be taken: adjust the parameters during the pretreatment process to solidify part of the pigment, and adopt multi-stage filtration during the leaching process to reduce the evaporation and stripping temperature. The structure of the stripping tower was modified to reduce the processed pigment; the refining process adjusted the operating parameters based on the sample experiment to obtain the crude oil quality data, and the partial bleaching equipment was modified.Refined packaging oil is produced according to acid value (KOH) 0.05 mg/g, color Y5~6/R0.5~0.6; bulk oil acid value (KOH) 0.07~0.08 mg/g, color Y7/R0.7. The consumption of bleaching clay of reserve soybean oil and acid value (KOH) 2~3 mg/g crude oil is reduced by an average of 30% on the basis of the original 17~24 kg/t, and the shelf life of packaged oil is also significantly extended.

Soybean Oil Processing

The quality of crude oil is different, and its impurities, acid value, peroxide value, moisture, etc. will affect the color change of the oil.  Excessive use of clay in the oil bleaching process will cause isomerization of the oil structure, increase in trans acid in the oil, and decrease the oxidation stability of the product oil. The color of the grease has turned back. Analysis believes that the pigments in soybean oil can be divided into natural pigments, processed pigments and storage pigments. Different measures are taken in the soybean oil refining and refining process, the temperature and vacuum degree of the leaching evaporation process are adjusted, and some bleaching equipment is modified, which can reduce the consumption of white clay, remove the pigment in the oil at the lowest production cost, and improve the finished product. The oxidation stability of the oil.

1. Sources of pigments in vegetable oils

The pigments in vegetable oil are mainly derived from natural pigments, processed pigments, and storage pigments.

1.1 Natural pigment

The natural pigments in oils mainly include chlorophyll, lutein and carotene. Due to the existence of these pigments, the oils show different colors. For example, chlorophyll makes the oil green, lutein makes the oil yellow, and carotene makes the oil red. Most of these fat-soluble pigments enter the oil during the oil preparation process.

1.2 Processing pigments

Some pigments in fats and oils are formed during the processing of fats and oils. During processing, oils and fats are subjected to mechanical, moisture, oxygen and high temperatures, and proteins, sugars and other components undergo complex physicochemical changes, resulting in polymerization, oxidation and hydrolysis reactions to produce aldehydes, ketones, lower fatty acids, oxidative degradates, peroxides, epoxides and polymers. The amount of phospholipids and metal ions in soybean oil is positively correlated with coloration in storage and processing. Soybean phospholipids have poor oxidative stability and are brown when oxidized. Metal ions are oxidation inducers. Metal ions mainly refer to Fe2+, with Fe2+ greater than or equal to 0.2 mg/kg, promoting the oxidation of oils and fats and producing processing pigments. The activity of metal ions Cu>Fe>Cr, Cb, Zn, Pb>Ca, Mg>Al. In Cu and Fe ions content more than 0.01 mg /kg and 0.1 mg /kg, respectively, can promote the oxidation of oil and grease, soybean oil containing phosphorus greater than 25 mg /kg, the concentration of iron ions will increase and oxidation stability becomes poor. High temperatures and metal ions cause the oxidation of tocopherols in the oil to produce dimer and tocopherylquinone. The total amount of γ-tocopherol and γ-tocopherol dimer (γ-TED) in soybean oil is greater than 550 mg/kg, and no re-coloring and acidification will occur. Chlorophyll is converted to the red variant of lutein at high temperatures, and free fatty acids (FFA) and iron ions produce dark iron soap. Residual phospholipids in soybean oil after deodorization of phospholipids occurred pyrrolidization (4,5-epoxy-2-heptenal and multiple amino phospholipids react to form polymer pyrrolidic phospholipids), which is a carbon-ammonia reaction, producing non-enzymatic browning, causing recoloration.

1.3 Storage pigments

Oil deteriorates during storage, proteins, sugars, phospholipids and gums hydrolyze pigments, which are suspended in the oil with a positive charge. Long-term storage of oil and grease, impurities within the oil become nuclei, the formation of trace crystals. When the oil is in contact with iron in the storage tank, the generation of oxides is accelerated as the temperature rises. The color of the oil deepens as the storage time increases.

 2. Prevention and control measures for color deepening during soybean oil processing

2.1 Pre-treatment process control

2.1.1 Conditioner

Phospholipids are combined with proteins, sugars, pigments, etc. Under normal circumstances, the content of bound phospholipids is more than that of free phospholipids. Consider soybean protein denaturation temperature greater than or equal to 80 ℃, according to the original grain soybean moisture in 10% ~ 13%, heating layer with 0.1 MPa steam, the discharge temperature at 65 ~ 70 ℃, out of the tower soybean water content of about 10.5%, retention time of 40 min. soybeans from the top of the material to the first layer, heated, the temperature from the ambient temperature to 45 ℃, soybean internal moisture from the center to the surface transfer and coalescence, falling to the next layer of 100-120 ℃ hot air, surface moisture is blown away, falling to the second layer of tempering layer heating, moisture continues to transfer from the center to the surface and coalescence, to the surface of the hot air layer of water is blown away. In the moisture transfer phospholipids absorb water and swell, leaching and stay in the meal. The more this heating, hot air blowing, heating, hot air blowing cycle, the more soybean moisture reduction, while the longer the phospholipids absorb water. Soybeans in the conditioner retention 35 min, the discharge temperature of 55 ℃, compared with soybean retention 40 min, the discharge temperature of 70 ℃, the amount of phosphorus in the oil increased significantly. So the recommended soybean retention time of 40 min, the discharge temperature of 70 ℃, the opening degree of hot air according to the feed discharge moisture to determine.

Soybean Oil Processing

2.1.2 High moisture expanding

Adopt high-moisture expanding to make the phospholipid passivate and leave in the soybean meal. There are three elements for maturation of expanded material, they are temperature, pressure and moisture. If the moisture of the flakes entering the expander is 10%, to reach the moisture of extruded material 12%, add 20 kg/t of steam with moisture to the expander, and the temperature of materials at the outlet of expander is 115~120℃,the moisture is 12%. The steam with high moisture enters the flakes and deeply moistened under screw extrusion at 3.0 MPa. The flakes are instantaneously matured, the cell membrane is destroyed, and the phospholipids and oils free out. Under high moisture, the released phospholipids absorb water and expand then aggregate, adhering to the pigments together to form ether insoluble matter and stay in the soybean meal. The phosphorus content of the extracted crude oil decreased from 850-900 mg/kg to 550-600 mg/kg, and the color of extracted crude oil became lighter significantly.

2.2 Extraction process control

In the evaporation and steam stripping process of extraction, reduce the temperature and reduce the processed pigment.

2.2.1 The filtration of extracted crude oil

The generally designed tandem hydrocyclone, if the nozzle diameter is small, which will cause blockage when the impurity of miscella fluctuates;if the nozzle diameter is large, the solid impurities enter the evaporation system, causing coking and carbonization when heated, and forming film on the surface of heat transfer tube, it will affect rising film of miscella. Therefore, design the first hydrocyclone, tank filter, and the second pipeline wire mesh filter to reduce the solid impurity content in the miscella.

2.2.2 Evaporation system

In operation, the concentration of miscella at the outlet of the first evaporator is 70% ~75%. According to the partial pressure principle of multiphase liquid, the temperature of  the second evaporator should be increased to about 115℃, if the residence time of high concentration miscella in oil-oil heat exchanger and evaporator is too long, it is easy to form scale, which causes the heat transfer efficiency to decrease. When the temperature is increased, the temperature of the stripper should be 115-120℃. Tocopherol is partially oxidized to chroman pigments under the high temperature of the second evaporator and stripping tower, and there is high-temperature pigments generated by scaling when the film formation of oil and grease takes long time. In the evaporation system, the first evaporator and the second evaporator have two separated vacuum system. During operation, the temperature of the first evaporator is adjusted to 60~62℃, the vacuum degree is 39~40 kPa, and the oil temperature out from the second evaporator is 93~95℃, the vacuum degree is 34~35 kPa, the oil temperature at the outlet of the stripping tower is 95℃, the vacuum degree is 35 kPa, the solvent content in extracted oil is 30-50 mg/kg, and the color of oil is lighter obviously.

2.2.3 Selection of Stripper Tower

It adopts inclined orifice plate type stripping tower and sieve plate type stripping tower.

Inclined orifice plate stripper: The oil falls from the top and flows smoothly without accumulating phospholipids. Cleaning holes are opened on the two sides of the shell and cleaned once every two months.

Sieve plate stripper: 7-layer sieve plate is designed to make the oil and steam fully contact; direct steam enters the bubble generation part from the bottom for final stripping. The steam discharged from the steam jet pump of the dryer enters the oil level of the stripper for secondary use.

Observe the soluble content of these two kinds of stripping tower oil by using: Control the oil outlet temperature to 95℃, and the content of the oil is less than 50 mg/kg; Control the oil output temperature to 105℃, and the content of the oil is less than 20 mg/kg. The temperature requirements for the mixed oil of the two stripping towers are about 15℃ lower than that of the dish-type stripping tower, which saves the amount of steam and the crude oil is light in color.

To be continued…

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