Color control measures in soybean oil production and processing

Technology News 2024-04-25 10:32:28

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.

The quality of crude oil is different, and its impurities, acid value, peroxide value, moisture, etc. will affect the color change of the oil. The consumption of clay in the Pearl River Delta oil plant in Guangdong is generally 17-24 kg/t. 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.

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.

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 (Zhenjiang Sinograin), 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 to p 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.

2.3 Control in Refining Process

According to market requirements, control the acid value (KOH) of finished product packaging oil 0.05 mg/g, and color R0.5~0.6; control the acid value (KOH) of bulk oil 0.07~0.08 mg/g, color R0.7. During the production process, it was discovered that when the phosphorus content of refined oil exceeds 20 mg/kg, the color R value increases by 0.2-0.3 within 3 days; Brazilian soybean oil and American soybean oil are mixed in a 2:1 ratio, and the color is Y6/R0. 6. The acid return of refined oil is less than 0.02 mg/g.

2.3.1 Acid refining, alkali refining, deacidification and dephosphorization

Phospholipids and FFA will stick to the adsorption points of the clay surface during bleaching, and the bleaching efficiency of high phosphorus, high acid value, high residual soap, and high floc oil is low. Phospholipids have greater alkali extraction ability than FFA in the alkali refining process, and react to form salts that are insoluble in fats, which will mutually adsorb and condense with the nascent soap particles to form larger soap masses and be separated and removed. Acid refining mixer should use a powerful mixer, adding 75% phosphoric acid, low-concentration acid can increase the contact area and improve the mixing effect more than high-concentration acid, and remove phospholipid calcium and magnesium salts. The soapstocks has surface adsorption and damage to phenolic chromophore groups. The surface affinity of soapstocks produced by the reaction of low-concentration lye is affected by the water film, which has weak effect on the chromophore groups and poor bleaching ability. However, the surface area of soap feet formed by too high concentration of lye is too small, which affects the adsorption of pigments, and only a suitable concentration of lye can have a good bleaching effect.

Soap nibs absorb part of the pigment in the oil, and will not desorb and return to the oil under heating temperature conditions. Therefore, master the concentration of alkali addition, the amount of alkali addition, the initial temperature and the final temperature, and alkali refining can reduce the color by 30%. NaOH reduces the oil peroxide value better than KOH and Na2CO3, which can significantly reduce the acid value and peroxide value of the oil. In the process of alkali refining and deacidification, the soapstock adsorbs the combination of Fe2 + and phospholipid and processes the degraded pigment together and enters the centrifuge for separation. For soybean oil with high reserve and heat loss rate, alkaline degumming and deacidification have better effect than hydration degumming. The dephosphorization efficiency is proportional to the concentration of lye and the amount of super alkali. The chlorophyll content of soybean crude oil is 1000~1500 μg/L , Alkali refining can remove 25% of chlorophyll and 50% of processed pigments. Under conditions, mixing with American soybean oil can have a good refining effect. When the oil-soap particles are obviously separated during the reaction, heating destroys the dispersed state (soap particles), releases the surface affinity of the soap particles, adsorbs pigments and other impurities, and promotes the further flocculation of the soap particles. In order to avoid the peptization of soap particles and the desorption of adsorbed components, the faster the heating to the final temperature, the better, and the heating rate is recommended 1°C/min.

(1)Acid refining and alkali refining of direct extraction oil

Main operating parameters of acid refining and alkali refining: acid refining coagulation time is 40-60 min, the concentration of alkali added is determined according to crude oil with different acid values, the oil and alkali are mixed at 80-85℃, and the alkali refining reaction time is 6-15 min. The acid value (KOH) of the oil from first centrifuge is 0.1~0.2mg/g. Alkali refining oil is washed with about 3% hot soft water, added with citric acid chelated metal ions, The selected washing water is guaranteed to be free of metal ions, 90% desoaping can be achieved by first washing, and 90% desoaping by the second washing. Control the acid value of neutralized dry oil (KOH) 0.15 mg/g, residual soap content 41.0 mg/kg, phosphorus content 9.50 mg/kg, and water content 0.1%.

(2)Acid refining and alkali refining reserved soybean oil

Most of the oil reserved are US soybean oil, Argentine soybean oil and domestic soybean oil, with very few Brazilian soybean oil. The acid value (KOH) is 2~2.5mg/g, the peroxide value is high, the phosphorus content is 120~200 mg/kg. During 280℃ heating test, If there are few precipitates and show red when the temperature is raised, refining is relatively easy; if there are many precipitates and black, refining is more difficult. In the alkali refining, the mass fraction of lye is 14%-18%, the excess alkali is 15%-20%, the neutralization temperature is 65-70℃, and the oil temperature entering the centrifuge is 85℃. It is required that the acid value (KOH) of the oil from the first centrifuge is 0.03~0.05 mg/g, the soap foot has a pungent odor. During the washing stage, add 10% to 15% water.

For poor-quality crude oil (FFA over 3%), a low-temperature two-step alkaline refining process is adopted. The main operating requirements are as follows:

(1)Add phosphoric acid: add 0.05%~0.2% food grade phosphoric acid (75%~85%), the oil temperature is 70~80℃, enter the powerful mixer, enter the acid reaction tank and stay for 15~20min.

(2)Low temperature alkali refining: Co

As bleaching temperature increases, the acid value of the oil will increases, the temperature has a critical point for the influence of the acid value of the bleached oil. If the temperature exceeds the critical point, the FFA increases greatly. At 100℃~105℃, 30 min, 120℃, 5 min, the color of the oil will rise, the acid value will rise, the oil will be oxidized, and it will smell of clay. Too high bleaching temperature and too long time will cause the oil to turn back to acid.

For normal soybean oil bleaching, adopts 105℃~112℃, for reserve oil, 100℃~110℃.

(2)Time: After the adsorbent is saturated for 25 minutes and the time is extended, the oil oxidation speed will increase. If the adsorption speed is exceeded, the oil color will rise and the acid value will rise.

(3)Stirring: The design of the length, speed and number of the stirring blades during adsorption and bleaching should ensure that the oil and the adsorbent establish a dynamic and uniform contact and adsorption balance, and avoid long partial contact time or partial static. Under negative pressure, it is mixed vigorously, but it should not cause oil splash.

The mixing of the white clay pre-mixing tank is the key. The steam jet is linear, and there are dead corners, which cannot form turbulence; the length of the mechanical mixing blade must be long enough to form turbulence on the surrounding inner walls. Mechanical mixing should be equipped with high-quality mechanical seals, and the mixing intensity should be high to avoid heterogeneous mixing.

(4)Addition ratio: As the amount of activated bleaching clay increases, the concentration of activated bleaching clay in the oil reaches saturation, the adsorbent particles affect each other, and the change in bleaching rate becomes smaller. For refining Brazilian soybean oil, the ratio of activated bleaching clay to palygorskite is 1:2 or the ratio of activated bleaching clay, filter aid and palygorskite is 1:1:2. palygorskite for refining American soybean oil. Brazilian soybean oil with a heat loss rate of 15%, the ratio of activated clay to palygorskite is 1:1. Refined National Oil Storage (5 years), the ratio of activated bleaching clay to palygorskite is 4:1 or 5:1, Refined National Oil Storage (2 years), the ratio of activated bleaching clay to playgorskite is 2:1, determined according to the sample test Add 0.1%~0.5% activated carbon. The ratio of adsorbent addition is considered from cost, color and acid reversion. The choice of decolourant depends on cost, activity, oil residue and waste bleaching clay disposal.

(5)Bleaching process

Adopt the re-bleaching process: first pass the oil to be bleached through a bleaching filter (the network is covered with filtered oily bleaching clay), and perform pre-bleaching while removing residual soaps, phospholipids and other mucilage. Generally, the decolourant can be reduced by 2 to 3 kg/t (Calculated by activated bleaching clay). Two-stage counter-current bleaching: compared to adding activated clay at a time, it can save 15%-40% of the amount. Bleaching by filter press: The effective concentration of adsorbent is high, which is close to countercurrent beaching.

2.3.2.3 Adjust bleaching equipment 

The bleaching tank has large volume and insufficient stirring intensity. A premixing tank with long and narrow compartment structure is considered, which combines mechanical stirring with steam stirring.The oil baffles and overflows in the bleaching tank, and "first in first out" is implemented. The bleaching agent is added according to the bleaching requirements. The temperature is controlled at 80-95 ℃ and the time is 5-10 min. the oil flows into the bleaching tank and is retained for 10-20 min before filtering.The bleaching tank body is provided with an oil distributor, which is arranged close to the oil inlet. A number of guide plates are arranged along the axial direction and inclined downward. The guide plates are staggered along the axial direction. The outlet of the upper guide plate is located above the lower guide plate, and the inclination angle of the guide plate is 5 °~ ten °.The length of the oil inlet pipe of the oil distributor is designed to be 5-10 cm. The oil distributor has an inlet and 2-4 branch pipe outlets to avoid oil stacking and sputtering, and then the oil is guided by a number of guide plates arranged in layers to extend the time.The bleaching tank has three layers and the bottom is cone bottom to avoid oil deposition and short circuit in the bottom and outlet pipeline.The volume of the temporary storage tank for bleached oil can ensure that there will be no overflow during the filter switching and the uncolored oil won't directly enter the bleaching tank, keep the filtering speed of the bleached oil into the filter stable, and reduce the fluctuation of the oil flow during the filter switching.

2.3.3 Deodorization

Generally, the combination of packed column and plate column is selected, the deodorization temperature is 245 ~ 255 ℃, the time is 88 min, and the trap temperature is 45 ℃. However, the deodorization temperature of the reserve oil is 255-260 ℃, the time is 90 min, and the capture temperature of the trap is 40-42 ℃, so the trans acid seriously exceeds the standard. In order to avoid excessive trans acid content, excess alkali was added during alkali refining, and the temperature was reduced to 245 ℃ during deodorization to extend the deodorization time and control the increase of trans acid.

During deodorization, carotene is decomposed into small molecules and volatilized at high temperature, but new pigment and pigment solidification may be produced.

3. Prevention and control results of Color and luster deepened during soybean oil processing

Adjust the operation of the conditioner and the bulking machine in pretreatment workshop, improve the mixed oil filtration system in the leaching workshop, increase the filter, reduce the temperature of the second evaporator and the stripper, increase the vacuum, and improve the structure of the stripper. Reduce processed pigments.In the alkali refining, take advantage of different soap particles to adsorb processed and stored pigments can achieve better results, while the ability of activated clay to remove processed and stored pigments is weaker than that of natural pigments, and the amount of clay added in bleaching is more increased. It will cause the instability of fat fatty acid and cause color and acid reversion.At the end of the deodorization process, extend the bleaching time for 15-20 minutes to have a good thermal bleaching effect. If the time is too long, the oil and fat oxidation stability will decrease, and the deodorization temperature will produce trans acid after the temperature exceeds 250°C.

For small sample experiments on soybean oil from different sources to detect phosphorus content, acid value, chlorophyll content, peroxide value, 280 ℃ heating test, and fine-tune the process parameters.In the first half of 2019, the amount of white clay added will be reduced by an average of 3% to 4% on the basis of the previous 2-3 years.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 17~24 kg/t, and the shelf life of packaging oil was prolonged obviously.  

The degree of chlorophyll removal in each refining process: neutralization 25%, bleaching (0.25% white clay) 75%, (0.75% white clay) 90%, and a small amount of deodorization. The chlorophyll content of refined oil is 10-20 ng/kg, which is yellow. If it reaches 50 ng/kg, the oil is green and gray. Control the degree of soybean oil bleaching in the refining stage: neutralization R7~8; bleaching R2~2.3; deodorization R0.3~0.6. The color (Y6/R0.6) and acid value (KOH) of refined soybean oil decreased from 0.05 to 0.06 mg/g to the color (Y5/R0.5) and acid value (KOH) 0.05 mg/g, and the addition of white clay increased by 0.8 %. The color is controlled at (Y6 /R0.6) to achieve that the packaging oil does not return to acid and color. The acid value (KOH) of the bulk oil is 0.07~0.08 mg/g, and the color is Y7/R0.7.

4. Conclusion

The bleaching of activated clay brings pollution to the factory, and the waste clay is flammable and causes environmental pollution,which is the focus of mandatory management by the environmental protection and safety supervision department, and also the weak place of safe and clean production in refining workshop. How to reduce the consumption of white clay and find alternative methods for white clay bleaching are hot topics in the industry. Nanomixer used in acid refining and alkali refining can reduce the amount of phosphoric acid, alkali, and clay, and increase the yield of alkali refining. At present, several refineries in China have been installed and waiting for commissioning. Shandong Boxing Xiangchi Oil Plant is commissioning , but it has not achieved the expected results, and the specific debugging results remain to be observed.

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