From the soybean oil refining production process, we investigated the changes in the content of γ-tocopherol, phosphorus, metal ions and other trace components affecting soybean oil color reversion during the refining process, as well as the effects of the addition of auxiliary materials and deodorization conditions on the color reversion of finished soybean oil during the storage period, the results showed that: γ-tocopherol had the greatest loss in the deodorization section, and it was recommended that the deodorization temperature should be below 250 ℃, while the fine adjustment of auxiliary materials and steam extraction pressure had little effect on this. It is recommended to set the key link of phosphorus content control in the degumming section to reduce the pressure and production cost of phosphorus removal by adsorption of white clay in the bleaching section; metal ions can be removed by adsorption of soap foot and bleaching agent in the normal process of deacidification and bleaching; the loss rate of γ-tocopherol, phosphorus content by deodorization and the storage period are all related to the loss rate of γ-tocopherol. There is a positive correlation between the colour reversion of finished soybean oil and the colour reversion of metal ions; the ratio of bleaching agent types has no significant effect on the colour reversion of soybean oil from different origins; while high citric acid addition has an inhibiting effect on the colour reversion of soybean oil stored for one month, the inhibiting effect is not obvious with the extension of storage time; the temperature of deodorization and steam pressure of extraction have little effect on the colour reversion of US soybean oil, but have a significant effect on the colour reversion of Brazilian soybean oil. The effect of deodorization temperature and steam pressure on colour reversion in US soybean oil was not significant, but on colour reversion in Brazilian soybean oil was obvious.
Influenced by lifestyle habits, consumers still prefer lighter-coloured oils when buying refined oils such as soybean oil. In fact, soybean oil, sunflower oil, corn oil and other oils are light yellow in colour after refining processes such as degumming, deacidification, bleachingand deodorization, and then gradually darken in circulation and storage due to the effects of light, temperature and humidity. Colour reversion after refining is a common problem for vegetable oil producers, mostly in refined soybean oil, refined corn oil and refined rapeseed oil, with soybean oil and corn oil in particular being the most serious . Therefore, how to effectively control color reversionand improve the quality of fats and oils is still a challenge for the fats and oils industry. In general, the main causes of color reversionare: (i) raw materials; (ii) oxidation, isomerisation or polymerisation of low molecular pigments; (iii) auto-oxidation and isomerisation of oils and fats; (iv) metal ions, phosphorus and vitamin E in oils and fats; and (v) processing and storage conditions.
This article is based on the soybean oil refining production process, to explore the changes of γ-tocopherol, phosphorus content, metal ions and other trace components in the refining process and the amount and type of decolorising agentadded to the ratio, the amount of citric acid added, deodorization temperature, vacuum pressure and other process parameters adjustment on the stability of soybean oil colour during storage, in order to provide some reference value for solving the problem of soybean oil color reversion.
- (1) Changes of γ-tocopherol during the refining process and its effect on the color of finished soybean oil in different storage periods.
Vitamin E is a common fat-soluble vitamin in vegetable oils, mainly in the form of tocopherols and tocotrienols, tocopherols have α, β, γ, δ 4 monomers, of which, γ-tocopherol content in the highest percentage of oil, itself and its oxidation products is an important factor leading to grease back to color, in micro-oxygen conditions, γ-tocopherol can be oxidized to generate 5-tocopheryl-γ-tocopherol (γ-TED ) and 5,6-o-quinone-γ-tocopherol (tocopherol red), which is a deep red substance and can cause the re-coloring of oils and fats. Therefore, this experiment mainly investigated the changes of γ-tocopherol content in soybean oil of different origins during the refining process and its effect on the color of finished soybean oil at different storage periods.
the γ-tocopherol content of U.S. soybean degumming oil was 688-740 mg/kg, Brazilian soybean degumming oil was 482-641 mg/kg, and Argentinean soybean degumming oil was 416-479 mg/kg. Compared with Brazilian and Argentinean soybean degumming oil, the γ-tocopherol content of U.S. soybean degumming oil was higher, indicating that the γ-tocopherol content of raw oil from different origins was different. During the refining process, it was found that the γ-tocopherol of soybean oil from different origins was less lost in the deacidification and bleachingsection, while the greatest loss was in the deodorization section, indicating that the deodorization section was the most important stage of γ-tocopherol loss.Under the same conditions of other process parameters, when the deodorization temperature increased from 245 ℃ to 255 ℃, the γ-tocopherol loss rate of deodorized oils from the United States, Brazil and Argentina showed an increasing trend with the increase of deodorization temperature; under the same temperature conditions, the γ-tocopherol loss rate almost did not change significantly by fine-tuning the steam pressure of the stripping; adjusting the additions of auxiliary materials, such as the ratio of white clay to alabaster, citric acid Adjustment of auxiliary additions, such as the ratio of bleachingclay to gravimetric clay, citric acid, etc., did not have significant effects on the γ-tocopherol content in the refining process, indicating that the retention rate of γ-tocopherol was most related to the deodorization temperature in the refining process. Tocopherols are natural antioxidants, and in refining production, the deodorization temperature should be focused on, and in order to maximize the retention of tocopherols and reduce losses, it is recommended to control the deodorization temperature below 250 ℃. Currently, the oil industry are using Rovipon colorimetric method to measure the color of fats and oils. The red value (R) of common edible oilcolor is generally between 1 and 9, and within this range, the allowable error of red value is ±0.3. When the oilcolor|ΔR|≥0.3, theoiltesting equipment can accurately distinguish the change, and at the same time, consumers can also obviously distinguish the difference of oil color, so|ΔR|≥0.3 can be used as the judgment standard of good oilcolor return .
- (2) The change of Phosphorus in the refining process and its effect on the color of soybean oil in different storage periods
The color reversion phenomenon of first-grade soybean oil has a certain relationship with the phosphorus content, and the higher the phosphorus content, the color reversion is easier. Studies have pointed out that the phosphorus content of refined oil should be less than 1 mg/kg, the phosphorus content of bleachedoil should be less than 2 mg/kg, and the phosphorus content of degummed oil should be less than 12 mg/kg. In practicalproduction, when the phosphorus content of the finished oil is greater than 5 mg/kg, the rate of oil color reversionis significantly accelerated. Regarding the phospholipid causing the color reversion of the oil, there are currently two explanations: one is that the phospholipid can form a colored complex with the remaining sugars and proteins in the oil, or be oxidized and decomposed to produce colored substances, resulting in the color reversion; the second is the non-hydratable phospholipid Phosphatidylethanolamine occurs non-enzymatic browning in the oil refining and deodorization stage, that is, phosphatidylethanolamine undergoes pyrrolization, which causes the color reversion.
Under normal production parameters, the removal of phosphorus in soybean oil is mainly reflected in the neutralization section and the bleachingsection, and the deodorization section can only remove about 13.3-23.1% of the phosphorus in the oil.
Phospholipids in oils are mainly divided into hydrated phospholipids and non-hydrated phospholipids. The hydrated phospholipids account for the main part and can be basically removed in the degumming section, while acid refining degumming is mainly to remove non-hydrated phospholipids. Therefore, in production, the key link in the control of the phosphorus content in the oil should be placed in the degumming section to ensure that the oil is in full contact with phosphoric acid, so that the non-hydratable phospholipids in the oil can be converted into hydrated phospholipids as much as possible, which can reduce the pressure of reducing the phosphorus content in the bleachingsection, reduce the amount of decolorizer, and reduce production costs.
- (3) Changes of metal ions in the refining process and their effects on the color of soybean oil in different storage periods
The metal ion content in the refined oil after deodorization is closely related to the color reversion. When the phosphorus content of the deodorized oil is 0, the iron ion content is less than 0.15 mg/kg, and the copper ion content is less than 0.1 mg/kg, as long as the method is appropriate, the oil can be stored for a long time, there will be no color reversion problem. If the metal elements are not removed completely, the automatic oxidation of the oil will be promoted during the storage process, and the quality of the oil will be deteriorated. At the same time, metal elements can react with lipids, non-hydratable phospholipids, fatty acids, and some pigments to generate fatty alkyl radicals, phospholipid metal complexes, fatty acid iron salts, chelating compounds and other substances, which can accelerate the color recovery of oils. Zheng Liyou’s experiment found that iron and copper ions are extremely significantly correlated with the color red value.
- (4) Effects of bleaching agent addition amount and species ratio on the color of soybean oil in different storage periods during refining.
At present, the most common oil bleaching method used in industry is adsorption decolorization,commonly used adsorption decolorizers are activated carbon, bleaching clay and attapulgite, bentonite and so on. In the process of bleaching, if the amount of decolorizer is not appropriate and the quality indexes (mainly including activity, metal ions, particle size, etc.) does not meet the requirements, it will affect the stability of the refined oil, shorten the period of deterioration and rancidity, and cause the oil reversion.The addition amount of decolorizing agent added affects the bleaching effect. Too much addition of decolorizing agent can easily cause oil oxidation and isomerization, which will cause trace hydrolysis and oxidation of bleached oil, increase the content of polar substances such as conjugated dienoic acid, and deepening the color.The physical and chemical properties of different kinds of decolorizers are different, and the best bleaching effect can be obtained by choosing a suitable decolorizing agent.
- (5) Effect of decolorizer ratio and additive amount on color change of soybean oil.
under the condition of consistent decolorization, type ratio and deodorization temperature, as the amount of decolorizing agent increases, the color red value of bleachedoil decreases overall. After the refined oil is stored for a period of time, the ΔR of American soybean oil increases, but below 0.3, the ΔR of Brazilian soybean oil does not change much, but both are greater than 0.3, indicating that Brazilian soybean oil is more likely to color reversionduring the 18-month storage period. Under the same conditions that the added amount of decolorizer and the deodorization temperature, adjust the ratio of decolorizer, and after storage for a period of time, the color of the endproduct oil will appear color reversion, but the effect is not significant.
- (6) Effects of deodorization temperature and stripping steam pressure on the color of soybean oil in different storage periods.
adjusting the deodorization temperature between 245 ~255 ℃ and stripping steam pressure between 0.1 kPa~ 1.0 kPa has no obvious influence on the Δ R of American soybean oil stored for 10 months, but has a great influence on the Δ R of Brazilian soybean oil stored for 7 months, which indicating that the stability of American soybean oil is good,but Brazilian soybean oil stability is poor, easy for color reversion.
The color reversion of oil is a very complicated problem. At present, it is still not possible to accurately explain which one or several factors are the main reasons for the color reversion. To control the color reversion of the oil well, it is necessary to focus on the raw materials, the pressing process, the quality of crude oil and the refining process are comprehensively analyzed and research. And during the refining process, the changes of free fatty acids, γ-tocopherol, phosphorus, heavy metal ions and other trace components also have an important impact on the stability of edible oil.To determine whether a batch of soybean crude oil is suitable for the production of finished products and whether it will occur colorreversion , the production department and the quality control department need to make a comprehensive analysis according to the production and testing indexes of each link, rather than conclude with a single index.
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