color and lustre
Vegetable oil is usually light yellow or light green and other different colors, which is due to the existence of oil soluble pigments such as carotene, chlorophyll, lutein, vitamin E oxide. Different oils and fats with different processing methods have different colors. Each kind of vegetable oil has its normal color range and value. For example, according to the national compulsory standard, the first grade peanut oil shall not exceed yellow 25 red 2, the first grade soybean oil shall not exceed yellow 70 red 4, and the first grade rapeseed oil shall not exceed yellow 35 red 4. If the value exceeds this range, it indicates that the oil is not refined well or has deteriorated.
280 ℃ heating test
It is a simple method to identify the content of phospholipid. When the content of phospholipid in oil is high, flocculent precipitate will be produced after heating; When the oil is rancid, the color of the oil becomes dark or black. According to the national compulsory standard, the color of grade I peanut oil, soybean oil and rapeseed oil should not be darkened and no precipitates should be found in the 280 ℃ heating test; The color of secondary oil is allowed to be darker, but not black, and micro precipitates are allowed.
Smoke point
It is one of the indexes to measure the thermal stability of oil when it is heated in air. Smoke point is caused by the existence of some substances with relatively low boiling point, such as free fatty acids, monoglycerides, unsaponifiable substances, etc. Generally, the smoke point of low refined oils is much lower than that of salad oils and high-grade cooking oils. The national recommended standard stipulates that the smoke point of salad oil shall not be lower than 215 ℃.
Acid value
It is one of the important marks of oil refining degree and quality. In addition to the fact that rice bran oil and coconut oil may have high content of free fatty acids, the high acid value indicates that the refining degree of oil is low, or some factors, such as high temperature, excessive water content, containing some metal ions or long-term storage and air contact oxidation, lead to the deterioration of oil. Oils with high acid value are not suitable for storage or consumption. According to the national compulsory standard, the acid value of first grade oil shall not exceed 1.0mgkoh/g, that of second grade oil shall not exceed 4.0mgkoh/g, and that of salad oil shall not exceed 0.3mgkoh/g.
Peroxide value
The hydroperoxides produced by the oxidation of oil and oxygen in the air are the primary products of automatic oxidation of oil. They are highly active and can be rapidly changed and decomposed into aldehydes, ketones and oxides, resulting in rancidity and deterioration of oil. Therefore, hydroperoxide is a sign of the degree of oxidation of oil in the early stage. Hydroperoxides are harmful to human health, and oils with high peroxide value are not suitable for consumption. The national compulsory standard stipulates that the peroxide value of peanut oil, sunflower oil and rice bran oil shall not exceed 20meq / kg, that of rapeseed oil, soybean oil and cottonseed oil shall not exceed 12meq / kg, and that of salad oil shall not exceed 10meq / kg.
cgv
When the hydroperoxides formed in the process of oil oxidation gather to a certain extent, they will decompose and polymerize to a certain extent, and the decomposition products will produce many carbonyl compounds, such as aldehydes and ketones. This index is an important hygienic index, which is used to evaluate the quality of oil. According to the national compulsory standard, the carbonyl value should not exceed 20 mEq / kg, and the salad oil should not exceed 10 mEq / kg.
Residual amount of solvent in leached oil
The extraction method is often used in the preparation of edible oil. The solvent is No.6 solvent with hexaalkanes as the main component. The solvent contains alkanes, cycloalkanes, olefins and aromatic hydrocarbons. Among them, aromatic hydrocarbons are more toxic, while alkanes are less toxic. They mainly act on human central nervous system and cause imbalance of lipid substances in nerve cells. According to the national compulsory standard, the residual amount of solvent should not exceed 50 mg / kg.
