Color is one of the important quality indicators of oils and fats, and it is also the basis for the appearance of consumers when purchasing from the shelf, so it has become a routine item in the quality inspection of oils and fats. Oil processing enterprises also need to control the color of refined oil according to national standards, so as to obtain good decolorization conditions for oil processing technology, and then obtain maximum profits.
Commonly used methods for the determination of oil color include visual method, potassium dichromate method and Lovibond Colorimeter method. The visual method is to directly observe the color of the oil with the human eye; the potassium dichromate method is to compare the oil sample with the prepared potassium dichromate sulfuric acid standard solution, and the color value when the color is equal is the potassium dichromate method Color value; the Lovibond Colorimeter method is to compare the color of the standard color glass sheet with the oil sample, and the depth of the color is represented by the number marked on the required standard color glass sheet. This method is currently an internationally accepted inspection method. Among them, the Lovibond Colorimeter is most commonly used.
my country's national standard stipulates that the Lovibond Colorimeter method (GB 5525-1985) is used to measure the color of oil. For example, in the actual measurement of raisin oil, yellow is the reference value and red is the control value. First, fix the yellow to 35, and then adjust the red glass (sometimes Add blue glass flakes), when the two parts in the field of vision have the same color, the numbers of various glass flakes are the measured value of the grease color. This method is more accurate in determination, but requires experienced inspectors to obtain better reliability and reproducibility.
The principle of the Lovibond Colorimeter method for vegetable oil color is to compare the measured oil sample with two magnesium carbonate reflective sheets and three standard color sheets of red, yellow and blue with different depths under a certain light source. The instruments used are Lovibond ratio meter, Gardner Colorimeter and colorimetric tube (Fig. 2-1).
The basic operation process is as follows: level the instrument, install the observation tube and magnesium carbonate sheet, and check whether the light source is in good condition. Take the clarified sample and pour it into the colorimetric tank to reach a distance of about 5mm from the top of the colorimetric tank. Place the cuvette in the Colorimeter. First fix the color value of the yellow glass flake according to the regulations, turn on the light source, and move the red glass flake to adjust the color until the color of the flake is exactly the same as that of the oil sample. If the oil color is turquoise, blue slides need to be added. At this time, move the red slides until the number of blue slides reaches the minimum value. Write down the number of yellow, red or yellow, red, and blue slides. The respective totals are the color and luster of the oil sample to be tested. The test tube is filled with the sample to be tested. If the sample is slightly cloudy it should be filtered. Compare with glass standards to determine which standard is closer to the sample in terms of hue and saturation with negligible differences in hue.
At present, there are many deficiencies in the measurement of color and luster, and subjective factors often occupy a large proportion. Therefore, it is imperative to develop an objective detection method. The image processing method is developed in line with this trend . A new method for determining the color of oils and fats. In 1999, Sun Fengxia, Zhou Zhanming and others successfully measured the color of peanut oil, rapeseed oil, cottonseed oil, soybean oil and sesame oil by using computer image processing method. The artificial subjective error was avoided by thousand image processing, and the same oil was measured many times at the same time. The reproducibility and repeatability of the color are good, so the measurement results of this method are objective.
