Abstract : The change rule of physical and chemical indexes of aged edible vegetable oil was studied. The refractive index of long-term stored soybean oil, peanut oil, blending oil and rapeseed oil before and after aging was determined. The results showed that the refractive index of the four common vegetable oils increased significantly compared with that before storage. The determination of the refractive index of oil and fat is fast, convenient, safe and reliable, and can be used as an effective auxiliary means to determine whether the oil is rancid and aged.
Key words : edible vegetable oil, refractive index, temperature, formula
From the above analysis, the ratio of peanut oil to soybean can be quickly judged by the refractive index to judge the quality of peanuts.
Refractive index is one of the basic inspection items of vegetable oil. The refractive index of vegetable oil is determined. Abbe refractometer is a tool for measurement and is common in refractive index detection.
[Task Objectives]: 1. Master the method for determining the refractive index of fats and oils.
2. Determination of soluble solids using Abbe refractometer
[Instrument]: ( 1 ) Instrument preparation: Abbe refractometer, beaker, plastic dropper
( 2 ) Ethanol and pure water
( 3 ) Preparation of other auxiliary products: cotton wool, tweezers
( 4 ) Abbe refractometer DR-A1
[Detection steps]
1. Connect the instrument and turn it on. Select the BRIX value (or nD value) measurement mode.
2. Turn the prism switch, open the secondary prism, take the absorbent cotton with tweezers, and wipe off the surface of the main prism and the secondary prism. Drop a drop of pure water on the main prism and turn off the last prism.
3. Observe the bright and dark field of view from the eyepiece and adjust the measurement knob so that the light and dark boundary line just passes through the intersection of the intersecting lines. Look at the LCD display value is 0.0%, if not 0.0%, then do zero calibration. (Make sure the zero is correct)
4. Dry the prism and drop the dropper sample onto the main prism to close the last prism. Adjust the measurement knob to make the light and dark boundary line pass the intersection. If the boundary line is blurred, rotate the color difference compensation knob to make the boundary line clear. The nD value can be displayed directly on the LCD screen without having to look at the scale reading.
5. At the end of the measurement, take the absorbent cotton with tweezers and dip the alcohol to clean the surface of the main prism and the secondary prism.
It is recommended to configure a constant temperature water bath when measuring samples, and the measurement results will be more accurate.
4. As a result, the LCD display data is calculated as the refractive index value under the temperature measurement condition. If the measured temperature is not at 20 ° C, it must be converted to the refractive index (n20) at 20 ° C according to the following formula.
Refractive index (n20) = nt + 0.00038 × (t-20)
Where: nt--the refractive index of the oil temperature measured at t ° C;
t--The oil temperature when measuring the refractive index; 0.00038--The oil temperature is in the range of 10-30 °C, and the correction coefficient of the refractive index is 1 °C.
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