Identification of vegetable oils and fats (4)

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24. To identify the true and false identification of soybean oil, it is necessary to first understand the quality characteristics of soybean oil, and the normal quality characteristics of soybean oil have changed, indicating that the quality of soybean oil has changed. In the case of oil purchases at the fair, most of the oil was mixed with fakes, and rice soup was generally added. The method for identifying adulteration is as follows:

(1) See brightness: good quality soybean oil, texture clear and transparent, no turbidity phenomenon. If the oil is turbid, it indicates that it is adulterated.

(2) Smell smell: Soybean oil has bean-flavored, non-bean-flavored oil, indicating that it is adulterated.

(3) Look at the precipitation: good quality soybean oil, processed through multiple procedures, the impurities have been separated, the bottom of the bottle will not have the phenomenon of precipitation of impurities, if there is precipitation, indicating that the soybean oil is coarse or mixed with starchy substances.

(4) Test the moisture: pour the oil into the pot a little. When heating, if the oil emits buzzer sound, it means there is water in the oil. When you choose oil in the market, you can also drip a few drops of oil on waste paper. When you ignite and burn, if you emit buzzer sound, it means that the oil is mixed with water.

In order to accurately identify the presence of other oils incorporated in vegetable oils, chemical tests can be used to identify them.

Take 5 ml of the oil sample in a test tube, add 3 ml of trimethylolmethane (2 ml) and 2% potassium nitrate solution, and shake the tube vigorously to make the solution milky. If the creamy body is lemon yellow, it means the presence of soybean oil. If it is slightly yellow, there is peanut oil and sesame oil.

25. Identification of Edible Oil Blended with Cottonseed Oil In the farmers' markets of cotton-producing areas, it has been found that useful crude cottonseed oil is incorporated into edible oil for sale. People who eat this kind of oil will experience food poisoning. Identification of vegetable oils with sensory methods for the incorporation of seed oil: oil bubbles appear green or brown-yellow, and the oil is heated and rubbed on the palm of the hand to smell the cottonseed oil.

Chemical identification method: Take 5 ml of oil sample into a test tube, add 1% sulphur powder carbon dioxide solution, dissolve the oil, add 1-2 drops of pyridine (fire 5 ml caramel), put the test tube in saturated salt, Slowly heat until the salt solution has boiled for 30 or 40 minutes. Take it out and observe. If the oil sample is red or orange, then the oil is mixed with cottonseed oil. The depth of the general oil color is related to the amount of cottonseed oil incorporated. It is blended with cottonseed oil and has a deep color. Blending more than 0.2% of cottonseed oil into edible oil can be detected by this method.

26, identification of edible oil mixed with mineral oil

In the farmers' market, it has been found that the use of mineral oil in the sale of edible oil has seriously jeopardized the health of consumers.

(1) Sensory identification method

1 Look at color: After blending with mineral oil in cooking oil, the color is deeper than pure edible oil.

2 smell smell: when smelling with the nose, can smell the unique smell of mineral oil, even if the cooking oil is mixed with less mineral oil, can also make the original cooking oil smell thin or disappear.

3 oral tests: oil mixed with mineral oil, bitter taste into the mouth.

(2) Chemical identification method

Take 1 ml of oil sample, place it in an Erlenmeyer flask, add 1 ml of potassium hydroxide solution and 25 ml of ethanol, and then connect the conical flask to the air condensing tube for reflux and saponification. After about 5 minutes (shock during saponification should be added) After heating evenly), add 25 ml of boiling water, shake and observe. If the oil sample is turbid, it means mineral oil or rosin is mixed in the oil.

In addition, mineral oils can also be detected by fluorescence since mineral oils have a fluorescent reaction and edible oils have no fluorescence reaction. The method of detection is that the oil and the known mineral oil are dropped one by one on the filter paper and then irradiated under a fluorescent lamp. If the oil sample reflects the fluorescence of the mineral oil, it indicates that the edible oil contains mineral oil.

27, identification of edible oil mixed with salt water

Some unscrupulous traders use a certain percentage of salt water in the cooking oil to increase the weight of the oil.

(1) Sensory identification method

1 Look at the color: For the cooking oil that enters salt water, it loses the color of pure oil and makes the color lighter.

2 see the transparency: Because salt water is brighter, after people eat oil, make the concentration of edible oil lower, the oil is more light and bright. 3 oral tests: For salted cooking oil, there is a salty taste in the mouth.

4 hot test: cooking oil into the salt water, heating the pan, will send a buzzer sound.

(2) Chemical identification method

Take 107 ml of oil sample, place in a separatory funnel, use 30 ml distilled water, 20 ml, 10 ml extraction, and the aqueous phase, and then wash the aqueous phase with a small amount of petroleum ether, the water phase into the porcelain evaporator, add One milliliter of a 5% potassium chromate solution was titrated with a 0.1 mol/L silver nitrate solution to determine the content of sodium oxide. At the same time, the same kind of cooking oil without salt water was used as a blank control to determine the amount of salt water.

28, identification of edible oil mixed with rice soup

In the farmer's market, it is common to incorporate rice soup in cooking oil. Cooking oil mixed with rice soup is harmless to the human body, but it can make the oil worse, which is not conducive to the use of stir-frying. (1) Sensory identification method

1 Look at the color: Regardless of the vegetable oil, the white rice soup will lose its original color and lighten its color. In summer, oil and rice soup are divided into two layers.

2 see transparency: rice soup is a starchy paste, lack of transparency, once the oil for people, so that the purity of the oil to reduce the refractive index increases, poor transparency.

8 smell smell: Every kind of pure edible oil, has its own smell of the oil, such as sesame oil has sesame oil flavor, soy oil has bean flavor. The smell of smelling oil is light or disappears for the cooking oil that is in the rice soup.

4 hot test: the rice oil into the cooking oil, wok heating, it will send a buzzer sound.

(2) Chemical identification method

Take 5 ml of the oil sample and place it in a test tube. Add 1 to 2 drops of iodine reagent. The oil sample shows a blue reaction, indicating that the oil has entered the rice soup.

The iodine reagent was prepared by taking 10 g of potassium iodide and 5 g of iodine and dissolving it in 100 ml of distilled water and diluting it with 50-fold dilution.

29. Identification of edible oil mixed with castor oil

The edible oil is mixed with castor oil. The sensory basic method is to keep the oil sample static for a certain period of time so that the vegetable oil and the castor oil are automatically separated into two layers. The vegetable oil is in the upper layer and the castor oil is in the lower layer.

Basic chemical methods, there are the following two:

(1) Color reaction method

1 Take a few drops of the oil sample and place it in a ceramic colorimetric plate. Add a few drops of sulfuric acid. If it is light brown, it means that the oil is mixed with castor oil.

2 Take a few drops of the oil sample and place it in a porcelain cuvette, and then add a few drops of nitric acid. If it is brown, it means that the oil is mixed with castor oil. (2) Anhydrous ethanol test method

The test of edible oil mixed with castor oil is based on the fact that castor oil can be mixed with anhydrous ethanol in any proportion, while other common vegetable oils are not easily soluble in ethanol.

Take 5 ml of oil sample and place it in a graduated centrifuge tube with a stopper. Add 5 ml of absolute ethanol and plug it. Vibrate vigorously for 2 minutes. Remove the stopper and centrifuge for 5 minutes. Remove the centrifuge tube and let sit for 30 minutes. Observe the lower oil layer of the centrifuge tube. If the oil layer is less than 5 ml, the castor oil is mixed with edible oil. This test method can detect 5% of castor oil incorporation. If more castor oil is used in edible oil, the volume of oil layer in the lower part of the centrifuge tube will be less.

30. Identification of Edible Oil Mixed with Tung Oil

The chemical identification of tung oil incorporated in vegetable oil can be done in the following ways:

(1) Nitrous acid method: Take 5 to 10 drops of oil sample, place it in a test tube, heat 2 ml of petroleum ether, dissolve it, and add 1 g of sodium nitrite to crystallize and 1 ml of 5 mol of sulfuric acid to shake the tube. The mixture was thoroughly mixed and placed for a while. The color was observed. If 1% Tung oil was blended in the oil, the oil sample became white and turbid. If 2.25 Tung oil was blended in, the oil sample appeared white floc.

(2) Sulfuric acid method: Take 2 drops of oil sample on white porcelain plate, add 1 drop of sulfuric acid. If the oil is mixed with tung oil, blood red clot will appear, the surface will shrink, and the color will gradually deepen.

(3) Antimony trioxide method: Take 1 drop of oil sample into a test tube, add 1 ml of 1% ruthenium trioxide methane solution, mix and shake, and put it in warm water at 40°C for 5 to 10 minutes. Red, indicating oil mixed with tung oil.

(4) Picric Acid Method: Take a sample of 1 ml of oil and place it in a test tube. Add 3 ml of saturated picric acid glacial acetic acid. Mix and shake. If the oil sample is red, it indicates that it is mixed with tung oil.

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