Key words: melamine, pseudoprotein principle, Kjeldahl method, solubility. The recent "Sanlu milk powder" incident caused an uproar, and the fundamental reason was the additive "melamine" in milk powder.
What is the intention of adding melamine to milk powder? How to identify melamine with simple experiments? These are all issues that people care about, and this knowledge is what we need to study as consumers. The "milk powder incident" has had a great impact at home and abroad, resulting in many infants being poisoned and suffering from kidney calculi. Through consulting data, theoretical analysis and experiments, it is concluded that melamine is added by using the principle of false protein, thus reducing the cost and deceiving consumers. 2. Family members can detect melamine by a method similar to cooling crystallization to reduce the possibility of poisoning.
Theme and discussion: Melamine, chemical formula C3H6N6, referred to as triamine, also known as 2,4,6-triamino-1, 3,5-triazine, 1, 3,5-triazine-2,4,6-triamine, 2,4,6-triamine.
First, the principle of pseudoprotein
Melamine is added to milk and milk powder mainly because it can pass off as protein.
All foods should be tested for protein content according to regulations. However, in order to make huge profits, merchants add a lot of water to milk, which will reduce the content of protein. Therefore, the merchants took advantage of the loopholes in protein's monitoring method and escaped punishment.
Protein is mainly composed of amino acids, and its average nitrogen content is about 16%. Because the direct detection of protein is too complicated, the determination of protein content in milk in food industry adopts Kjeldahl method. The principle is simple: protein contains nitrogen, so the sample is treated with strong acid to release the nitrogen in protein. By measuring the nitrogen content, the content of protein can be calculated.
Through calculation, the nitrogen content of melamine is 66.7%, which is more than four times that of protein. White and tasteless, it is an ideal protein fake. The national standard for fresh milk is 100 ml ≥ 2.95g, the protein content of all brands of milk powder is 15-20%, and the average nitrogen content in protein is 16%. If the protein content of a qualified milk is 3% and the nitrogen content is 0.48%, the protein content of a qualified milk powder is 18% and the nitrogen content is 2.88%. The nitrogen content of melamine is 139 times that of milk and 23 times that of milk powder. Adding 0. 1g melamine to every 100g milk can theoretically increase protein by 0.625%. It is estimated that the content of test protein in plant protein powder and feed will increase by one percentage point, and the cost of using melamine is only 1/5 of that of real protein raw materials. Melamine, as a white crystalline powder, has no smell and taste, and it is not easy to be found after doping.
Milk powder is toxic because it contains melamine, which may be added directly to milk powder or to raw milk.
This is the so-called "pseudoprotein principle".
Second, home testing for melamine.
Melamine is soluble in hot water and slightly soluble in cold water. Based on the law of melamine solubility changing with temperature, it can be roughly detected by a method similar to cooling crystallization.
Third, the experimental report
【 Experimental Name 】 A small experiment to detect melamine at home.
[experimental date] 2008- 12-2
[experimenter] liu ling
[Objective] By comparing different milk powders, the method for detecting melamine was determined.
[Experimental instruments and medicines] Four glasses, two pieces of black cloth, chopsticks, refrigerator, Sanlu milk powder (borrowed from a neighbor's house), Hongxing milk powder, hot water, clear water and a glass of cold water.
【 Experimental Steps and Phenomena 】
1. Put the same amount of Red Star milk powder and Sanlu milk powder into two glasses respectively, add the same amount of isothermal boiling water (less water than usual) into the two glasses respectively, and fully stir the two cups of milk with chopsticks to completely dissolve the milk. There is no obvious difference between the two cups of milk, and the color of Red Star milk powder is slightly darker. Label the outer walls of two cups of milk for identification.
Put two cups of milk in the refrigerator at the same time and let the milk cool for one hour.
3. Take out two cups of milk, carefully observe the two cups of milk, and find that there is a small amount of precipitation at the bottom of Sanlu milk powder cup, but there is no obvious change in Hongxing milk.
4. Prepare an empty cup, cover the mouth of one cup of milk with a black cloth, fix the cloth tightly by hand, turn the cup upside down and let the milk filter into the empty cup through the black cloth. Similarly, filter another glass of milk with another black cloth.
5. After filtering, compare two pieces of black cloth. It was found that there was a small amount of white massive solid on the black cloth of Sanlu milk powder, but there was no obvious solid on the black cloth of Hongxing milk powder. Close the black cloth and rinse it repeatedly with clear water. Open it. There are several white crystals on the black cloth of Sanlu milk powder. Pour the white crystal into a glass of cold water and the solid will sink to the bottom.
【 Experimental description and conclusion 】 Melamine is soluble in hot water and slightly soluble in cold water. After cooling the hot melamine solution, the melamine solid will precipitate into white crystals, whose density is greater than that of water. Therefore, the sediment filtered by Sanlu milk powder is likely to be melamine. Melamine can be detected at home by this method.
[Experimental Evaluation and Discussion] This is a life experiment that is not difficult and can be done by everyone without professional instruments. It only needs a cool head and careful observation, and does not need profound theoretical knowledge or excellent experimental operation level, so it has broad application prospects. The disadvantage of this method is that it still can't rule out the possibility of other substances and can't effectively prove that the precipitate is melamine. The prospect is that it can still effectively prevent the consumption of dairy products added with melamine, so that people's lives can be more guaranteed.