Science popularization: How is melamine added to milk?

Many people drink milk to replenish calcium, but if you take a closer look at the labels on the packaging of domestic fresh milk, you will generally not find the calcium content listed. The labeled nutritional components only include two items: fat and protein. Fresh milk is divided into whole milk, low-fat milk, and skimmed milk, each with different fat contents. And in today's world where fat is considered a health killer, most people do not care whether the fat content meets the standard. Protein is the main nutritional component in milk, and the packaging of fresh milk always indicates that the protein content is ≥2.9 grams per 100 milliliters, indicating compliance with the national standard for fresh milk (≥2.95 grams per 100 milliliters).

The protein content of raw milk is generally above 3%, so it usually meets the national standard unless water is added to the original milk. To avoid someone selling water at the price of milk, it is necessary to test the protein content when purchasing raw milk. Based on the chemical properties of proteins, there are several testing methods, each with its own advantages and disadvantages. The Kjeldahl nitrogen determination method, which is widely used in the food industry and has been designated as the national standard, is one such method. This method was invented by the Dane Johan Kjeldahl in the late 19th century. Its principle is quite simple: proteins contain nitrogen elements. By treating the sample with strong acid, the nitrogen in the proteins is released. By measuring the nitrogen content, the protein content can be calculated. The nitrogen content of milk proteins is about 16%. According to the national standard, multiplying the measured nitrogen content by 6.38 gives the protein content.

Therefore, the Kjeldahl nitrogen determination method does not actually measure protein content but rather infers protein content by measuring nitrogen content. Clearly, if the sample contains other nitrogen-containing compounds, this method will not be accurate. In normal circumstances, this is not an issue because the main components of food, except for proteins, do not contain nitrogen (other main components like carbohydrates and fats do not contain nitrogen). Therefore, the Kjeldahl nitrogen determination method is a very accurate way to measure protein content. However, if someone adds other nitrogen-containing substances to the sample secretly, they can deceive the Kjeldahl nitrogen determination method and obtain falsely high protein content readings, passing off watered-down milk as original milk.

A commonly used nitrogenous substance to impersonate protein is urea. However, the nitrogen content of urea is not particularly high (46.6%), and dissolving it in water emits a pungent ammonia smell, making it easily detectable. Moreover, a simple detection method (the Griess reagent method) can determine whether urea has been added to the milk. Thus, later, counterfeiters switched to melamine. Melamine has a high nitrogen content of up to 66.6% (the higher the nitrogen content, the more protein it can impersonate), is white and tasteless, and there is no simple detection method (high-performance liquid chromatography, a high-tech method, must be used to detect it), making it an ideal protein impersonator. Melamine is an important chemical raw material, widely used in producing synthetic resins, plastics, coatings, etc., and its current price is approximately 12,000 yuan per ton. During the production of melamine, waste residue containing 70% melamine is produced. The counterfeiters use this melamine residue to impersonate protein. Some "biotechnology companies" promote "protein essence" online, which is essentially melamine residue, adding it to feed and dairy products to impersonate protein.

How is melamine added to milk? There are two possible routes. One is adding it to the original milk at the milk station. This has certain limitations because melamine is slightly soluble in water, with a solubility of 3.1 grams per liter at room temperature. That means 100 milliliters of water can dissolve 0.31 grams of melamine, containing 0.2 grams of nitrogen, equivalent to 1.27 grams of protein. From this, we can calculate that to meet the requirement of ≥2.95 grams of protein per 100 milliliters, 100 milliliters of milk can have a maximum of 75 milliliters of water added (and 0.54 grams of melamine). Another route is adding melamine during the powdered milk manufacturing process, which is not limited by solubility, allowing for any amount to be added.