Traditional preserved eggs are mainly made of soda ash, lime, salt and tea. In order to make protein in fresh eggs quickly solidify and shell, it is necessary to add some lead oxide. In this way, preserved eggs contain a certain amount of lead. If you eat this kind of preserved egg often, the lead in it will slowly accumulate in the human body, which will damage your health.
Now the pickling process of preserved eggs has been improved. Copper sulfate and zinc are used instead of lead oxide, and lead compounds are not used, hence the name "lead-free preserved eggs". However, because there are trace amounts of lead in fresh eggs, air, water and soil, the lead content in preserved eggs cannot be zero. But as long as it is controlled within the standards set by the state, it is safe to eat.
Firstly, five kinds of preserved eggs were broken and stirred evenly with a blender, so that the protein and yolk were evenly mixed together to get five samples.
Then, weigh out 0.5g sample, add nitric acid, and put it into a "microwave digester" for digestion.
Digestion is divided into three processes: digestion at 120℃ and 10 atmospheric pressure for 3 minutes; Then digested at a high temperature of 150℃ and at 20 atmospheric pressure for 2 minutes; Finally, it was digested at 180℃ and 30 atmospheric pressure for 3 minutes.
After digestion, when the temperature drops below 80℃, it can be taken out of the digester.
At this time, as soon as the lid of each sample is lifted, a yellow smoke will come out. Director Jin said it was the smog of nitrogen oxides. After the yellow smoke dispersed, we saw that the digested preserved eggs had become transparent liquid like water.
Followed by constant volume. The digested sample was washed several times with deionized water and transferred to a 25 ml volumetric flask.