Chemical Engineering in Daily Life – Question 34: When cooking dumplings, should you remove the lid to cook the filling or keep it on to cook the noodles? Standard answer already available

The Northerners are better at making dumplings; Southerners tend to buy them ready-made, as there’s no fun in that process. -------------------------- Is there enough space in the dumpling? With a lid on, the external pressure is high, which can easily crush the dumpling ? ? ?

The main difference is that by closing the lid, the pressure inside the pot increases, causing the temperature to rise more rapidly； Moreover, the temperature increase when the lid is opened is slow and uneven

When cooking dumplings, is it true that you remove the lid to cook the filling and keep the lid on to cook the dough? It’s always been cooked this way. But frozen dumplings cannot be done this way!

Right. After covering a pot of boiling water, the pressure inside the pot becomes higher than atmospheric pressure. As a result, the dumplings in the pot expand, and an air layer forms between the wrapper and the filling, which hinders heat transfer. Over time, this can even cause the dumplings to burst. Therefore, it is necessary to add cold water to lower the temperature and pressure while cooking the dumplings. When the lid is opened, the pressure inside the boiler equals that of the outside environment, causing the dumplings to deflate; that’s why the lid is opened to cook the filling.

In my opinion, covering the pot is used for cooking noodles when the water is boiling, and it’s used to cover the filling while cooking it.

The principle is that once the dumplings are put into the pot, they are cooked with the lid on; this increases the pressure inside the pot, causing the temperature to rise rapidly and the water to boil intensively. As a result, the wrappers cook quickly and do not stick together easily； The advantage of opening the lid and cooking at this stage is that the dumplings with cooked skins, as they keep rolling, have their skins often exposed above the water surface, which prevents them from sticking, while the filling inside still manages to cook properly.

But frozen dumplings cannot be done this way! Why

Yesterday on the streets of the city, I saw people buying zongzi and yuanxiao, and it made me laugh uncontrollably. Today is the Mid-Autumn Festival, but I can’t help wanting to make dumplings by myself. But I don’t make dumplings more than 3 times a year either.

Standard answer: First, dear netizens, imagine the temperature distribution in the temperature field of a boiling pot. Add water to the pot, bring it to a boil, and then put the dumplings into the pot. At this point, it needs to be heated for a while longer before the water boils again; of course, the dumpling wrappers and fillings also heat up at the same time. The issue we are discussing is what happens after the water boils again. Since the liquid used for cooking dumplings is already a \"soup\" rather than water, and it contains many surfactants, foam forming during boiling is a serious problem; this is also why it’s necessary to lift the lid of the pot from time to time. When the pot lid is on, or in other words, when the pot is covered, there are three phases inside the pot: vapor, liquid, and solid. The lowest temperatures are found inside the dumpling filling and near the lid of the pot, where the gas phase is present; however, the temperature is relatively uniform there. What does it mean for food to be cooked? It actually refers to the length of time it is heated at a certain temperature. The same goes for dumplings. Everyone should notice this phenomenon: when making dumplings, there is always some air inside them, no matter what. As long as it is heated, the equation PV=NRT definitely applies, so the dumplings will surely swell up. Especially after the water used for making the dumplings boils again for a short while. In this way, the dumplings as a whole have a density lower than that of water, so they float. In other words, a part of the dumpling wrapper is exposed to the air. If the pot lid is on, near the gas-liquid interface of the boiling water, the temperature is around 100+°C with little difference, so the dumpling wrappers (the so-called \"flour dough\") are cooked almost evenly. This is what is known as “cooking noodles with a lid on.” If the lid of the pot is removed, although the water is boiling, there is a significant temperature difference between the gas-liquid interface and the outside of the pot. In a regular pot, unless it’s extremely sophisticated, the temperature of the gas phase – which is a mixture of air and water vapor – located near the surface of the water, that is, above those puffed-up dumplings, may not be very high; it might be only 70–95 degrees. In such cases, the surface of the dumplings exposed to this gas phase will not be heated adequately. Even though the dumplings are boiling, the dough of the dumplings that is exposed to the gas phase for a moment isn’t properly cooked. And what about the dumpling filling? It should be noted first that the temperature of boiling water is around 100 degrees Celsius (even if the heat is not strong enough or the pot is too shallow, the temperature of the water on the surface may be below 100 degrees, or even 95 degrees). Water boiling simply means that the temperature at the bottom of the pot is high enough to create bubbles, and these bubbles have not disappeared by the time they reach the surface of the water. So the dough and filling of the dumplings that are underwater are both cooked. During the cooking process of dumplings, the dough wrapper and the filling separate almost completely; the filling collapses into a ball, with the air inside naturally rising to the top of the filling. In this way, when the lid of the pot is open, the upper part of the dumplings that are exposed comes into contact with a gas phase at a relatively low temperature, while the lower part is separated by an air layer (a mixture of air and water vapor); the thermal conductivity of the gas phase is certainly much lower. As a result, the dough of the dumplings that stay on the surface does not get properly cooked. It makes sense to explain it this way: when cooking dumplings, the filling is cooked with the lid off, while the noodles are cooked with the lid on. Specifically, the surface is in closer contact with water, while the filling is still separated from boiling water by a layer of skin. It then comes down to what it means to be \"cooked.\" In fact, it’s easy to tell whether the dough used for dumplings made from wheat flour is cooked or not; if it isn’t boiled in boiling water for a sufficient length of time, it will have a raw taste and stick to the teeth. No mother would be that careless (a newly married future mother might, but she would still taste it). As for dumpling fillings or meatballs, it’s hard to determine whether they are raw or cooked; everyone prefers a tender texture, and this tenderness can actually be achieved by heating them at 80-90 degrees for a certain period of time. Cooking dumplings is a very simple task; just do as your mom tells you to. However, when analyzed using chemical engineering principles, it turns out to be quite complex, involving issues such as density differences, thermal conductivity, heat transfer, phase equilibrium, and temperature gradients. One must think carefully about things in life and at work; otherwise, one can only end up as a worker or an operator.

The original poster got it wrong; it’s the exact opposite! ! ! ! ! ! ! ! ! ! ! ! !