Why is there foam when cooking meat

When cooking meat, everyone will find that there will be a lot of foam floating on the soup during the stewing process, which looks very dirty. It is actually very simple to remove the foam. You can wash the meat several times or soak it for a while before cooking, then blanch it before cooking. This way, there will be very little foam during cooking. So, why does foam appear when cooking meat?

This layer of foam is formed by blood remaining in the capillaries in the meat. Careful friends will find that the amount of foam produced when cooking different parts of meat is different. This is because the number of capillaries in different parts is different. Meat with clear patterns, such as the meat on the back of pork, has clear patterns and fewer capillaries, making it easier to wash away residual blood, so there is less foam when cooking soup. However, it is very difficult to clean the blood remaining in meat with complex patterns and rich capillaries, such as meat from the legs and shoulders, so more foam is produced when cooking soup.

Why does blood turn into foam when heated?

This requires analysis of the composition of the blood. Below I will use pig blood as an example to explain why pig blood forms foam after heating. At this point, we need to understand the principle of froth formation: food foam is usually a dispersed system formed by bubbles in a continuous liquid phase or a semi-solid phase containing a soluble surfactant. As shown in the figure below. Simply put, it's the same principle as when a child blows soap bubbles, and it's the same principle as when you beat egg whites and create bubbles.

We all know that pig blood is rich in hemoglobin, which accounts for 18.9% of fresh blood[1]. Hemoglobin is a highly soluble protein that can form stable foams. In pork broth, the gas is air or CO2, and the continuous phase is pork broth. When water boils, the air dissolved in the water is released. When the temperature rises to 100°C[2], hemoglobin denatures, destroying its secondary, tertiary and quaternary structures. As a result, hydrogen bonds and disulfide bonds are opened, exposing the polypeptide chains. The polypeptide chains can bind to the bubbles in the soup, causing the protein to expand in volume and form foam.

After saying so many professional terms, in fact, this layer of foam is hemoglobin combined with gas, which increases the volume of hemoglobin and thus forms foam.

Do you understand? So this foam is harmless. No matter what form hemoglobin becomes, it is still just protein. Don't be deceived by its appearance. However, from the perspective of a seasoned foodie: this layer of foam has a fishy smell and a bad taste, so it is recommended to remove it.

How to remove this layer of foam?

1. Soak the ingredients in clean water for a long enough time and change the water several times;

2. Some ingredients need to be blanched in boiling water before making soup, and then cooled down before making soup;

3. After the above treatment, some ingredients will produce some foam when stewing the soup. At this time, you can use a colander (there are also mesh spoons specially used for skimming foam) and other tools to skim off the upper layer of foam!