The dispute between "hard tofu" and "soft tofu" is because of this addition!

Audit expert: Wang Guoyi

Postdoctoral fellow in Nutrition and Food Safety, China Agricultural University

Tofu is a delicacy on our table and a treasure of taste left by our ancestors. As early as the Han Dynasty, the Chinese had mastered the preparation method of tofu, adding gypsum to soy milk to obtain tofu that is "white as pure jade and fine as solidified fat".

After thousands of years of development, tofu and its products have become an indispensable ingredient on the Chinese table. It is not only delicious, but also has high nutritional value and is deeply loved by people.

Tofu source: pixabay

But how much do we know about tofu, a food that we are so used to? Do you know how tofu is made? From soybeans to tofu, how did it become what we see today?

The production process of tofu is well known. The main raw material for making tofu is soybeans. The production process can be roughly divided into several steps: "soaking beans - grinding - filtering - boiling - adding pulp - pressing into shape".

In the process of making tofu, soybeans as raw materials need to be soaked for a period of time, and then a certain proportion of water is added to grind them into soy milk when the beans swell to 2-3 times their size. Then the impurities (i.e. bean dregs) in the soy milk are filtered out, the soy milk is heated to cook, and the foam is skimmed off to make it smooth and pure, which can improve the taste of tofu.

Soybean, soy milk, tofu source | pexels

After preparing the hot soy milk, the next step is to add some brain.

The process of adding brine to form tofu pudding is called dipping soy milk. The key to turning liquid soy milk into solid tofu is adding brine. As the saying goes, "brine and tofu, one thing conquers another." The soy milk that has been added with brine will slowly precipitate and gradually form semi-solid tofu pudding.

Source of tofu making | China News Service

After solidification, tofu pudding is smooth and brittle, with low strength and cannot be formed. In order to obtain tofu with greater hardness, the last step is to press and form it. Pressing is to squeeze out the water in the tofu pudding. Before pressing, the tofu pudding needs to be broken up to destroy the original protein arrangement structure and release more water. The amount of water can affect the taste of tofu. If you want to get firm tofu, press more water. If you want soft tofu, press less water.

How does liquid soy milk become solid tofu? The key lies in the seemingly magical process of "adding brine". Adding brine means pouring brine into the cooked soy milk to achieve the effect of precipitation and coagulation.

Brine, also known as bittern, is the remaining part after making salt from seawater or salt lake water. It contains magnesium chloride, magnesium sulfate, sodium sulfate and other ingredients. These ingredients are all electrolytes. When dissolved in water, they will form many free-moving positive and negative ions, which can act as a coagulant.

Soy milk is rich in protein, but protein molecules are large and not easily soluble in water, which makes the cooked soy milk a colloidal solution with high-speed movement of protein molecules.

Organic structures can be divided into typical organic structures that repel water molecules, "hydrophobic groups", and atypical organic structures that are close to water molecules, "hydrophilic groups", based on their reaction to water. The same is true for proteins, which have both typical organic structures dominated by carbon-hydrogen structures and atypical organic structures dominated by carboxyl groups losing hydrogen ions.

Tofu source: pixabay

The hydrophobic groups are close to each other, while the hydrophilic groups are more willing to contact water molecules. As a result, many structures with hydrophobic groups inside and hydrophilic groups outside are formed in soy milk. The hydrophilic groups will tightly hold the water molecules to form a set of water films. Such protein structures are "colloid particles". They are very small, and because the water film blocks and the outer groups have the same charge, they repel each other, and the proteins cannot condense.

When a coagulant is added to the colloid, the electrolyte formed by the coagulant in the water will break the original situation. The ions in the electrolyte will neutralize the charged ions on the surface of the colloid particles, causing the protein to coagulate. When the coagulated particles are large enough, precipitation will occur. The previously dispersed proteins coagulate with each other under the action of the brine to form tofu brain. By squeezing out excess water and increasing the density between the proteins, tofu with moderate hardness and softness can be obtained.

Brine is not necessarily required for “adding brine” and brine is not the only coagulant. Other coagulants such as gypsum, ester acid, citric acid, etc. can have the same coagulative effect.

Different coagulants will affect the taste and nutritional value of tofu, but the production principles are similar. Brine tofu has a soft and tough texture, strong bean flavor, and low water content, commonly known as "old tofu" and "northern tofu". Gypsum tofu is tender and smooth, with a high water content, commonly known as "tender tofu" and "southern tofu".

Source of rennet tofu丨A shopping website

In addition, there is a kind of boxed tofu on the market, which is smooth, tender, white and delicate. The coagulant of this tofu is different from the traditional one. It uses a new chemical substance - gluconolactone, which is also an example of technology used in food processing.