Case solved, why do we have to lick the lid when drinking yogurt? !

Yogurt is a common dairy product that many people love to drink. Yogurt has a variety of flavors to meet the dietary needs of different people. However, there has always been a controversy about yogurt: " Do you lick the lid when drinking yogurt? "

When you open the lid of many yogurts, there will be a thick layer of yogurt on the inside. Some people will eat it all, but some people think that this is a sign of being "stingy and too frugal."

Source: A shopping website

So, should we lick the layer of milk on the lid of yogurt? Where does it come from? Why do some yogurts stick to the lid but others don't? Today, I will explain it in detail .

Why does yogurt stick to the lid?

Why does yogurt stick to the lid? This is actually related to the "wetting ability" of liquids on different materials.
For example, when we drop a drop of water on the glass, the water will soak the surface of the glass, and we think that the glass is a "hydrophilic" material; on the contrary, if we drop a drop of water on the screen of a mobile phone, the water will exist in the form of a water droplet and will not wet the screen of the mobile phone. At this time, we think that the material of the mobile phone screen is a "hydrophobic" material.

Hydrophobic materials

Source: pexels

The material surface and the liquid droplet will form a "contact angle" of different sizes. When the attraction between the molecules inside the liquid and the attraction between the molecules at the interface between the liquid and the solid material pull each other, an "angle" will be formed on the gas-liquid surface and the solid material surface. This is the contact angle.

Source: Zhu Zhong et al., "Magical Surface Infiltration"

Where γSG, γSL and γLG are the interfacial energies of solid/gas, solid/liquid and gas/liquid, respectively. θc is the angle between the tangent line of the droplet at the triple point and the bottom surface.

The larger the contact angle, the weaker the ability of the droplet to wet the material surface; the smaller the contact angle, the stronger the ability of the droplet to wet the material surface.

Source: Wikipedia

However, yogurt is a liquid that forms a smaller "contact angle" with the yogurt lid, that is, its wetting ability is stronger. Therefore, during transportation and storage, the yogurt can easily soak into the inner surface of the lid, which is called "sticking to the lid".

So, since yogurt has strong wetting ability, why don’t some yogurt products stick to the lid?

2

Why do some lids not have the word “奶” on them?

In fact, whether different yogurt products are covered or not is also related to the yogurt processing technology.

Source: pixabay

In my country's national food safety standards, yogurt is defined as "a dairy product made from raw cow's milk, raw goat's milk or milk powder, which is sterilized, inoculated with thermophilic Streptococcus and Bulgarian Lactobacillus, and then fermented."

The process of making yogurt includes two steps: "fermentation" and "filling". The order of these two steps determines whether the yogurt will stick to the lid.

Yogurt that is first filled and then fermented is called set yogurt , such as the old yogurt on the market. Its production process is to fill the raw materials into the yogurt cup first, and then complete the fermentation in the cup.

Source: unsplash

The yogurt produced in this way has a thicker texture and is not easy to stick to the surface , so it usually has to be scooped out with a spoon before eating.

The yogurt that is fermented before being bottled is called stirred yogurt . Most of the yogurt we often eat is this type. Its production process is to ferment the raw materials in a large fermentation tank, stir them continuously during the process, and finally fill them into yogurt cups.

The yogurt produced in this way is more fluid than solidified yogurt and is prone to shaking during transportation. In addition, the lid of the stirring yogurt cup is made of hydrophilic material, so it is easy for the lid to stick.
Generally speaking, yogurt sticking to the lid is due to the infiltration of yogurt and the surface of the lid, which is quite common. In addition, some people also believe that yogurt sticking to the lid tastes better. Is this true?

Source: pixabay

3

Why does yogurt with a lid taste better?

The yogurt that gets stuck on the lid will, under the action of gravity, aggregate its "casein" in the form of a three-dimensional network, which has high viscosity and is not easy to fall off, while the whey (whey protein, water) will be easily diluted out of the casein network.

In other words, the yogurt on the lid that we eat is "purified" yogurt with less water. Its viscosity, protein and fat content are higher than the yogurt in the cup, so the taste is more mellow.
However, the statement that "the covered yogurt is the essence" is not very reliable.

After all, no matter how the yogurt on the lid is purified, it still contains the same nutrients as the yogurt in the cup, but the protein concentration is different.

As a common dairy product, yogurt can provide us with rich daily nutrients, such as protein, calcium, phosphorus, magnesium, B vitamins, etc. Moreover, because yogurt has undergone a fermentation production process, many proteins and lactose are broken down into small molecules, which are easier to digest and absorb than milk.

Source: unsplash

According to the "Dietary Guidelines for Chinese Residents 2022", each person should eat a variety of dairy products every day, with an intake of more than 300 grams per day. For people who are lactose intolerant or do not like the taste of milk, yogurt is a very good source of dairy products. It should be reminded that yogurt often has a high sugar content. You can choose products with low sugar content through the ingredient list, but such products will be more acidic.

So, it’s totally fine to drink yogurt and lick the lid. After all, such nutritious food should not be wasted.