Why does yogurt stick to the lid of the package?

Due to not very gentle movements, the yogurt can splatter and settle on the flap of the jar that acts as a lid. But why does it stay there attacked like a stalactite? And why does it seem to be thicker (better “viscous”) than the yogurt that remains in the jar? To answer these questions we must take into consideration the composition and structure of yogurt, its physical properties and the microscopic structure of tab. Let’s start by saying that there are no specific studies that explain the thickening of yogurt on the tongue, but by combining some knowledge of physics and chemistry we can get an idea of ​​how these factors interact with each other.

Composition and structure of yogurt

Yogurt is obtained by adding specific families of bacteria to milk, Lactobacillus bulgaricus And Streptococcus thermophiluswhich break down lactose into lactic acid in a process called lactic fermentation. This step is not only responsible for the acidity of the yogurt, but is fundamental for its structure And consistency. From a physical point of view, yogurt, like milk, is a colloidi.e. a heterogeneous mixture of microscopic particles (diameter 10^–9 but 10^–6 m) finely dispersed in an external phase. Yogurt is in fact composed of over 80% water, in which fats, proteins, sugars, vitamins and minerals (such as calcium and phosphorus).

The gelatinous consistency of yogurt comes from the interaction between the whey proteins (denatured during the heating process) and the caseina hydrophobic protein that tends to form micelles, droplets isolated from the aqueous environment. Bacterial fermentation and acidic pH lead to the formation of disulfide bonds between proteins and hydrophobic bonds between casein micelles. The result is the formation of a gelatinous lattice inside which the whey is trapped.

The separation of the whey causes the yogurt to thicken

Heterogeneous systems such as colloids by their nature are unstable systems which tend to separate over time. It’s a phenomenon called syneresis and it also happens in yogurt. When it remains still for a long time or if it is not stored at the right temperature (4°C), the liquid part of the yogurt, i.e. the whey, tends to escape the grid formed by proteins and casein and to emerge on the surface of the yogurt.

It’s nothing to worry about, it’s a natural and non-harmful processIn fact, all you need to do is stir the yogurt to restore its consistency. (ATTENTION: we are talking about a small separation of phases: if the yogurt changes color, smell or the consistency is particularly different from the usual one, it may have gone bad or is not suitable for consumption, so it is better to throw it away.)

The same thing happens to yogurt that sticks to the tab: it may happen that after a certain period of time, the whey separates from the gelatinous part and “falls” into the jar, thus making the remaining part denser.

But why does a part remain stuck to the tongue?

The colloidal nature of yogurt is one of the reasons why it sticks to the tab and does not fall into the jar: being more viscous than any liquid, yogurt tends to adhere more to surfaces.

Furthermore, a second physical property also comes into play, the surface tensioni.e. the cohesion force of the particles present on the external surface of a liquid. It’s the same one that causes water drops to form on the windshield or those of dew on the leaves. Thanks to this surface tension, drops or splashes of yogurt that end up on the lid tend to stick to it.

We may need to consider another factor: since jarred yogurts are often fermented directly in the package, it is possible that the carbon dioxide produced during the fermentation process exerts some pressure which helps to hold up the yogurt that has stuck to the tongue.

Adhesion depends on the tab: but does it hurt to lick the yogurt lid?

Yogurt stuck to the tongue doesn’t seem like a problem to which we should dedicate more than a few minutes of our attention, but in reality this leads to large waste because many consumers tend to throw away the dirty yogurt tab.

Today, the tabs are made from a layer of aluminum covered with a very thin layer of plastic, to prevent the aluminum from coming into contact with food (let alone with an acidic food like yogurt). If we lick the yogurt on the tongue, the risks of taking aluminum are minimalbut prevention is better than cure. To overcome these problems, yogurt manufacturers are doing research to improve the microscopic structure of the tabs and make them more water-repellent.

A smooth texture means more contact surface with yogurt and therefore also more adhesion strength. On the contrary, rough and irregular structures reduce the contact surface and tend to reduce adhesion and surface tension. Taking inspiration from lotus leaves, known for their water repellency, tabs are in fact being developed with microscopic fractal structures capable of breaking up yogurt drops.

Again from plants, this time the carnivorous ones Nephentesanother strategy arrives to increase the water repellency of the tongues and surfaces in general: the production of water-repellent sprays based on natural waxes to be applied on surfaces so that liquids and semi-liquids, such as yogurt, do not stick and slip off.