Sometimes, when we open a jar of yogurt, a surprise awaits us: that lump stubbornly stuck to the center of the tab, which threatens to dirty our fingers or make a mess by splattering on our table. Have you ever wondered how it happens? And why is it present in some jars and not in others? There are those who have studied the issue in depth, arriving at formulating a solution. Obviously these are Japanese, a people who seem obsessed with jar caps (after all they are the inventors of self-closing jam caps): the Morinaga Milk company, part of the larger Morinaga group, leader of the packaging industry based in Tokyo, a few years ago launched the revolutionary Toyal Lotus flap, an aluminum lid guaranteed to never present stains and lumps of yogurt when opened. How did they do it?
Splashes and jolts
The first thing to understand is how a yogurt stain forms on the inside of the tab that closes the jar, and why it is present in some packages, and not in others. And it’s easy to say: yogurt is a colloid (more precisely, in this case, a sol), a suspension of microscopic particles that contain fats, proteins, sugars and many other substances, suspended in a liquid material (water), and this gives it a liquid but at the same time viscous consistency, which means that it tends to splash when shaken, and to stick to the surfaces with which it comes into contact.
If yogurt containers, once packaged, are shaken during transportation or storage, spills that hit the flap tend to stick to it. They stick to it due to surface tension. And if the packages are stored for a long time in non-ideal temperature conditions, they can undergo a process called syneresis: the separation of the whey, which drips downwards falling into the jar, and the solid part composed of proteins such as casein , which hardens on the lid. It is in these cases, therefore, that we will find the yogurt encrusted on the tongue.
It’s all about water repellency
Once you understand what the problem is, you can then find a solution. To prevent the yogurt from adhering to the tab when the jar is subjected to some stress, it is necessary to increase the water-repellent properties of the material it is made of as much as possible. In this way, the splashes will naturally slide down, and we will not find lumps when we open the jar.
The goal can be achieved in many ways, and the one created by the Morinaga Milk researchers is inspired by what happens in the natural world. Various elements of the plant world, such as lotus leaves or broccoli flowers, are in fact practically impossible to get wet, and this is because they have a microscopic structure that determines a very high level of water repellency: they are defined as fractal, or hierarchical structures, and are determined by presence of irregularities, ridges and valleys, composed of other smaller ridges which in turn have sub-ridges, and so on. This conformation minimizes the adhesion of liquids, and therefore prevents the yogurt from sticking to the tabs.
While waiting for these new technologies to spread to Italy too, all that remains is to keep the lumps on the yogurt lids. Another question then arises: is it safe to lick them from lids? And unless once you open the jar the look or smell makes you suspect that it has gone bad, the answer is: absolutely yes. Syneresis is a completely natural process, regularly exploited in the production of cheese, and causes nothing more than the formation of a harder, and absolutely edible, rind on the surface of the tongue.