Yoghurt is having a moment. The Greek variety has gained popularity so quickly and unexpectedly – it’s now a multi-billion-dollar industry – that no one has yet figured out exactly what do with all the leftover whey. And there’s been much ink spilled about the supposed health benefits of consuming its live bacteria. It’s also easy to make yogurt at home, and curious DIY yoghurt makers have even begun sharing their experiences on YouTube, literally putting the goo under the microscope and wondering aloud, “What the heck is going on here?”

As it turns out, the answer is plenty of fascinating chemistry.

Yoghurt making is a controlled curdling process. Basically, you're forcing the milk to go off in a very specific way. To lay the groundwork for the right final texture, commercial manufacturers agitate the milk in a device a lot like a washing machine. This changes the microscopic structure of the milk by breaking up its big fat globules into lots of little ones. Then milk proteins form skins around each glob. This means that when the milk startles to curdle, and the proteins start to stick to each other, there will be a more even distribution of fat throughout the yoghurt. 

Then the temperature is turned up. The heat helps kill off any stray bacteria that are in the milk, and it also starts the job of unfolding the proteins so that they can form the molecular mesh that lies at the heart of yoghurt.

The temperature the milk should be heated to – and for how long – depends on the desired flavour. In commercial operations the milk often stands at 85C for 30 minutes or between 90C and 95C for five minutes. The makers of a kitchen appliance for making yoghurt at home note, “Yoghurt made from milk kept below 170F (76C) is thinner and tastes fresh, a little fruity and more tart, while yoghurt made from milk held at 195F (90C) for 10 minutes is noticeably thicker and tastes less tart and somewhat creamy/nutty/eggy.”

Everything from the heating of the yoghurt to the protein content of the milk can affect the final texture

Once the heated milk has been brought down to human body temperature – around 37C – it's ready for what many think of as the defining process in yoghurt: fermentation. At this temperature the two most common yoghurt-making bacteria, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, will thrive. As they grow, they take lactose, a milk sugar, and ferment it into lactic acid. The acid builds up and the pH of the milk drops.

The milk proteins notice the change. Up to this point, they have been packed together, either around fat globules or in their own small clusters that are stabilised by a salt called calcium phosphate. But this salt dissolves at low pH. As a result, the clusters start to loosen. Down, down, goes the pH, and the proteins in the clusters are freed to begin linking to each other, forming a mesh. They trap water and fat globules in the chains of clusters and proteins. Milk has become yoghurt.

When the fermentation process is arrested by cooling the yoghurt down, the result is a gel. In strained yoghurt, like Greek yoghurt, there is an extra step that involves breaking the gel by stirring and then separating out water, sugar, and proteins in the form of whey. That creates a texture that's more creamy than gel-like – but even without straining, everything from the heating of the yoghurt to the protein content of the milk can affect the final texture.

And you'd better believe that all of yoghurt’s potential textures have been studied by food scientists and food companies. The number of devices used for testing these qualities is slightly humorous, though you can see why it would be convenient to have objective measurements.

There’s the consistometer, a gadget with a reservoir at the top of a ramp that lets you essentially race yoghurts, timing how long it takes them to ooze a given distance. Then there’s the viscometers, some of which resemble tiny standing mixers, and the penetrometer, a device that involves simply dropping an object on the yoghurt from a pre-defined height and seeing how far it goes in. (All of these, of course, can be used for almost anything whose consistency you'd like to measure.)

Amateur yoghurt-makers with microscopes may be pleased to learn they are in good company. Commercial manufacturers also use microscopes to assess the structure of their yoghurt's gel, albeit often ones of higher magnification. Using fluorescent tags, they can see the clusters of proteins, the bacteria caught in the mesh, the fat globules, and all the other pieces assembled in that delicate chemical dance that make yoghurt what it is.

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