Why we intentionally introduce bacteria to create your favorite snack

Science · Published 27 October 2026 Swipe more like this →

In short: To make yoghurt, you must first perform a rigorous biological cleansing. You sterilize equipment and heat milk to extreme temperatures to kill off every living microorganism. Then, in a deliberate paradox, you introduce specific 'good' bacteria to transform the liquid into a solid.

The process begins with a series of industrial-grade preparations: sterilization, pasteurization, and homogenization. Pasteurization involves heating milk between 85 and 95 degrees Celsius for 15 to 30 minutes to eliminate unwanted pathogens. Homogenization follows, mechanically breaking down fat droplets into smaller sizes to ensure they remain suspended throughout the mixture rather than sinking and creating a lumpy texture. Every piece of equipment used in this production must be sterilized first. This step ensures that no unwanted microorganisms contaminate the batch before processing begins.

Once the milk is cooled to an optimal range of 40 to 45 degrees Celsius, 'starter cultures' are introduced. These consist of lactic acid bacteria, specifically species like lactobacilli and streptococci. These microorganisms perform fermentation by consuming lactose—the primary sugar in milk—and converting it into lactic acid. During this multi-hour process, the bacteria also begin to digest milk proteins. These specific lactic acid bacteria are also utilized during cheese manufacturing. They act as the primary agents for converting milk into a different food product.

The transformation from liquid to solid is a result of chemical shifts. As lactic acid accumulates, the pH of the mixture drops to approximately 4.4. This acidity causes the milk proteins to coagulate, a process where a liquid turns into a more solid state. This drop in pH serves a dual purpose: it creates the signature tangy flavor and provides an acidic environment that prevents 'bad' bacteria from growing. The process concludes when cooling the mixture to 5 degrees Celsius halts the bacterial activity, preventing the yoghurt from becoming too solid. The fermentation period requires many hours to complete successfully. This duration allows the bacteria enough time to work at their fastest rate within the temperature range.

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