Dairy products can be really nasty when they spoil, and even with modern fridges and preservatives, many of us are quick to throw out a tub of yogurt if it seems even slightly dodgy. Considering all this, it's hard to imagine the fridge-less folk of the 1800s bothering with such spoilable products, but they did. What's even more interesting is that they used the exact same technique we still use today: lactic acid fermentation. For us, fermentation is about making new food products like creamy yogurts and spicy kimchi. Back then, it was mostly about preservation. When you had more milk than you could drink within a few days, you needed to make some butter or cheese to avoid wasting it.

It's no secret that lactic acid fermentation has been used for a long time, but the recent discovery of some 130-year-old butter cultures in Denmark has shown just how little our processes have changed over time. That's not to say they haven't improved, however, since the old cultures were contaminated with multiple types of troublesome bacteria that we can successfully avoid today. As you might know, fermented milk products like yogurt and butter have existed for much, much longer than a century, with records dating back over 7,000 years. That's because milk naturally contains lactic acid bacteria, which can trigger the fermentation process spontaneously. That said, it's much harder to control the process this way.

Read more: Here's What Bologna Actually Is (And Other Facts That Might Surprise You)

Bacteria are everywhere, including in our food. They reproduce rapidly and consume components like sugars and starches, converting them into something else. A lot of the time, these conversions are not good for us. Bacteria breaks down food products and causes the kinds of ruined textures, bad odors, and discoloration that make food inedible.

Some bacteria work in our favor, and the lactic acid variety is a prime example. When you add lactic acid bacteria to milk, raise the temperature, and wait for a few hours, it will convert the sugars to lactic acid. This acidifies the milk and makes it harder for spoilage microorganisms and pathogens to survive, extending shelf life for up to three to five weeks. The newly produced acid also brings other changes, including a thick, creamy texture and tart taste that aren't very milk-like at all. That is why it was given a new name: yogurt.

You can also churn fermented milk to make butter. However, that isn't the most common way to do so today. We usually use a machine to separate the fat (buttercream) from the liquid, and then churn that cream into butter. If you want a creamy spread that was made with lactic acid, you'll need to buy cultured butter that has gone through a stage of fermentation before being churned. Compared to the regular thing, it has a deeper flavor that can really make a big difference when you cook with it.

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