McDonald's attracts plenty of customers — it said in its operations manual years ago that it sold 75 hamburgers a second — many people have expressed concerns about its burgers' ingredients.

A few years ago, a man named David Whipple conducted an experiment with a McDonald's burger. He left it in a cupboard for 14 years and found it in a similar condition as when he bought it.

Business Insider was given the opportunity to take a tour of a McDonald's factory in Germany to find out how McDonald's burgers are made.

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Nowadays, it seems we're getting more and more critical when it comes to ingredients. From organic ingredients and excess sugar to "E numbers" (or food additives) and salt, the list of contents to worry about seems to be growing exponentially.

McDonald's attracts plenty of customers — it said in its operations manual years ago that it sold 75 hamburgers a second — but the fast-food giant is by no means off the hook when it comes to this sort of scrutiny.

In 1999, a man named David Whipple started an experiment to see how many preservatives there were in a McDonald's burger. In 2013, he showed the world his burger 14 years after putting it in a kitchen cupboard — and it still looked almost exactly the same.

But Keith Warriner, the program director at the University of Guelph's Department of Food Science, said McDonald's hamburgers' not rotting had little to do with preservatives.

"The reality is that McDonald's hamburgers, french fries, and chicken are like all foods and do rot if kept under certain conditions," he said. "Essentially, the microbes that cause rotting are a lot like ourselves, in that they need water, nutrients, warmth, and time to grow. If we take one or more of these elements away, then microbes cannot grow or spoil food."

McDonald's also said in a statement this year that there were no preservatives in its fries or several of its burgers. But many people are still fixated on the notion that a McDonald's burger is pumped full of them.

To see how the burgers are made, Business Insider toured a McDonald's factory in Günzburg, Germany, where an average of 5 million burgers, from the Big Mac to the Quarter Pounder, are produced every day.

Here's how they're made.