osi meat supplier mcdonaldsKim Bhasin / Business InsiderThe USDA inspector gets a parking spot right up front.

I recently visited the McDonald's headquarters in Oak Brook, Il., to see how the fast food juggernaut does business.

As part of the tour, I was given a look at the production lines at OSI — a McDonald's meat supplier and a long-time piece of its supply chain.

The OSI plant in West Chicago is the birthplace of the McRib, the legendary pork sandwich that's making its return across the nation today.

The technology used to create the McRib was "cutting edge" back when it was invented, explained OSI operations manager Darren Lange.

OSI only runs its McRib pork patty lines seasonally, because the sandwich isn't available all the time at McDonald's. Luckily, they were running the McRib patties on the day I was there.

The scale of the place is impressive. The plant produces a whopping five million beef patties a day.

I wasn't permitted to take photos inside the plant, so unfortunately there are none to share. I can, however, describe to you what it was like.

The pork starts off in a vat on a pallet, which is then hoisted up in the air and dumped into the grinder, where it comes out as normal-looking ground pork — as you'd see in any supermarket, but in a giant pile.

Workers fill up large bins as the pork comes out of the grinder, which are inserted into the next machine in line.

Then, the McRib takes shape. 

The pork goes into custom-made molds, and the rib-shaped meat plops down onto the next conveyor belt in rows of three.

A quality control worker keeps watch over this belt. He's constantly removing little pieces of pork and snatching away any patties that come out misshapen.

After that, the patties get a spritz of water to combat dehydration and are flash frozen in a massive freezer.

The freezer is absolutely huge. It's around 75 feet long and freezes the patties in around two minutes at -60 degrees Fahrenheit.

Once the patties pop out the other side, it's time for boxing. But getting the seemingly endless run of McRib patties into boxes is harder than it sounds though.

While it only takes a couple of workers to box the beef patties on each line, McRib patties are more difficult because they have a strange shape. They're little ridged parallelograms, which aren't easy to organize in a rectangular box. So there are a lot more workers at the end of the McRib line, scurrying to handle the pork patties.

The boxed-up McRib patties are automatically separated from boxes of beef patties on yet another series of conveyor belts, where a worker tapes them up and sends them along to a palletizer and freezer at the end of the line.

The entire process takes around 45 minutes.

From there, they're sent to McDonald's stores around the region to be slathered in barbecue sauce and chomped down by McRib fanatics.

We've been assured that all McDonald's beef is indeed 100 percent beef until it makes it to the restaurants, where salt and pepper are added right before consumption. The McRib's pork, on the other hand, isn't quite 100 percent pork, because the supplier adds water and a seasoning (salt, pepper, sage, rosemary extract and dextrose).

One thing that immediately struck me, having never been behind-the-scenes at a meat supplier, was the amount of care taken to prevent contamination of the meat.

Everyone wears hairnets and special coats, and if you have facial hair, you have to wear a mask. I had to wear rubber boots that go over your shoes. In order to go onto the production floor, visitors have to wash and scrub their hands in two different machines, then walk slowly over a device that cleans the underside of their boots. Visitors can't wear jewelry either, for fear that something could drop into the meat.

Every night, once all the work is done, a sanitation shift cleans up the plant. It's four hours of intense washing of all the machinery, plus additional checks and balances to make sure that everything's okay. The on-site USDA inspector, with his fancy front-row parking spot, does a walkthrough each morning to assure that there's no residue or anything else left behind.

It's quite the operation.

Disclosure: McDonald's provided travel and accommodations for the trip.