I visited a McDonald's test kitchen to try its biggest menu overhaul in a decade. The burger patties are a big improvement, but the new Big Mac is too sweet.

It was my first time at the McDonald's UK head office. I was expecting some more glitz and glamor from a company with a $204 billion market cap, but the flag was a nice touch.

The test kitchen is modelled on an actual McDonald's restaurant, with a similar layout and the same equipment you would find in an average store.

McDonald's has overhauled its buns. The new recipe is described as "a buttery, brioche-style" bun with a "soft and pillowy" texture. McDonald's has also scaled back the number of sesame seeds on each bun.

They look great, taking on a shine thanks to a light glaze. It's worth noting that while they are "brioche-style" they do not contain any egg, milk, or butter like a traditional brioche does.

The beef patties at McDonald's are now getting a "tighter sear" on the grill. The idea is that by exposing them to more heat, you seal more moisture inside the patties for a juicier bite and a stronger grilled flavor.

The onions found on hamburgers, cheeseburgers, and the Big Mac are now being added to the patties during the grilling process rather than after. This way they soften up more and take on some charred flavor from the grill and the meat.

The lettuce used at McDonald's stores isn't changing, but the way it is handled is. It will now only be kept out of the refrigerator for a maximum of 30 minutes, and kept at colder temperatures once out to retain more crunch.

The iconic McDonald's American cheese slice isn't getting any recipe changes, but McDonald's claims the hotter sear on the patties makes for more melty cheese than before.

Armed with all of the information I needed about the new recipes and cooking methods, it was time for me to try some new and old style burgers to see how the changes stacked up.

The visual differences were pretty striking. The new bun looks much more appealing thanks to its glossy finish and darker toasty color. It was clear that the cheese had melted more in the newer burger, with the slice's edges drooping while the original burger's slices stayed rigid.

The Double Cheeseburger is one of my favorite McDonald's menu items so I was apprehensive to try a new version, but it was a definite improvement. The bun has more bite to it now with a satisfying bready chew, and a stronger toasted taste.

The beef was also great, with an almost caramelized flavor thanks to the hotter sear. The only underwhelming change was the onion, which was more mildly sautéed than "charred."

The buns were strikingly different once again. McDonald's say the newer versions have fewer sesame seeds, but I wouldn't say it's a drastic change.

The Quarter Pounder also doesn't benefit from grilled onions. It still has the larger onion pieces compared to the Big Mac and Cheeseburger options, but they're not added during the cook process. The only change is that they, like the lettuce, are being held out of the fridge for less time to maintain crunch.

Somewhat disappointingly, I barely noticed any change to the taste of the Quarter Pounder with Cheese. The texture of the new bun added to it again, but the meat didn't seem to benefit much from the new sear approach.

The sear is more noticeable on the thinner patties, but any charred flavor on the outside of the quarter pounder got lost in the thick, meaty inside as soon as I started chewing. If you were a fan of the original burger, you shouldn't be concerned about the new version.

The new Big Mac looks great. The glossy bun catches the eye once again, and the reduced sesame seed count emphasizes that even more.

It also seemed to stand up straighter than the older version, possibly due to the colder lettuce and new bun giving it more structural integrity.

Biting into the new Big Mac ultimately led to disappointment. There were some definite areas of improvement — the sandwich felt hotter and fresher, the beef was juicier, and I was still enjoying the chewier bun.

The main issue for me was the brioche flavor added to the new buns. When combined with the other Big Mac ingredients, specifically the Big Mac sauce, it created a strange, overly sweet taste in my mouth which I just couldn't seem to get past.

The original Big Mac is, in many ways, a perfect burger. The sauce is slightly sweet but with plenty of tang to balance it, something that is lost in the new version. The bun was slightly too spongy, but in hindsight I see why that was the case.

The new bun seemed less able to withstand the toppings, which kept sliding out of the back of the burger every time I took a bite, while the old sandwich held firm. The new bready, buttery brioche-style version didn't have the requisite give needed to squeeze the fillings tightly enough to contain them.

Overall I'd say the menu changes represent a step in the right direction for McDonald's. The new method of cooking the beef patties was probably my favorite change. All the new patties I tried felt hotter, juicier, and had a better grilled flavor than the many, many McDonald's burgers I've eaten in the past.

McDonald's also seems to be placing more emphasis on producing fresher food, with small changes to things like lettuce storage and grilled onions actually making a noticeable difference to the end product.

A company of the size and reputation of McDonald's doesn't roll out significant changes without good reason. They put the new items through a rigorous testing process including plenty of tastings with members of the public, so they must be pretty confident people will enjoy the new offerings.

I will certainly still be dining at McDonald's in the future, although part of me will be hoping they at least scale back the buttery sweetness of those new Big Mac buns.