After six years, Burger Theory’s Union Street store is closing. Its final day of trade (which is expected to be this Saturday) comes 18 months after a minor rebrand and a major menu shake-up, which saw a switch to kangaroo-based patties and subsequent outrage on social media.

“Basically, we’re pretty tired,” says co-owner Rob Dean, who also runs an outpost at Flinders University. “We’re just shrinking down. We moved out of Adelaide Uni last year, too. I guess it’s consolidation after a crazy few years.”

The Union Street site (which evolved from the Burger Theory food truck) had been on the market since late last year. Dean confirms a buyer has been found, but he’s not revealing any names publicly yet. He says the decision to sell was two-fold.

“For one, my co-founder Dan [Mendelson] has gone back to Canada. He moved back for personal reasons … the other one is it just wasn’t performing as well as we’d like it to,” he says.

Last January the pair made the switch from their signature house-ground premium beef to “smashed” kangaroo-meat patties (with a percentage of beef fat to help bind them). Their reasoning was financial (the price of beef had jumped) and environmental, but it was also about making a better-tasting burger, Dean told Broadsheet last year. “We genuinely think it’s better than our previous burgers. If we’re thinking that but hedging our bets, I don’t think it’s ‘gonna cut it. Sustainability is a great by-product, but we’re still trying to make the best burger.”

Beyond the meat, there were changes to the burger style too. They’re smaller (as are the prices) and the patties are squashed on the grill and cooked well-done (rather than medium), forming a caramelised crust. The kitchen also switched from a brioche bun to imported potato buns (the same used at popular US chain Shake Shack). And the pickles were replaced with karkalla (aka pigface – a native succulent).

The result, at least according to this writer, was a superior burger. But the evolution saw a downturn in revenue. “There’s probably quite a few things we got wrong with the change, in terms of our messaging … how we did it,” says Dean. “It also kind of got hijacked as soon as we launched it,” he says of the uproar. “It was just overwhelming.”

Online backlash ranged from concerns for animal welfare and food safety to the less common, “What will it be next – possums?”

Despite the growing presence – and acceptance – of kangaroo meat on restaurant menus all over Adelaide, Dean acknowledges the different, perhaps broader market of a burger joint.

“It’s in a lot of the higher-end, progressive places, and one of the things we really wanted to do was democratise it a bit. And make it more mainstream. I mean, there’s nothing more mainstream than a burger. Maybe we underestimated the cultural hang-up of kangaroo meat, which seems to still be there.”

It’s worth noting that Burger Theory’s Flinders site hasn’t experienced the same downturn in business. “I’ve been trying to work that one out myself,” Dean admits. “I guess one thing is it’s a bar, so … we have a 50/50 split on food and beverages, whereas the other sites were more heavily food focused. For whatever reason – I don’t know if it was less attention or focus on [the roo], but – we’re selling as many burgers as we ever have.”

The Flinders outpost will continue to trade. And Dean’s not ruling out a similar concept in the future. “We’re working with a couple indigenous-food suppliers to see if maybe we look at partnering with one of them and trying it again. I just don’t know when that might happen.

“I’m pretty stubborn but I still think we made the right call. Obviously we regret it didn’t have the impact we hoped it would. But we don’t regret the idea. The fact there’s this bountiful resource we’re not making use of properly... The way the world’s heading, I think it’s still the right move to make in terms of our values and principles. It just hasn’t quite paid off. But I don’t think we’re giving up on it.”

“There’s a lot of noise around [plant-based alternatives] Beyond Burgers and Impossible Burgers, which is great, but ... switching to kangaroo is one that doesn’t involve millions of dollars of scientific research,” he continues. “It’s a resource that’s there. I still think it’s the right way to go. I think we got a few things wrong in the execution. And maybe we were a little bit early.”

It’s not all doom and gloom though, he adds, acknowledging the “loyal clientele” who supported the change. “It will be a shame for them that we don’t have something for them to go to straight away. We move on from this one and then work out what’s next. So there’s a bit of hope, I just don’t know what it will look like when we do come back.”

Burger Theory Union Street will trade until this weekend.