Costco members complain its butter changed and they're switching brands. Here's what is behind the debate.

A recent post on social media site Reddit has kicked off furious online debate about a kitchen staple from Costco.

Devotees of the membership-only retailer are complaining that Costco's house brand of butter is suddenly causing their baked creations to fall apart, prompting speculation that the company changed the formulation of its Kirkland Signature "blue box" sweet cream butter.

To be considered butter, cream products must meet a federal standard set by the U.S. Department of Agriculture. It must be made exclusively from milk or cream, or both, and contain at least 80% milkfat. 

A box of four, one-pound packages of Kirkland butter costs about $17.40, according to Instacart. That's considerably less expensive than Land O'Lakes butter sold at Walmart, which for the same amount runs about $24. But some bakers say they're switching allegiance after encountering problems while baking with the Kirkland-brand butter.  

Costco did not reply to CBS MoneyWatch's request for comment about whether its Kirkland butter formulation had changed.

Variables that affect butter

In the U.S., butter contains between 80% to 82% fat. European butters typically have higher fat content, which yields denser baked goods. Experts say other variables, including butter's water content, could be at play in the Costco butter debate. 

"The amount of water that's incorporated or that is separated out when you churn cream is a little bit variable," Rosemary Trout, a professor of culinary arts and food science at Drexel University, told CBS MoneyWatch. 

If the water content in butter is too low, dough can come out too crumbly. 

"The structure of fats in butter is also really important, as they're going to impact the way it melts and how soft it's going to be," Trout said. 

Additionally, the quality of the milk that's used can affect how butter behaves when using it to bake. Trout said she does not know whether Costco changed its butter recipe or formulation.

"Hesitant to bake with it"

Some people are also saying they've stopped buying Costco butter and have switched to other brands because it was no longer doing the job.

"My mom and I have been Costco 'blue box' salted butter loyalists for some time," wrote the Redditor, who posted under the username momster0519 and who kicked off the debate. 

"We use butter for our pie crust recipe and that crust would not hold up! 2 batches just crumbly and could not get it to roll," momster0519 posted. 

She tried the same recipe with a different brand of butter and it worked, she said, leading her to claim that Costco's butter had somehow changed. 

"I'm hesitant to bake with it for any recipe now," the Redditor wrote. 

"All sorts of trouble"

A baker who shares videos on TikTok as @theloafandcrumb also suspected Costco's butter was the reason her Swiss meringue buttercream frosting didn't pass muster. So she swapped it for another brand, while otherwise leaving the recipe unchanged else. Success.

"So I've been experiencing all sorts of trouble with my Swiss meringue butter cream that I've been making for years the same exact way. Have not changed a single thing and as of, just about two months ago it decided it's no longer going to work," she said in the video. 

She pointed to the Costco butter as the culprit, saying that when she added it to the cooked egg whites, the concoction refused to whip into frosting. "It used to work wonderfully and it no longer is," she said. 

Trout, the food expert, said that butter with high water content would also also hinder baking. 

Other commenters added that they'd recently experienced similar issues with Kirkland Signature butter, saying it caused their cookies to spread or bake differently. Cookies or other baked goods that don't stick together or that are too crumbly could mean the butter did not have enough water in it, according to Trout. 

A writer for food website and social network Allrecipes.com also investigated Costco's alleged butter formula change, but could not definitively conclude that the butter had changed since she "wasn't able to compare the current version to a previous version."

Her best guess: Kirkland Signature Sweet Cream Butter could have less fat, and therefore higher water content, than it previously did or compared with other butter brands.

"But, if that is true it doesn't seem to be enough to warrant boycotting Costco's butter," the writer concluded.