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

A post on social media site Reddit that was shared just after Thanksgiving has kicked off a widespread online debate about a kitchen staple from Costco, one of the largest big-box retailers in the U.S.

Costco devotees are complaining that their preferred retailer's own brand of butter is suddenly causing their baked creations to fall apart, leading them to speculate that it 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 package of four, one-pound boxes of Kirkland butter costs about $17.40, according to Instacart. That's considerably less expensive than Land O'Lakes butter sold at Walmart, for which an equivalent amount would set you back about $24. But some bakers say they're switching allegiances 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.

Butter variables

In the U.S., butter contains between 80% to 82% fat. European butters typically have higher fat contents, which lead to more dense baked goods. There are other variables, including butter's water content, which experts say 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 a butter's water content is too low, it could cause dough to be too crumbly, for example. 

"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 added. 

Additionally, the quality of the milk that's used can impact 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 simply wasn't 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 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 turn out the way she'd expected. So she swapped it for another brand, while keeping everything else the same, with positive results. 

"So I've been experiencing all sorts of trouble with my Swiss merengue 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 identified 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 professor, said a butter with a high water content would make a product like that harder to make.

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 are too crumbly could mean the butter did not have enough water in it, according to Trout. 

Does it warrant a boycott?

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

Her best guess is that Kirkland Signature Sweet Cream Butter could have less butterfat, and therefore a higher water content than it did previously, or compared to other butter brands.

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

Megan Cerullo

Megan Cerullo is a New York-based reporter for CBS MoneyWatch covering small business, workplace, health care, consumer spending and personal finance topics. She regularly appears on CBS News streaming to discuss her reporting.