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.