Costco's new Kirkland Signature olive and rosemary sourdough bread has proved popular with shoppers, but it has reignited debate over what genuinely counts as "sourdough". Purists argue that true sourdough should be leavened only with a live sourdough culture, with no commercial yeast, vinegar or other additives used to mimic the flavour or acidity, meaning the label is being applied loosely by some retailers and manufacturers.

According to standards cited from the Real Bread Campaign, authentic sourdough relies solely on wild fermentation for both rising and taste, without shortcuts. Critics have questioned whether Costco's version, and similar mass-produced loaves, meet this bar, while many customers on social media have said they simply enjoy the bread regardless of technical purity. The disagreement reflects a wider tension between artisan bread-making traditions and the growing commercial use of the "sourdough" label on supermarket shelves.