As a former buyer for a major supermarket chain in the midwest, I question your statement that "salted butter often sits on grocery store shelves longer than unsalted does". Salted butter outsells unsalted by a wide margin which leads to faster turnover. The only way to check freshness at the store is to look for sell by or use by dates on the package. Even a sale price can work both ways ... markdown for a slow seller or a special brought in because the manufacturer has a large (fresh!) supply.

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