The Costco Nuts Customers Often Have To Return

Costco's Kirkland Signature bulk nuts are a perfect example of the quality and quantity of items you can get for a killer deal at the retail giant. However, you may want to think twice before purchasing the brand's 3-pound bag of walnut halves. According to recent member reviews and Reddit threads, customers frequently return it for smelling rancid and tasting bitter — a concern that echoes some Coscto recalls that almost affected millions. 

"I buy a bag of these walnuts about once every three weeks and for the last several months every bag has been stale and rancid." This comes from a 1-star review left on the product's webpage, which, unfortunately, is joined by hundreds of other low-rated reviews that claim similar experience. Some even mention mold: "Right upon opening the bag, there is a strong earthy odor, and the walnuts have a noticeably bitter and rancid taste. Some even appear to show signs of fungus or mold."

The issue was further spelled out on Reddit with threads titled, "Why do my Kirkland walnuts smell stale?" and "Why are Kirkland walnuts soft and bitter?" All of which were, again, met with comments about similar experiences, as well as possible insight into the cause of the taste and smell problems.

Walnuts have a mild but notable tang due to compounds —  tannins and catechin, specifically—  in their skin, and when stored improperly, these compounds produce a bitter taste that signifies rancidity. Walnuts also contain polyunsaturated fat, a heart-healthy fat that oxidizes or break downs when exposed to heat, light, or oxygen. The immediate result of oxidization is a bad smell and/or taste, but there are also long-term side effects to eating nuts that have "gone bad": oxidized nuts can be linked to heart disease and cancer.

Many reviewers noticed the walnuts had gone bad the moment they opened their non-expired bag. However, the potential factors that are involved within the journey walnuts undergo to get to Costco locations — from harvesting to packaging and cross-country transportation, often in hot temperatures — make rancidity almost inevitable.

Unless Costco implements specialized chilling methods for nuts similar to those used for perishable items during shipping to stores or directly to customers, the quality of Kirkland walnuts may vary. Regardless, for long-term storage up to a year, it's advisable to store walnuts in the freezer to prevent oxidation. If your walnuts are opened and still fresh, make sure to store them in a freezer-safe bag or container before freezing.

It's important to note that Kirkland Signature walnuts weren't a part of the eight nuts at Costco ranked worst to best, and according to the product's webpage, it's actually a customer favorite, amassing an impressive 4.8 stars from over 5,000 5-star reviews. The most recent review to address rancidity was wrote in late 2025, so it may just be that there's a few bad bags within a batch every now and then

If you encounter a stale or moldy bag of Kirkland Signature walnut halves upon opening, you can take the spoiled product back to any Costco location for a 100% refund. They won't be phased by the return, since nuts are one of the Costco food items customers return the most.