Yes, you can apply for an exemption from their expectation - keep in mind that an Expectancy is not an obligation or compelled performance.
Interestingly enough I am presently (actually this afternoon) working on an exemption for a client right now.
You basically need to prove why an X-Ray would be unacceptable in your process and that the alternate is a metal detector.
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Texture, density, etc come into play.
The specific so called "requirement" is for finished product - our client shipped their raw product where the killstep is actually done by the end user - be it a store or a customer of a store that takes it home and cooks/bakes it. It becomes a finished product once it is processed at store or home.
.... ha ha ha, and there's the out. How things are worded in all these expectations by various companies is amazing to me, it's like shall & must as compared to should, would, could
Keep in mind that you will need an actual Costco generated exemption in hand and not just a letter from an "expert." for the audit.
All the Best,
All Rights Reserved,
Without Prejudice,
Glenn Oster.
Glenn Oster Consulting, LLC
Consultants for SQF, ISO-certified payment systems, Non-GMO, BRC, IFS, Lodging, F&B
http://www.GlennOster.com -- 774.563.6161