Mari,
Yes, we have been discussing the logic of things, but this may be more about habit, emotions, and attachment. The teaching of not wasting food from the grandfather would weigh significantly. Perhaps we can hope that, as evidenced by his asking you if the food is safe, he has suspicions brewing and these might slowly convince him.
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My father has emm "antiquated" ideas about cooking food (meat) and leaving it sit in the pan all day, then reheating for 2 mins and eating. Most of the time it's ok but it gets scary sometimes.
My late uncle's wife had a thing about not wasting food -- she ate a potato that had actually sprouted a plant, because she didn't want to waste it. We don't know if she was sick (probably) but no ER was involved. She was
more than lucky with the dose of solanine she got.

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Just saying... he's not the only one.
It's hard to live with these things, though. 
I hope your husband doesn't hurt himself. 
waves