"opened in the fridge." this is perhaps the worst thing you could do. Moisture is your enemy.
An air tight container, preferably with as much air as possible removed, is how to go. Then there's no difference between storing in the fridge or in a cupboard (ie. out of the light).
I tried storing beans in a vacuum sealed bag resealed after each time and noticed zero difference. Maybe I just go through a bag too quickly.
Beans are a lot more forgiving when it comes to storage. Once you grind them, however, the grounds absorb moisture and oxidize relatively quickly.
I just leave my beans in their bag and store it in the cupboard, but yeah a bag lasts about 2 weeks. We came across a half used bag of beans that we'd forgotten about and had been sitting in a cupboard for a year. The coffee made from those beans was terrible.
Fridges remove moisture. It's why you cover things you place into the fridge.
Low humidity is not the same as no humidity and the relatively drier coffee grounds absorb that moisture leading to flavor change.
Look at a graph of temp vs moisture for hvac. We keep our fridge will below normal fridge temps. Moisture levels are significantly impaired. It also helps with odors and storage longevity. Cold air simply can't hold enough moisture to become an issue, in general.
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