I shudder to think of the thousands of tons of baking soda that have been wasted because of this myth. This was a very successful marketing ploy by you-know-who!
But let me back up for a moment. From a chemical point of view, baking soda can in fact neutralize some odors that might be found in your fridge. It works with some odors, such as rancid butter, fish, and decaying meat, but not on others (garlic, onion, mold and mildew). The problem is getting the odor to the baking soda.
Folks will just put an opened box of baking soda in the fridge. At best they will remove the entire lid. The odors will at best contact the top surface of the baking soda, which soon gets “used up.” The rest of the box is wasted and does you no good.
For baking soda to be effective at all you would have to spread it out in a thin layer on a large plate. But that’s a spill waiting to happen, and who has the space in their fridge?
Activated charcoal pellets are much better at absorbing odors than baking soda, but it is sort of pricey and again must be spread out in the fridge so that the odors can get to it, making it impractical for most people.
Bottom line? The only really effective way to prevent food odors in your fridge is to wrap smelly food, throw out stale stuff, and clean the fridge now and then. But you knew that all along, right?