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I am curious about how baking powder, both types (with and without a high-temperature aluminum salt), loses its oomph.

Assuming a constant room temperature, is it time only? Exposure to air/atmospheric humidity? Both?

For example, what are the expected differences between a never-opened canister vs one with a typical plastic lid opened/closed with occasional use, over time?

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It's mostly due to moisture absorption from the air. Basically, baking powder is a mixture of a base - baking soda (sodium bicarbonate or similar) and a weak acid, usually tartaric acid (potassium bitartrate) and/or monocalcium phosphate or a similar acid.

Alone and dry the acid and base are more or less inert and won't break down substantially at room temperature, but if you mix the two together and add moisture, the acid and base solubilize and react. This is what happens when you add them to your baking, and is exactly what you want to happen to get the bubbles of carbon dioxide produced to rise your batter. The heat of the oven causes a faster reaction and expands the bubbles to make bigger spaces in the baking, resulting in the rising effect.

However, at room temperature both the acid and the base are hygroscopic to some extent, so they absorb moisture from the air over time and even this tiny bit of moisture results in a small bit of the reaction and degradation of the mixture.

There's a bit more explanation in the wikipedia article on Baking powder.

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  • Most interesting. Thanks. Moisture from the air and not temperature. In that case, sounds like refrigeration to slow degradation would be problematic - unless perhaps kept in a sealed laboratory-style desiccator.
    – revans19
    Commented Sep 10, 2023 at 2:52
  • @revans19 Yes, thermal degradation above about 80 C for the bicarb I think. Refrigeration will cause more water absorption. Best just to buy what you need and keep as well sealed as you can.
    – bob1
    Commented Sep 10, 2023 at 2:58
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    Note that the room temperature has an indirect effecet on the amount of water in the air. The same humidity percentage of air means vastly different quanitities of water at different temperatures. Idealls you want storage in a dry and cool place.
    – quarague
    Commented Sep 10, 2023 at 6:38
  • @quarague good point, but doesn't change the message from the answer.
    – bob1
    Commented Sep 10, 2023 at 9:47
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    @GentlePurpleRain: Every time you open the refrigerator door, warm (and, depending on your local climate, likely humid) air flows in to replace the cold air flowing out. As the air cools down, the moisture in it will condense on all cold objects and surfaces inside the fridge. (Top loading refrigerators lose much less cold air when opened, making them not only more energy efficient but also less prone to accumulating moisture, but unfortunately they tend to be less convenient to use for most applications.)
    – Ilmari Karonen
    Commented Sep 11, 2023 at 18:30

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