Can nine year old baking powder be harmful?

If you mean harmful to humans, the answer is no. Absolutely not.

If you mean harmful to the cookies, the likely result is they won’t rise since the baking soda is way outdated.

I wonder if you could sell it off as an antique ha.

It’s doubtful but as @Buttonstc has already stated, it won’t be of much use in baking.
I liken it to using oils – that is to say, aromatherapy oils – they have an expiry date, not because they go off and aren’t fit for use, because you ‘could’ still use them – but they’d certainly lose their effectiveness over time.

Think of it this way:

When you put fresh baking powder in something and bake it, a chemical reaction occurs and the products of that reaction (CO2 and, basically, salt) end up in the baked goods.

In old baking powder, the reaction has already been slowly happening in the can over all those years. So when you add it to your dough or batter, you’re not putting anything in that wouldn’t have ended up in there anyway. You’re just not getting the benefit of all of the CO2 that fresh powder releases, because it got slowly released long ago in the can.

This question and the fact that both baking powder and baking soda are referenced prompted me to research what, if any, is the difference between the two. Here it is:

Both baking soda and baking powder are leavening agents, which means they are added to baked goods before cooking to produce carbon dioxide and cause them to ‘rise’. Baking powder contains baking soda, but the two substances are used under different conditions.

Details here

Your cookies will be flat, but tasty. Just serve them as if they are supposed to be that way and enjoy!

Nine year old baking powder? Mazel tov! Throw it out, but it won’t do any harm other than not being of any use at all. Tits on a bull. Decorative perhaps, but useless.

But it does give you that satisfying feeling of having followed the recipe exactly as written ~~

:D

@Sunny2

That’s what Julia would do :)

Dropped fish or chicken? flat cookies? just it as if it turned out exactly as planned.

What happens in the kitchen, stays in the kitchen :)

^^ Exactly. You can’t always get away with it, but you can try.

I would say not harmful but toss it out. It may of absorbed odors in that time. You don’t want your cake smelling like a litter box or room of old shoes. It’s so cheap just throw it out!