I have a "best friend" baker friend who makes the most excellent cakes and she made the cake for my parents' 60th anniversary. Two-tiered white cake with pineapple filling and beautifully and elegantly decorated. There was a silver foil round bottom underneath, wooden dowels throughout. During the celebration we ate the top tier. a few days later we cut into the bottom tier and YUCK! it tasted like bug spray or chemicals or gasoline. It was delivered by my friend and never moved from the spot we served it. It didn't come in contact with any chemicals before or after delivery that we are aware of. It looks so beautiful and you can smell the chemical smell somewhat, but the taste....horrific...like it actually made the tip of my tongue feel numb/tingly. What could have caused this? The whole bottom tier of the cake is wasted now.
I’m so sorry to hear about your parent’s anniversary cake going bad.
When a cake develops a chemical taste and/or odor it’s usually caused by a chemical reaction from metals and acids.
From your description of the cake, filling, and the aluminum foil wrap, my guess is these were the culprits.
Baking powder is sodium bicarbonate (baking soda) mixed with an acid to control the rate of activation. Baking soda is an alkaline, so it requires an acid to fully activate. Two common acids used to make baking powder are sodium aluminum phosphate or sodium aluminum sulfate. Both of these can impart a chemical taste. Aluminum free baking powder is available. In the future you might request that your cakes are made with the aluminum free baking powder.
Cake batters usually include both baking powder and baking soda If a cake batter does not contain enough acid to fully activate all of the baking soda, then the residual baking soda will also leave a bad chemical aftertaste.
Acidity ingredients, like buttermilk, can cause a chemical reaction if it comes in contact with reactive metals. You mentioned the cake was wrapped in aluminum foil. Aluminum, steel (not stainless steel), copper, and iron are all reactive metals, so they cannot be used with acidic ingredients.
You mentioned a pineapple filling; pineapple is highly acidic and the aluminum foil wrap would definitely react with the acids in the pineapple.
Reaching a 60 year wedding anniversary is pretty remarkable. I hope your parents enjoy many more years together