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I have a few cans of diet soda that with a best-before date in 2023, and while some of them taste more-or-less normal, others taste bitter, almost like alcoholic beverages.

Obviously, there's no actual sugar to ferment into alcohol (just artificial sweeteners), so that can't have actually happened. Additionally, they were in sealed cans that had been sitting in a fridge since they had been purchased.

What sort of chemical reaction could be responsible for this change in flavour? The USDA says that they should still be safe to drink, but I'm curious about the chemistry and I can't find anything aside from maybe a few references to artificial sweeteners decomposing into formaldehyde at high temperatures which probably wouldn't apply here, right?

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  • You might be tasting the caffeine if the sweetener has broken down
    – Joe
    Commented May 27 at 15:57
  • It would certainly help if you included the ingredients of the soda in question. Which artificial sweetener is used, aspartame, sucralose, stevia, etc.?
    – NSGod
    Commented Jun 2 at 20:09
  • @NSGod Pepsi Max. The ingredients are listed on their website here: pepsimax.com.au/products/pepsi-max
    – nick012000
    Commented Jun 3 at 1:04
  • The taste of fizzy drinks changes noticeably over time, with some of the more subtle flavours being lost first.  I suspect that what you can taste is simply the acid and more persistent lemon flavours remaining after the others flavours and some of the sweetness have gone.
    – gidds
    Commented Oct 28 at 15:44

1 Answer 1

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The contents of "soda" cans is acidic because when CO2 dissolves in water it produces carbonic acid. The acid dissolves the aluminum to produce aluminum carbonate. I had a forgotten box of soda water cans which, when discovered 3 years later, were empty. You could see tiny pinholes in the can walls.

The bitterness in your cans may be due to the low pH of the carbonic acid. According to https://en.wikipedia.org/wiki/Aluminium_carbonate aluminum carbonate is poorly characterised. The only reference to taste I could find was on Reddit which is ... well, Reddit.

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    Aren't aluminium cans coated in plastic to prevent the metal from leeching into the drink?
    – nick012000
    Commented May 30 at 8:02
  • @nick012000 I think the coating is imperfect and/or erodes over long time periods.  (I too have had many-years-old cans leak with pinholes.)
    – gidds
    Commented Oct 28 at 15:38
  • Additionally contents of soda cans are also acidic because most Soda contains phospheric acid.
    – davidgo
    Commented Nov 3 at 3:04
  • @davidgo ... its only cola-type drinks that contain phosphoric acid, to balance the sweetness. Clear carbonated drinks like soda water, 7-Up and Sprite have no phosphoric acid, but they may contain citric acid.
    – Woody
    Commented Nov 3 at 10:21

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