I have this well-rated chocolate cake recipe that I have not baked yet. It's ingredients (among others) are

  • 175g self-raising flour
  • 1 tsp bicarbonate of soda

My understanding is that soda is needed to make the cake fulffier, and it only works when it is dissolved in vinegar (or there is something acid in the batter, e.g. kefir). And there are no acidic ingredients in this recipe (sunflower oil, self-raising flour, cocoa powder, bicarbonate of soda, caster sugar, golden syrup, eggs, semi-skimmed milk).

So what's its purpose here?

  • 2
    I'm not a baker but a chemist, sodium bicarbonate can release CO2 under acidic conditions, but it also decomposes if heated. – xngtng Apr 30 at 13:39
  • 1
    Two-in-one questions are discouraged here, so I would recommend you remove the bonus question and ask it separately (although my one-line suggestion would be to look up a substitution for self-raising flour and apply this, considering it separately to the soda in the recipe). – dbmag9 Apr 30 at 13:48

Both cocoa powder and milk are acidic, and will be reacting with the soda here. Bicarbonate of soda will react with any acid, not only vinegar.

There is also a process called thermal decomposition, where the soda releases CO2 under high temperature without needing an acid, although less than it would in a reaction with an acid and leaving a compound with an unpleasant flavour. See Wikipedia for more detail:

Heat can also by itself cause sodium bicarbonate to act as a raising agent in baking because of thermal decomposition, releasing carbon dioxide at temperatures above 80 °C (180 °F), as follows:[16]

2 NaHCO3 → Na2CO3 + H2O + CO2

When used this way on its own, without the presence of an acidic component (whether in the batter or by the use of a baking powder containing acid), only half the available CO2 is released (one CO2 molecule is formed for every two equivalents of NaHCO3). Additionally, in the absence of acid, thermal decomposition of sodium bicarbonate also produces sodium carbonate, which is strongly alkaline and gives the baked product a bitter, "soapy" taste and a yellow color.

In your recipe, I imagine the soda reacts with the acidic ingredients to avoid this important taste.

  • Thank you! Would you have any guidelines for what the temperature/quantity of acidic ingredients should be, to avoid unpleasant metallic taste? – Yulia V Apr 30 at 14:01
  • 1
    No idea, I'm afraid. I would just aim to follow a recipe which someone else has tried successfully. – dbmag9 Apr 30 at 14:07

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