Baking Powder vs Baking Soda

(Sodium Bicarbonate)

Both baking powder and baking soda in baking get used interchangeably but should they? To understand this we need to know what their chemical structures are, and why they work as levening agents. To start off a levening agent is a chemical that helps the rise of a batter or dough, creating air pockets that give softer, less dense mouth feel. Baking powder and baking soda contain the same levening agent sodium bicarbonate. Sodium bicarbonate is made from the atoms carbon (C), oxygen (O), hydrogen (H) and sodium (Na) linked together through bonds. However the sodium is held close to the rest of the molecule through a different kind of bond to the rest of the atoms. This bond is referred to an ionic bond. An ionic bond is the same as holding oppositely charged magnets together; they stick.  In order for sodium bicarbonate to be used as a levening agent we must react it with something to cause it to break up into carbon dioxide gas (CO_{2 (g)}) and water (H₂O _(l)). Where the carbon dioxide gas creates “air pockets” in the batter when baked which in turn will lead to a lighter, softer mouth feel when the cake is eaten. 

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  To react the sodium bicarbonate we must use something that is safe to eat and preferably something that is already in the food preventing a change in flavour.  Luckily our cake batter is a liquid due to the water in eggs, milk, buttermilk, etc. which starts the reaction. When the sodium bicarbonate is dissolved the sodium atom is lost because water molecules have a small negative charge, which screens the negative charge of the carbonate part of the molecule. Now that sodium is gone the rest of the molecule is vulnerable for attack by lurking chemicals!

  
  

  

   (Water Shielding the Sodium Atom) 

  
  

  What do I mean by lurking chemicals? I mean acids. Acids are commonly found in food, everything from buttermilk to fruits, but what are acids and what are they doing in our food? Just as easily the sodium was lost in sodium bicarbonate by just dissolving in water; the same can happen when a loosely bonded positively charged hydrogen is hydrated, it falls off the molecule. And that’s all an acid is (for this purpose), a positively charged hydrogen floating around in water.

  However if this positive hydrogen finds the negative bicarbonate they bond much more strongly together than the sodium did because hydrogen is much much smaller than the sodium, therefore we can say the charge density on the hydrogen is far greater. But as a consequence of the positive hydrogen bonding with the negative bicarbonate a chain reaction is set off, causing the bicarbonate to decompose, breaking down into water and carbon dioxide. But from what I just said you need strong acid, an ingredient that will lose a hydrogen atom when dissolved in water, but not all cakes have acidic ingredients in them like buttermilk or fruit, so where is the acid source? And this is where the difference between baking powder and baking soda lays. Baking soda is pure sodium bicarbonate and baking powder is sodium bicarbonate and an acid in the form of cream of tartar. In baking powder the acid and sodium bicarbonate are dry therefore cannot react; they must be dissolved in water to allow a reaction to occur.  Now we understand the difference between baking soda and baking powder we can use them when appropriate. If you know you have a strong acid in your batter like buttermilk you know you can get away with only using baking soda to yield a good rise. However if you know that the batter contains no strong acids it may be worth using baking powder where you know you’re more guaranteed a good rise. 

  
  

  

  (Reaction Mechanism of Sodium Bicarbonate with an Acidic Hydrogen)

  
  

  A draw back that affects both baking powder and baking soda is that as soon as they are dissolved they can react with acid and if the batter is not in the oven you are losing precious carbon dioxide, and therefore rise. To prevent this and further ensure a decent rise you can buy and use double action baking power which contains two acids; cream of tatar  and sodium aluminium phosphate or sodium aluminium sulphate, or sodium acid pyrophosphate. These additional acids do not react with the bicarbonate until a critical high temperature is reached, which in baking terms means you get two rises, ensuring a deliciously light cake! But only if your cake has a neutral pH; if you have buttermilk or an acid in your batter it doesn't matter that your heat sensitive acid will not react until your critical temp, all your bicarbonate will have already be gas and water.

  
  

  Happy Baking!