Download The chemistry of baking powder
and more Study notes Chemistry in PDF only on Docsity! KitchenChemistry The chemistry of baking powder Read the passage below, which explains the chemistry of how certain foods ‘rise’ during cooking and answer the questions that follow. Sponge cakes made with (right) and without (left) baking powder Many food products such as bread, sponge cakes and buns have a honeycomb structure which contains many bubbles. During cooking these bubbles are formed by a gas and the mixture ‘rises’. In some cases the gas is air which is whipped into the mixture before cooking and which expands during cooking. In other cases the gas is carbon dioxide. This can be formed either from the fermentation of sugar aided by yeast (as in making bread) or by chemicals that react to form carbon dioxide. Baking powder SS08 The chemistry of baking powder page 1 of 4 The most common chemical to do this is sodium hydrogencarbonate, NaHCO3 (more commonly called sodium bicarbonate, bicarbonate of soda or just ‘bicarb’). This can form carbon dioxide in two ways: • on heating 2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(l) • on reacting with an acid, such as hydrochloric acid (HCl) NaHCO3(s)+ HCl(aq) → NaCl(aq) + CO2(g) + H2O(l) Cooks do not use a strong acid such as hydrochloric acid; they use instead a weak acid such as potassium hydrogentartrate (potassium hydrogen-2,3-dihydroxybutanedioate or potassium hydrogen-2,3-dihydroxysuccinate), also called cream of tartar. The formula of this acid is: The gaseous product of the reaction is the same as with hydrochloric acid – carbon dioxide – but it is produced much more slowly. Potassium hydrogentartrate is a solid and this means that it is possible to mix it with the sodium hydrogencarbonate without the two reacting – they only react in the presence of water. This dry mixture is the basis of baking powder. The reaction is: One problem with the use of potassium hydrogentartrate is that it is very soluble in water. So as soon as it becomes wet (when milk is added in a cake recipe, for example) it dissolves and reacts. This risks all the gas escaping while the cake mix is still liquid and before it goes in the oven. Most baking powders nowadays are so-called ‘double acting’. This means that, along with the sodium hydrogencarbonate, they use a mixture of potassium hydrogentartrate and calcium dihydrogendiphosphate (CaH2P2O6), which is also a solid acid. The potassium hydrogentartrate dissolves and reacts almost immediately (which makes the dish ‘rise’ on mixing) while the calcium dihydrogendiphosphate is slower to dissolve and will not react until the mixture is in the oven and the gas bubbles are trapped by the cake as it bakes. SS08 The chemistry of baking powder page 2 of 4 H HO C COOH HO C COOK H H HO C COOH HO C COOK H H HO C COONa HO C COOK H + NaHCO3 → + CO2 + H2O The honeycomb structure of bread is caused by gas bubbles
