At a cooking lesson, a French chef once answered to this question. They advised leaving the unknown samples out overnight to detect the difference. If the sample is firm in the morning, it is cake. It’s a biscuit if it’s soft. Voila!
Because of water loss, a cake gets tougher as it matures. But a biscuit becomes softer because it absorbs water.
According paraphrase Mrs Beeton, a Victorian cooking writer, “genuine sponges contain no fat,” merely eggs (which do include some fat) and sugar whisked to an emulsion with some flour gently mixed in so as not to damage the mixture’s bubble structure. All other types of cake contain added fats.
It’s incredible how many different food products can be manufactured with the same four fundamental ingredients: wheat, sugar, eggs, and oil. The variety is the result of several factors, including the relative amounts of the ingredients in the recipe, preparation technique and, to a smaller extent, cooking method.
The light, easily crumbled texture of cake relies on a few things. The structure is made up of two big molecules: starch (flour) and protein (eggs). They produce a lattice-like pattern when heated with moisture. Sugar and fat partially cover the holes, but some air spaces remain, resulting in a light texture. Too much sugar and fat will cause the starch-protein lattice to collapse, producing a heavy cake.
Butter was traditionally used as the fat, and it had to be mixed with sugar for a long period (up to an hour in some ancient recipes) to absorb air. Eggs have to be pounded for a long period to include as much air as possible. The hydrogenation of vegetable oils in the early twentieth century generated softer fats that could be whipped with sugar much more readily, resulting in lighter cakes.
The cake batter expands during cooking as the raising ingredient (sodium bicarbonate) creates carbon dioxide and the water converts to steam. In the meanwhile, the egg proteins coagulate, and the starch granules absorb water and gel. Both of these procedures cause the batter to set.
The lightness of sponges is dependent on beating eggs and sugar together for at least 10 minutes to include a lot of air and create a thick, mayonnaise-like mixture. The flour and melted fat are then carefully combined so that no air is displaced. The sponge must be baked immediately.
Biscuits typically comprise simply wheat, sugar, and fat, with a trace of egg. Without the starch-protein lattice, the result will inevitably be firmer.
Isabella Van Damme
Cakes, sponges, and cookies are all examples of heat-induced protein and starch networks that include varying amounts of air. Cake and sponge both contain a large amount of air and have a fine crumb structure.
Basic biscuits are made using oil, sugar, and flour, but no eggs or other rising agents are used. As a consequence, biscuits include a small amount of air, which is often trapped in cracks or irregular air cells rather than the more spherical air cells seen in cake and sponge. Biscuits have a gritty, crumbly texture because fat, sugar, and wheat combine to produce aggregates.