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The Science of Baking: Flour 101 + The Advantages of Cake Flour
Consider this the first of many random explanations on the mechanics of baking. Aside for having a love for baking and pastry making, my other hobbies include being a ridiculous nerd for science - especially for biology and chemistry. And so, over the years, I've accumulated some knowledge in the form of a small red spiral notebook, the contents of which include biochemical breakdowns of important reactions that occur in the baking process, which I intend to share with you all. Please do enjoy, and try not to judge – I had few friends in high school. Hopefully by the end of these Hubs, our little “Science Baking Quest” will have imparted on you enough knowledge to make your own recipes.
I think it's fitting that the first of these entries correlate to the first page in my shamelessly effeminate red notebook. The topic addressed on this page was the very first question I found myself with a desperate need to have answered: just what the hell is cake flour, and why does this recipe (for an old-fashioned Vanilla Bean Cake) call for it?
To answer this, we have to first look at the role of flour in the baking process. Of a cake, that is; breads are another Hub. Every ingredient in a cake has a specific function, which can be classified into one of the following general areas: structure, tenderness, moisture, leavening, and flavoring, most of which have some overlap. Moreover, you'll find that almost all cakes share the following core ingredients: flour, eggs, sugar, fat (butter, shortening, oil), salt, milk, chemical leavening, and flavorings. For now, let's focus on the wonders of flour.
To understand the role flour plays in cake baking is to understand what flour is. In standard definition, a flour is a finely ground powder made of pulverized cereal grains. Wheat flour, as opposed to rye or maize flours (¡arriba, Hispanos!), is the common, commercially used variation we use in the production of most baked goods. Now, wheat is a multi-tasker, as most of the core ingredients are, with regards to the general categories of function defined earlier. Flour's main function in a cake is in providing rigidity to its structure. The hand it plays in structure also sticks itself in the areas of tenderness, moisture, and even leavening (and flavoring, if you feel the need to add a few pounds of raw flour to your cake).
Pardon my wandering mind (they don’t put enough Ritalin in the tap water, anymore), we were discussing the nature of flour before we can discuss its purpose. There are three key molecules found in wheat (and hence found in flour) that are ridiculously important to its function in cake baking: starch, glutenin, and gliadin. These molecules should be vaguely familiar if you've studied organic chemistry (or have ever dreamt about Alton Brown). Starch, the generic name for the polysaccharides amylose and amylopectin, is a massive sugar complex found naturally in plants, made up of a giant network of condensed and connected glucose molecules. The compact union of these glucose molecules, added to the massive amounts of these connections present in starch, provide rigidity in plant cell structure, and, logically, would provide rigidity in the structure of a compound that it comprises (let's say Mr. Stay Puft, or a cake batter). Now, glutenin and gliadin are two protein complexes that combine to form the composite glycoprotein network known as gluten, which should be familiar to anyone who has even marginal experience in baking. Gluten is what makes stuff tough. They tell you not to overmix when baking sweets and treats because the gluten might build up and turn your cookie into a chewy brick. And they tell you to mix like hell when making biscuits because you want a lot of gluten, which gives biscuits their form and familiar texture.
Fun, random, and almost entirely useless side note: celiac disease is a condition often billed as an "allergy" to gluten in which the immune system reacts erratically to its presence. This may result in a slight anaphylactic reaction or in the gluten not being absorbed at all by the digestive system. New episode of House, anyone?
Suffice it to say that gluten is really, really important in terms of the functions of flour in a cake. You can see how a large, expansive network of sugars and proteins can affect structure, much like layers upon layers of bricks and stone providing support for a building. Er, perhaps a better metaphor would be the structure of a naturally occurring mountain. Rocks don't fill in every space; that's why you get air pockets and caves. But the rigidity of each component of the mountain and the locked way in which they connect provide a sound structure. Keep those air pockets and caves in mind.
In terms of tenderness, flour is a toughener. The more flour you have, the more proteins you have, the more gluten you get, the tougher the structure becomes. And a really rigid structure means a really tough cake (think the arrangement of carbon atoms in a diamond). Now, in terms of leavening and moisture, let's go back to the metaphor of the pockets and caves (which I may very well have made up - I'm no good at Earth Science). Carbon dioxide is bad for the ozone layer, but it may very well be God in Cakeland. CO2 bubbles are released in several small reactions during the baking process, from the fermentation of sugars to the reactions catalyzed by chemical leavening (which is another, very fun post) to the basic process of glycolysis in cellular respiration. The main outcome is that these bubbles and gasses give rise to the cake. What this has to do with the gluten and starch complexes is that within their networks, CO2 bubbles become trapped, thereby giving the cake rise (cool, huh?). Unfortunately, this has undesirable effects in terms of moisture, as these networks make it easy for flour particles to absorb H2O molecules, leaving behind a dry cake. In fact, gluten has been cited as one of the reasons why things go stale.
And so, with this basic understanding of flour, we can answer the question we started out with: what is cake flour, and why do we use it when making cakes, as opposed to something like bread flour, or even all-purpose flour? I'd say the biggest thing to remember when justifying paying an extra dollar for cake flour is that it is made from soft winter grains, which contain less glutanin and gliadin than all-purpose flour; cake flour generally contains anywhere from 7 to 9 g of protein per cup of flour, as opposed to 10-12 g in all-purpose. Less protein in the grains results in less gluten developing in a cake, which means what, class? Yes, that's correct: a more tender cake as the product. Fortunately, structure isn't compromised because in lieu of more protein, cake flour contains more starch than AP, which stabilizes the cake. The second most important feature of cake flour is that it is well-bleached, which results in a few helpful characteristics. Chlorination in bleaching results in the surface of starch molecules being able to bind and retain more molecules of fat, sugar, and water, which creates more tiny, tiny bubbles. Furthermore, bleaching results in the flour having a lower (slightly acidic) pH, generally somewhere around 5.2. Remember organic chem? Cooking has as its goal the denaturation of a lot of proteins in raw food. What else causes proteins to denature aside from heat? Yes, I heard it in the back of the room: acidity and alkalinity. The lower pH causes the proteins in gluten to denature and set faster, thereby mellowing out the tough gluten particles and resulting in a richer, finer texture. Also (yes, there's more), cake flour is more finely ground than its brethren. Smaller particles mean more particles per unit volume in comparison to other flours, which, in turns, results in a greater surface area on which more, smaller reactions can occur (you have to love chemistry, man). This results in a smoother texture in the final baked cake.
Well, I hope that's all you'll ever need (or want) to know about cake flour. I hope you learned something new here that you may apply in the kitchen sometime soon. At any rate, you can't deny that it's at least sort of perversely fun to know these fun little facts, eh?
Until next time, keep living and happy baking.
-Michael (with apologies to Alton Brown, the food geek god).