We are in the middle of two weeks of pastry classes at school right now. Like most cooking schools, the FCI has a full program just for pastry training, but everyone in the culinary program goes through some of the basics of pastries. Many pastry preparations can be used to make savory (non-dessert) dishes, and in addition any chef who aspires to run a kitchen or other food service operation needs to have some understanding of pastries to better work with his pastry chef and to fill in when the pastry chef is not at work.
Most elementary pastry preparations that underlie any finished product are involve more chemistry and science than the usual off-the-cuff throwing together of interesting ingredients that you see on something like Top Chef. Where we taste and adjust dishes as we go in cooking, when preparing pastries we measure precisely and follow directions carefully. In addition to measuring ingredients to the right proportions to get the desired chemical and structural transformations take place, I'm learning that the precise application of heat is often even more important to many recipes.
The process of tempering chocolate is one of the more interesting examples of how varying heat within a small temperature range changes the properties of one of the most common ingredients used in sweets. Chocolate has a structure that consists of a couple of different kinds of crystals. One of the crystals, the "unstable" one, melts near room temperature, at anywhere from 59F to 82F; the "stable" crystals melt at 89F-93F. In addition to the desirable "melts in your hands, but not in your room" property of the more stable crystals, they also have a glossier appearance and a less gummy texture than the unstable crystals. If you melt chocolate, to use as a frosting for example, if it cools rapidly (as it will if spread in a thin layer, like a frosting) there will not be enough stable crystals formed before it cools below 82F and unstable crystals take over and determine the crystalline structure of the solid chocolate. To "temper" chocolate, which makes it look and feel nicer on things, the cook melts solid chocolate and then cools it to the point where crystals form but it can still be stirred, and then gently heats it to a temperature above the melting point of unstable crystals but below the melting point of stable crystals (about 88F) to eliminate any unstable crystals that formed. It is held at that temperature to allow enough stable crystals to develop that when it is further cooled there will be enough of them to establish the structure of all of the solid chocolate. This is something I've only read about and never done, and in the basic pastry training that the culinary students get at the FCI we will not be doing it. I've heard (but never seen with my own eyes) that one of my friends who is a trained pastry chef can go through this process just by feel, without using a thermometer. She can also always recognize that most chocolate-covered desserts, even in many upscale bakeries and fine restaurants, have had some wax added to the chocolate to make it more shiny and stable and look like properly tempered chocolate.
We've been through a couple of simpler careful heating processes in the past week during our pastry classes at school. The first was to make Génoise cakes, a very light sponge cake that rises and becomes airy through creating and cooking egg foam, without any chemical or other leavening agents (such as baking powder or yeasts). The first step in making Génoise is to whisk together eggs and sugar into something like a sabayon (which normally includes only egg yolks, not the whole eggs used in Génoise). As the eggs are whipped, they will increase in volume as air is incorporated into them, but the foam is not stable enough to survive the amount of heat and time it takes to bake the cake. To make them more stable, you heat the eggs and sugar as they are whisked. The heat causes the tightly-wound proteins in the eggs to lengthen into strands that can bond with each other and form a more stable network in which air can be trapped for a longer period of time. In many pastry sauces and creams, eggs can be heated as the binder in a liquid all the way to boiling and will not curdle (scramble) if they are handled properly and stirred constantly. But for Génoise, we want to heat the eggs just to the point that they can hold a foam structure long enough to last through the baking process. If they are heated beyond that, the cake will become tough and chewy as the network of proteins gets more solid. To achieve the result we want, the eggs must come to a temperature of at least 110F, and we usually heat them to a minimum of 115F for the best results, but if at any point they go over 120F, we'll throw the mixture away and start over. When the cake is baked (after adding a little bit of flour to make a batter out of the whipped eggs and sugar), the foam is just stable enough to last through the baking process, and in fact if you remove the cake from the oven and it is not baked all the way through, the uncooked batter will quickly deflate and you'll have what Chef Marc derisively calls "a flat tire." There is no recovery if this happens -- you just have to start over.
The second preparation we made that involved achieving a narrow temperature range was classic buttercream frosting. As our assistant instructor Chef Matthew (who worked for awhile as a pastry chef) told us, a colleague once told him that the only real purpose of Génoise is to serve as a platform for the delivery of buttercream. The frosting is made from only 3 basic ingredients: egg yolks, sugar, and butter. The frosting is not cooked and there is no liquid in which to dissolve the sugar, but we want the sugar to be soft and not grainy when the finished preparation is at or below room temperature. The solution is to heat the sugar to a stage where its crystal structure will be permanently changed and it would cool into a soft and pliable solid if left on its own and not mixed into our frosting. We'll get this result by heating the sugar to what is called the "soft ball" stage, between 234F and 240F. When it is in that temperature range, a bit of the sugar cooled in ice water can be rolled between your fingers into a soft but cohesive ball. If heated to just below that range, the sugar upon cooling will tend to spin fine threads rather than forming a solid lump or ball; if heated above that range, it will form crunchier and eventually rock-hard solids when it is cooled (this is how hard candies are made). To make buttercream, you beat egg yolks and then mix hot sugar brought to the soft ball stage into the eggs, then cool the mix enough that it will not melt butter, and whip a whole lot of butter into it to form a frosting.