This idea of Metastable State Foods came up at a going-away party for one of the lab members departing our lab. Specifically it related to what seems to be a tendency of the French to want to capture transition states in food. Also it serves as a way for me to introduce transition states and how they fit into this Molecular Yoga theme.
The primary French cuisine I’d claim to be a “Metastable Food” is the ever-difficult to make Macaroon! These are layered meringue cookies that need to be cooked to a precise temperature for a precise amount of time. Cook for too short of an amount of time and you end up with a soupy syrup. Cook for too long of a time and you end up with something that is solid as a rock. What you want to aim for, is a lightly cooked wafer that still has the airiness of the meringue batter but does not end up overcooked. We need a way to organize this information to better understand what is happening to our Macaroons!
There is a standard way to represent the progress of a reaction: Free Energy Diagrams (FED). We introduced the idea of Free Energy (FE) a few weeks ago. When molecules undergo a reaction, their movements, changes in structure, changes in heat, and etc are often associated with changes in their Free Energy. Let’s say that at room temperature there are three states of Macaroon: syrup, rock-hard, and perfect. These are three points on our Free Energy Diagram. For the sake of this explanation lets say that in the course of baking the syrup state has the highest unfavorable Free Energy. What you’ll notice is that if we fall “down” the slops in our FED the FE is decreasing or becoming more favorable. Most FED will represent changes in this way, dropping down to a lower point on the y-axis represents a favorable progression of the reaction on the x-axis. What we can use our FED to do is to try to understand the state our molecules or materials will be in after the reaction has gone to completion.
The reactions don’t always go directly from one state to another freely however. There can also be slight increases or “Free Energy Barriers” as we move along the progress of the reaction. This barrier prevents an otherwise favorable reaction such as moving from “syrup” to “perfect” from happening. One interesting thing that can happen is that if molecules cannot overcome the energy barriers they can become trapped in a middle state. This is called a “Meta-stable” state. This is where we find our Macaroons. If we put enough energy into a “syrupy” macaroon, say by heating it, it will become “perfect.” If we put even more energy into it, the “perfect” macaroon will overcome another energy barrier and turn into a “rock-hard” macaroon. By knowing the energy barriers between the states we can try to trap our cookies in delicious meta-stable state.
One last point to make with our meta-stable French confections is that we are talking about reactions going to completion and therefore equilibrium states. The heights of energy barriers will play into the kinetics of the reaction as well. So these diagrams might not necessarily tell us about what will happen if we bake for, say 20 minutes as opposed to 40 minutes. We’ll discuss more in the future about the relationship between Thermodynamics and Kinetics. For now what we’ve learned is that metastable states can play an important role in getting a molecule to be in the exact right configuration. I believe that a Molecule’s ability to hold a specific pose, a specific metastable state, could be very important not just for cooking but for biotechnology and medicine as well. That is a post for a later discussion!
Thanks for reading and feel free to comment below!