This week's dish gracepoint food science tip is about retrogradation or simply put the process which stale starchy foods become soft again.
Starches (amylose and amylopectin) bind to water when cooked and it's referred to as gelatinization. When hot, there is alot of free energy which allow the molecules to move around and the gel is in a disordered state. As the gel cools, it regains structure as it crystalizes and hardens resulting in the texture known as stale bread.
To make this easier to understand... for those of you who have made rock candy or have taken o-chem lab, imagine a sugar solution that is boiling. The solution flows easily when hot. As the solution cools, the molecules start clumping together and forms crystals. By the time it's room temp, the solution is hard, brittle, and jagged. On a microscopic level the starches are doing the same thing.
Bread that is fresh out of the oven have a moist, tender quality that quickly degrades the moment it is removed from heat. This is, to a certain extent, reversible. When a gel crystalizes, it can return to it's disordered state through reheating. The starches return to a gelatinized state and the process is called retrogradation. The result is a soft, moist food that is nearly the same quality as when fresh.
So what good is all this book knowledge if you can't apply it? If you bought french bread to serve with dinner or have old bread rolls you'd like to use, you can simply place in the oven at a low temp (I usually set it to 250F) and allow it to get warm. Just watch out that it doesn't toast too much. Once the internal temperature reaches 140F, or the temperature of a roast done "medium", the starches should have softened. Another way, which is quicker but not as good, is to place it in the microwave for about 10 seconds. I actually take french bread slices and do this and it comes out soft and chewy.
This process is not just limited to bread but explains why leftover rice becomes hard once it gets old (and cold) as well as doughnuts, bagels, pastries and why pizza fresh from the oven is always better. In all of these cases, you can simply reheat in the microwave and it will be (partially) restored
It works reasonably well and gives good results, but it is limited. Every time it is heated and reheated, there is loss of moisture as it evaporates. Without water to rehydrate the food, it will not properly retrograde. That's why many people add a little water to a bowl of rice, cover it and microwave.