Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.
AdventureInSoup asks via twitter:
Interpreting instructions is one of the most difficult parts of cooking. Sometimes there are important secrets hidden in basic instructions, and sometimes people just get bored writing the same thing over and over again. Even with something as well understood as basic temperature, we know that measuring and calibration is an issue. Then when we get to something as fuzzy as boiling, well…
Let's start with the easy bit: the boiling point of a liquid is the temperature at which it turns from a liquid to a gas. Very simple, good junior-high physics explanation. Maybe even elementary-school science, really. It's been a while. Therefore, one might think, if something is boiling, it must be at the boiling point. Likewise, if something is at the boiling point, it must be boiling. Stands to reason, right?
Yes. Aaaaand… no. There are two basic problems with that line of thinking. First is that there are situations in which a liquid can be at or above its boiling point and still be a liquid. The most common occurence of this is when you microwave a liquid in a very clean and very smooth glass container. Boiling doesn't just happen spontaneously. The bubbles want to form on something, and that something is usually an irregular point on the surface of your container. You've probably noticed when you're boiling water on the stovetop, you often see columns of bubbles in the beginning coming from a couple of points, as if there were a small leak in the bottom of the container. These points, when used for forming bubbles, are called nucleation points, and they make it easy for liquids to bubble and, consequently, boil.
In the microwave example, there are no nucleation points, this causes the liquid to heat to above its normal boiling point, also known as superheating. The instant something provides a surface on which to form bubbles, these bubbles will form quickly, like Mentos in diet coke. For the many of the same reasons as Mentos in Diet Coke, even. and as the water is above 100°C, it can burn you even worse than normal water could. Mind you, as the Mythbusters explain, it requires distilled water, because nucleation points can develop around items dissolved in water, and tap water always has things dissolved in it:
More importantly, however, we know that there are degrees of liquids turning into gas. After all, we have a difference between simmering and boiling, where bubbles are formed and liquid is turned into gas. Then there's a rolling boil and a full boil, which might be different from a regular boil. We should deviate a bit from the textbook definition of a boiling point to properly understand.
We know from the article about reducing sauces that liquids can turn into gasses without being at the boiling point. After all, that's what evaporation is, and liquid evaporates easily, if slowly, at room temperature. Let's go back to our kitten analogy from the reduction sauce article. We have a room filled with kittens, who represent water molecules, and there are little doors all around the edges. The kittens don't know about the doors, they only pass through them when they happen to be going fast enough that they would hit the wall, and find that they're through a door instead.
The speed of the kittens indicates the temperature of the water. If they're going slowly, then it'll be rare that one accidentally brushes against the wall and falls out. However, if they're at the boiling point, then that means that they're going so fast that it doesn't matter where they are, they're going to fast to stop before they would hit the wall and either bounce off and keep going*, or instead go through the door.
Remember that not every kitten has to be going at it's maximum speed in order for the average speed of all the kittens to be at the "boiling point." Some kittens could be walking around, some could be taking a nap. However, as more and more kittens get riled up, it makes it harder for the other kittens to sleep or just walk around, so they all get into the act eventually. While it's only groups of kittens bouncing around at full speed, that's basically a simmer. When all of them get into the act, you've got yourself a rolling boil.
To my knowledge, there are no international or national governing bodies that regulate the differences between a simmer, a boil, a full boil, and a rolling boil. Chances are that the last two mean exactly the same to anyone who uses the terms, and many would have difficulty distinguishing between those and a boil.
However, in general, a simmer is when you barely have any bubbles at all. It's right at the edge between definitely liquid and maaaybe gas. A full or rolling boil is when you are making bubbles so quickly that no amount of sitting on the stove is going to make more bubbles come quickly. You are at maximum bubble creation. A boil would be between the two.
The reason you would use a rolling boil is when you don't want the boil to go away after you add some ingredient. A popular example is pasta: if you're going by boxed pasta directions, they'll suggest you get to a rolling boil then cook for a certain amount of time. This ensures that you'll have minimal temperature loss when you add the pasta, so that won't affect cooking time. Aside from that, I really don't know a reason why you need to worry about it. Boiling is rarely a precision act, so a minute more or less is probably not going to hurt much.
So, unless you are working from a finicky or unfamiliar recipe that you're not ready to mess with, and the boil seems vital to preserving the cook time, I wouldn't worry too much. If it's pasta or potatoes or similar, there are better ways to determine of the dish is done, so use one of the proper methods.
*- The walls are very springy and no kittens ever get hurt in this room. Just so you know.