"I've heard that if you add salt to eggs too far in advance, it can ruin their texture and toughen them up. When should I salt my eggs?"
I, too, have heard this tale. In fact, it was a legendary chef (who shall remain nameless) who told me several years ago, "Don't add salt to your eggs until right before you cook them, or it will thin them out and then toughen them up." Without questioning it, I've dutifully followed his advice ever since. (I am, however, willing to call out Gordon Ramsay for giving the same advice. Since he's made a career of shaming others, he totally deserves a potential lashing here.)
As we know by now, it isn't uncommon for such hand-me-down kitchen wisdom to crumble under scrutiny. Would this little tidbit of cooking lore hold up and forever vindicate Mr. Ramsay? It's an easy enough question to test out, so I started cracking some eggs.
Before we get to my test, let's review some egg fundamentals that may help us understand this better. So, what exactly happens when we cook an egg?
To simplify, let's focus on the egg whites (a.k.a. albumin), which are mostly made up of water and various proteins. In their normal state, those proteins are all folded up on themselves and tend to avoid each other. Think of them like a group of highly antisocial people who would rather curl up into fetal position on the floor than interact. If light rays were a group of joggers training for the marathon, and just happened to come across this odd, balled-up bunch, they'd have an easy time running around and through them. That's why raw egg whites are transparent: light has little trouble cutting a path through minimal obstacles like free-floating, scrunched-up proteins.
But there are ways to change the behavior of our uncommunicative proteins. The obvious one, of course, is to add heat. Heat works on the proteins in the egg white much the way alcohol might work on our group of uncommunicative people. Suddenly, they're willing to stretch out, interact with those around them, maybe even join hands and hug. But in the case of the proteins in albumin, once that hugging and handholding starts, it pretty much stays that way, like a crowd of people in a perpetual, iron-gripped embrace.
This joining of proteins is what firms up the egg and turns it opaque, since light can't get past these interconnected proteins as easily—just imagine the joggers trying to get through a group of drunkards who have handcuffed themselves together. The more heat you add, the tighter those protein bonds get, causing the egg to firm up more and more. Eventually, if you overcook the eggs, the bonds will get so tight that the proteins start squeezing out the water—a phenomenon you may have noticed if you've ever hammered eggs really hard. (Actually, a cooked egg doesn't have to stay that way: It is possible to un-cook an egg and return the solid whites to their original clear, fluid consistency, but it requires adding toxic chemicals that would render the egg inedible.)
Aside from heat, there are other things that can promote this protein linkage. One is acid, which is why some people say adding a splash of vinegar to egg-poaching water helps set the white more quickly. Another is salt. According to Harold McGee's On Food and Cooking, when salt dissolves into egg, it separates into charged ions that change the electrical environment enough to encourage coagulation. Based on that, it might seem logical to conclude that eggs with salt will coagulate more quickly, and therefore be more prone to toughening.
To find out what actually happens when you add salt to egg, I scrambled five side-by-side batches. Each batch contained three eggs and 1/8 teaspoon kosher salt, with the only difference being how long each was exposed to the salt before cooking: 60 minutes, 30 minutes, 15 minutes, 5 minutes, and 0 minutes.
The result was that all the scrambled eggs were nearly indistinguishable from each other. If anything, the eggs that sat with salt for the longest were more moist and tender than the eggs that were exposed to salt for less time, though I can't stress enough that the differences were incredibly subtle.
While salt certainly doesn't hurt the eggs (and may even help), there's no question that the most important factor when cooking eggs is the cooking technique itself. In the case of scrambled eggs, that means using gentle heat and taking the eggs off the heat a little early to account for carryover cooking.
Still, it seemed strange that my tests indicated that, if anything, salt can improve the texture of the eggs, which is the exact opposite of what we might expect from an ingredient that promotes coagulation. I turned back to McGee for an explanation. According to him, the reason that the salt doesn't toughen the eggs is that, while acting as a catalyst to make proteins bond at lower temperatures, it simultaneously functions as a buffer, preventing the proteins from getting too close to each other and reducing the risk of water being squeezed out.
Basically, to return to my analogy, salt is sort of like a therapist who's been introduced to our socially-challenged, group, with the mission of getting them off the booze while teaching healthier ways to be friendly—but not too friendly—with each other. I'd say Gordon Ramsay could maybe do with a little bit of pre-salting in his life too, if you know what I mean.