I have looked up numerous pasta recipes and all of them suggest adding a certain number of whole eggs and a certain number of additional egg yolks. I am just curious why we need only yolks? How would the pasta be different if I added a total number of whole eggs instead? Is it a matter of colour, or texture, or both? And more importantly how
It’s about what properties you’re looking for in the pasta. Serious Eats had a primer on fresh pasta a couple of years ago, and they discussed some of the differences:
Egg Whites, Egg Yolks, Water: Identifying the Best Source of Hydration
With my flour selected, it was time to test different sources of moisture. My first step was to make three doughs, keeping the hydration level as consistent as possible across the board. I used three equal measurements of all-purpose flour as my baseline; one batch got water, one batch got egg whites, and the third got egg yolks.** I added just as much as I needed to make the dough come together. This is what I wound up with; you can probably tell which is which. ** I stuck with large eggs for all my tests, and even weighed them to make sure that I was adding consistent amounts of water, protein, and fat to each dough.
(image) The water-only pasta (right) was a total bust—the noodles were bland, mushy, and...well...watery. And the egg white pasta (center) wasn't much better: Whites are almost 90% water, so, while the noodles weren't quite as bad as the water-based version, which literally fell apart and stuck to each other in a big, gluey mass, they definitely weren't winners. The yolks, on the other hand, made a beautiful, golden dough (left). Yolks contain about 48% water, 17% protein, and around 33% fat. More yolks will deliver more color, more egg flavor, and silkier noodles.
Unfortunately, that high fat content complicates things a little bit. Though it's not exactly scientifically accurate, you can think of that fat as making the gluten proteins all slippery, preventing them from building a strong network—when I tested this using different amounts of olive oil, I found that, sure enough, more oil made for softer, mushier, less elastic noodles. And, to complicate matters even further, I had a really hard time getting the flour and yolks to come together. It was a dry, tough dough that was difficult to mix and knead—not exactly beginner-friendly.
Difficulty aside, an all-yolk pasta may make great noodles, but it's not sufficiently elastic to use for stuffed pastas, which require a dough that can be rolled more thinly and is, quite simply, bendier. I needed to strike a better balance.
At this point, I knew there was no point in adding water—if I wanted additional moisture, egg whites were definitely a better bet. It seemed clear that my dough was going to require a combination of whole eggs and additional yolks. I ultimately settled on three yolks for every egg white.
What's that? You like softer, mushier noodles? Good for you. Add a teaspoon of oil to my basic recipe. Want a richer, eggier flavor and a more golden hue? Throw in an extra yolk and add a little more flour. This is your dough.
It is not actually required. I have never seen an egg pasta recipe which uses extra yolks, and Ruhlmann, a cook who wrote a book with prototypical recipe ratios, also uses whole eggs only. So you can certainly make (good) pasta without the extra yolks, and using extra yolks is just one possible variation. You simply have happened to come across recipes written that way.
If you use more yolks, you can expect a stronger eggy taste and a slightly gooier texture than with whole eggs. Possibly also a color difference, if you have dark yolks and use a lot of them, but you can't rely on that. If you need colored pasta, you should simply color it.