Cookie Science: How Do Eggs Affect My Cookies?

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[Photograph: Vicky Wasik]

Unless some dietary consideration has forced eggs from your diet, you've probably never stopped to reflect on how dang important they are to cookies. Most recipes call for one, and we add it without a second thought. By the time we've got a warm cookie in our grubby little paws, who's still thinking about eggs? Our focus is right where it belongs: on that buttery richness, those crispy, caramelized edges, and that wonderfully tender crumb.

If butter and sugar are the stars of the cookie show, then an egg is craft services, dutifully showing up to provide the fuel everyone else needs to get the job done: plenty of water, some lecithin (a powerful emulsifier), fat, and protein. The dough's principal actors just walk right up to the eggs and help themselves. Gluten goes for a little water, maybe some lecithin, too, and sugar rounds out its flavor with fat. Aeration gets a steamy lift from moisture, the Maillard reaction borrows yolks for browning, and texture all but sets its watch by protein coagulation.

To better understand the egg's role in cookies, and how altering the ratio of yolks and whites can impact a dough, I decided to conduct a series of controlled experiments. To keep my findings relevant, I wanted to avoid recipes with wild-card ingredients and techniques, so I stuck with the basics—my Soft and Chewy Sugar Cookies—allowing the information gleaned to be extrapolated into some general cookie concepts.

Separation Without Anxiety: How Egg Yolks and Whites Are Incorporated Alters Their Effects

Eggs are incredibly convenient. Nothing to measure or sift, no rounded teaspoons or tightly packed cups; just crack and go. One egg is the lowest common denominator in any recipe, and below that we'll divide a batch no further. Woe be unto the pastry chef who suggests baking cookies with a fraction of an egg!

Half an egg may be a 24-gram abomination, but we'll tolerate recipes that call for yolks and whites (then dutifully refrigerate the casualties). Maybe that's because whites and yolks are so cleanly split, or because such recipes promise something more: precision. Instead of requiring a one-size-fits-all approach to the needs of a dough, yolks and whites allow fine-tuning.

Whether a recipe calls for such an adjustment or simply one whole egg, the method of incorporation is almost always the same: slipping it in just after the butter and sugar are creamed up light and fluffy, with the mixer still running. It's the same principle that's behind homemade mayo, where constant whisking helps the emulsion expand.

My sugar cookies follow that standard format, so the only change from batch to batch was the amount of egg whites and egg yolks—or their absence.

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[Photograph: Sarah Jane Sanders]

I mean, what could better demonstrate an egg's role in a cookie than a batch made with none at all? That simple subtraction transformed my normally tender sugar cookies into dense, hyper-crumbly wads of dry shortbread that barely spread. Aside from the slight graininess of undissolved sugar, it was pretty tasty in a "stale Girl Scout Trefoil" kind of way, but definitely nothing I'd try again.

Moving on to the egg-white-only trials, I immediately ran into problems. Instead of smoothly incorporating into the dough, the watery whites would slip and slide against the butter, forming small curds. By the time sheer horsepower brought everything back together, the air cells carefully cultivated by creaming had been roughly beaten out.

Compared to a classic whole-egg recipe, one made from whites alone formed a heavier dough that gave up less moisture as steam, which would normally escape after helping those air cells expand and rise. The result? Moist, dense, and chewy cookies that spread thin, albeit to varying degrees.

(An aside: If the cross-sections look oddly dry, it's because the actual cookies were so soft, they collapsed beneath my knife. To examine their true crumb structure, I had to bake the samples several extra minutes.)

The cookies made with a single white spread closer to normal, but with a comparatively tight crumb. The two-white version puffed a fraction more (extra liquid = extra steam, thus a better chance of rising despite poor aeration), producing thick cookies that were outrageously chewy and tender—the uncontested favorite among my tasters in the white/yolk showdown.

The differences weren't strictly textural. Without the custard flavor of an egg yolk, the white-only cookies tasted more intensely vanilla, an asset I destroyed by attempting to enhance the chewiness provided by whites with the richness of a yolk. That adjustment made the cookie tender to the point of crumbliness, with a confusing flavor that was more bland than a whole-egg cookie and not as powerfully vanilla as the all-white versions.

It was a taste of what was to come, as each subsequent experiment with egg proved itself a new adventure in disappointment.

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[Photograph: Sarah Jane Sanders]

I'd had such high hopes, too, as the lecithin-rich yolks worked as an emulsifier, helping the butter and sugar whip up especially light and fluffy, without any hint of curdling. Yet using a single yolk was strangely similar to using a single white. The cookie spread poorly, with a dry and gritty texture that was only marginally more delicate thanks to proper aeration.

Like the two-white version, my two-yolk dough spread and puffed much like a normal cookie...on the outside, anyway. Inside, it had the texture of an over-baked muffin that was left to stale on the counter overnight. The sort you'd warm up and suffer through, pretending to be satisfied by the overall butteriness but painfully aware of every wasted calorie. On the plus side, the yolk-enriched cookies tasted exactly like a vanilla wafer, a potential perk in recipes that are baked until crunchy or crisp.

Desperate to harness that flavor in the context of a soft and tender cookie, I decided to try a version made with six yolks (three and a half ounces). It emulsified the dough rather nicely, and I figured the sheer volume of fat would force them into something moist. The aroma of yellow cake that filled my kitchen seemed promising, but the result was an alarmingly spongy cookie with a disturbingly savory edge. (Other adjectival pairs considered for that sentence: strangely/bizarrely, distinctly/confusingly, oddly/disconcertingly.)

Clearly, yolks have no business being left to their own devices.

Returning to the 50/50 split of yolks and whites that nature intended, I tried a batch made with two whole eggs (also three and a half ounces). That pushed the cookie dough into cake-batter territory, resulting in something like ladyfingers, with a delicate richness and fine-grained crumb.

Scaling back, I tried a version with one whole egg plus an extra yolk, which gave the sugar cookies a light but chewy texture and a flavor that reminded me of French vanilla ice cream. It was way too much personality for a simple sugar cookie, but throw some brown sugar and chocolate chips into the mix, and you'd have a winner.

Of all the experiments, I can recommend only two:

The double-white/no yolk combination was amazing. Wonderfully chewy, moist, and mild, making it a fantastic adjustment to any dough in which you'd like to highlight a more delicate flavor. Almond, butterscotch, coconut, lemon, and vanilla come to mind.

The double-yolk/one white combination provided a hearty flavor and texture that would stand up to any chunky or moist ingredients that might turn a denser dough to mush. Think: chopped chocolate, toffee, or other candy pieces.

What About Other Cookies?

It's worth repeating that these findings are all based on my Soft and Chewy Sugar Cookies, which involve creaming the butter and sugar together until light and fluffy. With alternate techniques, your mileage may vary—especially with wild-card ingredients, like molasses or baking soda, that can have a more profound impact on the texture of a dough.

In his quest for the ultimate chocolate chip cookie, for instance, Kenji discovered that while an extra white could make his cookies cakey, extra yolks turned them dense and fudgey—the exact opposite of my findings here. The big difference? Instead of creaming the butter, his recipe relies on whipped eggs and sugar to aerate the dough.

Everything makes perfect sense in the context of that technique. Egg whites entrap more air while contributing more water, encouraging steam and gluten formation: perfect conditions for lean cookies that are thick and puffy. Cakey. Yolks cut the water and throw in fat, hindering both gluten development and aeration, producing cookies that are dense, tender, and rich. Fudgey.

So, remember that there are no hard and fast rules when it comes to what eggs can do for cookies—it's all contextual. Generally speaking, when recipes call for melted butter or whipped eggs, yolks produce fudgier cookies, while whites make them cakey. When recipes call for butter creamed until light and fluffy, the reverse is true: Yolks tend toward cakey cookies, while whites make them fudgey.

To uncover any deeper truths, the experiments will have to continue with other recipes and techniques. Fortunately, I have a sweet tooth and a gallon of milk, so let the baking begin.