Chicken breast has far less fat and connective tissue than other parts of the bird, which leaves it with little capacity to hold onto moisture as it cooks; even a small amount of overcooking is enough to dry it out.
Why does the lack of fat and connective tissue matter?
The fat and collagen found in cuts like the thigh or drumstick melt slowly during cooking and add moisture and flavour back into the meat. Breast meat has very little of either, so it has no fat buffer to fall back on once its internal temperature passes the ideal point, and it starts drying out quickly.
What happens to protein fibres as they cook?
As meat cooks, its protein fibres tighten with heat and push the water inside them out. Up to a moderate internal temperature, this tightening is limited, but once the internal temperature crosses a certain threshold the fibres keep squeezing tighter and push out more and more water. Being a lean cut, breast meat has no reserve to make up for that water loss, so the result quickly turns dry and stringy.
Why does uneven thickness cause problems?
A chicken breast is thicker at one end than the other; the thin end is already overcooked well before the thick end reaches the target temperature. When the whole piece is cooked for a single length of time, the thin parts almost always end up dry while the thick part can sometimes stay undercooked.
Why does cooking fast over high heat raise the risk?
On high heat, the outside of a chicken breast quickly passes the desired temperature while the inside may not have reached it yet; extending the cooking time to make sure the inside catches up only dries the outside out further. Cooking over moderate heat, where the inside and outside temperatures move closer together, lowers the risk of drying out.
How can this dryness be reduced?
Pounding the piece to an even thickness, brining it in salted water before cooking, or cooking it over low-to-moderate heat while checking the internal temperature are all practices that meaningfully cut down on breast meat's tendency to dry out. What they have in common is not letting the internal temperature climb higher than it needs to.
Why does resting reduce dryness?
If a chicken breast is sliced the moment it comes off the heat, the juices trapped between the fibres run straight out onto the plate with the first cut. Letting the meat rest for a few minutes gives the heat time to spread more evenly through it and lets the fibres relax slightly and hold onto more of that liquid; this short wait noticeably cuts down how much juice ends up on the plate once it is sliced.
Why does bone-in breast dry out less than boneless?
Bone acts as a mass that spreads heat slowly to the meat around it, which lets the parts closest to the bone heat up more gradually. A boneless fillet has no such buffer, so heat moves through the meat faster and less evenly, which raises the odds of ending up with overcooked patches.