Cooked chicken can reveal disconcerting pink juices and meat around the bones. This raises concern about food safety, as we associate pink meat with under-cooking and we seem to worry about chicken more than other meats. However, these juices can be present even when the chicken has been cooked to a safe pasteurisation temperature. The juices do not necessarily indicate that the chicken was under-cooked, more likely it was a young bird.
Chicken producers want their chickens up to size and out to market in the shortest possible time. This is usually around 6-7 weeks. The bones of young chickens have not had time to mature – they are soft and porous. With cooking, juices in the bone marrow seep through the porous bone and collect under the chicken meat. These juices are pink, and so the meat appears underdone. Freezing raw chicken forms ice crystals that further break down the bone structure and increase seepage during cooking.
One of the main functions of bone marrow is to replenish the circulatory system with red blood cells, and hemoproteins (protein molecules containing iron) are an essential component of these cells. It is these proteins (or heme) that we notice in the juices because their iron tints them red. The juices are not blood and don’t contain red blood cells – these are too large to get through the bone. Marrow also replenishes white blood cells (immune system lymphocytes) and platelets (clotting agents). Useful stuff. Fish bones do not contain marrow.
The bone casing consists of minerals (apatite crystals that are mostly calcium and phosphorous) and collagen (a protein). With cooking, some collagen denatures (gelatinises) into gelatine which dissolves in the marrow juices and thickens it. If the cooked chicken is chilled, the gelatine sets the juices to the pink jelly that is often encountered with cold chicken.
It is the gelatinisation of bone collagen that makes bones an essential part of stock-making. Because this occurs more readily with younger animals, veal bones are often added to beef bones/meat to add the succulence of gelatine to a beef stock.
The pink juice/jelly is most noticeable in chicken because the flesh is light. However, it can also occur with meat from other young animals, such as lamb. Even in older animals, the bone near the heads of the long bones (trabecular bone) is especially porous, so pinkness can still occur near the joints.
With continued cooking, the juices will go from red to brown, however this is unnecessary from a food-safety perspective and the chicken can end up overdone. Note also that the components making up the juices are still there (and will be eaten), they are just not pink anymore.
We are right to pay attention to doneness around bones though. This is because it does take a while to cook meat that is near bone. Bone has a high mineral content (up to 70%) and therefore should conduct heat well (like a ceramic). However, it has an open honeycomb-like structure filled with air, and air is a very poor heat conductor (which I’ve mentioned before). It turns out that the effect of the air dominates, and bones act as insulators. This means that meat near bone is protected from the heat of cooking on the bone side, and so takes longer to cook.
If cooking with conventional methods (roasting, barbecuing, stovetop etc), then by far the most food-safe way to know when meat is cooked is to use an accurate probe thermometer – an essential tool for any kitchen. If using sous vide, bone-in chicken thighs/legs will be tender and juicy (and pasteurised) when cooked at 65C for 90m.