Although sous vide london broil said to “lock in the moisture” or “seal in the juices”, in fact, searing results in a greater loss of moisture than cooking to the same internal temperature without searing. The browning creates desirable flavors through the Maillard reaction.
The appearance of the food is usually improved with a well-browned crust. The contrast in taste and texture between the crust and the interior makes the food more interesting. Searing does not cause caramelization, which affects only sugars, or simple carbohydrates: the Maillard reaction involves reactions between amino acids and some sugars. Typically in grilling, the food will be seared over very high heat and then moved to a lower-temperature area of the grill to finish cooking. In braising, the seared surface flavors and colors the cooking liquid. In reverse searing, the order of cooking is inverted. The belief that searing meat “seals in the juices” is widespread and still often repeated.
This theory was first put forth by Liebig around 1850. The notion was embraced by contemporary cooks and authors, including Escoffier. Experiments to test the theory were carried out as early as the 1930s and found that the seared roasts lost the same amount of moisture or more. Generally more liquid is lost, since searing exposes the meat to higher temperatures that destroy more cells, in turn releasing more liquid.