The Importance of a Cold Start and Uncovered, Slow Heating

After the blanching or browning, the meat solids are started in an uncovered pot of cold water, which the cook brings slowly to a gentle simmer and keeps there, regularly skimming off the fat and scum that accumulate at the surface. The cold start and slow heating allow the soluble proteins to escape the solids and coagulate slowly, forming large aggregates that either rise to the surface and are easily skimmed off, or settle onto the sides and bottom. A hot start produces many separate and tiny protein particles that remain suspended and cloud the stock; and a boil churns particles and fat droplets into a cloudy suspension and emulsion. The pot is left uncovered for several reasons. Because this allows water to evaporate and cool the surface, it makes it less likely that the stock will boil. It also dehydrates the surface scum, which becomes more insoluble and easier to skim. And it starts the process of concentration that will give the stock a more intense flavor.