This is "whipped" or defibrinated blood, which has lost the power of coagulating spontaneously.

Since cold water extracts the albuminous juices, while boiling water hardens them into a leathery consistency, water used for stewing should be neither cold nor boiling, but of a temperature which will barely coagulate the albumen and retain it in the meat in as tender a condition as possible; i.e., about 134° to 160°.