For successful poached eggs it is essential that the
eggs are extremely fresh. If the eggs are more than 1
week old the protein in white breaks down and the white
starts to become watery (secondary) and will not hold
its shape when cooked.
The vinegar in the water helps maintain a good shape
by increasing the rate of the egg protein's coagulation.
When large amounts of poached eggs are required they
may be cooked and held in a bowl of cold water and then
reheated to order.