Even in Shakespeare’s day it was understood that solid particles in the blood would clog the arteries, leading to death. In describing how the poison caused his blood to coagulate (solid particles to form) the ghost uses two examples of food:

(1) to posset was to make a thick beverage, called posset, by adding some acidic alcohol (ale or wine) and spices to hot milk.

(2) to curd was to take the first step in cheese making which was to separate milk into the solid proteins (the curds) from the liquid (the whey), which could be done by adding vinegar which caused the milk to sour and coagulate.