Variations of head cheese exist throughout Europe and the rest of the world, with differences in construction and ingredients; for instance, one version pickled with vinegar is known as "souse". Historically, meat jellies were constructed of the cleaned (i.e., all organs removed) head of an animal, which would be simmered to produce a stock naturally gelatinous in nature; this would then congeal as the stock cooled. Meat jellies made in this manner were commonly a peasant food and have been made since the Middle Ages. Modern head cheese recipes may need additional gelatin, or more often require reducing, in order to set properly.