Far less well known than say West Country cider, plum jercum is supposedly a traditional Midlands beverage, associated with wassailing and all the pleasure and falling over that such events imply. Made along the lines of cider, but with plums to give the flavour and body, jercum is proof of man's ingenuity when it comes to making strong drink.
The plums are squashed and fermented like cider, with sugar added to give the resulting drink a bit more kick. At least two sources think that yellow plums would have been used originally, but there is no reason why purple varieties cannot be substituted.
Modern makers are likely to use improved brewing or wine yeast of some description, but the natural yeasts on the fruit's skin would have sufficed in past times, the product being less sophisticated but assuring the same end result.