Yonkem Yeast is a brand of boxed yeast.
Luigi, spurred by one of Slimer's ideas, baked a giant pizza as a gimmick to attract more customers to the Ritz Cafe. One of the ingredients he used was some Yonkem Yeast. Luigi advised Slimer not to use too much of it. However, Manx threw a whole box of Yonkem into the mix. The yeast rose and carried Slimer and Luigi out of the cafe. They fought back and let the pizza bake on the hot pavement.