Pasteur is someone's name. To be "boiling" means to be angry. Louis Pasteur was a microbiologist that discovered and encouraged pasteurization. Pasteurization is the process of using heat, or boiling something in order to remove disease causing microbes. The joke is centered on the word "pasteur" as it means to boil, and Pasteur is boiling angry about not being invited.
Pasteur was boiling about not being invited .