Sanitation and cleaning are easily confused, but both are important to nurturing your yeast. You want to make sure that every surface that touches your beer, from the boil kettle to the glass, is free of microbial wildlife. Without cleaning everything first, though, you’re missing the point. Dried gunk stuck to your carboy provides an irregular surface that the sanitizer can’t easily reach. Beasties can lurk in crevasses and wait things out. Later, while your beer is fermenting, they can reproduce and come into contact.
You need to remove dirt and other deposits from your equipment so the bacteria have nowhere to hide and your sanitizer can do its job. That’s why you should follow a two-step process of cleaning your equipment first, then sanitizing. Cleansers, such as Powdered Brewery Wash (PBW) or Sodium Percarbonate, will strip away dirt, dried beer and wort, and other residue. After a quick rinse, you can bring on a sanitizer such as Stellarsan to finish the job.