For new brewers, the fermentation process can look quite alien at times, especially when different batches of beer may behave differently due to different ingredients, fermentation temps, etc. Some people will dump perfectly good beer thinking it may be infected, when it actually isn’t. So before you decide whether your beer is a dumper, you will want to visually inspect and possibly even taste it to make sure it’s still worth keeping or not.
Foamy bubbles on top of the beer is called “krausen” and is NOT a sign of infection. It is completely normal and may either be a high krausen (very foamy) or a low krausen (just a thin layer of foam). Some beers may not even exhibit much of a krausen. As long as the gravity of the beer is dropping, it is fermenting. It all depends on the yeast, malt, hops, etc. So keep in mind that not all beer fermentations will look the same.
An oily sheen on top of your beer that may look kind of like thin white ice sheets with jagged edges is a sign of the beginning of an infection. This infection is usually caused by wild yeast such as Brettanomyces or wild bacteria such as lactobacillus. In some cases, it could be a combination of these or other bacteria/wild yeast. In more advanced infections, this layer of biofilm, called a “pellicle” may look very wavy, sort of like ramen noodles. Or it may look like bubbles with webbing coming off it. These are different types of infections, but cause by similar bacteria/yeast strains.
A pellicle is only formed in the presence of oxygen and is a way for the wild yeast/bacteria top protect itself from oxygen because they prefer an anaerobic environment. So you can still have an infection even without the pellicle formation if your beer was free of oxygen exposure during fermentation. In this case, the only way to know whether it is infected or not is to taste it. Don’t worry about getting sick because none of these wild bacteria, yeast or mold can harm you. If the beer tastes bad or rancid, you might wanna dump it. But in some cases, an infection can result in a pretty good tasting beer, albeit a sour one. Keep in mind that sour beers, which are all the rage right now, are brewed with these wild yeasts and bacteria.
If your beer was infected and you were using plastic equipment, such as our LBK or Brewmax fermenter, you may need to replace them with new equipment. Scratches inside the fermenters can harbor bacteria that is VERY difficult to sanitize. So when making sour beers with these wild yeasts/bacteria, it’s always recommended that you use glass, or at least dedicate your plastic to making sour beers only.
If your beer is infected with mold, which will be fuzzy and discolored (usually green but can be white or brownish – but always fuzzy), this can typically be skimmed off the beer. Mold only grows on the surface and will not penetrate the beer itself. Mold cannot survive the alcohol in beer. Fortunately, mold usually takes a long time to grow on beer so as long as you’re not leaving it in the fermenter for too long, you shouldn’t have this issue.
Proper cleaning/sanitizing, and the proper care of your plastic equipment (only soft cloths for cleaning, nothing that can scratch the plastic) will help prevent these types of infection. But always keep in mind that even with the best cleaning and sanitizing procedures in place, you can still get infections from time to time. Don’t let this discourage you. Learn from it and keep brewing.