Homebrewing Deconstructed: That infection looks like...

This is one of those things I read homebrewers say that makes me want to punch babies in the eye. You've seen it before: some homebrewer gets an infection, posts pictures asking what it is and people roll up proclaiming it's X bacteria or yeast based on the way the pellicle looks.

You cannot tell what an infection is by looking at the pellicle with the naked eye.

You cannot.

No. You can't.

There is simply no way to eyeball a pellicle and know what is underneath. Many bacteria and wild yeast create pellicles. They can create pellicles that look the same, especially if your infection is caused by multiple organisms. Lots of bacteria will make white or white-ish pellicles. You cannot just look at that and decide it's lactobacillus. You would have to take a sample, look at it under a microscope and be able to determine exactly what you have (which may be a difficult task itself). If you could, breweries dealing with infections could just brew a batch, wait for a pellicle to appear and then diagnose the problem by looking at it. Ever heard of a brewery doing that? No, I haven't either. They send cultures to a lab to diagnose under a microscope.

Part of what fuels this nonsense is the belief that the only organisms that can live in your beer is saccharomyces cerevisiae, brettanomyces, pediococcus, lactobacillus and acetobacter. This itself is untrue, but people think there's only three things that will form a pellicle and that means you have a one in three chances of guessing correctly. Well, that's not true at all. Lots of other stuff does live in beer and will form a pellicle. Some non-brettanomyces yeast can live in your beer and will form a pellicle. There are lots of bacteria that can also mix it up in your beer. Not just pedio and lacto.

It doesn't really matter whether you think you have a lactobacillus infection or a zymomonas infection. Your sanitation procedure should still be the same. However, you can avoid sounding foolish by not looking at pictures of pellicles and proclaiming to know what it is.