I recently bought some pre-poured agar plates partly for plating some mixed-culture beers to isolate some samples, and partly just to play around. I had a few plates left over that weren't going to be used, so I ran a little experiment.
I streaked 4 nutrient agar plates with dregs from Gueuze Fond Tradition, which was one of the beers I was culturing. I let them sit for about 10 minutes to dry. Then I sprayed each with enough of each test solution to visibly wet the entire surface of the plate. I let each plate sit for 2 minutes, then poured off any residual sanitizer. I then stored the plates upside-down to dry. The plates were stored at ambient room temperature, and the results are from 8 days after plating.
The 4 plates were sprayed with:
-Filtered tap water (control solution, and what I use to mix my Iodophor and Star San)
-Bacardi 151 rum (75.5% ethanol)
The results are pretty clear. The control (tap water) is the big loser and has a significant amount of growth. The ethanol has a surprising amount of growth. Iodophor has a handful of scattered colonies. And Star San is the big winner in this test with no growth after 8 days.
I have to admit, the results were surprising to me. Coming from a medical background, I am very comfortable with iodophors and ethanol for surface disinfection. Povidone-iodine, aka Betadine, is a very common surgical prep and is an iodophor (iodophors are a class of compounds, by the way). But Star San just is not as broad spectrum (regardless of what my results show), and would never be used in the medical field. It is commonly used in dairy and food-processing, however, and certainly has its benefits.
So what do I think is happening here? First of all, what makes Star-San useful in dairy and food processing is that it doesn't lose its potency in the presence of organic matter the way other sanitizers do. It is also much less volatile than ethanol. That is an important piece, since you need to ensure that your surface stays wet for the entire contact time for the sanitizer in question. And that is what brings me to the last piece - Star San has an added foaming agent. This really helps out in the contact time department.
So what are my personal takeaways from this experiment? The big one is that you can't make up for poor cleaning practices with sanitization. You must have a surface that is clean of all organic materials for your sanitizers to be effective. My second one is that I will continue to use Star San in my brewery, and that I will go back to using it as my preferred spray sanitizer. Iodophor has a 2-minute contact time and no additives to help it cling to surfaces. I think I'll take my chances with Star San. I will still use ethanol when I want broad-spectrum coverage (such as stepping up bottle dregs, where I'm starting from a small cell count), but I will be sure to soak the hell out of any surfaces in question and keep it wet by re-applying if needed.
Edit - finally able to get the images in line