When chilling your wort after boiling, there is a “danger zone” to be aware of — a range of temperatures that support the rapid growth of microorganisms (wort spoilers) in your chilling wort. The high end of the “danger zone” is the temperature at which wort-spoiling bacteria and wild yeasts can survive in wort. The low end is the temperature at which these “bugs” can survive and still reproduce rapidly. Above the high end, heat kills the contaminating microorganisms. Below the low end, they can still survive and reproduce, but are slowed somewhat by cooler temperatures. The range can be stated as 160–80 °F (71–27 °C), but these numbers are just estimates. There are countless numbers of bacteria species and it’s always possible that some wort spoilers could survive above 160 °F (71 °C) and the value of 80 °F (27 °C) hinges on how you define “rapid.”
Whatever the boundaries, the idea is sound — there is a range of wort temperatures that support the rapid growth of contaminants to the extent that they can cause problems in your beer. Knowing this, there are some steps that homebrewers should take to ensure a higher level of wort sanitation.
The simplest action you can take to minimize the extent of contamination is to chill your wort as quickly as feasible through this range. The good news is that homebrew wort chillers (most standard immersion chillers, counterflow chillers or plate chillers) are up to the task in most situations. Even if it takes up to an hour to get below 80 °F (27 °C), your beer is likely not going to be ruined. Still, if you shoot for a level of beer quality higher than “not ruined,” it pays to make an effort to chill as quickly as you can reasonably manage.
If you are cooling with an immersion chiller, swirling the chiller through the wort will speed cooling. (Wear heat resistant gloves to protect your hands from steam and the hot tubing on the side carrying cooling water away from the chiller.) If you live where groundwater temperatures are not very chilly, adding a pre-chiller — an immersion chiller sitting in an ice-water bath — can help immensely.
In Texas, my groundwater can be as high as 75 °F (24 °C), so I place a pre-chiller between my water tap and the wort chiller. I chill initially with straight tap water, because the temperature difference is large enough that initial cooling proceeds quickly. Once cooling slows, I place the pre-chiller in the ice bath to pick up the pace again.
If you use a counterflow wort chiller or a plate chiller, the wort passes through the danger zone while inside the chiller. Obviously, this means that you want to be certain that the inside of your chiller is clean and sanitized. If you use an immersion chiller, the wort passes through the danger zone in your kettle. While the kettle should be sanitary from boiling, the wort’s surface is exposed to airborne contaminants. For this reason, I like to cover my kettle when chilling with my immersion chiller. To do this, I place the lid on the kettle as loosely as possible in the last 5–10 minutes of the boil, so the steam sanitizes the (clean) bottom of the lid. (I actually rotate the lid a couple times to make sure the entire surface gets steamed.) When the boil is over, I start the chilling water, then close the lid. I also drape two (clean) T-shirts over the kettle. This offers some protection from airborne organisms falling into the wort. (It isn’t complete protection, though. As the headspace above the kettle cools, some air is going to be sucked in.)
One potential drawback to covering your kettle is that condensation from the wort (potentially containing the volatile compounds DMS) will drip from the lid back into your wort. Although this is a risk, I’d rather this happen than leave my wort exposed to the open air. By the end of the boil, especially a 90-minute boil, most of the DMS is boiled off. Still, when I move the lid, I try to let the drips fall outside of the kettle.
So, when chilling your wort, it pays move through the danger zone as quickly as possible and ensure that your wort is shielded from contaminants during this time. (Either ensure the inside of your counterflow or plate chiller is squeaky clean and sanitized or protect your wort from airborne contaminants when immersion chilling.) After chilling, getting the wort aerated and pitching promptly will also help minimize the impact of any wort contaminants.
The only real drawback to knowing about the danger zone is that that horrible Kenny Loggin’s song will get stuck in your head. So stop thinking about it. Now.