In my 9 years running our Concord, California showroom many customers have asked “What can I do to keep fermentation temperatures cool without having to buy a refrigerator or your temperature controlled conical.” One method, commonly discussed on the internet, is placing your carboy in a shallow pan filled with water and covering that with a t-shirt. The evaporation of water up through the t-shirt can cool you fermenter up to 5 degrees off the ambient temperature. However in the middle of summer this is just not enough. One of my favorite methods is placing my carboy or bucket into an ice water bath.
With this simple method, you can easily and accurately (with a little practice) control fermentation temperatures without buying an extra refrigerator or spending a lot of money. You do need to find or buy a large 20-40 gallon bucket and place your fermenter inside of it. A plastic garbage can or our WE509 (on morewinemaking.com) work perfect. Fill the large bucket or garbage can with water up to the level of your wort in your fermenter. Add ice every morning and every night for the first 5-7 days of fermentation. I suggest you freeze (3-4) 1 gallon milk jugs or something comparable (not glass!) and rotate them out. Exactly how much ice you need will be determined with a little trial and error.
If you want to ferment around, for example 68F, your goal is to maintain 62-63F in the water. Remember, yeast produces energy, which in turn creates heat. If your water bath temperature reads 62-63F, your actual fermentation temperature is probably 66-68F during peak fermentation. Use any thermometer to test water bath temperatures, but for even better control measure the actual temperature of the fermenting beer. You can use our carboy or stopper thermowell, FE612 or FE613, and a thin probe thermometer, MT395, to monitor your fermentation temperature precisely. Tip: fill thermowell with water after inserting thermometer.
The goal of this set up is to keep your yeast as temperature controlled as possible the first 4-5 days. The majority of cell growth and ester production comes at the start of fermentation. If your ferment is too hot at the beginning, your yeast will grow wildly and produce beer that has a rough finish and tastes alcoholic. Stress on yeast cells can also affect their permeability (ability to take in nutrients) as well as deplete their glycogen reserves, which they use for survival later in the ferment. Thus fast, wild starts can also result in stuck or stopped fermentations as you get closer to the end.
While maintaining the perfect set temperature all the way through fermention is idea, if the fermentation temperatures do rise above 68-70F later in the fermentation, after most of the sugar is consumed, far fewer off flavors are produced. By controlling the fermentation temperature closely for the first few days you allow for your yeast to do most of their work in a controlled situation. They like that and as a result reward you with great beer!