The Details About Fermentation
After reviewing ingredients and ingredient selection, its time to start looking at the equipment you will use with those ingredients. We’ll start off by looking at your fermenter and understanding what it really does. Basically, the fermenter has five jobs: contain the mash, keep out bugs and other stuff that can contaminate the batch, control the temperature of the mash, monitor the progress of the fermentation, and control the liquids and solids in the fermenter. There are exceptions to these uses, non-temperature-controlled fermentation and open fermentation being the two largest and most obvious exceptions, but even these processes need some idea of what is going on with temperature or be designed to control which bugs get in and out for accentuation.
Contain The Mash
Containing the mash may seem like an obvious job for your fermenter, but I’ve seen a number of fermenters that have leaks from the jacket or even the body of the fermenter—whether it’s from a cracked weld of a valve that doesn’t fully seal or seal easily. If you’re buying used equipment you need to inspect the welds and fill it with water to make sure that it doesn’t leak before you start using it. The volume your fermenter should contain needs to be about 20% more than you intend to fill it with to account for foaming. It is possible, if you’re using foam control agents, to decrease that extra volume and do a 500-gallon fermentation in a 550-gallon tank. What your fermenter is made of does matter. The most common fermenters are made of stainless steel or plastic but, as we learned from Bill Owens in his speech at ADI, don’t use galvanized steel! The reason for this is that the low pH of a fermentation can extract the zinc in galvanized steel and create rat poison. Stainless steel is a nice fermenter material because it is non-reactive and easy to clean but it is more expensive than plastic. It does, however, have a much longer lifespan than plastic. The closest to a problem that stainless steel has is when you’re using caustics to increase the pH and fermenting at a high temperature. If you go the plastic route, you want to be careful because plastics can leach, particularly at low PH levels and high temperatures. If you’re pushing your fermentation pH down to the 4-4.5 range, you don’t want to use plastic.
The next consideration is size. A 3:1 height to width ratio is about the tallest that you can go and it definitely minimizes any floor space concerns. Normally I see 2:1 ratios that allow for more tanks in your distillery without overstressing the yeast or needing a higher ceiling. So, for instance, a 500-gallon fermenter would be 3.5 ft. wide and 7 ft. tall at 2:1 or 3 ft. wide and 9 feet tall for 3:1. A 1:1 square takes up a lot of room since it’s 4.5 ft. by 4.5 ft. Personally, I prefer round fermenters because the heating and cooling is more even due to the fact that there are no corners. Also, the mixing is more even without any dead spots. Fermenters do naturally mix due to yeast motion, CO2 bubbling, and heat rising.
Keep Out Bugs & Other Contaminants
Bugs and microbes (even if you’re doing open fermentation) are desirable to minimize or to delay a secondary infection by a bug or microbe dropping in. If you are doing open fermentation, look for a narrower mouth for your fermenter to focus the CO2 out and create more pressure to keep out bugs and other crud. Something to keep in mind: a 500-gallon fermenter is going to release 10% (or about 4.35 pounds) of CO2 per hour. If you’re going the closed fermentation route, be aware of the backpressure if you have an airlock. Too much CO2 entrainment is bad for yeast and the more pressure on the vessel, the more CO2 will be entrained. To figure this out, look at the carb tables to highlight that each PSI of pressure on the tank creates 0.05 volumes of CO2 at 75 degrees Fahrenheit. Additionally, each inch of water creates about 0.04 psi of backpressure. So if you’re using a 5-gallon bucket, you’ve increased the CO2 levels by about 0.1 volumes.
Control The Temperature of Your Mash
Next, you need to look at how you’re going to control the temperature of your fermenter. There are two ways to do this, with a jacket or a cooling coil. First, you need to determine how much cooling you need to do. The surface area of your cooling surface will dictate how quickly cooling occurs. Generally, it is the yeast putting out the heat evenly through the fermenter. It’s technically hottest at the top because heat rises and most of us use top fermenting yeast, so that’s where the action is happening. For our 500-gallon 2:1 fermenter at 10% ABV, the yeast will output 1,353 BTU per hour. If we want our fermenter at 60 degrees Fahrenheit but the room is normally 75 degrees, that will add 140 BTU per hour to the tank, assuming it’s uninsulated stainless steel. That means that you need 0.12 tons of cooling to keep your fermenter at the desired 60 degrees. The school of minimal cooling tends to cause designers to shrink the cooling surface since that’s the most expensive part of the equipment, but the problem with that route is that temperature works as a gradient. So the uninsulated side will be influenced most heavily by the room temperature and this normally requires the jacketed side to be run colder to ensure that your average temperature is where you want it. On the flip side, a fully jacketed tank sees the hottest temperatures in the middle and coldest temperatures around the rim. This allows for a higher jacket temperature. A solution to this is to mix your mash with an agitator or a turbulent fermentation, but this can introduce oxygen, which is bad, and this is particularly hard if you have a square tank. Consider using multiple zones in the jack, especially for taller tanks, so that you can have temperature layering. The cone should be cold to increase the settling of yeast and other solids; the top is next coldest to mitigate the excessive heat and lower jackets cooled less. These should each have their own temperature probes.
To monitor the progress of your fermentation you are going to be taking representative samples. That does not mean just taking from the top or bottom; you should pull your samples from the center of the fermenter tank. You also want to be able to do temperature monitoring and check load cells. Ideally, you should have the ability to add chemicals, adjust the temperature, and utilize all the features we’ve talked about above.
Controlling The Contents In Your Fermenter
Finally, you need to think about how you’re controlling all your solids and liquids, especially if you are fermenting or distilling on the grain. If you intend to move the grain from the fermenter to your still, you really want a flat, sloped, or dished bottom. This makes it harder for the grain to plug the outlet and square corners allow for the grain to settle out, so it’s best to agitate the tank first to make sure that everything is well mixed. Now, if you are trying to ferment on the grain or solids but you don’t want them in your still, then a conical bottom is best since the solids will pack down there and you can rack from the top, above the conical portion. Finally, if you aren’t fermenting on the grain and don’t care about the yeast in your fermenter, then it doesn’t matter what shape you get—but flat bottoms are cheaper!
Meet The Guest
Dr. Diego Benitez is a scientist, entrepreneur, and active owner of Progress Brewing in South El Monte, California (Los Angeles County). He holds a