Fermenting on or off the grains simply means whether the grains are present during fermentation or not. Fermenting ON the grains means that the grain is still present. Fermenting OFF the grains means that the grains are separated before fermentation. This article is geared towards distillers, beers are mainly fermented without the grains.

Fermenting on the grains benefits:

1. If your mash has incomplete starch to sugar conversion and not all the starch was liquefied during mashing, you can add enzymes which will keep converting starch to sugar during fermentation. Glucoamylase will keep forming sugar from residual starch as the yeast produces ethanol and may help you increase ethanol yields. Forming sugar during ferment is commonly known as Simultaneous Saccharification and Fermentation (SSF).
   
   
2. Increased flavour- This one is in both the pros and cons section! If you want a super neutral vodka, fermenting on the grains isn’t the best idea. Your distillation column may or may not be able to separate some undesirable astringent flavours. If you are planning on a very complex whisky, you might want to ferment on the grains. Extract more tannin (polyphenols) and oils. Hotter ferment and hotter sparge (if you remove the grain before distilling) will extract more tannins. The new make whisky might be undrinkable for the first year. However, these compounds will react with chemicals from the oak barrel and those chemicals will oxidise through air contact through the breathable staves, creating new complex flavours. Tannins are an important factor in red wine production, creating structure in the wine and superior mouthfeel. In several years time you might be left with a complex masterpiece with a super long finish!
   
   
3. Less sparge water- ethanol is more miscible with water than sugar is. If you ferment on the grains, it requires less water to rinse the ethanol out of the grains than it does to rinse sugar off the grains. Keep in mind that temperature plays a roll here, so it isn’t necessarily a 1:1 comparison. If you ferment off the grains, lautering will be done while the grains are still hot, after mashing. Sugar is more soluble at higher temperature. When you are lautering a fermentation with grains still inside, it is normally done at ambient temperature or whatever temperature the fermentation finished at.

Fermenting off the grains benefits

1. Cleaner flavour. Removing the grains prior to fermentation will mean that less tannins (polyphenols responsible for astringency in high concentrations) and other possibly off flavours are avoided. Trub, fine solids left in during the fermentation such as amino acids and proteins can lead to increased hydrogen sulfide production, and a “sulfury” tasting spirit. Copper can be an important part of beverage distillation, as the hydrogen sulfide reacts with the copper, removing it from the spirit. There are also other ways to remove the sulfury flavour, such as running a tall distillation column in full reflux for a while to purge the gas before taking the heads cut. Hydrogen sulfide boils at negative 60 degrees C, so it will definitely come off first. Different yeast strains produce different amounts of sulfur. Hydrogen sulfide is a gas, so letting your spirit rest for a long time will allow the gas to dissipate.
   
   
2. No drunk cows- A lot of distillers give/sell their spent grain to cattle farmers. If you draw the sugary wort off the grains and then give them to a farmer, there will be zero alcohol content. If you ferment on the grains and don’t manage to sparge off all the ethanol, the cows might be stumbling around the paddock the next day! One moooo means they like the sugary grains better, two moooos means they like alcoholic grains, ask one next time you see one.
   
   
3. Easier pumping- separating the wort from the grains as soon as possible means that you don’t need a special pump which can handle high amounts of solids to transfer the grain, water and sugar mixture to the fermentation tank and then back to the lauter tank or still.
   
   
4. Similar to easier pumping, except easier to cool. Heat exchangers often have small gaps between the plates. Cooling a fermentation with the grain in often requires a special heat exchanger design which won’t clog up with solids.

An extra layer of complexity is added for distillers who ferment on the grains. Should you filter/lauter the grains off before pumping it into the still or not? Again, there is no right or wrong answer, simply different variations that give different results.

Distilling on the grains benefits:

1. Massive reduction in heating energy use. When separating the grains, brewers/distillers will generally sparge (which means to wash the grains) with 1.5 times as much water which was used for mashing. If it is a mash with 100 L of water, 150 L of water will be used to rinse all the sugars out of the grains. A total volume of 250 L of water, minus some trapped in the grain husks. If you ferment and distill with the grains still there, you only need to heat ~40% of the amount of water to boiling, a huge energy saving!
   
   
2. Practically perfect ethanol yield when distilled on a tall continuous column. When sparging grains, there comes a point where it is futile to wash any more sugar/ethanol out of the grains. You are just diluting the mixture too much, resulting in more energy usage to boil more water for a tiny amount of yield. If you ferment on the grains and then pump this directly into a 10-20+ plate continuous stripping column, the ethanol yield for this step will be very close to 100%. If you distill on the grains in a pot still, there is much more alcohol left over. This is really more of a benefit of a continuous stripping column vs pot still in terms of alcohol yield…..but a continuous stripping column pairs perfectly with fermenting on the grains. Continuous stripping columns usually aren’t practical in small diameters which are used by hobby distillers. The gaps in the plates will be small and clog with solids. Larger columns can handle the solids with ease (the grain can also be milled much finer if it is being fed into a continuous column to prevent clogging, though this makes it harder to recover the spent solids for animal feed use).
   
   
3. Full bodied complex flavour- as with fermenting on the grains, boiling with the grains after ferment will further increase the amount of tannin and other chemicals extracted. As mentioned before, this isn’t ideal for making a clean neutral spirit which is drinkable from day one if you don’t have a very tall still, but can be very desirable for a complex whisky or other spirit if you are willing to wait many years for the flavours to oxidise, react and mellow. If your distillation column is very tall, you will be able to remove flavours you don’t like and include flavours you do like.

Distilling on the grains drawbacks:

1. Burning solids to heating elements/hot surfaces. Grain husks, fine flour, yeast etc may burn to hot surfaces and be incredibly hard to clean off. If it does burn, it could produce horrible smoky flavours, and no i’m not talking about an easy way to fake peated whisky! Burning solids to an electric heating element can easily cause it to melt, if this happens the batch will be ruined and your element will need replacing. Distilling with the grains still in is never recommended for electric heating elements. If you have a steam jacketed still and a powerful agitator, you are safer, though some distilleries who distill on the grain have special agitators which scrape the bottom of the still to prevent burnt solids.
   
   
2. Off flavours caused by boiling with the yeast, tannins etc.

What do we do at Headlands Distilling Co.?

We ferment on the grains! Seacliff Vodka (which is also the base of our gins) is designed to be a vodka with complexity, not a neutral industrial spirit. However, our packed vodka column is very tall, with 37 theoretical plates. While more flavours are extracted from the grain during fermentation, we are able to separate out the flavours we don’t like and only include the flavours we do like, or the compounds responsible for structure, a more viscous mouthfeel without the harshness. It is a common misconception that a very tall column will only produce a very neutral spirit. This is partially true, it CAN produce a very neutral spirit, but if you keep the distillation running, the other components will start coming off in order of boiling point/polarity as well, then you can blend them back into the hearts if desired.

We remove the grains before distilling and then pump this into our energy/yield efficient continuous stripping column.