Originally By Tony Ackland
Temperature Control for FermentationsTemperature control is very important during fermentation. Yeast is a living organism, and will die if too stressed. Both alcohol and temperature stress it. With no alcohol around, it won't die until about 40 °C. At 14% alcohol, it will die at 33 °C, and at 25 °C if in 20% alcohol. So keep it below 25 °C at ALL times. Lower temperatures will also result in less volitiles. When the temperature has been kept below 30 °C the production of fusel oils is minimal, and is extremely small if kept below a maximum of 25 °C. This is where you get into a bit of a trade-off; if you keep it too cool, it will take heaps longer, with greater time for the risk of infection etc to set in. At 25 °C, it will take 3 days to ferment 0.24 kg/L sugar, but at 15 °C it will take nearly 2 weeks.
Higher fermentation temperatures will result in more fusels being formed. Jack advises ..
Technically, ethyl acetate and amyl acetate are considered esters, not fusels, but they act the same in the still. Fusel oils are formed by the ferment of amino acids- not sugars. There are two types of fusel oils; aliphatic and phenol. The aliphatic have a straight line structure and are volatile- they have a warming alcoholic/solvent note with fruity tones. They lead to definate harshness. Phenol types are involatile, aromatic alcohols with a madicinal flavor.
Lager yeasts fermented at the right temperature (cold) form less than half the fusel oils an ale yeast does at normal temp. (25mg/l against 70mg/l for an ale).
All yeast start to increase fusel oil production when sugar concentrations above 16% (sp.gr.1.065) are used.
Mutated and first generation (air-bubbled "lab-grown" yeast) tend to make more than recycled yeasts do- hence the Scotch distillers use of second hand yeast from the Dublin breweries).
The temperatures that I refer to, BTW, for the fast turbo yeast fermentations are between 80 and 90F (27 and 33C). And, in Making Pure Corn Whiskey I recommend a fermentation temperature range between 70 and 90F (21 and 33C) for the production of whiskey.
Now, the production of beer, wine, and whiskey (or any other flavour-positive spirit, for that matter) is a different story, because the congener profile formed during fermentation will pervade through to the finished beverage. This is clearly true of beer or wine where no distillation is done, so whatever is formed is with the substrate for the duration of its life cycle. Flavour-positive spirits undergo distillation but since certain families of congeners are retained this makes such spirits sensitive to the congener make-up of the mash, unlike grain neutral where everything but the alcohol is stripped out.
I have done extensive experimentation with whiskey-mash fermentation, including numerous different temperature regimens. I've even lagered whiskey mashes with bottom-fermenting lager yeasts for as long as 13 weeks. It produced an unhopped corn/rye all-grain lager that I swear I could have bottled and conditioned and consumed as a very light (and cheap) lager, and I'm confident that it would have been delicious and refreshing. However, when I distilled it, it was completely insipid. It simply didn't have enough esters to give it a significant flavour. It was pleasant enough, but it just wasn't whiskey, although when I rectified it into grain neutral it was very clean.
All of this would have been due to the long lager fermentation where the yeast literally consumes esters, aldehydes, and fusel alcohols during the late-phase fermentation cycle.
I have found that the best whiskey and schnapps flavours are in fact formed during a hot, fast, brisk fermentation, and that long languishing fermentations (i.e. other than lagering) are the ones that produce the less desirable flavours. Of course, this point would be subject to personal preference.
In another experiment, I fermented a corn mash with a wine yeast and let it ferment for over four months. The mash actually formed a sherry flor on top and oxidized and darkened and took on a very unusual but pleasant smell. When I distilled it, it had a distinct fruity/sweet fragrance and flavour that could only be described as a delicious liqueur. I haven't had time to return to this line of experimentation, but when I do I'd like to explore this further.
Just to clear up my use of the term "secondary fermentation", what I mean by that is the fermentation phase that takes place immediately following the high krausen phase. Wine and beer makers will recognize the pattern whereby their fermentations start out with a lag phase followed by a vigorous bubbling phase, often with foaming, then it settles down to just spurious bubbling. This vigorous fermentation is the high krausen phase, or primary fermentation. After that, the mash or must settles down to a spurious bubbling, this is the "secondary fermentation" in my parlance, and it usually takes one or two weeks for beer and one or two months for wine. After this, the beer or wine is left to age or lager (German for, "store in the cold"). In my terminology, the fermentation that naturally carbonates a beverage is called the "conditioning fermentation".
Just to recap, a mash intended for distillation only needs to undergo the high krausen phase in my standard processes.
The familiar rotten-egg smell ... is due to the formation of hydrogen sulphide, mercaptans, and dimethyl sulphide. All of these compounds are usually consumed later in the fermentation in the case of beers and wines, but with distilled mashes, any amount of contact with copper in the construction of the still will instantly remove it.
An easy way to maintain the temperature in cooler climates is to wrap a water bed heating pad around the fermentor, and tape the thermostat to the side of it before wrapping it all in a blanket. Other people just keep their fermenter in the hot water cupboard. Another way is to keep it in a small cupboard or box with a light wattage lightbulb to supply a lttle heat (but shield the bulb so that the beer doesn't become light-struck). Some even use immersion heaters like those for tropical aquariums - but these can be tricky to sterilise, you need to get the wires through the lid in an airtight manner, and if you lift them out of the brew without turning the power off, they can quickly overheat and burn-out (an expensive exercise in forgetfullness). Others yet put their fermentor into a larger drum/container, fill the gap with warm water & then use an immersion heater to keep the outer water warm.
If using the Turbo yeasts, pay particular attention to the temperature. These babies can raise the temperature of the wash by 5-8 °C, so don't add them until the wash has cooled to about 18-20 °C.
If you are fermenting large volumes, you may need to actually cool the wash, either by dropping in frozen 2L softdrink bottles of water, or getting fancy like big brewers, and running cooling water pipes through the fermentor. The larger the amount you are trying to ferment, the harder it wil become to control, yet it is critical that you try to keep it all at 25 °C plus/minus only 1 °C. You may find washes larger than 200L difficult to control & keep cool.
Ian writes ..