Originally By Tony Ackland
IntroductionFermentation is the conversion of sugar to ethanol and carbon dioxide by yeasts (wort to wash). Whilst doing this, it can create a range of flavours beyond what the wort started with. During fermentation yeast converts sugar into alcohol and carbon dioxide by feeding on a series of increasingly complex sugars, essentially breaking the sugar down into other compounds which enable it to grow. First on the menu is glucose, before moving onto maltose, then maltotriose. Depending on the strain of yeast, these sugars may be tackled at different rates, and not always strictly in sequence. Although sugars account for the majority of flavours, yeast works on various other compounds, including amino acids and fatty acids, which also contribute flavours.
Theoretically 10 kg of sugar will produce 6.5 L (5.1 kg) of ethanol and 4.9 kg (4900L) of carbon dioxide. In doing so, some energy is released too (about 2.6 MJ/kg of ethanol).
Yeasts are single-cell fungi organisms. The most important ones used for making ethanol are members of the Saccharomyces genus, bred to give uniform, rapid fermentation and high ethanol yields, and be tollerant to wide ranges of temperature, pH levels, and high ethanol concentrations. Yeasts are facultative organisms - which means that they can live with or without oxygen. In a normal fermentation cycle they use oxygen at the start, then continue to thrive once it has all been used up. It is only during the anaerobic (without oxygen) period that they produce ethanol.
Gil explains ....
The yeast does not consume sugar as food, but the other nutrients added to the wort. Mead making is an interesting experiment in this respect since unlike grape juice honey water will not in itself sustain yeast, and any half-decent distiller will do themselves a favour by mastering the technique of making such an environment more friendly.
Over the years I have learned to sustain the yeast in mead batches on a mixture of Vegemite and Epsom Salts, then aerate the wort thoroughly before activating the yeast and pitching. You can experiment with any number of nutrients and aerating systems to breed as much yeast as you want, but I have found the above mix avoids an off-taste in the finished mead and is easy to introduce to the colony.
The process implies two distinct fermentation phases. The primary fermentation takes place as the yeast breeds rapidly in the initially aerobic environment and the colony comes up to strength. Then the secondary fermentation takes place in the anaerobic environment thus generated, as the yeast strips oxygen from the sugar molecules in order to avoid suffocating.
Fermentation does not mean that alcoholic is being produced, only that the wort is in a ferment; that is, bubbling merrily.
Throughout both stages there is an abundance of carbon dioxide being exhaled which assists in maintain the anaerobic environment conductive to the production of ethyl alcohol. It does need to be kept in mind that it is not the yeast colony's intention to produce the alcohol, but ours.
All the yeast is trying to do is avoid suffocating in anaerobic conditions.
Beyond that it is fundamentally misleading to suppose that yeast is much interested in sugar, which can kill it the same as alcohol does, and here we must also recall that we are merely exploiting its ability to adapt to what are essentially hostile conditions.
My reference is A.J. Salle, "Fundamental Principles of Bacteriology", 3rd Edition, New York: McGraw-Hill, 1948.
Another book that must be read is Bill Mollison, "The Permaculture Book of Ferment and Human Nutrition", Tyalgum: Tagari Publications, 1993.
The influence of the yeast depends on the sugar concentration in the wort, the pitching temperature, and the rate of fermentation.
There are three phases to fermentation once the yeast has been added:
For more information about fermentation, see Fermented Fruits and Vegetables - A Global Perspective, and Brewing Yeasts.
Yeast produces 33 times more alcohol while reproducing than when resting (so most of the gains are in the first couple of days, then you're just relying on the large numbers of yeast finally present to slowly work their way through the remaining sugars)
Once the nutrients have run out, and the fermentation has become "stuck" or sluggish, it is then too late to provide either nutrients or new yeash. If this happens really early during the fermentation, then you're in trouble.This is because when a yeast is deprived of a nutrient, it grows as best as it can with what is available, and then growth comes to a halt. Those cells are then put together with less than satisfactory levels of (lets say) protein due to deficient nitrogen. Their enzyme content is less than adequate, and they don't metabolize well at all. Growing cells are ~33 x faster at ethanol production than non-growing cells. Supplementation at that point does not reinitiate growth in the older cells. By that time the medium is higher in alcohol and still deficient in some nutrients. Some cells may even have died. Even supplying the combination of BOTH nutrients and new yeast won't get the activity restarted again. So the trick is to ensure you have enough nutrients available at the start of the fermentation.
You end up with having grown about 2g per litre of yeast (eg 40g in a 20L wash) This is why you don't get the full 51.1% conversion of sugar to ethanol, and gives some idea of the amount of nutrients - particularly nitrogen - that you need to supply.
Bakers yeast will produce a maximum of around 14% alcohol, whereas the "turbos" can generate up to 20% alcohol. Obviously you'd use different amounts of sugar for either case. Its not that the Turbo makes a higher % from less sugar, its advantage is that it can handle the higher concentrations (first of heaps of sugar, then later, the high alcohol %), and hence you need proportionally less water. Hence you end up with more alcohol in your 20L wash, because you are able to put more sugar in. Only use the Turbo's if you're after a "neutral" alcohol. If you're trying to make a flavoured spirit, (eg corn whisky, brandy, rum, etc) then use a yeast which will help give you the flavour profile that you desire.
While making the ethanol, the yeasts will also make very small amounts of other organic compounds - including other alcohols, aldehydes, esters, etc. These are known as the "cogeners" or the "fusel oils". It is the presence of these that give the alcohol its flavour. So when trying to make a neutral spirit, we'd try to minimise their presence, but if making a whisky, rum, brandy etc, then we need a very small proportion of them present.
Fusel oil concentration in the wash can be up to around 0.24 g per Litre from 20% sugar (eg about 3.2 mL from a 20L wash)
Fusels increase depending on ...
* yeast strain (eg Saccharomysce cerevisia makes more than S.carlsbergensis)
* temperature (higher temp = more fusels)
* increased aeration and agitation (news to me ! so don't over-stirr it !)
* wort composition (nitrogen sources and readily metabolised sugars)
The most common limiting factor for yeast growth is a lack of nitrogen. Nitrogen is approx 9% of the cell mass. Most common form to add it is as the ammonium ion, as the sulphate and phosphate salts (phosphorus is approx 1-2% of the cell mass, and sulfur 0.3-0.5% so these are needed too - this is a nice way of getting all three in there). Add the ammonium phosphate at a rate of 25-50 gramms for a 25L wash.
The second most common limiting factor is a lack of oxygen, but it only needs it until high cell numbers are present (eg during the first day) (so make sure that you've aerated the wash well just prior to adding the yeast, but don't do this too much later in the game) "Splash filling" is enough to do the job.
Bacteria can double in number every 20-30 minutes, but yeast takes 3 hours (so guess which one will win the race if an infection gets started and you don't deal to it). Another technique to help with this is to use a lot of yeast - when using Bakers yeast, use at least 150g for a 20L wash. Note that using more yeast wont make the yeast work through to a higher % alcohol, but just enable it to get where its going, faster.
Theres a fair bit of choice available as to which yeast to use. I'm personally inclined to use the "Turbo" yeasts, which are pre-packaged with all the nutrients etc necessary. Thats because I'm only ever doing sugar-water washes for pure neutral spirits, and I find it easy, convienient, and reliable. I don't try and reuse it a second time, as I only distill every couple of months, and can't be bothered storing it for that long. If however you are doing more of a grain or fruit based mash, and interested in flavours, then consider some of the other yeasts.
How do you know when fermentation has finished ? Alex tells ..
1. There is no more bubbles coming to the surface.
2. There is no more hissing noise inside the vessel.
3. Gravity of the mash sinks equal or below 1.00
4. The mash does not tast sweet anymore.
5. It has been sitting in the bathroom for three weeks.
I understand that the much popular ... "turbo" yeast products are no more than specially packaged Saccharomyces strains that include the bare necessities (in nutritional terms) that yeast will need to barely ferment just one sucrose based batch. That's why you guys find the notion of re-pitching your yeast so alien. I believe turbos are a very good thing for the yeast industry and truly they deserved a break. But I find they could try to strike a more consumer wise equilibrium on pricing (IMO they're obscenely expensive). However there's a notion that I believe would make this group improve exponentially their distilled products (and that I haven't read about in any post so far) and it's that whatever you can do to enhance your wash's quality as a fermented product brings by itself a better spirit. I'm no fanatic on this. I don't drink my molasses wines, for instance (though my whiskey's beers are just as good as the product I sell commercially, sans the hops, of course). It's just little things you need to do to avoid the basic problems, like always boiling and quickly cooling the wash, aerating the cooled wash prior to inoculation, keeping the fermentation temp below 23 deg. centigrade, and the original sugar concentration below 17-19° Brix (1.070-1.079 s.g.), and of course, work sanitarily. That's all.