There are two forms of active yeast the instant, dry, powdered type and the active, moist variety which comes in blocks. Either one sort or the other will be obtainable from the baking section of your local supermarket or perhaps from a delicatessen and it makes little difference which you use. The powdered yeast is about three times as active, pound for pound, as the moist yeast in block form, so work out which of the two sorts is the best buy. If there isn't a great deal of difference in price choose the dry type because of its much longer shelf life but do check the "use-by" date to ensure that it is fresh. Dry yeast which has been in storage for several months without refrigeration and without being vacuum-packed could be useless. Of all the enquiries received from readers the most prevalent concern a failed fermentation, or one which refuses to go to completion. In most cases the cause of the problem has been traced to the yeast having lost its activity due to poor storage, and this is really self-evident because we only have yeast and sugar and there can be nothing wrong with the sugar.
To ferment 10 kg of sugar use 450 grams (1 lb) of the moist yeast in block form or 150 grams of the dry, powdered variety. In the first case, to prepare it for use you will need to make it into a cream. Use a stainless steel bowl and two wooden spoons. Break the block into walnut size pieces and let them stand for about 15 minutes in a small amount of water before attempting to cream them. The chunks of yeast will swell in the water and be far less sticky as a result. Work at it gently until a lump-free cream is produced and then pour the cream into the sugar solution. The dry powdered yeast can simply be sprinkled slowly on to the top of the sugar solution where it will disperse and sink.
With this amount of yeast and the time being allowed for fermentation (5+ days) there is no need to add nutrients. Also, do not be seduced by claims that special yeasts will produce 15% or more alcohol solutions because this does not mean that you get more alcohol, only that you can use less water and spend more money. It's the same old confusion about concentration and amount. The amount of alcohol you get is determined by the amount of sugar you have used and all the yeast does is convert this sugar to alcohol. It might reduce the fermentation time from 5 days to 3 days but it is scientifically impossible for a yeast, any yeast, to produce more alcohol than allowed by the equation at the start of this chapter.
When the temperature in the fermenter has reached 30 deg. to 35 deg. C. adjust the thermostat or light dimmer control to hold it in this range. For the next five days or so the only attention required is a periodic check of temperature.
The completion of fermentation can be judged in several ways. One is the absence of foam on the surface of the solution; this foaming may be quite vigorous at first but diminishes steadily with time until eventually the fermentation ceases and the beer looks dark and still. To confirm that it is complete, switch off the pump and look at the hydrometer. The original sugar solution will have had a specific gravity of about 1.06 and the hydrometer will be bumping up against the underside of the glass cover, but as the sugar is converted to alcohol the hydrometer will sink and the S.G. fall to about 0.99, below 1.00 because of the presence of alcohol with a S.G. of 0.8. With a little experience you will know exactly when to expect the fermentation to be complete (e.g. 5 days) and can make a closer examination at that time.
When fermentation is complete, switch off the pump and heater and remove them for washing. Reach down into the beer and remove the rubber stopper, substituting a short (perhaps 1/z-inch) length of 11/z-inch copper tubing in the drain-hole. This will act as a dam and help to hold back some (but not all) of the yeast when you transfer the beer to the still.
Allow the beer to stand for several hours or preferably overnight in order to give the yeast a chance to settle to the bottom of the fermenter. At the end of this settling period, connect a hose between the drain valve under the fermenter and the inlet at the base of the beer-stripper. With a 25 litre boiler for the still the beer must be stripped in three batches of about 16 litres each, so make a dipstick marked at three equal heights and use it to gauge when each 16 litre batch of beer has flowed into the still.
Note: Some yeast will inevitably get into the beer-stripper. It will do no harm, but be alert to the possibility that it may accumulate in the bottom of the boiler over a period of months and start to clog the drain valve. Back washing with water after each run is therefore quite important.
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