Fermentation is the biochemical process where sugar (C6H12O6) is converted to alcohol (C2H5OH) and carbon dioxide (CO2). Yeast enzymes called zymase effect this process.
Yeast is a living organism and is in the mould family of plant cells. Yeast has two primary modes of metabolism: aerobic; and, anaerobic. "Aerobic" means in the presence of oxygen, and "anaerobic" means in the absence of oxygen. In it's aerobic state, yeast multiplies and increases its population within the fermentable substrate. In its anaerobic state, yeast stops multiplying and begins converting sugar to alcohol and carbon dioxide.
This transition from aerobic to anaerobic metabolism is a very natural process in fermentation. Anyone who has made homemade wine or beer observes this every time they add the yeast starter. First, the wine or beer goes through an eight to 20-hour lag time, and then the fermentation appears to start. During the lag time the yeast is consuming the dissolved oxygen in the substrate and is multiplying very rapidly. This is the aerobic phase. When the dissolved oxygen is completely consumed the, now abundant, yeast population begins producing alcohol and carbon dioxide. This is the anaerobic phase. The carbon dioxide is observed as the profuse bubbling that is characteristic of fermentation.
In order for yeast cells to be healthy and viable, the yeast needs certain nutrients, particularly, in the aerobic phase where cell multiplication is taking place. Among the many nutrients, nitrogen and amino acids are very important to yeast growth. Some fermentation methods, such as the fermentation of straight sugar and water, can avoid supplying nutrients by adding a very large charge of yeast. This avoids the need for an aerobic phase of cell multiplication because a sufficient population of yeast cells is present from the outset. However, the yeast has limits under these conditions. A straight sugar and water substrate with no yeast nutrients will rarely attain more than 8% alcohol before the yeast dies off.
In the case of fermenting grain mash or crushed grapes, adding yeast nutrients is definitely not required. Grain mashes and grape juice are bristling with nutrients, and ferment very fast and completely.
Grain mash, fresh out of the mash pot has a copious supply of fermentable sugars and yeast nutrients, but has very little, if any, dissolved oxygen for the aerobic phase of fermentation. This is a result of the long rests at high temperatures. You'll notice that boiled water tastes very different from water that hasn't been boiled. This is because the boiling drives off the dissolved oxygen.
On the small scale of 30L of corn mash, the mash can be thoroughly oxygenated by pouring it vigorously from one fermentation pail to another about four to six times. Also, vigorous stirring and rousing with a large spoon or paddle will work.
One fairly creative method of aeration is to use an aquarium pump to bubble air through an aeration stone immersed in the mash for about 30 minutes. This concept scales up very well to larger mashing operations. The idea being, to use larger pumps and aeration stones.
In the fermentation of wine and beer, the ferment undergoes primary and secondary fermentations. The primary fermentation is the vigorous fermentation that takes place over the first few days after the yeast is added. The secondary fermentation is the long slow fermentation that follows the primary fermentation. The primary fermentation only lasts a few days, but the secondary fermentation will slowly tick over for weeks, months in the case of wine fermentation.
A mash intended for distillation only undergoes a primary fermentation. Grain mash fermentations are typically 72-80 hours long, and then they are distilled. In fact, a secondary fermentation would be very deleterious to the ester profile of the mash and would ruin the finished whiskey.
During the primary fermentation the yeast is consuming readily available fermentable sugars. When the fermentable sugars have been exhausted, the yeast metabolism changes and begins breaking down unfermentable sugars and other organic compounds and consumes them. This involves the secretion of very different enzymes such as permease that enable the consumption of unfermentable sugars (dextrins and polysaccharides). This altered chemistry results in the formation of a family of esters, which have come to be called the "dreaded esters" by the author. The dreaded esters have very nearly the same boiling point as the alcohol/water azeotrope (i.e. 78.15oC (172.67°F)), and are almost impossible to separate out by distillation. Therefore, they pervade into the finished whiskey and ruin its flavour.
The only use for whiskey laced with the dreaded esters, is to rectify it to 95% alcohol by redistillation in a high-separation still, and treat it with activated carbon to render pure alcohol to be used for making vodka, gin, or liqueurs.
In order to be sure to avoid the dreaded esters, a mash fermentation should be distilled as soon as the vigorous primary fermentation slows down to a slow spurious bubbling, regardless of how complete the fermentation was, generally no more than 96 hours after adding the yeast.
Specific Gravity (SG)
SG is the measure of the density of a given liquid relative to water. The SG of pure water is, by definition, 1.000. If a liquid were exactly twice as dense as water it would have a SG of 2.000.
For the most part, SG is used in mashing to determine the amount of sugar dissolved in the mash. Dissolved sugar increases the density of the mash. Water that is 1% sugar has a SG of
1.004, water that is 2% sugar has a SG of 1.008, and so on. Also, SG is used to determine the progress and the end point of fermentation. As the fermentation converts the sugar to alcohol and carbon dioxide, the SG of the mash decreases. It often decreases below 1.000 because the presence of the alcohol, SG .8, and the absence of the sugar render the mash less dense than water, SG 1.000.
The originating specific gravity (OG) of a grain mash should be between 1.060 and 1.070. If the OG is very much higher than about 1.070, the alcohol content during fermentation will exceed 10%. As the alcohol content exceeds 10%, the yeast metabolism changes slightly, which can adversely affect the flavour profile. Also, the fermentation can drag out and risk the formation of the dreaded esters. It is possible, however, that there are specific yeast strains that surmount this problem. Perhaps, one of the closely guarded secrets held by the commercial whiskey distilleries.
The 30L of corn mash described in the chapter on Mashing is formulated to yield an OG of about 1.065. If it turns out to be too high, it should be diluted with water until it's within the range 1.060-1.070. If it's too low, there's no problem. It will work just as well, except the yield will be slightly lower.
There are two ways to measure SG. The best way is to use a refractometer. A refractometer is used by taking a few drops of the mash onto the slide of the refractometer, and looking through the eyepiece to observe the reading. Refractometers present their measurements on the Brix Balling scale. This scale is simply the percent sugar content of the sample. For example, a Brix of 16 means the sample is 16% sugar. The Brix scale maps to the SG scale by: Brix / 250 + 1 = SG. To go the other way: (SG-1) X 250 = Brix.
The main advantages to using a refractometer are: it only requires a very small sample; the sample does not require temperature correction; and, the measurement is not affected by the presence of mash solids suspended in the sample.
The other way to measure SG is by using an instrument called a hydrometer. A standard winemaking hydrometer, available at home winemaking supply shops, is excellently suited for measuring mash SG. A hydrometer cylinder, also available at home winemaking supply shops, is used to collect a sample and float the hydrometer to take the reading.
The hydrometer works by floating in a sample of a liquid in the hydrometer cylinder. If the liquid is relatively dense, the hydrometer will float higher in the liquid. If the liquid is relatively less dense, the hydrometer will float lower in the liquid. The hydrometer has a scale along its stem, and the observer reads the scale just below the meniscus (i.e. the surface tension) of the liquid level to determine the SG.
The density of liquids varies with temperature. Most hydrometers are calibrated at 15.5oC (60°F). This means that in order to get an accurate SG measurement, the sample must be at that temperature. Another way to get an accurate measurement is to measure the temperature of the sample, take the hydrometer reading, and then use the temperature-correction table at Appendix B to make the necessary temperature correction to the hydrometer reading.
For example, if a sample of mash were 33oC (90°F) and the hydrometer reading was 1.060, the temperature correction for 33oC (90oF) from the table at Appendix B is +.005. You would then add .005 to the hydrometer reading of 1.060 and get 1.065. This means the hydrometer reading of the sample at 15.5oC (60oF) would be 1.065, the correct SG of the sample.
To measure the Originating Gravity (OG) of a grain mash, it's important to collect a sample of the mash that's almost devoid of mash solids. When a mash has completed its conversion rest and has settled for a while, there's a clear light yellow liquid on top of the mash. A sample of this clear liquid can be carefully collected in a hydrometer cylinder and measured with a standard winemaking hydrometer.
Be sure to measure the temperature of this sample and use the temperature-correction table at Appendix B to correct the SG to 15.5oC (60oF), or you can chill the sample to 15.5oC (60oF) before taking the reading.
The terminating SG (TG) can easily be measured with a refractometer, but cannot be accurately measured with a hydrometer until the fermentation is completely finished. This is because it's almost impossible to collect a clear sample of the mash that's devoid of mash solids until then. However, you can get along without an accurate TG, since all you really need to know is the change in SG over each twelve hour period or so. When the SG shows a very small, if any, change since the last reading then you can conclude the fermentation is finished.
When the fermentation is finished and you have strained the mash (to be discussed later), the strained mash can be left to settle for 30 to 60 minutes before it's transferred to the still. A fairly clear sample can then be collected from the surface and the TG can be measured with a hydrometer with reasonable accuracy. Remember to make the necessary temperature correction as when taking the OG reading.
The alcohol content can then be calculated as follows:
(OG-TG) X 1000 / 7.4 = % alc/vol E.g. (1.065-1.002) X 1000 / 7.4 = 8.5% alc/vol
(OB-TB) X 4 / 7.4 = % alc/vol E.g. (17-1) X 4 / 7.4 = 8.65% alc/vol
Also, most winemaking hydrometers have a potential-alcohol scale on them. By simply looking up the SG reading on the hydrometer that corresponds to the difference of the OG and TG (i.e. OG-TG), you can rotate the hydrometer and read the alcohol content off the potential-alcohol scale.
After the fermentation (described below) is complete, usually 72-80 hours (never leave it more than 96 hours from when the yeast was added), the mash can then be strained and transferred to the still.
For the 30L of corn mash, it can be strained using a large nylon or cotton straining bag, available at home beer and wine making supply shops. A 20L pail or 30L fermenter can be fitted with pieces of fine rope, like binder twine, so as to cradle a colander or straining basket near the top of the pail to support the straining bag. The straining bag is then opened up and placed in the colander or straining basket. The mash can be poured into the straining bag until it's full of mash solids. The straining bag can then be twisted closed and squeezed by hand. When most of the liquid has been wrung out of the mash solids, they can be discarded and the process repeated until the entire mash has been strained.
Approximately, 60% of the entire mash liquid will run off in the first pour into the straining bag. After the first pour and straining, the receiving pail may have become quite full and need transferring to another container.
Of course, this manual straining cannot be done on a large scale. For large mashing operations a pneumatic grape press used for winemaking works excellently. There are other forms of these grape presses, some are hydraulic, others use an Archimedes' screw. All work equally well, remove almost all the liquid, and operate on large volumes of mash very fast.
After the mash is strained, the spent grains are excellent fodder for any composter. The yeast benefits the composting activity. On the large scale, the spent grains can be dried in the sun (to eliminate the residual alcohol) and sold or given to a livestock farmer as a form of highprotein high-fiber livestock feed. The spent grains are high in protein and fiber because the starches (originally 50% of the grain mass) have been almost completely removed by the mashing and fermentation, leaving behind only the protein and fibre.
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