This section describes how to ferment the 30L of corn mash produced in the section on mashing.


• 30L corn mash from the Mashing section

150g (1/3 lb) bakers' yeast cake Equipment

• 2 30+L primary fermenters with at least one lid

1 30+L primary fermenter with lid, and an aquarium pump with an aeration stone

• 1 standard winemaking hydrometer and cylinder

1 Refractometer

• 1 floating dairy thermometer graduated from 0oC-110oC (32°F-230°F) Method

Initially, the mash will be in the pot on the stove with the lid on. The mash should have been left to settle for at least 90 minutes, possibly as long as overnight, and cooled to below 38oC (100oF). The mash solids will have settled out, and there should be 7 or 8 cm (3") of clear yellow liquid on top of the mash.

Measure the OG of the clear yellow liquid by using a refractometer, or by using a hydrometer. If using the hydrometer, measure the temperature of the sample before measuring the SG and refer to the temperature-correction table at Appendix B to correct for the temperature, or chill the sample to 15.5oC (60oF) before measuring the SG.

The OG should be in the range 1.060-1.070. If it exceeds 1.070, thoroughly stir the mash until it is mixed. Take a hydrometer reading of the mixed mash. This can be done by simply floating the hydrometer in the mash and reading the SG. This avoids having to collect a sample in the cylinder. Also, no temperature correction is necessary for this measurement. This value will be markedly higher than the OG measured in the clear sample, and will not be an accurate SG measurement of the mash. Now, small additions of tap water, say 250 ml at a time, can be made to the mash, and mixed in thoroughly until the hydrometer reading has decreased by the amount the OG exceeded 1.070. The OG can now be updated by subtracting the amount it was reduced by the water additions.

For example, say the OG measured in the clear sample was 1.075. This would mean that the SG of the mash would need to come down .005. After mixing the mash thoroughly, the straight uncorrected hydrometer reading may be, say 1.080. Now, 250-ml additions of water can be thoroughly stirred into the mash, and straight uncorrected readings taken until the reading has come down .005 points from the 1.080 (i.e. to 1.075). The OG gravity can be updated to OG - .005 = new OG (i.e. 1.075 - .005 = 1.070).

The mash produced in the chapter on Mashing is not likely to require such a correction if the quantities were adhered to.

Next, the mash needs to be transferred to a clean 30+L fermenter. The temperature should have cooled to below 38oC (100oF) before it is transferred. For the 30L batch, the mash can be left to cool until the temperature is below 38oC (100oF). For larger batch sizes, it will be necessary to employ an immersion chiller to force cool the mash.

Once the mash has cooled to below 38oC (100oF) it can be transferred. This can be done by pouring the mash from the mash pot to a clean 30+L fermenter. If the full mash pot is too heavy or too awkward to pick up and pour, the mash can be ladled with a one or two litre measuring cup into the fermenter until the volume is down to a manageable level.

For larger batch sizes, a grain pump can be used to make all transfers. Grain pumps are like sewage pumps in that they are capable of pumping liquids that are full of suspended solids. Some brewing operations use sewage pumps (new ones, never used for sewage) for making such transfers. Also, sewage pumps are sometimes used for Recirculating Infusion Mash Systems (RIMS) that constantly circulate a mash during the mashing cycle. This maintains a uniform temperature and provides the stirring function for large-scale mashing.

The mash is now ready for aerating (i.e. providing dissolved oxygen). For the 30L batch, you can vigorously pour the mash from one fermenter to another four to six times. This works extremely well. You can also use an aquarium pump to pump air through an aeration stone placed in the mash. The aeration stone should be left to bubble in the mash for about 30 minutes. The yeast can be added while the aeration stone is still bubbling in the mash.

On the larger scale, bubbling through an aeration stone can be used, perhaps using a larger pump and larger stone. Also, mechanically rousing the mash in a manner that causes splashing for 20 or 30 minutes will work well.

If the mashing apparatus is equipped with a RIMS, as discussed above, the RIMS can be set up so that the output splashes into the mash. Fifteen to 30 minutes of such splashing would thoroughly aerate the mash.

After the mash has been aerated, the yeast can be pitched (i.e. added to the mash). Place the fermenter where it's going to sit undisturbed for the next three days, and make sure the ambient temperature is between 21 and 33oC (70 and 90oF).

Making sure the present mash temperature is below 38oC (100oF), add the yeast. For the 30L batch, add 60g (1/3 cup) of active dried bakers' yeast or 150g (1/3 lb) of bakers' yeast cake, and stir. After 30 minutes the yeast will have hydrated in the liquid and can be thoroughly stirred in.

Within 60 minutes of pitching the yeast, vigorous fermentation will be evident (i.e. vigorous bubbling). The bubbling will continually rouse the mash throughout the fermentation, ensuring the mash stays thoroughly mixed. Leave the mash for 72 hours from when the yeast was pitched. It's helpful to stir the mash thoroughly every 24 hours, but not necessary.

After 72 hours the fermentation will either be complete and the activity will have diminished to a slow spurious bubbling, or the fermentation will still be active and only have slowed down slightly. If the latter is the case, monitor it every six hours or so until the fermentation is very slow and therefore, finished. Fermentation shouldn't take more than 84 hours.

When the fermentation is complete, it's important that the mash be strained, placed in the still, and heated to above 52oC (125oF) within 24 hours or off flavours (i.e. dreaded esters) may develop.

When the fermentation is complete the mash is ready to be strained. For the 30L of corn mash, it can be strained by hand using a straining bag as described above. Larger volumes will require a mechanical pressing using a device such as a pneumatic winepress, also described above.

After straining, the volume of liquid retrieved will be about 70% of the entire-mash volume. If the mash were mechanically pressed, the volume of liquid retrieved is closer to 80% of the entire-mash volume. In the case of the 30L of corn mash strained by hand, between 20 and 22L of liquid will be retrieved.

After the mash is strained and has settled for 30 to 60 minutes, a fairly clear sample of the mash liquid can be collected from the surface, and the Terminating Gravity (TG) can be measured.

With the OG and the TG, the alcohol percentage can be calculated using the formula:

After the completion of this fermentation step, you will have about 20L of fermented and strained corn mash at about 8.5% alc/vol, and you will be ready to proceed to the distillation step.

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