Fermentation is the process when the yeast eats the sugar and multiplys which gives the by-products of alcohol and carbon dioxide. The technical reaction is C6H12O6 => 2(CH3CH2OH) + 2(CO2). This is what happens in the fermenting bucket. The more fermentable sugars in the brew the more alcohol is produced. The carbon dioxide is released from the fermenter through the air lock. When fermentation is finished the yeast will drop to the bottom of the fermenter and bubbles will stop coming through the air lock. It is important that the beer is not bottled until the primary fermentation is finished. If the beer is bottled too early too much carbon dioxide will form in the bottle and it could then explode. This can be checked using the hydrometer, the readings required are usually shown in the kit instructions. When the beer is bottled, the bottles are primed with a small amount of sugar which starts the fermentation process again. If too much sugar is used to prime the bottle, this can cause “bottle bombs”. During this secondary fermentation process the carbon dioxide cannot escape and is forced into the beer causing carbonation which makes the beer fizzy, easy eh?
There are several factors that have an effect on the fermentation process and the end result. these are:-
1. The type of yeast being used.
2. The temperature of the beer.
3. The amount of sugars used.
Types of yeast.
There are several types of yeast that have different characteristics, so have a different effect on your finished brew. Brewing kits come with a yeast packet which can be just sprinkled on the wort in the fermenter just before the lid is put on. This is the same for a wine kit so you don’t have to worry about getting the right yeast. When you progress to making your own beer most recipes specify which type of yeast to use. You can get various types of ale yeast and lager yeast. It all depends on the type of beer you are brewing. I have found lager in the fermenter can sometimes smell “eggy” but don’t worry if this happens as this is normal, and don’t be tempted to dump the batch of lager. Lager yeast produces more sulphur which causes the smell. One the lager is bottled and left to “lager” (condition) in bottles for a few weeks, this smell will disappear.
The temperature control of the brew is an important factor in the quality and taste of the finished beer or wine. The first important temperature is when the yeast is added or pitched into the wort. When you top up the brew with water you must check the temperature is approx 21°c-27°c, and you can use cold/warm water or even ice to achieve this temperature. Once the yeast is pitched and the top is on the fermenter it should be placed somewhere in the house out of the way that has a constant temperature of 21°c-27°c. The most common reason of a bad batch of homebrew is poor temperature control during this period. I am lucky that my “brewing” cupboard under the stairs is a constant 22°c all year and during the night. A variation of temperature during fermentation can cause off flavours in the brew. There are products you can buy to aid temperature control such as heater mats or heater belt.
Lager yeast ferments at a lower temperature, with some as low as 9°c. I have made lager from kits and have kept the temperature around 19°c (as per instructions in kit), by sitting the fermenter in a large cold water bucket with a wet towel wrapped around it. The lower temperature will make the fermentation time longer.
Sugar and alcohol.
The amount of sugars put into the brew will affect the amount of alcohol produced up to a point. I sometimes make brews with a higher ABV (alcohol by volume), by adding slightly more sugar or malt extract. I prefer to add dry malt extract, as this also adds body to the finished beer. If too much white dextrose is used and the alcohol content is high you will lose the head retention and the beer will appear flat.
Some kits have 2 tins of extract and do not require extra sugar apart from priming the bottles to carbonate the brew. If you use a 1 tin kit then you will have to add sugar. I usually use 500grams of dextrose and 500grams of dry malt extract. This method has worked well for me so i would suggest you use it.
The method for calculating your alcohol content, is to measure the specific gravity (density) of the wort before fermentation, called the original gravity (OG) with a hydrometer and measure again after fermentation called the finishing gravity (FG). The hydrometer floats in the liquid and measurements can be taken off the side. The more sugar in the wort, the more dense the liquid, the higher the reading. After fermentation the liquid will have a lower density as alcohol is not as dense as water, so a lower reading should be evident.
There is a calculation using these measurements which will tell you the alcohol content (ABV) of the finished beer:-
(OG)-(FG)÷7.46= approx % of alcohol by volume (ABV). You should add 0.5% onto this figure to take into account the extra sugar used for priming the bottles.
EG. (1042)-(1006)=36 ÷7.46=4.82+o.5=5.3% ABV.