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The Fermentation Process of Beer and How It Affects the Beer
Fermentation is but one step in the overall process of brewing beer. Fermentation is the process of deriving energy from the oxidation of organic compounds.  In the case of beer a yeast converts sugars into alcohol through this process. This sugar does not occur naturally but comes from starches found in the grains, like barley, used in the malting process that are broken down by enzymes. This malt becomes the wort to which the yeast is added. After the yeast is pitched the fermentation process begins.
The fermentation of alcohol is the most time consuming step in brewing.  It can take up to two weeks to complete but there are methods available to reduce this time.  Fermentation is where the majority of the chemical reactions take place. The yeast undergoes several physiological changes. There is a build-up of unsaturated fatty acids and sterols at the start of fermentation, which are vital nutrients for the yeast.  The yeast consume these nutrients and deplete the amount of sugar as the fermentation progresses. During the fermentation processs, not only ethanol bu also a variety of complex flavour-active metabolites.  Fermentation is consdiered complete when the supply of sugar is almost completely converted to ethanol.
A variety of facots change how the fermentation process is performed. This includes factors from both before an after the fermentation itself begins. This paper will look at the mechanism of yeast, and how it converts the sugars into ethanol. It will also delve slightly in-depth into other factors such as the type of malt used, the aeration of the wort, how temperature and pressure will affect the process of brewing, the pitching rate, and others.
The Mechanism of Fermentation
Sugar is converted into ethanol after a process known as glycolysis first occurs.  Glycolysis consists of nine steps. In step 1 and 3 ATP is converted to ADP, and a phosphorous group is attached to glucose. At step 5 NAD
is converted into NADH+ H
. During step 6 and 9 ADP is converted into higher energy ATP. This process then works to split a single glucose into two pyruvate molecules. Glycolysis stops here and fermentation picks up with an extra steap. The pyruvate is then converted into acetaldehyde and carbon dioxide. The pyruvic acid is also converted into various other chemicals, like lactic acid.  Then acetaldehyde is further oxidized to form the final product, ethanol.
Figure 1. a) Glucose b) Pyruvate c) Acetaldehyde d) Ethanol
The sugar starts on the left as glucose and is eventually oxidized to become ethanol.
Alcohol fermentation usually occurs under aerobic conditions. Most yeast strains will further oxidize ethanol eventually leading to nothing except carbon dioxide and water if oxygen is present (aerobic conditions).  Saccharomyces cerevisiae, also known as Brewers' yeast, is a top-fermenting yeast used in some beer fermentations, this strain of yeast prefers aerobic conditions when proper nutrients are given.  The oxygen present helps to prouce unsaturated fatty acids and sterol which the yeast requires to reproduce. 
How the Malt Affects Fermentation
The malt is the source of the starch, which eventually becomes the sugar required in fermentation. The quality of the grain, and how the malting process is performed directly influence the fermentation process.  Various kinds of grain may be used, ranging from rice to wheat to barley to corn, although barley is typically used for beer fermentations. A higher quality of grain will allow for greater flavour development during fermentation. A low qualiy grain will give a tasteless sugary beer.  In general it is seen that the stronger the wort is, the less yeast is required during fermentation. 
The nitrogen content of the grain used in the malt is of importance. This is because the nitrogen content is a direct reflection of the amount of protein in the grain.  Brewing generally favours lower nitrogen content grains. Along with the fermentation process the malt helps to give the beer its final colour and appearance.
Strains of Yeast Used
The strain of yeast used affects the type of beer that is produced. The most commonly used strains are S. cerevisiae for brewing ales and S. pasorianus for brewing lager beers.  While these strains are generally used for either ale or lager their function can be changed. S. cerevisiae and S. pasorianus are only general strain names for the yeast. Many sub-species exist that are tailored to the brewer's individual need.
The difference between the two strains is how they ferment in the cask an the temperature at which the fermenting occurs. Ale strains of yeast ferment at the top of the vessel and are reffered to as "Top Fermenting", while lager yeast ferments at the bottom and are in turn known as "Bottom Fermenting".  Top fermenting yeasts will contain more esters than a bottom fermenting beer. The bottom fermenting beer's tase depends heavily on the eact strain used and the temperature at which fermentation took place.
How Oxygen Affects Fermentation
As yeast prefers aerobic conditions the addition of additional oxygen would cause changes in the fermentation process. Oxygen affects the amount of unsaturated fatty acids present in the fermentation. An increase of C16:1 and C18:1 fatty acids was found with increased levels of oxygen.  The viability of the yeast remained constant throughout and the oxygen did not cause the yeast to die out. However there was decreased production of esters, the flavour compounds. 
The Pitching Rate
The pitching rate is the rate at how fast the yeast is added (pitched) to the wort. Yeast physiology and activity before pitching and throughout fermentation is esssential for consistent fermentations with high quality beer as a product. Results have shown that increasing the pitching rate decreases the fermentation time required.  A pitching rate of double the normal rate produced a time reduction of 60%.  In fermentations with increased pitching rates the yeast the yeast population remains at a viable level.  Higher alcohols increase in concentration with a higher pitching rate as do esters, which shall be mentioned later.  The most imporant thing from this is that the beer maintains an acceptable level of quality with enhanced pitching rates.
Variations in Temperature and Pressure
The optimization of temperature in important in the control of the production of flavour-active compouns like esters and higher alcohols.  An increase in temperature affects various parameters in fermentaiton. An increase in temperature leads to a shorter fermentation time.  In lager beers it also sees an increase in higher alcohols. Ethyl hexanoate was produced at an increased rate during higher temperature fermentations, however acetate esters decreased in amount.  For ale strains, only phenyl ethyl acetate production increased. Most aroma-active compounds are volatile, and so an increase in temperature will eventually evaporate some of them, effectively removing them from the beer. 
While increasing pressure would be out of the ordinary, an increased pressure should give results similar to that of an increased temperature. This is because of the Ideal Gas Law and that temperature and pressure are directly proportional. However this is simply speculation and would require further testing to prove true.
Volatile compounds consist of higher alcohols and esters. These are different from the aromatic compounds which come from the malt and hops and are formed as byproducts of the yeast metabolizing  These directly affect many of the flavours in the beer. Increased concentrations of isoamyl alcohol cause the beer to become heavier. Isobutyl alcohol reduces the quality of the beer as a whole.  Increased concentrations of esters such as ethyl acetate, which has a fruity solvent-like taste and isoamyl acetate, which has a banana flavour, is also increased.  As mentioned under Variations in Temperature and Presssure, volaile compounds depend on the temperature. The production of which are suppressed under low fermentation temperatures. Volatile compound concentration can be estimated by measuring the volume of carbon dioxide gas produced during fermentation because the two are directly proportional. 
Most ester compounds in beer are formed during the fermentation stage, as previously mentioned. Esters give the beer it's flavour, and being able to control their formation is important.  Changing the temperature and pitching rate as mentioned above had a positive effect on ester formation for the most part. Esters give off various flavours. Isoamyl acetate, one ester produced during fermentation, has a banana odor to it.  Other flavours range from apple to pineapple, to even a hint of clover. Combining different yeast strains, changing the quantity of starting sugars, temperature or changing the pitching rate have the effect of producing new esters with different characteristics.
Four categories of yeast immobilization exist. All of them are based on the physical mechanism of cell localization.  There are both benefits and disadvantages to using immobilized yeast in the fermentation process. Immobilizes cells are protects against certain stresses and also more reistant to ethanol. Compared to mobile yeast cells there is an increase in the storage of glycogen and trehalose as welll as structural polysaccharides.  The downside to using immobilized yeast is that special fermentation tanks must be used to accomodate this feature. The use of immobilization techniques also affects the production of higher alcohols and flavour-active compounds.  It affects the production negatively, using immobilization techniques tends to decrease the higher alcohol production.
Secondary fermentations occur after the first fermentation has neared completion. A secondary fermentation is a continuaion of the original except in a new clean, clean, vessel. The beer is removed from the original fermenter and placed in a new one to remove any dead yeasts. The removal of the dead yeasts helps to prevent the formation of acetylaldehydes.  Secondary fermentation is often used as a form of conditioning, as well.  This conditioning enhances the flavour of the beer and gives it a better appearance.
Discussions and Conclusions
The process of brewing has existed for nearly as long as man has. Ancient races produced crude beers by following a process similar to what is still done in this age. Ancient beers were simple and contained naught but water, a simple malt such as honey (mead), and wild yeast strains responsible for the fermentation.
Since then the methods of brewing beer have become far more sophisticated. Modern beers are far more complex in creation. Various strains of yeast have been engineered to be more resistant to oxygen, to be able to survive with less nutrients, and to reproduce at a faster rate. We've learned techniques to bring out the full flavour of all the ingredients present in the beer. We've even learned how to manipulate those flavours and create beers with textures and smells and flavours that would have once been impossible. Modern science has allowed us to analyze all of these factors and come to the conclusions that we have arrived at.
All variables discussed here affect the fermentation process in various ways and all lead to a different outcome and in some cases a very differentt beer in the end. Adding oxygen and immobilizing the yeast hinders the growth of higher alcohols. While increasing the pitching rate or increasing the temperature promotes the growth of higher alcohols. Some inhibit ester growth for flavour and some promote it. This leads to great variety and freedom when fermenting and brewing beer. Not one of these changes are more beneficial than another in terms of the quality of the beer. In the end it becomes a personal choice for the brewmaster to decide how the beer is created.
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