Large Scale Brewing
Although there are thousands of different beers globally, almost all beer flavours produced commercially, are tied together by the same chain of chemical reactions.
The process of producing beer commercially involves the controlled fermentation of wort, a liquid comprised of sugars, sulphur and nitrogen compounds, and the residue of malted barley. Generally, fermentation is referred to the process by which glucose is converted to ethanol and carbon dioxide and can be shown chemically below. In reality this process is not quite this simple as a series of biochemical reactions are required for fermentation to occur.
C6H12O6 + 2PO43- + 2ADP → 2C2H5OH + 2CO2 + 2ATP
(Campbell, n.d.)
Until the late 1950s the only known method of fermenting beer was a slow batch fermentation process carried out in a single fermentation vessel. Large numbers of tanks were required to hold the fermenting batches of beer resulting in a gradual fermentation period of up to 3 weeks (Midwest Homebrewing & Winemaking Supplies, n.d.). This had a significant consequence on the quality of each beer as there was no guarantee that the beer would have a consistent flavour, something which is highly valued by brewers in these times of quality assurance.
Overview
The continuous fermentation process shown above (Figure 1) consists of three large vessels where the fermentation takes place. Most breweries use fermenting vessels that are constantly stirred and include inbuilt temperature and pH sensors which allow close supervision of the wort as it is fermented. After fermentation the fermented beer flows into a fourth smaller vessel to separate the yeast suspended in the beer known as the yeast separator. The suspended yeast is known as haze. As the beer is separated from the yeast it flows into the maturation vessel where brand diversity occurs. Once the desired flavour has been achieved the beer is then bottled and packaged.
The Process
Method
Figure 1 - Continuous Fermentation process
Hold up vessel
To allow the best possible environment for yeast growth, the wort is oxygenated in a similar way to the carbonation of soft drinks. The wort is rich is the nutrients required for yeast growth such as sugars, and so partially fermented beer is added to the yeast flow to reduce the nutrient levels and decrease stress on the yeast (Edinburgh, 2013). Although the yeast flowing into the holdup vessel is recycled, it is still in a fermentative state causing fermentation to begin as soon as it makes conatc with the wort. The partially fermented beer is also added to change the pH of the system and produce an environment optimal for glucose conversion. The natural pH of wort is 5.6 - 5.7. By lowering the pH to 5.1 - 5.2 the brewery can achieve fermentation in a shorter period of time (Freccia, 1997).
Additionally, by adding partially fermented beer, the brewer can alter the environment reducing the possibility of competing micro-organisms such as bacteria or wild yeast (yeast which differs from the yeast chosen by the brewery), minimising the chance of contamination. The mixture is generally held in the Hold up vessel for anywhere between 3 - 4 hours.
Continuous fermenter 1
This vessel is comprised of around 60% of the total volume of the system. The yeast/wort mixture in this vessel is constantly stirred to prevent inconsistency and build-up of yeast. A small amount of the partially fermented beer flows from this vessel back into the Hold up vessel to aid in the pH reduction. The mixture is held in this vessel for around 30 hours, varying from brewery to brewery.
Continuous fermenter 2
The second Continuous fermenter is usually around half the size of the previous vessel. While being much smaller this vessel is responsible for much of fine tuning of the fermented beer, including the amount of carbon dioxide and ethanol present. Beer is generally held in this vessel for any amount of time above 12 hours.
Yeast separator & Yeast washer
Unlike the first three vessels, this fourth vessel is not stirred. As the beer flows into the cone shaped tank the yeast settles at the bottom of the cone and is recycled back into the Hold up vessel. From there it is reintroduced to the wort and continues to ferment the available glucose. It is common for more yeast to be produced during fermentation (via binary fission, the division of bacteria into two smaller identical cells) than is required by the brewery (BBC, 2009). Any excess yeast flows into the Yeast washer where, in combination with distilled water, it is washed and sent on to be dried and sold as a by-product. The water used to wash the yeast can then be used to dilute the beer in the maturation vessel. This process can take up to 12 hours.
Maturation vessel
While it may seem the beer is ready for bottling and packaging, the Maturation vessel is often where brands acquire their distinctive tastes. Depending on the type of wort that is used by the brewer, different esters and alcohols can be produced and give the beer distinctive flavours (Figure 2, Figure 3). There are also many non-fermentable compounds in the wort that add to the flavour profile of a beer. The main purpose of this vessel is to remove the foul tasting by-product knowns as 2,3-butanedione, a ketone. This compound produces a toffee/butterscotch taste that, for most breweries, does not complement their desired flavour. This compound forms during fermentation. The Maturation vessel is brought to a lower temperature through a series of heat exchanges that, in combination with the lowered pH (as the wort is fermented), allow the yeast to absorb the ketone. In response, the yeast then produces a compound, 2,3-butandiol which, to the human mouth, is tasteless (Campbell, n.d.). The beer is commonly kept in this vessel for around 48 hours. Finally, the beer is filtered and packaged.
Figure 2 – Significant flavour-active esters found in beer
Figure 3 – Selected alcohols commonly found in beers