The fermentation process
The main factors that affect the fermentation rate and influence beer quality are:
- The amount of yeast used to inoculate or pitch the fermentation.
- Yeast cell viability and yeast quality.
- The level of dissolved oxygen in wort at pitching.
- Wort soluble nitrogen concentration.
- Wort fermentable carbohydrate concentration.
- British ales are traditionally produced in relatively shallow open circular or rectangular vessels, constructed of wood or stone.
- They use top-fermenting yeasts that rise to the surface and can be skimmed off.
- Traditional European lager fermentations use similar, albeit deeper, open vessels where the yeast ultimately collects at the bottom.
- Nowadays most modern brewery fermenters are closed cylindroconical vessels constructed of stainless steel, with a capacity of up to 200000L.
- They normally have cooling jackets and are used to produce both ales and lagers.
Advantages of cylindroconical vessels include:
- Improved fermentation rates.
- Greater flexibility, as they can be used to produce a
range of beers.
- Relatively low construction costs, requiring relatively little civil engineering work and occupying less land area.
- large fermentation capacity, giving benefits of economies of scale.
- lower running costs.
- Easy CO2 collection from the top of the fermenter and yeast removal from the conical base, where yeast collects at the end of the fermentation.
- Beer losses are reduced to a minimum.
- Efficient temperature control.
- Consistent product quality.
- Easy to clean using modern cleaning-in-place (CIP) systems.
- The same vessels may also be used for beer maturation following yeast removal.
- The dimensions of these vessels aid rapid fermentation rates, due to greater mixing as the CO2 bubbles rise through the deep fermentation.
- Fermentation rates are primarily controlled by adjusting the temperature.
- However, care must be taken as higher temperatures may cause flavour defects.
- Ale fermentations in these vessels take 2–3 days at 12–24°C, whereas in a traditional vessel they last for 4–7 days.
- Lager fermentations are usually performed at lower temperatures of 3–14°C. They last for 5–7 days in these cylindroconical vessels, compared with 8–10 days for a traditional fermentation vessel.
- Beer fermentations can be monitored by measurement of CO2 evolution, ethanol production, the amount of heat generated or decline in wort specific gravity.
- The specific gravity of worts for ales is normally around 1.040 and falls to about 1.008 by the end of fermentation.
- For lagers the initial gravity is often higher, at 1.050.
- Wort pH starts at around 5.2 and falls to about pH 4.0 in finished beer, whereas the number of yeast cells in suspension rises through the fermentation period and falls towards the end, as the yeasts flocculate.