The fermentation stage of brewing plays a pivotal role in the outcome of the whole process. In order to obtain a successful outcome, which is defined as the conversion of wort into green beer with a desired and consistent composition, it is necessary to manage the growth and metabolism of yeast. This requires that the initial temperature, wort oxygen concentration, pressure adjustments, nutrient and enzyme additions, and pitching healthy yeast at viable pitching rates with precision and repeatability. In theory, therefore, providing good control of the initial conditions at the start of fermentation and efficient attemperation thereafter, the process should proceed in a predictable manner.
Temperature directly affects yeast performance. Temperature should be precisely controlled for each stage of fermentation. The optimal temperature depends upon the yeast strain, the beer style, and the characteristics of the fermentation. In general, if fermentation is too cool, yeast performance will be slow with the potential for premature flocculation.
Temperature Control Solutions
Fermenting vessels must be designed to facilitate the measurement and control of temperature. In unsophisticated traditional vessels, temperature measurement may be performed off-line. Conversely, modern vessels are provided with wall-mounted thermometers. These are usually of a type which provide an output that can be used to provide automatic attemperation. In large capacity cylindroconical vessels the location and number of temperature-transmitting probes is of crucial importance to the maintenance of proper attemperation.
Typically, the yeast is pitched between 15 and 22 degrees C (59 and 72°F). Generally, yeast ale strains produce more esters and higher alcohols than lager strains. Beer specific gravity or density should be checked and recorded daily to monitor fermentation progress.
In lager fermentation systems, the yeast is pitched at higher temperatures between 7 and 8 degrees C (45 and 46°F), and after a couple of days the temperature is increased to 10 to 11 degrees C (50–52°F) and allowed to ferment for 5 to 14 days at 9 degrees C (48°F) (Kunze, 1996). After 3 to 4 days, at peak fermentation, the fermentation temperature is allowed to ramp up in order to facilitate a rapid reduction in diacetyl at 11 degrees C (52°F) for two to three days.
Wort fermentation in beer production is largely anaerobic, but when the yeast is first pitched into wort, some oxygen must be made available to the yeast (Section 11.4). Injection of air or oxygen into wort during vessel fill is typically done in-line during fill.
The rate of fermentation also can be controlled by the application of pressure. This is generated by restricting the dissipation of evolved carbon dioxide. Pressure fermentation is the process of fermenting beer inside a closed and pressurized vessel. Typical fermentations allow carbon dioxide to escape the fermenter through an airlock or blow-off tube. In pressurized fermentation, the fermenter is sealed and the carbon dioxide produced by the fermentation is trapped inside the vessel.
Nutrient and Enzyme Additions
Although wort composition is largely determined by the time it is cooled and pumped into the fermenting vessel, some additions may be made during fermentation. The most common nutrient addition is zinc, usually added as a solution of the hydrated sulphate and at a concentration of 0.05 to 0.15 ppm zinc (Zn2+). This metal ion, which may be limiting in malt worts, is an essential component of several yeast enzymes, notably alcohol dehydrogenase (Boulton et al., 2006).
All-malt worts typically contain all the minerals, vitamins, and nitrogen necessary for a successful standard fermentation; their availability decreases, however, in high-gravity fermentations in which high percentages of adjuncts are used, i.e., maltose syrup. This reduces the ability of yeast to grow and may result in stuck fermentations, slow fermentation rate, decreased biomass formation, and changes in the spectrum of flavor compounds (Section 12.7).
In craft brewing, it’s important to understand that the fermentation process, driven by the natural life cycle of yeast, is truly at the heart of the beer making process. In the brewhouse, a brewer has many tools to manipulate and construct wort as well as those available during fermentation as already discussed, but once sent to the cellar the yeast take over and do all the heavy lifting of making beer.
Yeast Pitching Rates
The pitching rate is defined as the concentration of yeast suspended in wort at the start of fermentation. An often-quoted pitching rate is 1 million cells per milliliter of wort per degree Plato. So, a 12-degree Plato wort would be pitched to a level of 12 million cells/mL. Correct yeast pitching is especially important in higher gravity brewing (>16°P) to avoid slow or stuck fermentations (Casey et al., 1983).
Methods in Pitching Yeast
Pitching yeast is most commonly stored in the form of a slurry in which the cells are suspended in beer derived from the previous fermentation (Section 12.9). The pitching rate of a subsequent fermentation is controlled by the metered addition of a known weight or volume of slurry, either directly added to the fermenter (batch), mixed with the wort in a starter tank prior to transfer to the fermenter, or continuously metered in-line into the cold, aerated wort stream on the way to the fermenter.
Determining the Quantity of Yeast for Pitching
There are two approaches in determining the amount of yeast to pitch. The first method, determine the proportion of suspended solids, expressed either as percent weight to weight or weight to volume. The second method of yeast slurry analysis is to determine the cell count per unit mass or volume of slurry.
Pitching Based Volume or Weight. A weight-based pitching is preferable to volume-based since volume varies with temperature, carbonation, liquidity of the slurry, and degree of yeast flocculation (Mallet, 1992). In order to have consistent pitching rates, the yeast concentration in the yeast brink must be known and then metered accurately. Usually, the brewer has established a relationship, based on fermentation performance, between the yeast cell concentration and total solids of the cropped yeast in the yeast brink.
Pitching Based on Cell Number. This method requires the brewer to determine the yeast cell concentration and the viability. This method has the benefit unlike the other methods to correct for the presence of non-yeast solids. Furthermore, it allows pitching rates to be controlled directly in terms of yeast count. Two methods are commonly used cell counting: (1) electronic particle counter, which gives a rapid and automatic measure of the suspended yeast cell count and (2) direct microscopic enumeration using a counting chamber such as the hemocytometer.
Topping Up (Drauflassen)
If the brewer doesn’t have enough yeast or if the brewer has a smaller mash tun and kettle than the fermenter, the brewer can employ a technique called “topping up” or drauflassen. Topping up, a technique common among German lager brewers, refers to putting one batch of wort “on top of” another.
Stratification. The practice of topping up a fermentation may lead to a condition called stratification. Two layers of liquid, the low fermenting beer and the freshly added wort, do not mix; they form distinct separated zones, each with its own temperature and composition.
Uninoculated wort is liable to contamination by many types of beer spoilage organisms—bacteria such as Pediococcus, spp. and Lactobacillus spp.; and wild yeasts such as Hansenula, Dekkera, Brettanomyces, Candida, and Pichia. Other Saccharomyces species may be present.
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