Chapter 12


Monitoring Fermentation

Wort is the liquid extracted from the mashing process during beer brewing. This dissolved sugar also includes other ingredients and is referred to as “extract.” To monitor the fermentation process, brewers typically measure density, specific gravity, or Plato over time, as it quantifies the concentration of the wort. Because the sugars in the wort are converted into alcohol during fermentation, the sugar concentration gradually decreases over time—this gives brewers a way to estimate the final alcoholic content of their beers, but also gives them more control and greater knowledge of the brewing process and ensuring a consistent quality of their final product.


Various scales exist for measuring density, the most popular of which are degrees Brix, degrees Plato (°P), and specific gravity (SG). Winemakers tend to use degrees Brix (°Bx) and professional brewers generally use degrees Plato or specific gravity.

Degrees Plato

Plato is a measurement of the concentration of dissolved solids in a brewery wort. Degrees Plato (°P) is used to quantify the concentration of extract (mainly sugars derived from malt but also including other soluble material in wort) as a percentage by weight.

Specific Gravity

Specific Gravity (SG) is a measure of the relative density of one material compared to another. For brewing, the reference material is water, so the specific gravity of wort or beer is simply the density of the liquid divided by the density of water.

Sampling Wort Gravity

Sampling of fermenting wort can be done manually or automatically using sensors. In open fermenters, samples of fermenting wort may be obtained with a dropping can or similar implement. Top-opening manway doors in deep vessels allow access to perform the same operation; however, it is more satisfactory to provide a hygienic, cleanable sample point.

Sampling from Fermenters

Before off-line analyses can be made it is necessary to remove samples from the fermenter. Closed fermenting vessels are fitted with sample valves to facilitate off-line analyses. It is essential that these are of good hygienic design and are properly maintained. Some stratification is likely towards the end of fermentation when mixing due to natural convection currents is limited.

Measuring Gravity

Brewers nowadays typically use hydrometers or digital density meters to monitor the fermentation process.

Hydrometer. Measurement of density is commonly made using a glass hydrometer (“saccharometer”) calibrated in one of the units described earlier. A hydrometer consists essentially of a weighted, sealed, long-necked glass bulb that contains a paper scale inside the stem, so that is calibrated to read density, specific gravity, degrees Plato, degrees Brix, or some other related measurement.

Density Meters. Given drawbacks in using a hydrometer, the brewing industry is increasingly adopting the use of density meters to provide the brewmaster with highly accurate measurements that will help to achieve greater product consistency. The working principle of a density meter is based on an oscillating U-tube made of glass.

Final Beer pH

Finished beer should have a lower pH than pre fermentation wort. The pH of the final beer will vary with the beer type but all malt lager beers are usually in the pH range of 4.25 to 4.6, with adjunct beers sometimes being as low as pH 4 (lower adjunct buffering capacity).

Relationships between Gravity and Extract

Original Extract (OE). Original extract is the mass in grams of sugars in 100 grams of wort, prior to fermentation, as measured on the °Plato, °Brix, or dissolved solids scale.

Original Gravity (OG). Original gravity is directly related to OE, with the exception that it is a measure of specific gravity at a given reference temperature prior to fermentation.

Apparent Extract (AE). Apparent extract or residual extract is related to final gravity, and represents that portion of the OE present as residual sugars which were not converted to yeast biomass, ethanol, or carbon dioxide during fermentation.

Final Gravity (FG). Final gravity or terminal gravity is a measure of specific gravity, at a given reference temperature, at the conclusion of fermentation and is directly related to AE.

Apparent Attenuation (AA). Apparent attenuation (i.e., apparent extract) is usually given as a percentage to describe the percent of malt sugar that is converted by the yeast strain to ethanol and carbon dioxide.

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