Beer Gravity

This is the density of wort, or unfermented beer, a measure of dissolved substances, mostly sugar that will be turned into alcohol, plus some miscellaneous unfermentable carbohydrates, proteins, and other material. It is a general indicator of the strength of the soon-to-be beer. There are a variety of beer scales from which to select, the most common international brewing units used to express this density include: Specific Gravity, Total Dissolved Solids (sugars), Brix, °Plato, Brewer's Points, Balling, and ABV.

BEER TAX & MEASUREMENT

IN THE US AND CANADA, BEER IS PRIMARILY TAXED BASED ON THE VOLUME REMOVED FROM THE BREWERY.

In Europe, however, taxation is country dependent and is either based on the °Plato per hectoliters (strength of original extract by volume) or, in some cases, the alcohol content per hectoliter.

For most tax purposes, it is often assumed that 1 °Plato ≈ 0.4 percent alcohol by volume. However, in the UK, beer tax is calculated by multiplying hectoliters by the declared alcohol by volume as determined through distillation.

Primarily, measurement is about quality control. For instance, even though beer in the US is taxed based on its volume, it is still industry best practice for the beer chemist or brewer to keep accurate records of beer measurements.

BEER MEASUREMENT TERMS


For those new to brewing, there are some common brewing terms that are important to understand with respect to beer measurement. These terms are covered only briefly here.

WORT

This is the liquid extracted from the mashing process during beer brewing. Wort contains the sugars to be converted to alcohol during fermentation.

SPECIFIC GRAVITY (D20/20)

This is the relative density of a fluid at a reference temperature, relative to the density of water at a reference temperature. It can be measured using either a hydrometer or refractometer with an appropriate scale, providing fermentation has not yet started.

ORIGINAL EXTRACT (OE)

OE 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. It can be measured directly on certain refractometers and hydrometers or converted from Original Gravity.

ORIGINAL GRAVITY (OG)

OG is directly related to OE, with the exception that it is a measure of specific gravity at a given reference temperature prior to fermentation. It can be measured directly on certain refractometers and hydrometers or converted from OE.

 

APPARENT EXTRACT (AE)

AE 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 CO2 during fermentation. It can be expressed interchangeably as °Plato, Brix, or dissolved solids and is usually determined through calculations based on OE, or specific gravity as read on a hydrometer.

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. Specific gravity readings can signal the end of fermentation when they stop moving. FG can be measured directly on a hydrometer or calculated based on OE and AE.

APPARENT ATTENUATION (AA)

AA is a measure of the amount of sugar consumed by yeast during fermentation and the extent to which the yeast has been able to successfully convert fermentable sugars into ethanol and CO2. It is calculated as the drop in extract during the fermentation divided by the OE.

DISSOLVED SOLIDS SCALE

THE DISSOLVED SOLIDS SCALE WAS CREATED SPECIFICALLY FOR BEER; IT IS NOT JUST A REPURPOSED SUCROSE SCALE, NOR IS IT BASED STRICTLY ON MALTOSE ALONE.

The scale is instead scientifically derived from a complex beer model with a sugar profile that is specific to wort. The sugar profile accounts for maltose, maltotriose, dextrose (D-glucose), fructose, and sucrose, with the remainder consisting of larger saccharides, and other material which is non-fermentable by standard brewer’s yeast (in general, saccharides larger than DP3).

The Dissolved Solids Scale represents the true mathematical relationship between actual wort dissolved solids and refractive index. This dissolved solids content is expressed in grams of sugars per 100 grams of wort and is equivalent to percent weight per weight. For most calculation purposes the measurement can be used in place of Brix or °Plato.

Measuring wort on the percent dissolved solids scale will give you a more accurate indication of true total sugar content of wort, rather than trying to estimate it using a Brix refractometer. Also contributing to more accurate readings is the fact that the dissolved solids scale can be temperature compensated specifically for wort, compared with sucrose compensation in a Brix refractometer.

BRIX SCALE (°BX)

THE BRIX SCALE IS PROBABLY THE MOST RECOGNIZABLE SCALE FOR MOST BREWERS IN THE US AND CANADA. IT IS SIMPLY THE PERCENT WEIGHT OF DISSOLVED SOLIDS (SUCROSE) IN A SUCROSE/WATER SOLUTION.

It is used in beer brewing because the Brix units are a convenient unit of measure found on many refractometers and hydrometers. Within the accuracy of most handheld refractometers, Brix is synonymous with °Plato. However, the Brix scale is used more accurately in the wine industry, since grape must is composed primarily of sucrose, whereas beer is composed primarily of maltose with very little sucrose.

The difference between the sucrose and maltose content is often estimated and used as the basis of a correction factor in many popular beer calculators. Although the Brix scale is based on the sucrose/refractive index relationship it is temperature compensated for wort, just like the other Pro-Brewing scales.

°PLATO SCALE

DEGREES PLATO REPRESENTS A UNIT OF MEASURE DATING BACK TO BREWERS FROM THE LATE 19TH CENTURY.

The oldest scale, Balling, was developed in 1843 by Bohemian scientist Karl Balling as well as Simon Ack. Adolf Brix corrected some of the calculation errors in the Balling scale and introduced the Brix table. In the early 1900s German Fritz Plato, and his collaborators, made further improvements introducing the Plato scale.

The intention of the scale was to convert specific gravity, as measured with a hydrometer, into the weight percent of sucrose, as a measure of total fermentable material. So a wort measured at 12° Plato has the same density as a water−sucrose solution containing 12% sucrose by weight, denoted as 12% Brix. For the brewer, it has an advantage over specific gravity in that it expresses the measurement in terms of the amount of fermentable materials.

The Plato scale is historically more popular in central European brewing and is very nearly equivalent to the Brix scale which is used predominantly in the US.

SPECIFIC GRAVITY SCALES

SPECIFIC GRAVITY IS THE RELATIVE DENSITY OF A FLUID AT A REFERENCE TEMPERATURE, RELATIVE TO THE DENSITY OF WATER AT A REFERENCE TEMPERATURE. SPECIFIC GRAVITY IS OFTEN MEASURED WITH A HYDROMETER.

In order to obtain an accurate reading from a hydrometer, the solution measured must be at the reference temperature of that hydrometer or the solution temperature needs to be measured and a correction applied to the reading.

Specific gravity readings are typically denoted in the form “D t1/t2,” where “D” represents relative density, t1 is the reference temperature of the solution, and t2 is the reference temperature of water. One problem faced by the brewer is that most brewing hydrometers are calibrated for a reference temperature of 60/60 °F and most reference data and calculations are based on a reference temperature of 20/20 °C (68°F). Therefore, their are two specific gravity scales, Specific Gravity (D 20/20°C) and Specific Gravity (D 60/60°F).

One more note about the specific gravity scales. During fermentation, yeast converts wort sugars into carbon dioxide and ethanol. As fermentation progresses, sugar content de-clines, while ethanol content increases. Since ethanol is less dense than water, and less dense than the sugar it is replacing, the specific gravity decreases with time, until it stops declining, signaling the end of fermentation.

BREWER’S POINTS

BREWER’S POINTS ARE A TYPE OF SHORTHAND USED BY BREWERS AND ARE THE EQUIVALENT OF ((SPECIFIC GRAVITY -1) X1000).

For example, wort with a specific gravity of 1.045 would be said to have 45 points. Using the old brewer’s rule-of-thumb, it is easy to estimate Brix of °Plato by dividing points by four.

For example, a brewer would estimate that a 45 point wort would have an estimated °Plato or Brix value of 11.25. Alternatively, multiplying Brix, °Plato, or dissolved solids by four would give you a rough estimate of points.

BALLING UNITS

IN 1843, KARL JOSEPH NAPOLEON BALLING CONSTRUCTED A METHOD AND A SERIES OF TABLES RELATING SPECIFIC GRAVITY TO THE PERCENT BY WEIGHT OF SUCROSE IN A SUCROSE WATER SOLUTION.

Balling, noticing that the specific gravity of a wort increased with the weight percent of dissolved materials in almost the same manner as if the dissolved materials were entirely sucrose, conducted all of his experiments with pure sucrose and water, as they were a good approximation to beer worts. After making up sugar solutions, Balling developed his table relating the density of the solution (at 17.5 °C) to the weight percent of sugar. He measured the weight percent in degrees Balling (°B), defined as the number of grams of sugar per 100 grams of wort.

Several years later, in the early 20th century, Dr. Fritz Plato improved upon the relationship and published his own set of tables, calling the corrected unit a degree Plato (°P). Balling units are seldom used anymore and for all practical purposes are the same as Brix and °Plato.

HYDROMETER

The hydrometer is based on Archimedes principle that a solid suspended in a liquid will be buoyed up by a force equal to the weight of the liquid that it displaces. Therefore, the lower the specific gravity of the substance, the lower the hydrometer will sink.

Hydrometers are usually constructed of glass or plastic. They often have a cylindrical stem with a measurement scale sealed inside and a carefully weighted bulb to make it float upright. In use, the hydrometer is gently lowered into the liquid until it floats freely. The reading is taken at the point where the surface of the liquid (the meniscus) crosses the scale on the stem of the hydrometer.

One important point to note is that whatever the unit of measure, the hydrometer scale is only valid at one particular temperature. Hence, they must be used in conjunction with a thermometer, and any variation from the reference temperature must be noted and compensated for.

REFRACTOMETER

A refractometer is an optical instrument designed to measure the concentration or mixture ratio of water soluble fluids. It measures refractive index, the speed at which light passes through a liquid. The denser the liquid the slower the light will travel through it, and the higher its reading will be on the refractometer.

Like hydrometers, many different scales are available that convert refractive index into a unit of measure that is more meaningful, i.e., Brix, specific gravity, °Plato, etc.

Although refractometers are also dependent on temperature, certain refractometers are made with automatic temperature compensation; in fact, you shouldn’t rely on results from a refractometer without temperature compensation. Brewers are likely to encounter two main types of refractometers; Traditional Analog and Digital Handheld.

FORMULAS

Plato to SG
(Plato/(258.6 – ((Plato/258.2) * 227.1 ))+1= SG

SG to Plato
((SG)² – (205.347)) + (668.7183)(SG) – 463.371=°Plato

Brix to SG
(Brix/(258.6 – ((Brix/258.2) * 227.1))) +1= SG

SG to Brix
(((182.4601 * SG-775.6821) * SG+1262.7794) *SG – 669.5622)=Brix

Information displayed by permission of Chronicle Books, and MISCO Beer Technical Bulletin, MISCO.com/beer. Copyright.