Taking Control of Specific Gravity
Brewing beer consistently and to style depends on controlling many variables. Specific gravity is a variable in almost every part of the brewing process, from selecting grain to bottling. Specific gravity influences many aspects of a beer’s flavor, from alcohol content to sweetness. Controlling it is a challenging but essential task.
Defining Specific Gravity
Specific gravity refers to the density of the wort or beer compared with the density of water. Density is simply mass divided by volume. The number it yields indicates the mass (the term scientists use, which is more technically correct than “weight”) per unit of volume — pounds per gallon, for example. Density varies with temperature because liquids expand as they are heated, meaning they occupy a greater volume at a higher temperature, and contract as they are cooled.
Most homebrewers use a hydrometer to measure specific gravity. A hydrometer is simply a sealed glass device that consists of a graduated stem attached to a lead-weighted bulb. When the hydrometer is immersed it displaces the liquid depending on the density. When you read the gravity, read the bottom of the curve, called a meniscus, that the liquid forms where it meets the glass.
Because the hydrometer is calibrated relative to the density of water, 1.000 equals the specific gravity of water. The four-digit reading for wort or beer can be expressed as “points” or even a percentage over water’s mass. For example, a measurement of 1.020 is commonly stated as “ten twenty” or 20 points of gravity. It also means that the solution is 2 percent heavier than an equal volume of water. Likewise, 1.040 is 4 percent heavier.
Because temperature affects density, hydrometers are calibrated to be accurate at a specific temperature. The reference temperature is 68° F or 20° C. A correction is necessary if you measure it at a different temperature.
Another way to quantify liquids in brewing is to measure as a percentage the mass of sugar the liquid contains. This is what the Balling, Brix, and Plato scales do. The Balling scale was created in 1843. The Plato scale, used by many professional brewers, was developed in 1900. It was considered an improvement on Balling, which uses a different reference temperature. However, the two differ only slightly and are accepted as equal for most purposes. Brix is a scale used in the wine, sugar, and soft-drink industries and is also equal to the other two.
These scales are constructed by weighing a reference liquid against liquids of equal volume but containing varying additions of sucrose. Degrees Plato (or Brix or Balling) measures the percentage of the total weight of the liquid accounted for by sugar. In a wort of 12° Plato, sugar makes up 12 percent of the total weight. Thus, the wort is said to consist of 12 percent sugar.
Degrees Plato can be measured using a hydrometer. It can also be roughly figured from specific gravity. Simply divide the specific gravity reading by four to get degrees Plato. So a reading of 1.020 equals 5° Plato. This conversion is only an estimate. It is most accurate at lower gravities and becomes quite inaccurate above 1.060.
Sugar content can also be measured with a saccharometer, which is a hydrometer graduated for measuring extract by weight, or with a refractometer, which measures the sugar by light refraction.
Extract
The purpose of mashing is to convert starches in the malt to sugars that can be used by yeast to create alcohol. Extract refers to the sugars yielded from the malt during mashing. Malt extract syrup and dry malt extract are the extracts produced by mashing, then condensed or dried. Extract is an important issue in controlling specific gravity.
For all-grain brewers, specification sheets for malts often give an extract value for the grain. Presented in extract points per pound per gallon, this can be used as a reference when formulating a recipe (see Typical Common Malt Specifications, page 47).
Malt Selection
The two main varieties of malt for brewing are two-row and six-row barley. These can be made from many different varieties, such as Harrington, Manley, Excel, and others. In the past two decades dramatic increases in malting technology and efficiency have led to superior malt products worldwide. Malt today is highly modified, with protein levels of 8 percent to 12 percent for two-row and 10 percent to 13 percent for six-row. Both also have high diastatic powers — they have the enzymes necessary to break down starches into sugars.
This is vital for the brewer to understand when choosing malts and formulating recipes for maximum gravity extraction and best system efficiency. Choose malt with protein as low as possible and the diastatic power as high as possible, keeping in mind that the higherthe protein value, the higher the diastatic power. This is because higher protein malts simply have more enzymes available.
Diastatic power is a measure of the strength of protein- and starch-reducing enzymes and theirpotential in mashing. It is measured in the United States in degrees Lintner (L°), not to be confused with Lovibond (°L). Very highly modified two-row barley might have diastatic power in the 40s or 50s. North American varieties and some German six-row pilsner malts might be in the 150s. Again, accompanied by a proportional amount of protein.
Mashing for Maximum Gravity
Preparing the grist (grain for the batch) for mashing is the first part of the brewing process at which the brewer has control over the extract of the malt and the subsequent gravity of the wort. The crush and the quality of crushed material in the mash are very important. Keep in mind that crushing the grain is just that, not grinding or pulverizing the grain.
The idea is to expose the starch-rich endosperm (food reserve of barley kernel) to allow conversion by the enzymes. A grind that is too coarse will not allow for adequate conversion or good color and flavor extraction. Conversely, grinding the grain too fine will create an excess of flour, which will result in a stuck mash and poor lautering efficiency. Both scenarios, in turn, lead to inferior worts in the boil kettle.
Dilution
Mash dilution is an important variable in gravity control because it directly affects the enzyme activity and, in turn, extract and run-off efficiency. Many articles describe mash thickness in nebulous terms such as “porridge like” or “medium thick,” but these terms have little meaning if they are not quantified. Dilution is measured as a ratio of water to grist in the mash tun. The most common dilution is a “medium thick” mash of 1.25:1, that is 1.25 quarts per pound of grist. The term medium thick is generally accepted to describe a mash ratio of 1.25:1 to 1.75:1.
A medium-thick mash is generally optimum for several reasons. A thick mash, say 1:1, keeps the enzymes very stable, even at temperatures higher than normal. But the downfall is that thick mashes often do not allow the water to adequately encompass the crushed grain, creating dry spots and poor extraction. This is partially due to the fact that the water cannot penetrate to hydrolyze (break down) the starch. Furthermore, thick mashes are difficult to lauter and run off.
A thin mash, from 2:1 to 2.5:1, on the other hand, produces great extract and run-off but is very unstable. The enzymes are spread out in the extra water, making them very vulnerable to temperature changes. This means that you have to be very accurate. The main concern is the enzyme beta-amylase, which is reduced above 149° F. Without a sufficient minimum of beta activity, the wort simply will not be as high in gravity.
A medium-thick mash, then, seems to be a good compromise as well as a safe bet for enzyme stability. A medium-thick mash of 1.25:1 or 1.5:1 is also just about perfect for the thickness of a good mash filter bed during lautering, which is essential for quality wort.
Mashing Techniques
The easiest and coincidentally most common type of mash is the single-infusion mash. This means basically that all of the water and grain go into the mash tun in one step at one temperature. It will generally steep in the saccharification temperature range of 149° to 156° F and at a pH of 5.2 to 5.4.
There are several advantages to this type of mash schedule. First, it saves time. Second, it can be done in a mash tun that needs no heating capabilities, making it easy for homebrewers to build and keeping the equipment cost low. Third, it is uncomplicated and only requires one vessel.
The process is really quite efficient and quick. Mashing at 149° F for 60 minutes is usually sufficient to break down all of the complex carbohydrates to sugars. An iodine test will tell for sure. A short recirculation before heating to mash-out (168° F) or sparging helps to make the run-off more uniform and establish the grain bed for best filtration. This invariably improves your extract efficiency.
While there are good reasons to choose decoction mashing or temperature program (step) mashing — a desire to use traditional methods or for flavor implications, for example — gravity shouldn’t be an issue. If low gravity is the problem with your single-infusion mash, make sure you’re hitting your mash temperatures and mash times accurately and that the grain is properly ground.
Boil
A typical wort is expected to have a 10 percent to 15 percent reduction in volume due to evaporation during a standard 90-minute boil. The brewer must compensate for the evaporation to control the original gravity of the wort after the boil (see Calculating Evaporation, page 46). The most elementary method is to run extra wort volume into the kettle during lautering or, in the case of extract brewing, provide extra water to be boiled off.
Gravity has a slight effect on hop utilization. As gravity increases, hop utilization — the amount of hop bitterness transferred into the wort — decreases. As a general rule, utilization begins to decrease in wort with a gravity of 1.050 (12.5° Plato) or more.
Yeast and Fermentation
Fermentation is the stage in which gravity is reduced through the natural action of the yeast on the sugars in the wort. As yeast turns sugars into alcohol, the specific gravity decreases. Yeast selection is a key to hitting the desired final gravity.
Attenuation is the percentage reduction in the wort’s specific gravity during fermentation. The sugars are metabolized by the yeast into ethyl alcohol and carbon dioxide. The ability to attenuate varies from strain to strain, making it an important factor in recipe design. For example say you are making a Scottish ale. You want it to have a sweet taste characteristic of the style. So the final gravity should be fairly high, meaning more sugar is left in the beer to create sweetness. You need a yeast strain with a low attenuation percentage.
A beer with a high original gravity needs to have a yeast with enough attenuative power to finish the fermentation. Otherwise, too much sugar will be left in the beer and the final gravity will be too high for the desired characteristics. Yeast companies provide the attenuation percentages for most of their yeast strains.
Gravity is also affected by the overall health of your fermentation. Pitch plenty of yeast that is viable (contains few dead cells). To keep your yeast viable, make sure it has been handled properly, the package isn’t too old and, if you repitch yeast, it hasn’t been used too many times. Also, be sure the yeast you pitch gets plenty of oxygen at the time you pitch it.
Temperature can be used to control gravity during fermentation. If the temperature is cold enough, the yeast will cease to be active and fermentation will stall. Brewers sometimes crash-cool beer to stop fermentation at a specific point, usually a fairly high final gravity.
Homebrewers can transfer the carboy to a refrigerator or chest freezer or pack ice around the carboy. The gravity will continue to decline until the yeast becomes inactive. This temperature is different for each yeast strain, but most ale yeasts halt completely around 40° F. Lagers halt closer to 33° F.
Following the proper procedures and keeping track of the numbers can help you control gravity to make the best beer possible.
