Gravity, Plato, and Brix
Measuring sugar content is an important factor in brewing. Knowing your numbers will help keep the beer balanced, as well as know if the beer has finished fermentation and what the alcohol content of your brew is. Three common measures of sugar content are gravity, brix, and plato.
Gravity is commonly used by homebrewers, and is a measure of density. A reading of 1.000 is the density of distilled water at 60°F. Sugar and other dissolved solids increase the density of the solution. Typically, beer will have a gravity between ~1.040 to 1.090 before fermentation, and end between 1.010 to 1.020 after fermentation. Often, dry ciders, wines, and meads will have a final gravity less than 1.000. This is because alcohol is less dense than water, and with no residual sugars in solution, the finished product has a gravity below that of distilled water.
There are three terms to know for gravity, each referring to the specific time the gravity is taken. Original gravity is the gravity taken after the wort is cooled and in the fermenter, just before the yeast is added. Final gravity is the gravity taken after the fermentation has finished. Specific gravity is the gravity taken at any specific time. Homebrewers are mostly concerned with the original and final gravities; if you know both these numbers, you can easily calculate your alcohol. The basic formula is as follows:
ABV = (OG-FG)*131.25
Here, OG is the original gravity, FG is the final gravity, and ABV is alcohol by volume. Keep in mind this will yield a close approximate, not an exact value, for the alcohol.
Plato is a measure of the weight of malt sugars in solution expressed as a percentage. It is mostly used by commercial brewers, as it is more accurate for their purposes. Roughly speaking, 1° Plato is equivalent to a gravity of 1.004. This figure becomes less accurate, however, at amounts over 13° Plato.
Brix and Balling
Brix is very similar to Plato; it refers to the percentage of sugar in an aqueous solution. It is mostly used by winemakers, and (similarly to Plato), 1 brix is roughly equal to a gravity of 1.004. Balling and brix are closely related, with the only difference between the two being the calibration temperature. Brix is calibrated to 68°F, while balling is calibrated to 59°F. The difference here is very minor.
Homebrewers use the gravity scale mostly as a function of tradition. All homebrew recipes use gravity, and homebrewers express their numbers in terms of gravity.
There are two tools used to measure gravity, each suited to a specific operation. Hydrometers can be used for both pre-fermentation readings and post-fermentation readings. Refractometers should only be used for pre-fermentation readings, and are mostly used by all-grain brewers.
Hydrometers are the most common way to measure gravity. A hydrometer is floated in the wort or finished beer. The meniscus (top, clear layer of the liquid) crosses the scale at a specific point, and the number on the scale is written down. The top of the scale reads 1.000, and the scale runs down the hydrometer with increasing gravity, thus the higher the gravity, the more of the hydrometer will stick out of the liquid when floated. Because reading the scale requires getting fairly close to the hydrometer, most brewers prefer to take a sample of the wort or beer and fill a hydrometer jar. This is a long, skinny cylinder that allows the reading to be taken in a relatively small amount of sample.
Hydrometers can be used at any time, both pre- and post-fermentation, making them a staple in the homebrewers brew-house. They are inexpensive, but typically made from thin glass with weights on the bottom. Be careful with your hydrometer, as they are prone to breakage if dropped even small distances. It is likely to break over time, so it’s usually best to keep a back-up on hand.
Refractometers are a secondary tool that can measure both brix and gravity. They require only a small amount of liquid for a reading and are much sturdier than hydrometers. They work using properties of refraction, and as such are specifically calibrated for sugar-in-water solutions. Thus, they are great for pre-fermentation readings, but are become extremely inaccurate for post-fermentation readings, as alcohol will throw off the scale. Correction calculators exist on the internet; however they will typically have some degree of inaccuracy.
To use, simply squeeze a couple drops into the end piece, and slide the cap on top so liquid is displaced evenly through the end piece. Hold up to the light and look through the eye piece. Inside you will see a scale with gravity and brix. Blue light will cover the top part of the scale, and clear light below. The junction between blue and clear light will be the reading.
Knowing your gravity is important. It will help you replicate your process and have predictable results. If your original gravity is off, it can usually be corrected easily enough. Dilute the wort with extra water if your OG is too high, or use a little extra extract if your OG is too low.
For all grain brewing, knowing your post-mash gravity is useful for making sure your process is going as intended. If the gravity ends up too low, a small flaw in the process may have occurred. This will help with fixing the problem or knowing what to expect on future brews. All-grain brewers tend to check gravity after the mash (called pre-boil gravity), as well as after the boil (called post-boil gravity).
Finally, checking gravity is also the best way to judge if fermentation has completed. After fermentation has begun, yeast will continue to ferment all the sugars they can. Towards the end, carbon dioxide production will slow, and the airlock will bubble more slowly. After fermentation has completed, the airlock may continue to bubble due to pressure or temperature changes forcing dissolved CO2 out of solution. Beginning brewers may confuse this for fermentation continuing.
The best method for judging fermentation completion is to check the gravity when it seems fermentation has finished. Wait a few days, then re-check the gravity. If the two measures are the same, fermentation has completed. If the second reading is lower than the first, fermentation is still occurring.