In brewing and fermenting we fuss about gravity. Standing around a fermenter, number dropping about gravity is equivalent to standing around an office water cooler gossiping about management changes. So our weird culture established, what are we talking about? We all know that gravity keeps our feet on the ground, so how is this phenomena linked to brewing gravity? It really isn’t, other than both concepts coining their word “gravity” from the same Latin root “gravitatem” meaning weight, heaviness, pressure. In alcohol manufacturing, we talk about specific gravity which is literally measuring the “heaviness” of our batch in comparison to water. So why do we care about how heavy we are anyhow? Is specific gravity more accurate and guess-a-gravity an alternative? We also go on about final gravity. Is final gravity when you hit the ground dead?

Appreciate our terms can be confusing. Let me break it down. In brewing we measure what we can to check things are going to plan. One thing we take note of is the dissolved sugars in our batch. This indicates to us the possible final alcohol we will achieve and the reduction of it indicates how well our yeast are performing. We measure sugar relating to fermentation in various units: Balling (°Bg), Plato (°P), Brix (°Bx), Oechsle (°Oe), Klosterneuburger Mostwaage (°KMW), Normalizovaný Moštomer (°NM), Baumé (Bé°) and Specific Gravity (SG). All of these have different scales and different regional and industry niches.

What we are measuring in all of them is relative density (mass of a unit of volume) of a substance as a ratio to a reference material. In this case, the density of our sugar water compared to water. Specific gravity is the unit to measure this density ratio. So how did we end up with so many contenders? Well early 1800’s Karl Balling then Adolf Brix then Fritz Plato all decided to take the specific gravity scale and prepare tables of specific gravity of a known unit of sucrose. Balling did it to 3 decimal places, Brix to 5, and Plato to 6. A classic: one out doing the other, out doing the other. Their idea was all the same, people would take specific gravity readings and then use “their” table to look up how much sucrose was in their solution. Thing is in fermentable liquids such as wort and wine’s equivalent must, there are a lot of particles in there other than just sugars and water, and sucrose isn’t even the most common sugar in there. As already stated, maltose is for wort and glucose and fructose is for must.

Here’s a table showing the breakdown of sugars in 6 samples of barley only wort. Sucrose only ranges from 3-13%. A more useful beer sugar measuring index would have been to base it on maltose rather than sucrose, regardless of how many decimal places you can calculate.

Source: Researchgate.net - Content of the total fermentable sugars present in the wort

The soluble sugars of grape must contain about 95% glucose and fructose, with the remaining 5% being sucrose with a touch of pentose. In the wine world scales were created with the similar idea of being based on the specific gravity. In wine’s case, that’s the specific gravity of the sugar of must while also factoring in other components in the must, compared to water. Must is the equivalent of beer’s wort. Ferdinand Oechsle’s scale is widely used in the German, Swiss and Luxembourgish wine-making industries. The Klosterneuburger Mostwaage (°KMW) is used in Austria. The Normalizovaný Moštomer (°NM) is used in Czech Republic and Slovakia. The Baumé scale is occasionally used in France and by USA brewers and is a little different. This scale wasn’t made exclusively for wine and as such was based on a known amount of sodium chloride dissolved into water.

Therefore, all of these scales, whether specifically created for wort, or must, or general use, only give a rough estimate of the fermentability of a liquid. For this reason, I go back and use the standard specific gravity, and rather than being concerned with estimating the sugars in the wort, my focus is placed on the total change in wort density. Going with specific gravity then, important brewing measurements become:

Specific Gravity (SG) – the measuring unit for the density of the wort as a ratio to water
Original Gravity (OG) – the specific gravity reading of the wort taken just before fermentation
Final Gravity (FG) or Terminal Gravity – the specific gravity reading of the wort taken at the end of fermentation

Our main focus is creation of alcohol so having an indication of the amount of sugar fermented by our yeast is paramount. We get this from subtracting our final reading from the value of our original reading: Original Gravity less Final Gravity (OG – FG). All the reading taken (and often daily) between these two figures is just so we can reassure ourselves the yeast is happy and doing its thing.

Gravity: Let's get Specific and Final about it