The body of a beer defines how it flows over the surfaces of your tongue and palate. A light beer will flow similar to water, while a dark and heavy beer will flow more like syrup.
The Original Gravity (OG) of a beer is the specific gravity of the wort after the Brew House process, and before the Cellaring process (simply just before fermentation). It is used to determine how much sugar is available for fermentation. Darker or stronger beers will have a higher OG, while lighter or weaker beers will have a lower OG.
The Brew House process starts as water, which has a 1.000 specific gravity. During the Brew House process, water dissolves sugars and proteins by mashing malted cereal grains. When the Brew House process ends, the wort has an OG of roughly 1.040 to 1.130 (depending on style & recipe). Malt sugar is the main contributor to original gravity. Grain protein is the next highest.
The Final Gravity (FG) of a beer is the specific gravity of the beer after fermentation in the Cellaring process. It can be used to determine to what degree fermentation was successful (how much alcohol was created). During the fermentation step, yeast convert the dense sugars into ethanol and carbon dioxide. The CO2 is released, and ethanol has a specific gravity of 0.789, so as fermentation proceeds, the specific gravity of the wort decreases. Most beer styles have a final gravity around 1.010.
Darker or stronger beers usually have a higher FG. A beer with a higher FG will have more residual sugar, have a heavier body, and flow more like syrup. Lighter or weaker beers usually have a lower FG. A beer with a lower FG will have less residual sugar, have a lighter body, and flow more like water.
A beer might have increased residual sugars for a couple reasons. First, yeast cannot process complex sugars. Dark malts produce more complex sugars than light malts. Complex sugars can be intentionally created in the mashing step of the Brew House process, by mashing at the higher end of the temperature range. Second, some yeast strains have lower attenuation. They will process less sugar during fermentation, before going dormant, than a yeast strain with higher attenuation.
So, how does a brewer create more or less alcohol in beer? Increase or decrease the malted grains, respectively.
Specific Gravity is a measurement of density of a solution/material relative to water. Density equals mass/volume and is dependent on the ambient temperature and pressure of the surrounding environment. A lower ambient temperature will increase the density of a gas/liquid/solid. A higher ambient pressure will increase the density of a gas/liquid/solid.
The way beer feels in your mouth also depends on the dissolved gases added during conditioning in the Cellaring process. Carbon Dioxide is the most common gas added to beer, while Nitrogen is the second most common.
Carbon Dioxide adds a prickly feel to beer, especially in beers with high volumes of carbonation.
Nitrogen adds a smooth feel to beer,
A couple silly beers actually have Helium dissolved, a low density gas that elevates your voice a couple octaves.
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