Professional brewers use an hydrometer to determine the state of fermentation. How?
The hydrometer measures the density or specific gravity of a liquid, which can be simply referred to as “gravity”. Water has a gravity at 20°C of 1.000. When sugars (such as malt extract and table sugar) are dissolved in water the gravity increases. For example when 1.7 kg of a home brew kit is added with 1 kg of sugar to water to a volume of 23 L, these solids increase the gravity of the water to about 1.038. To simplify things brewers remove the decimal point from these numbers and they become 1000 and 1038. Each unit is a “degree of gravity”. During fermentation the gravity drops from an Original Gravity (°G) of about 1038° to a Final Gravity (FG) or Present Gravity (PG) of about 1005°. This is because the sugars (denser than water) are converted into alcohol (less dense than water), carbon dioxide gas (CO2) that mostly bubbles out of the fermenter and yeast (which sinks to the bottom). At a PG of about 1005° the yeast has fermented all the fermentable sugars. The remaining solids a
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