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If you’ve brewed your own beer or made your own cider you’ll have probably come across the term: specific gravity. It’s a tool that’s commonly used to determine how much alcohol you’ll get in your final drink. But what does it actually mean? And how do you determine it?
It starts with density
Before digging into specific gravity, we have to look at a basic property: density. The density of an object, e.g. a marshmallow or a peanut, is the weight of a specific volume of that material. The volume of a material is how much space that material occupies, it could for instance be a liter or a gallon. Mathematically it is expressed at:
ρ = m / V
ρ = density (we’re expressing it as kg/l)
m = weight/mass (we’re using kilograms)
V = volume (we’re using liters)
Objects with a very high density weigh a lot, even for a small volume of the material. The opposite is also true, an object with a very low density only weighs very little for that same volume.
A good reference: water
The most common ingredient in foods is water. Just about any food contains at least some water. Pure water has a density of about 1 kg/l. That means that 1 liter of water weighs 1 kilogram*. This is what makes weighing and measuring water so easy. You know that 1 liter weighs 1 kg, so volume and weight measurements are generally interchangeable.
Water is often used as a reference point for density values and as such, we’ll do so in this article as well. So let’s have a look at a few others and see how they compare to water.
*This statement isn’t 100% accurate, in reality, it deviates slightly, however, for most practical purposes in a kitchen it’s perfectly fine to use. Also, keep in mind that this works for kilograms and liters, as well as grams and milliliters. If you use ounces or pounds the one to one translation doesn’t work.
Very low density foods
All around you, you’re surrounded by air (unless you’re reading this while in space ;-). Air is a great example of a very low-density material. 1 liter of air weighs a lot less than 1 kg. As a matter of fact, that same volume of air weighs only 1/1.000th of what that same volume of water would weigh. This is a good thing, imagine having heavy air pushing down on us all day.
Foods that contain a lot of air tend to have a very low density as well. Marshmallows, foam on top of a beer, or a French meringue, but also breads. They all contain a lot of air and as such have a very low density. Even very large marshmallows or meringues aren’t that heavy.
Low density foods
Remember how oil floats on top of water? This is because oil has a lower density than water. A product with a lower density weighs less for the same amount of volume. As a result, it will slowly move on top, whereas the ‘heavier’ liquid sinks down.
The density of most oils in food is around 0,9 kg/l. This is a little less than the 1,0 kg/l for water.
High density foods
You can increase the density of water by dissolving other ingredients into the water. Dissolving the other ingredients will increase the weight of the water more than it does the volume (though the volume will change as well). When discussing specific gravity in more detail below we will focus on sugar solutions. Sugar solutions are known to be denser than pure water. The more sugar is dissolved in water, the denser it becomes.
Every other food that sinks when dropped in water will have a higher density than water. A thick dense brownie, a block of jaggery, a chocolate bar, cocoa powder (it’s why it sediments in chocolate milk).
Comparing densities = Specific gravity
It’s not always easy to measure the density of a substance. Instead, it can be easier to compare the density of a substance to a reference point. In the case of liquids (which we’ll focus on for the rest of this article) that reference is generally water.
The specific gravity (also called relative density) is the measure that’s used to compare the two densities. Mathematically it’s expressed as:
specific gravity = s.g. = density test liquid / density reference liquid
If the density of the test liquid is larger than that of the reference (e.g. a sugar solution), the specific gravity is > 1,0. If the density of the test liquid is smaller than that of the reference (e.g. oil), the specific gravity is <1,0.
Temperature is important
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Keep in mind that the density of a liquid is very temperature-dependent. Hot water has a lower density than cold water. As a result, the specific gravity can also be temperature dependent.
When do you use Specific Gravity?
A common use of specific gravity is, as we mentioned at the start, for making alcoholic drinks such a beer, wine, and cider. With any of these drinks, sugars in the grains, grapes, or apples are converted into alcohol by yeast (through fermentation). The more sugar the starting liquid contains, the more alcohol these yeasts can make from the sugar (up to a certain limit). As a result, the amount of sugars present at the start of the process is an important factor in determining how much alcohol will be formed.
It’s expensive and cumbersome to analyze the sugar content of a liquid. However, it’s a lot easier to measure the relative density/specific gravity of a liquid. By assuming that it’s mostly the sugars that influence the density of the starting liquid, you can make a reasonable estimate of the sugar content of a liquid by measuring the specific gravity/relative density!
Calculating alcohol content
When brewing beer or making apple cider, the density of the liquid will change during fermentation. In the beginning, you’ll have a lot more sugars than you will at the end. Brewers generally assume that all sugars that have disappeared have been converted into alcohol. As such, once you know how the density of your liquid changes you can make a reasonable estimate of the alcohol content of your drink. Calculators exist for this and do depend slightly on the process and ingredients used.
How can you measure specific gravity?
You don’t need complicated tools to measure the specific gravity of a liquid. A commonly used, simple tool, is the hydrometer.
A hydrometer is often a weighted glass tube (see photo). You simply place the hydrometer in the liquid you want to analyze. Depending on the density of the liquid it will sink in quite low, or will float quite high. The hydrometer will sink lower in less dense liquids and remain floating higher in more dense liquids.
Hydrometers are calibrated for a specific purpose. For instance, they can be calibrated for a specific temperature, or a specific type of liquid (though most are made to compare to water).
Measuring the specific gravity is a perfect example of using one property (density) to find out something about another property (sugar and alcohol content). Since food and drink are so complicated, you’ll see this done quite often! It’s a way to learn something about our food, without having to do the more complicated science behind it! It’s what food scientists do quite regularly.
Randy Mosher, Mastering Homebrew, 2015, Chronicle Books, p. 32-33