The "Pyler says" series explores excerpts from Baking Science & Technology, a textbook that teaches readers a range of baking and equipment concepts. The following passage is from Chapter 7: Fundamental Bakery Batter Processes.
Specific gravity measures the amount of air incorporated into a batter and determines the tenderness, grain, texture and volume of a finished cake. The more air incorporated, the lower the specific gravity.
It is calculated by dividing the weight of the batter by the weight of an equal volume of water. While this ratio may be simple, it is a delicate balance that can be impacted by a change in ingredients, even down to the character of the flour or shortening type; mixing or equipment design; and temperature. When it comes to shortenings, the viscosity differences between liquid and plastic will have a significant impact on specific gravity and must be accounted for, as well as the presence of emulsifiers.
Bakers can measure specific gravity manually by measuring a cup of batter, leveled off with a straight-edged spatula and weighing the same cup full of water, and then following the simple equation of dividing the weight of the batter by the weight of the water. Today’s controls on continuous mixers can monitor batter specific gravity in real time and adjust for deviations.
If a cake’s specific gravity is above the optimum range, that cake will typically have less volume, denser grain and tougher eating qualities. A specific gravity below the optimum range will be very fragile with dips on the surface and a crumbly crumb.