Density is the mass per unit volume of a material at a specified temperature and in vacuo and is the absolute scientific standard/reference.
Specific Gravity is the RATIO of the mass of a given volume of material to the mass of an equal volume of water (both at a specified temperature) measured in vacuoand is a dimensionless value
Apparent Specific Gravity is the same as above, except that measurements are made in air where a slight buoyancy effect occurs
Relative Density is an alternative title to Specific Gravity and used increasingly in its place.
Apparent Density is as above, but for measurements made in air, where there is a slight buoyancy effect.
To illustrate the differences, the figures for a 60° Brix material are:-
Density in vacuo = 1.28646 gm/ml
Apparent Density = 1.28544 gm/ml
Specific Gravity in vacuo = SG2020 = 1.28873
Apparent Specific Gravity = SG2020 apparent = 1.28908
In other words, a 1000 Litre volume of 60° Brix material has a weight in vacuo of 1286.46Kg, but 1285.44Kg in air. Very often SG values are used interchangeably, on the assumption that the density of water is 1.0000 leading to errors. For the example of the 1000 Litres of 60° Bx product the ‘weight’ from SG2020 is 1228.73Kg and 1289.08Kg from the SG2020 apparent figure.
At GMD we (prefer to) report values in terms of a weight/unit volume rather than a value based on the weight of an equal volume of water. We use the Apparent Density scale as the practical units for measurement in air in the real world.
Interconversions: SG2020 apparent x 0.997174 = Apparent Density at 20°CSpecific Gravity in vacuo SG2020 x 0.998234 = Density at 20°C in vacuo]
The essential oil in a fruit product has a lower density than that of the juice. For high oil containing products the oil content must also be taken into consideration when calculating the density.