The rehydration ratio is the drained mass of a rehydrated sample divided by the dry mass used, measured under stated conditions. A ratio of 5 means one kilogram of dried product yielded five kilograms of drained rehydrated product.
The number is only meaningful with its method attached, because every variable in the test moves the result: water temperature, contact time, the water to product ratio, whether the mixture is agitated, the mesh size used for draining, how long it drains, and whether the surface is blotted before weighing. Two honest laboratories can differ by a third on the same lot simply by draining differently.
Be careful with the definition as well. Some specifications use rehydration **capacity**, expressed as water absorbed per gram of dry matter, and some use a rehydration **coefficient** against the original fresh mass, which folds in the drying yield. These are three different numbers.
A usable specification names the definition, the water temperature, the contact time, the water to product ratio, the agitation, the draining method, the sampling method and the acceptance limit. Set the limit from at least three lots of the actual product rather than from a published figure, and record the trial as an annex to the specification.