A method of sequential isoelectric solubilisation of animal by-product comprising the steps of isoelectric solubilisation of the animal by-product in one of an acid or alkali solution to provide a first solubilised protein fraction, separating the first solubilised protein fraction from unsolubilised animal by-product, isoelectric solubilisation of the unsolubilised animal by-product to provide a second solubilised protein fraction, and separation of the second solubilised protein fraction from unsolubilised animal by-product. The protein from the first and/or second solubilised protein fractions is recovered by drying or precipitation. In one embodiment, the acid and alkali soluble protein fractions are proportionally combined to neutralise the composite fraction and precipitate protein.