Please have a look at some relevent PDFs, worth reading...

Protein solubility is a thermodynamic parameter defined as a soluble protein concentration that is in equilibrium with a crystalline solid phase influenced by the conditions of pH. Kjeldahl Method of Nitrogen can be used Determination of protein. Proteins are important constituents of foods for a number of different reasons. Determining their overall texture, e.g., tenderness of meat or fish products. It is usually considered to be the standard method of determining protein concentration. Digestion converts any nitrogen in the food. In order to evaluate protein change during Cantonese sausage processing, meat proteins were classified into five fractions on the basis of their solubility, namely the non-protein nitrogen (NPN), the water-soluble protein (sarcoplacmic proteins), the salt-soluble protein (myofibrillar proteins), the alkali-soluble fraction, and the alkali-insoluble fraction. As processing time proceeded, changes in the composition of Cantonese sausage proteins.

Thanks sir