The total protein content of the sample is sufficient.
Although the saturated concentration of ammonium sulfate was added to 85%, less than 10% of the protein was precipitated.
Would the presence of viscous substances in the sample solution affect it? And make it difficult for the salted out proteins to precipitate.
Add the AS slooooowly under constant vigorous stirring (but without foam production) and keep the sample on ice the whole time. Cold rooms chill only the operator, not the sample ;-(. The addition should take at least 30, better 60 min, then keep the sample cold and stir at least for another h (or o/n) to complete the precipitation.
Also remember that AS corrodes Al and Ti rotors, wash them thoroughly immediately after spinning AS-containing samples in them and then dry them up-side down.
In addition to the recommendations made by Engelbert Buxbaum , precipitation happens as the protein molecules are forced to interact between them as ammonium sulfate is added. You mention a viscous sample, from a theoretical point of view, the encounter of two protein molecules will be hindered by the high viscosity, diffusion will also be affected by the viscosity of the medium. Hence the recommendation of give enough time.
in addition to the (2) sensible and insightful recommendations its also very important to follow charts for the addition of (NH₄)₂SO₄ as the amount added is not intuitive ! DNA viscosity can be ameliorated by sonication or shearing through a small gauge syringe needle and any decrease in viscosity helps centrifugation bring down the proteins. Good luck!
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