of from 7.5 to 8.5, at a preferred reaction time of from 0.5 to 5 h, more preferably of from 0.5 to 3 h and especially preferably of from 0.5 to 1 h, the molar ratio of reactive polymer ester:protein being preferably of from 1:1 to 70:1, more preferably of from 5:1 to 50:1 and especially preferably of from 10:1 to 50:1.