Highly viscous, protein rich samples: such matrices are guaranteed to be high on the list of pH measurement nightmares. The electrode has to be chosen with respect to the:

• diaphragm - which should be as dirt-resistant as possible;
• electrolyte solution - which shouldn't denature the protein (3 mol/L KCL denatures quite well);
• viscosity of the sample: highly viscous samples (cream, yoghurt, butter) - use a "spearhead" electrode. Diary drinks with medium viscosity : use a robust diaphragm (ground joint)!