The nature of protein-polysaccharide nanoparticles prepared using two fabrication methods was compared: Type 1 particles were formed by creating beta-lactoglobulin nanoparticles, and then coating them with pectin; Type 2 particles were formed by heating beta-lactoglobulin and pectin complexes together. Protein nanoparticles (d = 180 nm) were created by heating beta-lactoglobulin above its thermal denaturation temperature (T-m) at pH 5.8. Type 1 particles were then formed by mixing these particles with high methoxy (HM) pectin under conditions where pectin adsorbed to the protein (pH < 6). Type 2 particles were created by heating beta-lactoglobulin-HM pectin electrostatic complexes above T-m at pH 4.75. At pH 4.5, Types 1 and 2 particulates had similar charge (-33 mV), protein content, and shapes (spheroid), however, Type 1 particulates were larger (d = 430 nm) than Type 2 particulates (d = 300 nm). The influence of pH, ionic strength and protein:pectin mass ratio (r) on the physical stability of the two types of particles was tested. A weight ratio of 2:1 (protein:pectin) gave good pH stability of the particles against aggregation by imparting more surface charge. Type 2 particles had a higher electrical charge, better stability to aggregation at lower pH values (pH < 4), and better stability to aggregation at high salt concentrations (200 mM NaCl) than Type 1 particles. These differences suggested that Type 2 particulates had a higher surface coverage with pectin, thereby reducing their tendency to aggregate. These results have important consequences for the design of biopolymer nanoparticles based on thermal treatment of proteins and polysaccharides. (C) 2009 Elsevier Inc. All rights reserved.