Peptide size control is important for obtaining desirable functional properties so that these peptides can be better utilized. Proteolytic enzymatic modification of soy protein isolates (SPI), followed by ultrafiltration, is an effective way to fractionate these proteins into peptides with controlled molecular size. SPI was predenatured by mild alkali at pH 10 and heated at 50°C for 1 h prior to partial hydrolysis by papain at pH 7.0 and 38°C for 10, 30, and 60 min (PMSPI10, PMSPI30, and PMSPI60). The hydrolysate PMSPI60 was further fractionated by ultrafiltration with a stirred cell and disc membranes (100-, 50-, and 20-kDa molecular weight cut-off) into one retentate (R100) and three permeates (P100, P50, and P20). Molecular weight distribution, surface hydrophobicity (S 0 ), protein solubility (PS), emulsifying activity index (EAI), and emulsion stability index (ESI) of the control SPI (without added papain), hydrolysates, and ultrafiltrates were investigated. Significant increases (P < 0.001) in S 0 , PS, EAI, and ESI were observed in the hydrolysates. Peptides in the permeates had higher PS and EAI but lower S 0 than the peptides in the retentate and hydrolysate. Soy protein peptides that were prepared from SPI by papain modification and ultrafiltration had lower molecular weight, higher solubility, and higher emulsifying properties. They could find use in products that require these properties, especially in the cosmetic and health food industries. JAOCS 75, 845-850 (1998).Proteolytic enzyme modification of proteins is an effective way to improve their functionality. The peptides produced by partial proteolysis of proteins have smaller molecular size and less secondary structure than the original proteins. They contribute to increased solubility, decreased viscosity, and significant changes in the foaming, gelling, and emulsifying properties when compared with those of the original proteins (1). To obtain desirable functional properties in soy protein hydrolysates, hydrolysis must be carried out under strictly controlled conditions to a specified (generally low) degree of hydrolysis (2). A limited degree of hydrolysis usually improves the emulsifying and foaming capacities, whereas excessive hydrolysis often causes loss of some of these functionalities (3). Therefore, peptide size control is essential if optimal and reproducible changes in functional properties are to be achieved (1).Ultrafiltration has been successfully used to separate proteins (peptides) from salts and sugar and to fractionate proteins (peptides) from each other (4). Partial hydrolysis of soy protein isolates (SPI) (5-9) generates soy protein peptides with different molecular weight distributions that can be separated with ultrafiltration membranes of different pore sizes.To understand the basis of peptide functionality, it is essential to know the fundamental properties of the peptide, including protein surface hydrophobicity, solubility, and emulsifying properties. This study investigated these fundamental properties of soy protein pep...