Surface Film Pressure of/3-Lactoglobulin, (x-Lactalbumin and Bovine Serum Albumin at the Air/Water Interface Studied by Wilhelmy Plate and Drop Volume M. PAULSSON AND P. DEJMEK ~ Department of Food Technology, University of Lund, Box 124, S-221 O0 Lund, Sweden Received December 21, 1989; accepted November 27, 1991 The surface film pressure (H) of 13-1actoglobulin, a-lactalbumin, and bovine serum albumin was studied in simulated milk ultrafiltrate (SMUF) and in water at concentrations from 10 -6 up to 1% (w/ v) at times from 30 s to 14 h and the results were analyzed with regard to adsorption transport and kinetics. In SMUF at low concentrations, fl-lactoglobulin was the most "surface active" protein. There was little difference in the surface film pressure between 13-1actoglobulin and a-lactalbumin at high con- centrations. Bovine serum albumin showed the lowest surface activity, but did not reach a constant II, even after 14 h. As the pH approached the isoelectric point, the surface film pressure increased, and in the case of bovine serum albumin rI increased faster. In water, however, the surface film pressures were lower than in SMUF, and for bovine serum albumin II developed more slowly. The transport to the interface was found to be controlled by diffusion only for a small concentration range of approximately 10-4%. It was controlled by initial flow disturbances at higher concentrations and free convection at lower concentrations. The rate of increase of the surface film pressure was not a simple function of surface film pressure and bulk concentration under any of the conditions studied. © 1992 Academic Press, Inc. INTRODUCTION Milk constitutes a major part of the diet in the Nordic countries. In Sweden, 10-15% of food expenditure is used for milk and milk products. Milk-based ingredients, particularly milk proteins, are common in nondairy foods as well. Skimmed milk is a stable dispersion of pro- tein aggregates, termed "casein micelles," in milk serum. Milk serum is principally a mix- ture of salts (38 mMK, 22 mMNa, 11 mM Ca, 32 mMCI, 12 mMphosphate, and 9 mM citrate ( 1 )) and dissolved proteins. The major proteins of serum, 13-1actoglobulin (3.2 g/li- ter), a-lactalbumin ( 1.2 g/liter), and bovine serum albumin (0.4 g/liter), are usually called whey proteins. The whey proteins can affect surface properties of milk and milk products, particularly when the dominant caseins of milk are not soluble or their transport is lim- To whom correspondence should be addressed. ited. They can also affect the colloidal prop- erties of products in which isolated whey pro- teins or whey is used as a functional ingredient. The amount of published surface film data on pure whey proteins is limited. The reports by Mitchell et al. (2), Graham and Phillips (3), and Tornberg and Lundh (4) involve phosphate buffers. However, because calcium is known to strongly affect the solution be- havior of the proteins, a more milk-like en- vironment was chosen for this study. MATERIALS AND METHODS Materials The investigated whey proteins were all purchased from Sigma Chemical Co.: /~-lac- toglobulin (/3-Lg) (L-6879, lot no. 111F- 8025 ), a-lactalbumin (a-La) ( L-6010, lot no. 52F-80751), and bovine serum albumin (BSA) (A-0281, lot no. 63F-9350). fl-Lacto- globulin was desalted using dialysis against 0021-9797/92 $3.00 Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved. 394 Join'hal of Colloid and Interlace Science, Vol. 150, No. 2, May 1992