The Effects of Caseinate Submicelles and Lecithin on the Thin Film Drainage and Behavior of Commercial Caseinate Fiona A. Husband and Peter J. Wilde 1 Institute of Food Research, Norwich Research Park, Colney Lane, Norwich NR4 7UA, United Kingdom Received December 11, 1997; accepted May 1, 1998 The drainage behaviour (stratiﬁcation, thickness, and mobility) of thin foam ﬁlms stabilized by commercial caseinate was studied in 10 mM phosphate buffer at pH 7.0. Thin ﬁlms of commercial caseinate drained in a stepwise manner, with steps of similar thickness. The drainage was rapid, temperature sensitive, and chaotic, and the surface mobility of caseinate thin ﬁlms also showed temperature sensitivity. The stepwise drainage is thought to be due to the layering of lecithin-caseinate submicelle com- plexes. Lecithin-stabilized thin ﬁlms showed similar drainage be- havior and temperature sensitivity. However, the ﬁlms were ap- proximately 66% thinner than caseinate ﬁlms, and surface diffusion was very rapid. Removal of lipid from caseinate dramat- ically affects the thin ﬁlm drainage properties and reduces tem- perature sensitivity. Reconstituted caseinate (i.e., extracted case- inate reconstituted with lipid), showed thin ﬁlm properties similar to the commercial caseinate. Caseinate supplemented with lipid showed thin ﬁlm drainage characteristics similar to caseinate, and surface mobility similar to lecithin. The presence of lecithin in caseinate thin ﬁlms causes an increase in mobility, drainage, and stratiﬁcation, along with a decrease in thin ﬁlm thickness. This demonstrates that lecithin, possibly partially bound to the casein- ate, is present at the interface disrupting protein-protein interactions. © 1998 Academic Press Key Words: caseinate; lecithin; thin ﬁlms. INTRODUCTION Caseinate is a widely used food ingredient, employed uni- versally for its good emulsifying and foaming properties (1). The four main caseinate proteins are  s1 -,  s2 -, - and - casein; other constituents include lactose, salt, and fat. The casein proteins are present in the form of 10 –15 nm diameter spheres, referred to as submicelles. There is now a good fundamental understanding of the functional properties (foam- ing and emulsifying) of these components (e.g., (2, 3)). Func- tional properties depend both upon the interfacial layer com- position (protein and lipid in this case) and any interactions or competitive adsorption between components. Commercial ca- seinate typically contains 0.5–2.0% fat. The fat composition of commercial caseinate does not necessarily reﬂect the fat com- position of milk, but depends upon the processing conditions. Two optional processing conditions, which directly affect fat composition are a) production of caseinate from skim milk (fat content 0.04 – 0.1%) and b) addition of lecithin to improve caseinate dispersibility (4). Thin ﬁlm drainage, stability, thick- ness, and mobility provide signiﬁcant information for under- standing foam and emulsion stability. Stratiﬁcation and rapid drainage of commercial caseinate thin ﬁlms have previously been reported (5), however the stratiﬁcation (or stepwise thin- ning) was assumed to be solely due to layering of casein submicelles. Stepwise thinning of foam ﬁlms differs from the gradual thinning normally observed in protein-stabilized foam ﬁlms, in that the ﬁlm thins in discrete steps. These steps appear as darker (thinner) regions appearing and spreading within the foam ﬁlm, previously only observed in surfactant or lipid ﬁlms above the CMC (7, 13). Several such steps may occur during the drainage process, until the ﬁlm reaches its equilibrium thickness. Lipid-stabilized thin ﬁlms display chaotic and rapid drainage (6), in contrast to protein-stabilized ﬁlms, which typically have very slow drainage (7–9). The presence of lecithin may affect the protein’s thin ﬁlm behavior, as has been observed with other surfactants and lipids (8, 9, 11, 17, 18, 20). In this paper we investigated the role of lipid on the thin foam ﬁlm behavior (drainage, thickness, and surface mobility) of caseinate. This was done by comparing a commercial case- inate that was spray-dried with lecithin (manufacturers speci- ﬁcation) with a) lipid-extracted caseinate, b) lipid-reconstituted caseinate, and c) lecithin. MATERIALS AND METHODS Surface chemically pure water (surface tension greater than 72.8 mN/m at 20°C) was obtained by steam distillation of demineralized water from potassium permanganate. Commer- cial caseinate (Alanate 191), (composition 93.1 wt % protein, 0.3% lactose, 1.1% fat, 3.8% moisture, remainder ash), which had been spray-dried with lecithin, was obtained from the New Zealand Dairy Board. Egg lecithin (P5394) was purchased from Sigma Chemical Company (Dorset, U.K.). N-(7-Nitro- benz-2-oxa-1,3-diazol-4yl)-1,2-dihexadecanoyl-sn-glycero-3- phosphoethanolamine (NBD-PE) was purchased from Molec- 1 To whom correspondence should be addressed. JOURNAL OF COLLOID AND INTERFACE SCIENCE 205, 316 –322 (1998) ARTICLE NO. CS985622 316 0021-9797/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.