Impact of protein self-assemblages on foam properties I. Nicorescu a, * , A. Riaublanc b , C. Loisel a , C. Vial c , G. Djelveh c , G. Cuvelier d , J. Legrand e a GEPEA, ENITIAA, CNRS, UMR 6144, Rue de la Géraudière, BP 82225, 44322 Nantes Cedex 03, France b BIA-INRA Nantes, Rue de la Géraudière, BP 71627, 44316 Nantes Cedex 03, France c CLERMONT UNIVERSITÉ, ENSCCF, LGCB, avenue des Landais, BP 206, 63174 Aubière Cedex, France d UMR SCALE, AgroParisTech, 1 rue des Olympiades, 91744 Massy Cedex 03, France e GEPEA, University of Nantes, CNRS, UMR 6144, 37 bd de l’Université, BP 406, 44602 Saint Nazaire Cedex, France article info Article history: Received 2 May 2009 Accepted 26 July 2009 Keywords: Whey proteins Heat-induced protein aggregates Protein aggregate morphology Protein foamability Foam stability abstract The influence of dynamically heat-induced aggregates on whey protein foams was investigated as a func- tion of the thermal treatment applied to WPI using a bubbling technique. The aim was to determine the interplay between the size/shape/proportion of the heat-induced aggregates and the properties of protein foams (formation and stability). Results showed that insoluble protein aggregates were highly branched and cohesive, whereas soluble aggregates were constituted by subunits, associated by hydrophobic bonds and formed by a-La and b-Lg monomers linked by disulfide bridges. Using the bubbling procedure, pro- tein aggregates were shown to slow down significantly foam formation. However, the rate of foam for- mation remained nearly unchanged for wet foams when the amount of insoluble aggregates was inferior to 5% and when their size remained lower than 100 lm. Similarly, protein aggregates did not seem to affect the destabilisation kinetics of wet foams, regardless of amount, size, shape and proportion. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Whey proteins are compact globular proteins ranging from 14 to 10 3 kDa (Eigel et al., 1984). The major proteins in whey are b- lactoglobulin (b-Lg), a-lactalbumin (a-La), bovine serum albumin (BSA) and immunoglobulins (Ig). The functional properties of whey proteins isolate (WPI) may be classified into two main groups: hydration-related (dispersibility, solubility, swelling, viscosity and gelation) and surface-related properties (emulsification, foam- ing and adsorption at fluid/fluid interfaces). A key physicochemical property of the whey proteins is their abundance of sulfhydryl amino acid residues that allows them to form intermolecular covalent bonds during the high-temperature processing of WPI solutions. Although intermolecular disulfide bonding is essential for form- ing heat-induced whey protein aggregates and for stabilising foam structures, the ability of these proteins to develop their functional properties depends on a large number of factors, such as pH, ionic strength, concentration and temperature (Morr & Ha, 1993). For example, above 65 °C, the apparent order of denaturation of indi- vidual whey proteins is Ig > BSA > b-Lg > a-La. However, their respective rates of denaturation are strongly influenced by pH, io- nic composition and total solids: e.g. Hill (1988) showed that heat- denatured a-La and b-Lg interact by ionic and disulfide bonding to form insoluble aggregates at pH between 4.5 and 4.6 or with the addition of some ionic compounds. Similarly, Kulozik (2008) dem- onstrated that the size and the internal structure of the whey pro- tein aggregates depend on temperature and composition, such as lactose content. For Morr and Josephson (1968), due to denatur- ation, whey proteins formed primary aggregates of about 200 kDa, the growth of which depended on salt ions. For Li-Chan (1983), the greatest impairment of protein solubility and the great- est loss of reactive –SH groups was reported at pH higher than 7 after heating at temperatures above 80 °C because of the prevailing protein–protein interactions; in this case, disulfide and hydropho- bic bonding were mainly responsible for the heat-induced forma- tion of insoluble aggregates. Recently, several studies (Grácia-Juliá et al., 2008; Ikeda & Morris, 2002; Mahmoudi et al., 2006; Mahmoudi, 2007; Schmitt, Bovay, Rouvet, Shojaei-Rami, & Kolodziejczyk, 2007) analysed the respec- tive effects of pH, ionic strength and temperature on the morphology of WPI soluble aggregates. For example, Mahmoudi (2007) showed that at 0.1 M NaCl, a self-organisation phenomenon of the protein aggregates was observed when heating at 100 °C, while no rear- rangement occurred below 80 °C; the spherical-shaped aggregates induced by heating above 100 °C were associated into bunch of grapes in which each spherical protein aggregate was stuck to the others by ionic bonding at high sodium chloride content. Schmitt et al. (2007) showed also that the combination of pH adjustment and NaCl addition to WPI could produce soluble aggregates upon heating at 85 °C for 15 min with physicochemical properties and 0963-9969/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2009.07.028 * Corresponding author. Tel.: +33 251785469; fax: +33 251785467. E-mail address: nicorescuirina@yahoo.com (I. Nicorescu). Food Research International 42 (2009) 1434–1445 Contents lists available at ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres