Milk proteins differentiation and competitive adsorption during spray-drying

Abstract

A non invasive method was developed to differentiate native whey proteins (NWI) and native micellar casein (NMC) at the surface of high protein milk powder. Surface analyses of the powders were performed by X-ray Photoelectron Spectroscopy (XPS). With this tool, it was impossible to differentiate casein and whey proteins by their C_1s, O_1s and N_1s signature; the atomic percentage being similar. But, minerals at the surface of these proteins were significantly different. As a consequence, a calibration curve was obtained with known mixes of both proteins and was used for their differentiation.

Competitive adsorption between casein and whey proteins during spray-drying was studied by a combination of different analytical methodologies (XPS, FTIR, zetasizer). The influence of the outlet air temperature on dairy proteins competition at the surface of the particle was investigated. For this purpose, three formulations were studied: 80/20, 50/50 and 20/80 (casein/whey proteins) and spray-dried at 80 and 130 °C (outlet air temperature). In formulations spray-dried at 80 °C, caseins were found overrepresented at the surface in comparison with the bulk composition. At 130 °C, the repartition between caseins and whey proteins was mostly the same at the surface and in the bulk. It was concluded that low outlet air temperatures allow the migration of surface-active components at the surface. As the spray-drying temperature increased, the surface may solidify and immobilize the proteins quickly, the surface being closer to the bulk composition.

Introduction

Milk proteins are important proteins widely used as ingredient in the food and dairy industries. Their functional characteristics and nutritional qualities make them very interesting components (Bart & Schlimme, 1988). Milk proteins can be classified in two categories according to their structure: flexible proteins for caseins and globular proteins for whey proteins (Dickinson, 2001). Caseins present a disordered structure and include proteins α, β, κ-casein, the mixtures sodium caseinate, micellar casein…. Whey proteins contain disulphide bridges, tertiary structure, and preserve their globular molecular shape even after adsorption on an interface. Therefore, their very different molecular structure had a strong impact on their physicochemical and interfacial properties. Caseins are prompt to adsorb at liquid/air interfaces, whereas, whey proteins are less surface-active due to their close-packed globular organization (Fox & McSweeney, 1998). Moreover, whey proteins show important denaturation at temperatures above 70 °C, whereas caseins are not sensitive to temperature. As milk proteins are very perishable products, there are often converted by spray-drying into stable products such as powders. Milk proteins are also spray-dried to reduce transport cost and/or to facilitate handling. Nowadays, milk protein powders have achieved great economic importance and present a wide range of tailor-made applications.

During spray-drying, the liquid protein preparation is changed into a powder by removing almost all of its water content. The influence of the drying liquid composition on the powder surface composition has been extensively studied by X-ray Photoelectron Spectroscopy (XPS) (Fäldt and Bergenstahl, 1996a, Gaiani et al., 2006, Kim, 2008). All these authors show that the composition of the powder surface is highly different from the bulk composition. Indeed, when a surface-active component is present in the drying liquid (such as a protein, a phospholipid…) the powder surface was covered by this component to a large extent (Gaiani et al., 2007, Kim, 2008). From these results, some authors suggested that the powder surface reflects the air–water interface of the drying droplet (Fäldt, 1995, Landström et al., 2000, Landström et al., 2003). The spray-drying process causes also a range of structural and physicochemical transformations which in turn may influences some functional properties of the powders: the wettability (Fäldt and Bergenstahl, 1996b, Gaiani et al., 2009), caking (Nijdam & Langrish, 2006), flowability (Onwulata, Konstance, & Holsinger, 1996) or also oxidative stability (Hardas, Danviriyakul, Foley, Nawar, & Chinachoti, 2000). When a mixture of surface-active components (proteins) is mixed, competitive adsorption is a common phenomenon. It has been shown that caseins with a high surface activity and a flexible disordered structure absorb and spread at the air interface quickly in comparison with a more compact and globular protein like whey proteins (β-lactoglobulin). The adsorption process at the interfaces has been suggested to include three main steps. The transport of proteins toward the interface happened first, and then an attachment of the protein at the interface occurred. Finally, structural rearrangements in the adsorbed state may follow (Dickinson, 2001). This phenomenon is well documented and a rich amount of information is available in the field of emulsions (Dickinson et al., 1988, McClements, 2004) and foams (Marinova et al., 2009, Zhang and Goff, 2004). Commonly, when it is adsorbing at air–water and oil–water interfaces, casein has been found to be more competitive than a globular protein (whey proteins). However, it is complex to compare studies where the adsorption has reached equilibrium (emulsion, foam…) with results where the adsorption is interrupted (during spray-drying) after a fraction of a second (Fäldt, 1995). As a consequence, very little is known on competitive protein adsorption in the drying droplet (Landström et al., 2003). Some authors developed an original fluorescence quenching method to determine the fraction of protein at the powder surface (Landström et al., 1999, Landström et al., 2000, Landström et al., 2003). Nevertheless, the surface properties of the proteins may be modified by the quenching introducing a bias in the results.

The objectives of this paper are first to develop a non invasive method allowing the discrimination of caseins and whey proteins at the powder surface. Then, the influence of the spray-drying temperature on powder surface composition was investigated by analyzing mixes containing different ratios of casein and whey proteins. Concurrently, some physicochemical properties of the powders were determined in order to a better understanding of the mechanisms between casein and whey proteins competition at the powder surface.