Influence of protein concentration on surface composition and physico-chemical properties of spray-dried milk protein concentrate powders Grace M. Kelly a, b , James A. O'Mahony b , Alan L. Kelly b , Thom Huppertz c , Deirdre Kennedy a , Donal J. O'Callaghan a, * a Teagasc Food Research Centre, Moorepark, Fermoy, Co. Cork, P61 C996, Ireland b School of Food and Nutritional Sciences, University College Cork, Cork, T12 Y337, Ireland c NIZO Food Research, PO Box 20, 6710 BA Ede, The Netherlands article info Article history: Received 23 February 2015 Received in revised form 2 July 2015 Accepted 6 July 2015 Available online 17 July 2015 abstract Surface composition, moisture sorption behaviour and glasserubber transition temperature (T gr ) were determined for spray-dried milk protein concentrate (MPC) powders over a range of protein contents (35 e86 g 100 g 1 ). Surface characterisation of MPC powders indicated that fat and protein were prefer- entially located on the surface of the powder particles, whereas lactose was located predominantly in the bulk. Moisture sorption analysis at 25  C showed that MPC35 exhibited lactose crystallisation, whereas powders with higher protein contents did not and continually absorbed moisture upon humidification up to 90% RH. The GAB equation, fitted to sorption isotherms of MPCs, gave increases in monolayer moisture value (m m ) with protein content. T gr , measured with a rheometer, decreased significantly (P < 0.05) with increasing water content and increased with increasing protein content (P < 0.05). In conclusion, increasing protein concentration of MPCs resulted in altered surface composition and increased m m value and T gr values. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Milk protein concentrates (MPCs) are high-protein spray-dried powders derived from defatted milk by means of membrane sep- aration (Chandan, 2011). Ultrafiltration (UF) and diafiltration (DF) are applied to skimmed milk, followed by evaporation and spray- drying (Mistry & Hassan, 1991), resulting in powders with protein concentrations ranging from 35 to 86 g 100 g 1 , with a concomitant reduction in lactose levels as protein content increases (Havea, 2006). In recent years, MPCs of different protein contents have been produced and incorporated into a wide range of products. For example, MPC powders are often used to standardise the protein content of milk for cheese-making, and are also used in recombined cheese, infant milk formula (IMF), dairy-based beverages and sports and nutritional foods. There are issues in the handling and storage of dried milk pro- tein ingredients, e.g., powder blockage in spray dryers, powder silos and hoppers, and shelf-life issues involving caking. These issues can be related to surface composition and moisture levels, which affect key functional properties, e.g., stickiness, wettability, bulk density and flowability of powders (Kim, Chen, & Pearce, 2002; Nijdam & Langrish, 2006). Non-uniform distribution of components, e.g., on particle surfaces versus their interior, can occur through various mechanisms. Spray-drying involves the rapid removal of water from a concentrated dispersion, during which milk components are concentrated, as moisture evaporates from the droplet surface (Kim, Chen, & Pearce, 2009), causing concentration gradients of solutes. Osmotic forces cause dissolved milk components to migrate toward the surface, partially replenishing the aqueous phase (Birchal, Huang, Mujumdar, & Passos, 2006). Another influ- ence is hydrophobicity, whereby hydrophobic molecules, such as fat, will preferentially take up position on the surface. One effect of this migration is that concentrations of lactose, fat, and protein at the surface of spray-dried dairy powders have been found to be different from the bulk composition (F€ aldt & Bergenståhl, 1996; Kim et al., 2002; Nijdam & Langrish, 2006). Powders with high * Corresponding author. Tel.: þ35 32542205. E-mail address: donal.ocallaghan@teagasc.ie (D.J. O'Callaghan). Contents lists available at ScienceDirect International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj http://dx.doi.org/10.1016/j.idairyj.2015.07.001 0958-6946/© 2015 Elsevier Ltd. All rights reserved. International Dairy Journal 51 (2015) 34e40