Dissolution of milk protein concentrate (MPC) powders by ultrasonication Noel A. McCarthy, Philip M. Kelly, Patrick G. Maher, Mark A. Fenelon ⇑ Food Chemistry & Technology Department, Teagasc Food Research Centre, Moorepark, Fermoy, Co. Cork, Ireland article info Article history: Received 10 September 2013 Received in revised form 3 November 2013 Accepted 4 November 2013 Available online 12 November 2013 Keywords: Ultrasonication Milk protein concentrate Dissolution Particle size abstract The aim of this study was to determine the effects of ultrasound on the dissolution properties of milk pro- tein concentrate (MPC) powders. A high intensity ultrasound (20 kHz; power 70.2 W) was used to dis- solve MPC powder and the dissolution rate was compared to conventional methods of dispersion (e.g., stirring). Rehydration of MPC by stirring at 50 °C complied with the 2-step dissolution model whereby the disappearance of larger particles coincided with the formation of smaller ones over time. MPC dis- persal by sonication dramatically accelerated this process, achieving >90% levels of powder solubilisation. Temperature rise (>70 °C) during sonication causes protein denaturation and aggregation, but this can be alleviated using thermal dissipation. Overall, the use of a combination rehydration process for MPC pow- ders involving (i) conventional dissolution (stirring) for 10 min at 50 °C followed by (ii) ultrasonication (<50 °C) for 1 min (energy density 21.1 J/mL) can achieve rapid powder dissolution. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Milk protein concentrates (MPCs) represent enriched forms of the naturally occurring casein and whey protein complement in defatted (skimmed) milk. MPC is typically produced by applying membrane separation processes such as ultrafiltration (UF) and diafiltration (DF) to pasteurised skim milk followed by evaporation and spray drying (Mistry and Hassan, 1991). The resulting protein concentrations may range from 40% to 85%, with a commensurate reduction in lactose levels as protein content increases (Havea, 2006; Sikand et al., 2011). Applications for MPC include infant milk formula, dairy-based beverages, sports and nutritional foods. Rehydration of MPC powders is a time consuming process and is significantly affected by several parameters, such as the heat- treatment of the skim milk, protein and mineral content, spray dry- ing temperatures, storage conditions of the subsequent powder (i.e., time and temperature) and the rehydration process (i.e., tem- perature of reconstitution and mixing conditions) (Gaiani et al., 2010, 2009, 2007; Havea, 2006; Mimouni et al., 2010; Richard et al., 2012). A temperature of 50 °C is regarded as optimal for decreasing MPC hydration time (Fang et al., 2011). The rehydration of MPC powders envisages four phases to the reconstitution pro- cess (Forny et al., 2011): (i) wetting of the agglomerate which in- cludes the penetration of the liquid into the pore system due to capillary forces; (ii) immersion of the agglomerate in the liquid; (iii) dissolution of the solid bridges between the primary particles followed by dispersion of the primary particles within the liquid volume; and (iv) dissolution of soluble primary particles and asso- ciated reduction in size. Prolonged powder storage time and higher temperature post- drying increase rehydration time, particularly in high protein MPC powders (Anema et al., 2006; Fang et al., 2011; Havea, 2006). Powder particle dissolution is a rate limiting step during rehydration due to casein micelle fusion caused by hydrophobic interactions (Horne, 1998). Jeantet et al. (2010) reported that >14 h stirring at 30 °C is required to transform a native micellar casein powder into a solution (4.8% w/w). High-speed mixers are commonly used to disperse and hydrate MPC powders and emulsi- fying salts (e.g. trisodium citrate, sodium hexametaphosphate, etc.) may also be employed in certain circumstances where formulations permit the addition of such minerals. Increasing mechanical agitation during powder dissolution helps to decrease rehydration time by promoting turbulence and facilitating the sol- ubilisation of powder particles in the liquid media (Richard et al., 2013). Calcium chelators improve solubility and dissolution of high protein MPC powders by binding calcium ions, thus improving casein micelle dissociation. Given the food formulation constraints associated with chemical mediation approaches, the current study used the novel technology of ultrasonication as a physical ap- proach to improve dissolution of MPC powders at relatively low temperatures. High intensity ultrasonication is a non-thermal processing tech- nique, with a frequency range between 20 and 100 kHz, and 10 and 1000 W/cm 2 of power (Kaltsa et al., 2013; Shanmugam et al., 2012). Ultrasonication has a wide range of applications in food 0260-8774/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2013.11.002 ⇑ Corresponding author. Tel.: +353 25 42355. E-mail address: mark.fenelon@teagasc.ie (M.A. Fenelon). Journal of Food Engineering 126 (2014) 142–148 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng