Effect of proteins on calcium phosphate deposition in turbulent flow as a function of surface properties R. Rosmaninho * , L.F. Melo LEPAE, Departamento de Engenharia Quı ´mica, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal Received 8 January 2007; received in revised form 26 April 2007; accepted 26 April 2007 Abstract In order to reduce milk fouling during heat treatment, several approaches have been evaluated and one of them, the modification of the surface properties of the stainless steel used in heat exchangers, is analysed in this work. Fouling behaviour of stainless steel surfaces (2R) and modified stainless steel-based surfaces obtained by TiN sputtering was analysed. The combined effect of b-lactoglobulin and the surface energy properties of the materials on the deposition behaviour of calcium phosphate was evaluated. The experiments were per- formed in turbulent flow regime in a heat flow cell where the fouling thermal resistance was continuously monitored. The presence of protein was responsible for changing the overall fouling curve independently of the surface nature, as well as for the appearance of two distinct growth periods. The second growth phase occurred after different delay times depending on the roughness, surface compo- sition and surface energy values of the deposition material. The resistance to cleaning of the different deposits was also evaluated and it was proved to be dependent on the surface energy parameters, probably as a result of the formation of different deposit structures. The main conclusion is that when protein is present, the non-modified 2R surfaces present the lowest amount of deposit, but the higher energy surfaces are the ones to retain less amount of deposit after the cleaning process. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Fouling; Calcium phosphate; Whey protein; Surface energy; Modified stainless steel surfaces 1. Introduction The present work is focussed on the effect of whey pro- teins on the fouling behaviour of calcium phosphate under turbulent flow on different stainless steel modified surfaces. The importance of this study emerges from the fact that the deposition of activated proteins and calcium phosphate are the two major mechanisms of fouling during the heat treat- ment of milk, which results respectively from protein dena- turation and from the decrease of calcium phosphate solubilization upon heating. Calcium ions, however, not only contribute to the process through calcium salts depo- sition but they also influence the denaturation and aggrega- tion of the proteins prior to deposition, following a process not yet totally understood [1,2]. Considerable discussion has been happening on whether fouling is primarily deter- mined by reactions in the bulk or at the heated surface. In fact, Belmar-Beiny et al. [3] outlined two simple models based on wall and bulk reactions and showed that phenom- ena in the bulk are the ones to control the process. More recent work from Morison and Tie on milk simulating solutions [4] proved on the other hand that fouling is mainly caused by surface reactions of both calcium phos- phate and protein. In the initial stages of milk processing, individual whey protein molecules are adsorbed onto the stainless steel heating surface covering most of the surface with a protein monolayer, after which the deposition of aggregates formed in the bulk, both as calcium phosphate and as whey protein particles, occurs [5]. Consequently, it seems that proteins are the ones to start the fouling process, as defended by Morison and Tie [4] and by Belmar-Beiny 0894-1777/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2007.04.012 * Corresponding author. Tel.: +351 22 5081685; fax: +351 22 5081449. E-mail address: roxane@fe.up.pt (R. Rosmaninho). www.elsevier.com/locate/etfs Available online at www.sciencedirect.com Experimental Thermal and Fluid Science 32 (2007) 375–386