Effect of high pressure processing on rheological and structural properties of milk–gelatin mixtures Anastasia Fitria Devi a,b , Li Hui Liu b , Yacine Hemar c,⇑ , Roman Buckow b , Stefan Kasapis a a School of Applied Sciences, RMIT University, City Campus, Melbourne, Victoria 3001, Australia b CSIRO Animal, Food and Health Sciences, Werribee, Victoria 3030, Australia c School of Chemical Sciences, The University of Auckland, Auckland Central, Auckland 1142, New Zealand article info Article history: Received 4 January 2013 Received in revised form 20 March 2013 Accepted 21 March 2013 Available online 3 April 2013 Keywords: Skim milk Gelatin High pressure processing Viscosity Phase separation abstract There is an increasing demand to tailor the functional properties of mixed biopolymer systems that find application in dairy food products. The effect of static high pressure processing (HPP), up to 600 MPa for 15 min at room temperature, on milk–gelatin mixtures with different solid concentrations (5%, 10%, 15% and 20% w/w milk solid and 0.6% w/w gelatin) was investigated. The viscosity remarkably increased in mixtures prepared with high milk solid concentration (15% and 20% w/w) following HPP at 300 MPa, whereas HPP at 600 MPa caused a decline in viscosity. This was due to ruptured aggregates and phase separation as confirmed by confocal laser scanning microscopy. Molecular bonding of the milk–gelatin mixtures due to HPP was shown by Fourier-transform infrared spectra, particularly within the regions of 1610–1690 and 1480–1575 cm 1 , which reflect the vibrational bands of amide I and amide II, respectively. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction One of the first applications of static high pressure processing (HPP) on a food product was reported by Hite (1899) on milk for extending its shelf-life. Since then HPP has been utilised not only for extending shelf life of perishable food, while ensuring its safety and nutrients, but also for modifying food macromolecule func- tionality (Knorr, Heinz, & Buckow, 2006). The extent of macromol- ecule rearrangements affected by high pressure is dependent on its intrinsic properties, physicochemical environment, applied pressure and temperature, holding time and decompression rate (Michel & Autio, 2002). The structure–function properties of foods are mainly governed by two groups of macromolecules: proteins and polysaccharides, consequently appointing them as the essential constructional materials and suitable components to model foods, which are com- plex systems (Morris, 2007). When two biopolymers are mixed, commonly the destabilisation force is bigger than the stabilisation force. As a result, unstable solutions are often encountered, which lead to either segregative or associative phase separation. The type of phase separation taking place in a mixed gel is determined by the procedure of gel formation that guides final application (Walkenstrom & Hermansson, 1997a). Less likely, synergetic inter- actions between biopolymers may occur, being recognised by a substantial increase in yield stress and elastic modulus in mixtures (Schmitt, Sanchez, Desobry-Banon, & Hardy, 1998). Gel fabrication by HPP may create textures with unique proper- ties that cannot be achieved by its thermal counterpart. The latter favours gel formation through disulfide bonding, whereas pressure treatment leads to structure development mainly via hydrogen bonds, although disulfide bonding can occur if the HPP is per- formed at sufficiently high pressure (e.g. 500 MPa) or at relatively high temperatures (Totosaus, Montejano, Salazar, & Guerrero, 2002). Weak bonds, such as hydrogen, electrostatic, van der Waals and hydrophobic forces can be destabilised by HPP, due to their limited energy levels. However, covalent associations of biopoly- mers are usually not affected under high pressure (Boonyaratanak- ornkit, Park, & Clark, 2002). Gelatin, a proteinaceous material derived from collagen, is an important macromolecule able to impart a variety of textures (Poppe, 1992). Interaction between gelatin and other food macro- molecules is an attractive research field, since the resultant knowl- edge is used to model novel food product development (Djagny, Wang, & Xu, 2001). In addition, there is an increasing demand to explore the functional properties of mixed biopolymer systems through HPP in an effort to add value to formulations. Clearly, mix- tures of gelatin and milk proteins, which find application in the food industry, can be of interest in this regard. However, publica- tions on the phase behaviour and structural properties of these binary mixtures involving high pressure processing are scarce (Hemar, Liu, Meunier, & Woonton, 2010; Walkenstrom & Hermansson, 1997a, 1997b). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.03.074 ⇑ Corresponding author. Tel.: +64 9 373 7599; fax: +64 9 373 7422. E-mail address: y.hemar@auckland.ac.nz (Y. Hemar). Food Chemistry 141 (2013) 1328–1334 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem