Contents lists available at ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres Development of iron-rich whey protein hydrogels following application of ohmic heating – Effects of moderate electric fields Ricardo N. Pereira a,⁎ , Rui M. Rodrigues a , Emir Altinok b , Óscar L. Ramos a,c , F. Xavier Malcata c,d , Paola Maresca e , Giovanna Ferrari b,e , José A. Teixeira a , António A. Vicente a a CEB - Centre of Biological Engineering, University of Minho, 4710-057 Braga, Portugal b Department of Industrial Engineering, University of Salerno, via Giovanni Paolo II, 132, Fisciano, SA, Italy c LEPABE – Laboratory for Process Engineering, Environment, Biotechnology and Energy, University of Porto, Porto, Portugal d Department of Chemical Engineering, University of Porto, Porto, Portugal e ProdAl Scarl - University of Salerno, via Ponte don Melillo, 84084 Fisciano, (SA), Italy ARTICLE INFO Keywords: Whey protein isolate Electric fields Iron Cold gelation Transmission electron microscopy ABSTRACT The influence that ohmic heating technology and its associated moderate electric fields (MEF) have upon pro- duction of whey protein isolate cold-set gels mediated by iron addition was investigated. Results have shown that combining heating treatments (90 °C, 5 min) with different MEF intensities let hydrogels with distinctive micro and macro properties – i.e. particle size distribution, physical stability, rheological behavior and micro- structure. Resulting hydrogels were characterized (at nano-scale) by an intensity-weighted mean particle dia- meter of 145 nm, a volume mean of 240 nm. Optimal conditions for production of stable whey protein gels were attained when ohmic heating treatment at a MEF of 3 V ∙ cm −1 was combined with a cold gelation step using 33 mmol ∙ L −1 of Fe 2+ . The consistency index of hydrogels correlated negatively to MEF intensity, but a shear thickening behavior was observed when MEF intensity was increased up to 10 V ∙ cm −1 . According to trans- mission electron microscopy, ohmic heating gave rise to a more homogenous and compact fine-stranded whey protein-iron microstructure. Ohmic heating appears to be a promising technique, suitable to tailor properties of whey protein gels and with potential for development of innovative functional foods. 1. Introduction Proteins are important food ingredients due to a unique combina- tion of biological nutritional and functional properties. They provide essential amino acids, and exhibit a range of dynamic and functional properties, with a particularly high versatility for processing. They can be used to form networks, structures and to interact with other com- ponents, thus improving quality attributes in foods (Yada, Jackman, & Smith, 1994). Whey proteins, such as β-lactoglobulin (β-lg) that constitutes more than 50% of the total protein in whey, followed by α-lactalbumin (α lac) and bovine serum albumin (BSA) that re- present 20% and 5%, respectively, are good examples of functional proteins (Nicorescu et al., 2008). Their ability to interact and form aggregates determines the functional and technological properties of products derived therefrom, such as whey protein isolate (WPI) and whey protein concentrate (WPC). Due to their particular structures and physicochemical properties, β-lg and whey protein aggregates have been extensively employed in applications, namely for gel formation, emulsions and foams stabilization, films and coatings, and encapsulation of labile and/or active compounds (Nicolai, Britten, & Schmitt, 2011). The functionality of a protein is closely related to its structure and is usually triggered by an initial step of unfolding that causes disruption of its secondary and tertiary structures, thus altering the surface exposure of amino acids. This phenomenon eventually produces an increased interaction potential (Foegeding & Davis, 2011), and lead to formation of small oligomers that may further interact to form aggregates, and ultimately a continuous network called gel (Totosaus, Montejano, Salazar, & Guerrero, 2002). Extent and rate of aggregation are strongly dependent of environmental conditions such as protein concentration, temperature, pH, ionic strength and ion type (Bryant & McClements, 1998). Cold gelation is a two-step process: unfolding of the protein is first promoted by heating the solution above the denaturation temperature; while the second step is carried out at room temperature, and consists either of changing pH or adding of salts. In salt-induced gelation of globular whey proteins or whey protein isolate, salts such as NaCl and CaCl 2 are commonly used. Slow addition of these salts to a heat http://dx.doi.org/10.1016/j.foodres.2017.05.023 Received 26 January 2017; Received in revised form 2 May 2017; Accepted 25 May 2017 ⁎ Corresponding author. E-mail address: rpereira@deb.uminho.pt (R.N. Pereira). Food Research International 99 (2017) 435–443 Available online 29 May 2017 0963-9969/ © 2017 Elsevier Ltd. All rights reserved. MARK