Contents lists available at ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres Effect of Ohmic heating on functionality of sodium caseinate – A relationship with protein gelation Thais Caldas Paiva Moreira a , Ricardo N. Pereira b, ⁎ , António A. Vicente b , Rosiane Lopes da Cunha a a Department of Food Engineering (DEA), Faculty of Food Engineering (FEA), University of Campinas (UNICAMP), Rua Monteiro Lobato, 80; Campinas-SP, CEP: 13083- 862, Brazil b Centre of Biological Engineering (CEB), University of Minho (UMINHO), Campus Gualtar, 4710 Braga, Portugal ARTICLE INFO Keywords: Ohmic heating Protein functionality Sodium caseinate Acid systems Moderate electric fields Water holding capacity ABSTRACT Sodium caseinate (NaCAS) is widely used in the food industry to provide nutritional and functional benefits. This work deals with the effects of applying moderate electric fields (MEF) of different intensity - ranging from 2 V·cm −1 to 17 V·cm −1 - on the physical and functional properties of NaCAS solutions during Ohmic heating (OH) at 95 °C. Self-standing gels were produced regardless the heating technique applied (i.e. conventional or OH), and these gels were much more prone to physical rupture when compared with the ones produced from unheated NaCAS. Interestingly, OH treatment formed gels with lower values of strain at rupture and water holding capacity than unheated samples; this pattern was not observed for gels obtained through the conven- tional heating treatment (at 0 V·cm −1 ). These effects may be linked with disturbances of the distribution of random coil structures and enhanced solubility of NaCAS at its isoelectric point, reducing aggregation and impairing the development of a more compact protein network. Results show that OH presents potential to be used as volumetric heating tool for NaCAS solubilization and for the production of distinctive acidified systems. 1. Introduction Caseins are an important source of protein in functional foods and are particularly important in preventing osteoporosis and reducing hypertension (Huppertz & Patel, 2012; Snyder et al., 2007). Caseins in milk consist in α s1 -, α s2 -, β- and κ-caseins, and differ by phosphoseryl groups (amount and distributions) and precipitation sensitivity in pre- sence of ionic calcium (Lucey, Johnson, & Horne, 2003). They are present in micellar form and are stabilized by coating κ-caseins; moreover, caseins are partially unfolded in solution, forming structures easily identified through experimental techniques. Generally, caseins are hydrophobic and negatively charged, thus providing a steric im- pediment that allows colloidal stability. Such stability depends on physiological conditions, hydrophobic bonds, cross-linked peptides or even ionic bonds (Gunasekaran & Solar, 2012; Normal, 2000; Walstra, Wouters, Geurts, Wouters, & Geurts, 2005). Milk caseins can be obtained through precipitation by κ-casein cleavage, followed by precipitation at pH 4.6 and addition of calcium in excess (casein precipitation), with a final addition of ethanol or a heating step at high temperatures (Kinsella & Morr, 1984). Further- more, it is important to highlight that caseins are quite stable to high temperatures since they do not coagulate at 100 °C for 24 h, and are resistant for 20 min under 140 °C, at natural milk pH (around 6.7) (Fox & Mcsweeney, 1998). However, isoelectric casein shows low solubility in water, but it can be converted into caseinate through dispersion of this protein in water and pH adjustment (pH ≈ 6.7) with alkali addition. Usually NaOH is used forming sodium caseinate (Thompson et al., 2008), which is stable to heat (140 °C for 15 min, at pH 7) and it is an effective emulsifier, thickener, and foaming agent. Sodium caseinate is usually applied in baked food, breakfast cereals, meat products, coffee whitener, whipped toppings, instant breakfast, desserts, puff snacks and cheese analogs (Kinsella & Morr, 1984). It is possible to produce casein/caseinate gels from the destabili- zation of caseins through enzymatic or acidification processes or using a combination of both. Although there are some differences between these processes, the inhibition by low pH is the main restriction in the subsequent employment of bacterial cultures (e.g. for cheese produc- tion) (Kuhn, Picone, & Cunha, 2009). During caseins acidification, the phosphoseryl residues and carboxyl groups change their ionized state due to their proton affinity, once at neutral pH caseins have negative charges and are neutral near the pI, which leads to particle aggregation and to the establishment of a gel structure (Broyard & Gaucheron, 2015). These gels are affected by https://doi.org/10.1016/j.foodres.2018.08.087 Received 25 April 2018; Received in revised form 27 August 2018; Accepted 27 August 2018 ⁎ Corresponding author. E-mail addresses: rpereira@deb.uminho.pt (R.N. Pereira), avicente@uminho.pt (A.A. Vicente), rosiane@unicamp.br (R.L. da Cunha). Food Research International 116 (2019) 628–636 Available online 28 August 2018 0963-9969/ © 2018 Published by Elsevier Ltd. T