Antioxidant properties of tofu whey concentrate by freeze concentration and nanofiltration processes Silvia Benedetti a , Elane Schwinden Prudêncio b,⇑ , Graciele Lorenzoni Nunes b , Karina Guizoni a , Lara Alexandre Fogaça a , José Carlos Cunha Petrus b a Departament of Chemistry and Food Engineering, Technology Center, Federal University of Santa Catarina, Trindade, 88040-970 Florianópolis, SC, Brazil b Departament of Food Science and Technology, Agricultural Sciences Center, Federal University of Santa Catarina, Rod. Ademar Gonzaga, 1346, Itacorubi, 88034-001 Florianópolis, SC, Brazil article info Article history: Received 18 July 2014 Received in revised form 26 November 2014 Accepted 12 March 2015 Available online 27 March 2015 Keywords: Isoflavone Tofu whey Freeze concentration Nanofiltration Antioxidant activity abstract The objective of this study was to add value to tofu whey by enhancing its potential as a source of isoflavones which are associated with the antioxidant activity of soybean. The concentration processes used were the block freeze concentration and the nanofiltration. The concentrates obtained from these processes were evaluated for their isoflavone content and antioxidant activity. The concentrated fluid obtained from freeze concentration showed an increase in the isoflavone contents for all stages. It was observed retention of isoflavones in the ice, however the concentration efficiency remained above 80% in all stages. The nanofiltration process promoted the concentration of b-glucosides and malonyl gluco- sides conjugates, but it was not possible to concentrate the aglycones. The values for the antioxidant activity of the concentrated fluid obtained from each freeze concentration and nanofiltration stage, mea- sured applying FRAP and ABTS assays, were significantly higher than that of the tofu whey. Moreover, the antioxidant activity was significantly correlated with the isoflavone content of the tofu whey samples. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Soybean is one of the most important legumes consumed and has long been used as a protein source in Asian countries. It con- tains several nutrients and functional compounds including phytic acids, saponins, oligosaccharides and isoflavones (Kwak et al., 2007), besides being an excellent source of vegetable oil in Latin America countries (Hirakuri and Lazzarotto, 2011). It can be com- mercialized in their natural state or as processed foods, such as the soymilk and tofu (Kwak et al., 2007; Kim et al., 2008). Tofu is the main processed soybean product in the world, pro- duced through the coagulation of soymilk and during the process- ing a liquid by-product called tofu whey (TW) or sunmul is generate. Large amount of tofu whey is produced every year, which is generally considered as by-product and used as animal feeds, fertilizer or simply discarded (Matemu et al., 2009). Due to its high organic matter content can result in serious environmental problems. However, as in the case of the soybeans, this residue contains notable concentrations of low molar mass substances, such as isoflavones (Kim et al., 2005). It is well known that the main source of the biologic activity of soybean food products is isoflavone, since they exhibit estrogenic, antioxidant, antiosteoporotic and anticarcinogenic activity (Cornwell et al., 2004). Many studies have demonstrated that soy- bean food products, including non-fermented products, present antioxidant activity (Kwak et al., 2007; Kim et al., 2008). Although many studies have been demonstrated that soybean has antioxidant capacity and this stimulated the development of health products with functional properties (Devi et al., 2009), very little is known about antioxidant activity of tofu whey. It is important to consider that to attain advances in the food industry effective, low-cost and environmentally–friendly tech- nologies that preserve the functional properties need to be found (Aider et al., 2009). Reports in the literature have shown that it is possible and advantageous to concentrate the phenolic compounds of vegetal extracts through freeze concentration (Boaventura et al., 2013; Belén et al., 2013) and nanofiltration (Conidi et al., 2011; Cassano et al., 2013), both processes being able to preserve the functionality. The freeze concentration process promotes the concentration of liquid food products by means of freezing with the subsequent separation of part of the frozen water from the liquid product. This technology has been considered promising for the http://dx.doi.org/10.1016/j.jfoodeng.2015.03.021 0260-8774/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +55 48 3721 5366; fax: +55 48 37219943. E-mail address: elane.prudencio@ufsc.br (E.S. Prudêncio). Journal of Food Engineering 160 (2015) 49–55 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng