Antioxidant properties of polyphenols incorporated in casein/sodium caseinate films Ahmed Helal a, b, * , Davide Tagliazucchi a , Angela Conte a , Stéphane Desobry b a Department of Agricultural and Food Sciences, University of Modena and Reggio Emilia, Via Pietro Mascagni 8/1, 42100 Reggio Emilia, Italy b LIBio, Universite de Lorraine, ENSAIA, 2 avenue de la foret de Haye, 54500 Vandoeuvre, France article info Article history: Received 13 September 2011 Received in revised form 21 November 2011 Accepted 1 December 2011 abstract Radical-scavenging activity (RSA) of sodium caseinate (NaCAS) films with 0e30% added casein and antioxidants was measured. Tannic acid and catechin were added to the films as model antioxidants. RSA was measured using 2,2-diphenyl-1-picrylhydrazyl and 2,2 0 -azino-bis(3-ethylbenzothiazoline-6-sulfonic acid); the two methods gave similar results for RSA measurement. Film casein content most influenced initial RSA, while increasing casein level resulted in an apparent decrease of RSA due to quenching. During storage, a good stability of RSA was observed. The surface antioxidant activity is of primary interest for food contact materials; a decrease in film surface RSA occurred during the first 20 days of storage, followed by an increase in surface RSA during the remainder of the 90-day storage period, especially at high relative humidity. This phenomenon was due to plasticizing of NaCAS, with possible network alteration over long storage times. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Recently, production of edible films has gained considerable interest for many reasons, such as consumer expectations of health, food quality and convenience. It has also become essential for packaging companies to have more proactive attitudes to reduce the environmental impact of packaging wastes (Siew, Heilmann, Easteal, & Cooney, 1999) and to develop environmentally friendly materials having high potential use in the food industry (Morillon, Debeaufort, Capelle, Blond, & Voilley, 2000). In this regard, milk proteins are considered as a suitable material for production of edible films. In addition to excellent nutritional value, milk proteins are known to possess numerous functional properties that make them excellent materials for edible film-forming agents (Mezgheni, D’Aprano, & Lacroix, 1998). Sodium caseinate (NaCAS) is a water-soluble polymer obtained by the acid precipitation of casein (Audic & Chaufer, 2005). Caseinate films that exhibit resistance to thermal denaturation and/or coagu- lation impart stability to edible protein films over a wide range of pH, temperatures, and salt concentrations. Caseinate films are suitable for use as coating material for some food products, such as cheese, vegetables and fruits, because of their transparent, flexible, and bland nature. Furthermore, they are used as microencapsulating agents of flavours and medicines (Khwaldia, Banon, Perez, & Desobry, 2004). Due to the structure and amino acid sequence of casein, it appears that the mechanism of film formation involves hydrogen bonding, electrostatic interactions and hydrophobic forces (McHugh & Krochta, 1994). Polyphenols and flavonoids in food items are liable to be degraded upon coming in contact with air and thereby lose their biochemical and nutritional properties. To protect these compounds in food, it is necessary to incorporate them into films that can protect them from degradation. In recent years, interest has increased in antioxidants that can be eaten in the regular diet. Polyphenolic compounds are known for their strong antioxidant effects (Lu & Foo, 2000; Meyer, Heinonen, & Frankel,1998; Sanchez- Moreno, Jimenez-Escrig, & Saura-Calixto, 2000). Owing to this property, they are beneficial as they reduce the risk of diseases associated with oxidative stress and also protect the human body from the harmful effects of free radicals and reactive oxygen species (Malik et al., 2011). Antioxidants also slow down the progress of many chronic diseases and lipid peroxidation (Gülçin, Mshvildadze, Gepdiremen, & Elias, 2004; Lai, Chou, & Chao, 2001; Pryor, 1991). Furthermore, polyphenols have been widely used as food additives to protect food nutrients against oxidative degradation (Gülçin, Berashvilli, & Gepdiremen, 2005). Similarly, tannic acid (TA), which possess antioxidant activity, is an example of a polyphenol (Andrade et al., 2005; Gülçin, Huyut, Elmastas, & Aboul-Enein, 2010) abundantly present in several beverages, including red wine, beer, coffee, black tea and green tea, * Corresponding author. Tel.: þ39 0522 522060; fax: þ39 0522 522053. E-mail address: ahmed.helal@unimore.it (A. Helal). Contents lists available at SciVerse ScienceDirect International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj 0958-6946/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2011.12.002 International Dairy Journal 25 (2012) 10e15