Antioxidant properties of polyphenols incorporated in casein/sodium caseinate films

Abstract

Radical-scavenging activity (RSA) of sodium caseinate (NaCAS) films with 0–30% 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′-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.

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 coagulation 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 and Foo, 2000, Meyer et al., 1998, Sanchez-Moreno et al., 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 et al., 2004, Lai et al., 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 et al., 2010) abundantly present in several beverages, including red wine, beer, coffee, black tea and green tea, and many foods such as grapes, pear, banana, sorghum, black-eyed peas, lentils and chocolates (Chung et al., 1998, King and Young, 1999). Flavonoids are polyphenols that are present in vegetables and fruits. Catechins (CAT) compose an interesting class of flavonoids ubiquitously found in fruits such as plum and apple, as well as in tea and red wine (De Pascual-Teresa, Santos-Buelga, & Rivas-Gonzalo, 2000). These are recognized as potent anticancer, anti-allergy and antioxidant agents (Kondo et al., 2000). CAT can comprise up to 30% of the dry weight of freshly picked tea leaf; the highest concentration of CAT is found in white and green teas, while black tea has substantially fewer amounts due to its oxidative preparation. Catechins are regarded as the most powerful antioxidants among plant phenols; it is reported that epigallocatechin gallate (EGCG), a catechin compound extracted from Korean green tea, is more active than vitamin C or vitamin E (Vinson, Dabbagh, Serry, & Jang, 1995). Radical-scavenging activity of tannic acid and catechin was attributed to the hydroxylation degree in their structure (Maqsood & Benjakul, 2010).

The instability of antioxidant molecules reduces their nutritional value. Entrapment of antioxidants in macromolecules is a good way to reduce oxidation due to limited oxygen access to molecular reactive sites. In the present study, caseins and caseinates were chosen to protect polyphenols against rapid oxidation. Recently, the effects of milk on antioxidant capacity of tea flavonoids was investigated (Dubeau, Samson, & Tajmir-Riahi, 2010), and described the complexes obtained when β-lactoglobulin (Kanakis, Hasni, Bourassa, Tarantilis, & Polissiou, 2011) or caseins (Hasni et al., 2011) were mixed with catechin and other polyphenols. This work was undertaken to investigate the effect of protecting TA, as an example of phenols, and CAT, as an example of flavonoids, against oxidation during storage at different relative humidities by entrapping these molecules in macromolecular films of different casein (CAS)/NaCAS ratios.