Antioxidant properties of polyphenols incorporated in casein/sodium caseinate films

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.