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Triggering Mechanisms for Ascorbic Acid-Incorporated Whey Protein Films

By
Theeranun Janjarasskul and John M. Krochta
University of California, Davis, CA

2008 Poster Presentation (PDF)

Incorporation of ascorbic acid (AA) in whey protein isolate (WPI) films has been shown to increase the film’s ability to protect foods from oxidation. However, triggering mechanisms for the O2-scavenging function of AA-WPI films have not been investigated. Our objective was to investigate triggering mechanisms for activating the O2-scavenging function of AA-WPI-films, including change of water activity (aw), pH and/or metal catalyst presence. Film-forming solutions were prepared with 5%(w/w) WPI, 5%(w/w) glycerol, and 0.2M AA. The pH of film-forming solutions was adjusted to 2.0 to inhibit initial AA O2-scavenging ability. Food model systems were prepared using agar-based gels with aw = 0.30, 0.50, 0.70 or 0.95 and pH 3.00, 4.63 or 7.40, with or without presence of Cu2+. The kinetics of oxygen scavenging of AA in WPI films were measured by placing RH-conditioned AA-WPI test films in contact with corresponding aw model food systems for varying times in controlled RH chambers. Test films were analyzed for residual AA content by HPLC. Rate of oxygen scavenging of AA in WPI films was observed to follow first-order kinetics. The film aw was found to be a rate-limiting factor controlling diffusion of triggering agents from the food models, hence the oxygen-scavenging rate of AA. At high aw levels, oxygen scavenging occurred at a desirably rapid rate. Food models with pH 7.4 produced the fastest oxygen-scavenging rate. The slowest rate at pH 3.0 was due to the lower concentration of O2-sensitive ascorbate monoanion. The presence of Cu2+, at high aw level, accelerated AA oxidation. The desirable activation of oxygen scavenging in AA-WPI film at high aw and neutral pH in presence of metal catalyst suggests that such AA-WPI films can be used to protect foods that have the highest vulnerability to oxidation.